Dual chamber pacemaker with adaptive atrial escape interval
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to cardiac pacemakers and more particularly to a dual chamber pacemaker which varies the pacer V-A escape interval in response to the timing of the patient&#39;s atrial activity. 
     2. Description of the Prior Art 
     The first pacemakers were asynchronous. these VOO mode pacemakers are typified by U.S. Pat. No. 3,057,456 to Greatbatch. Such pacers operate independently of intrinsic heart activity and pace the ventricle at a metronomic rate. Although such pacemakers provide a ventricular pacing rate sufficient to support life, these pacemakers can compete with intrinsic ventricular rhythms which can prove hazardous to the patient. 
     The VVI mode or ventricular demand pacemaker, as typified by U.S. Pat. No. 3,478,746 to Greatbatch, was introduced to reduce competitive pacing with naturally occurring ventricular rhythms. This ventricular inhibited form of pacemaker provides a ventricular sense amplifier responsive to ventricular depolarizations. The ventricular sense event recycles the V-V timer of the pacemaker. At the conclusion of the V-V time interval, the pacemaker stimulates the ventricle unless inhibited from doing so by a ventricular sense event occuring within the V-V interval. 
     The VAT mode pacemaker as typified by U.S. Pat. No. 3,254,596 to Keller provides an atrial sense amplifier responsive to atrial depolarizations which starts an A-V timer within the pacemaker. At the conclusion of this A-V time interval, the pacemaker generates a ventricular pacing pulse which is delivered to the ventricle. This stimulus is synchronized with the naturally occurring atrial beat. Therefore, this atrial synchronized form of ventricular pacemaker provides a prosthetic conduction pathway mimicking the natural pathway of the heart by providing a paced ventricular rhythm in response to and synchronized with the detected atrial rate. A defect of this pacing modality is the ability to compete with ectopic ventricular activity. An ectopic ventricular beat will be sensed in the atrium and treated as though it were an atrial event. This will result in the generation of a ventricular pacing stimulus a fixed A-V delay after the atrially detected ventricular event. Although such competitive pacing is considered harmless when the A-V delay of the VAT pacemaker is short, such a pacing regime can result in delivery of a ventricular pacing stimulus into the vulnerable period of the R-waves which is undesirable. 
     The VDD mode pacemaker, as typified by U.S. Pat. No. 3,648,707 to Greatbatch, overcomes this drawback of the VAT mode pacemaker by providing a ventricular sense amplifier responsive to ventricular depolarizations for inhibiting an otherwise scheduled ventricular pacing event by the pacemaker. In operation, the VDD pacemaker may sense atrial depolarizations through an atrial sense amplifier which starts an A-V timer. The A-V timer schedules the generation of a ventricular pacing pulse one A-V interval after the detected atrial signal. The scheduled stimulation, however, may be interrupted or inhibited by the detection of a naturally occurring ventricular event detected by the ventricular sense amplifier. In the absence of either atrial or ventricular rhythms, the VDD pacemaker will pace the ventricle at a metronomic rate. However, in the presence of a sinus atrial rhythm, the VDD pacemaker will pace the ventricle in synchrony with the detected atrial rhythm. 
     The DVI mode A-V sequential pacemaker, as typified by U.S. Pat. No. 3,595,242 to Berkovits, provides for stimulation in both the atrium and ventricle while providing for ventricular sensing. In this form of pacemaker, a ventricular sense event initiates a V-A timer as well as a V-V timer. At the expiration of the V-A interval, the pacemaker generates an atrial pace event, and at the expiration of the V-V interval, the pacemaker generates a ventricular pace event. If a ventricular event occurs during the V-A or V-V time, the pacemaker is inhibited and recycled so that the V-A and V-V timer restart. This bifocal form of pacemaker preserves the hemodynamic advantages of sequential atrial and ventricular contractions. 
     The DDI mode pacer, as typified by U.S. Pat. No. 3,747,604 to Berkovits, includes a V-A and V-V timer and further includes an atrial sense amplifier for inhibiting the generation of an atrial pacing stimulus at the conclusion of the V-A interval if a naturally occurring atrial depolarization occurs within that time interval. This DDI mode of pacemaker, unlike the VAT or VDD pacemaker, does not resynchronize or restart the pacemaker&#39;s timer upon the sensing of arial events. Therefore, it does not provide the atrial tracking response exhibited by VAT and VDD devices. 
     Recently, dual sense-dual pace or DDD mode pacemakers, as typified by U.S. Pat. No. 4,312,355 to Funke, have been introduced to compensate for many of the disadvantages of the preceding forms of pacemakers and for providing more efficacious therapy in the event of an intermittent block during atrial pacing. The DDD pacemaker provides an atrial sense amplifier to generate an atrial sense signal that synchronizes the pacemaker by initiating an A-V timer. The A-V delay schedules the delivery of a ventricular pace event unless a ventricular sense event occurs within the A-V interval to inhibit or cancel the scheduled ventricular pace event. This form of pacemaker also provides a ventricular sense amplifier for initiating a V-A timer in response to a ventricular sense signal which schedules the generation of an atrial stimulus at the conclusion of the V-A interval unless an atrial sense event occurs to cancel the ventricular pace event within the A-V interval. 
     This prior art DDD form of pacemaker is represented by the state diagram of FIG. 2. Assuming that the machine is in the atrial observation state 24 (AOT), then an atrial sense event (AS) causes state transition 11 which causes the machine to enter the A-V delay state 12 where the A-V delay timer starts to time out an atrial-ventricular escape interval. If the A-V delay timer (AVD) times out (AVTO), then state transition 14 takes place moving the machine to the ventricular pacing state (VP)16. In this state, the machine delivers a pacing stimulus to the ventricle of the heart. At the conclusion of the pacing pulse, the machine moves via transition 18 to the refractory state 20 during which time the post ventricular-atrial refractory period (PVARP) is timed out. At the conclusion of the post ventricular-atrial refractory period (PVARP), the machine moves via transition 22 to the atrial observation state (AOT). In this state, the pacer is responsive to the atrial sense amplifier as previously described. If atrial activity is not sensed, then the machine will leave the atrial observation state as indicated by state transitiion 26 whereupon entering the atrial pacing state 28, the machine generates an atrial pacing stimulus. At the conclusion of the pacing pulse reflected by transitition 30, the machine re-enters the A-V delay state 12. If a ventricular sense event occurs during this state as indicated by state transition 32, the machine re-enters the refractory state 20. 
     This state machine description depicts the interaction of the pacer with the heart in response to the various cardiac events which may occur during the various timing cycles of the pacer. 
     Each of the previously discussed pacing modalities incorporates escape intervals defined as the time period extending from a sense event to the scheduled generation of a succeeding paced event, which are fixed at a discrete value during the operation of the pacer. Modern versions of each of these prior art pacemakers provide for the remote programming of such escape interval under the direction of a physician. However, once programmed, the intervals are not altered by pacemaker events. 
     Pacemakers have been proposed and built which alter the escape interval of the pacemaker in response to pacemaker detected cardiac events. An example of one such pacemaker is U.S. Pat. No. 4,052,991 to Zacouto as well as U.S. Pat. No. Re. 28,341 to Gobeli. The Zacouto machine is a ventricular pacemaker with ventricular sensing which alters the V-V escape interval of the pacemaker depending upon the timing of naturally occurring ventricular events. This pacemaker measures the time interval from a ventricular pace event to a subsequent ventricular sense event and sets the escape interval of the pacemaker to that value. In operation, a Zacouto orthorhythmic pacemaker has a nomimal V-V escape interval which results in the metronomic pacing of the ventricle at that rate in the absence of detected atrial ventricular activity. When a ventricular sense event occurs, the orthorhythmic pacemaker may provide an escape interval longer than the nominal escape interval which Zacouto refers to as negative hysteresis. Alternatively the pacer may provide for a shorter escape interval than the nominal value, which Zacouto refers to as positive hysteresis. Consequently, the orthorhythmic pacemaker represents a pacemaker in which the escape interval of the pacemaker is altered on a beat-to-beat basis in response to the detection of ventricular cardiac activity. 
     The Gobeli pacemaker is a VVI device that exhibits positive hysteresis under the nomenclature of Zacouto since the escape interval of the pacemaker is lengthened in response to a detected ventricular sense event. 
     More recently, pacers have been proposed which alter the stimulation escape interval based on the evoked QT time interval. See, for example, U.S. Pat. No. 4,228,803 to Rickards and No. 4,305,396 to Wittkampf et al. 
     BRIEF SUMMARY OF THE INVENTION 
     The pacemaker of the present invention shares many structures and functions with the previously described pacemakers. However, unlike prior art DDD pacers, this pacer&#39;s V-A interval (VAI) may vary in response to detected atrial activity. Additionally this pacer alters the pacing behavior or stimulation regime of the pacer depending on the temporal relationships between an atrial sense event and the preceding ventricular event. The objective of this pacemaker is to maintain physiologic cardiac contraction patterns over a wide frequency range and to achieve this goal in the presence of a wide variety of conduction faults and arrhythmias. 
     The functional characteristics of this pacemaker which permit the realization of these goals include the computation of an atrial stimulation back up rate (ASBU) which adapts the intervention frequency of the pacemaker to the observed spontaneous atrial rate; sense factor (SF) which governs the pacer&#39;s atrial synchronizing behavior; and intelligent P wave treatment at upper rate (IPTUR) which governs the pacer&#39;s action in the presence of competitive rhythms. 
     The feature denominated SF for sense factor is very desirable in instances where the patient exhibits intermittent atrial function due to disease processes such as sick sinus syndrome or atrial flutter. In the past, such patients would not be candidates for atrial synchronized pacers such as the DDD, but would be best served by a DVI pacer. This is an unfortunate result since many patients may be denied the benefit of atrial synchronized rate responsiveness because of occasional or transient atrial dysfunction. Sense factor addresses this problem by providing the physician with a parameter which can adapt the synchronizing response of the pacer to the disease condition. In operation, the atrial observation time (AOT) of the pacer is bifurcated into two intervals denominated T1 and T2. The ratio of these two times is the physician selected sense factor (T2/T1=SF). Atrial sense events which fall within T1 are not used to resynchronize the pacer yielding a DVI-like behavior, while atrial sense events falling within T2 resynchronize the pacer yielding a DDD-like behavior. Consequently SF permits the pacer to exhibit a smooth transition between DVI and DDD behavior dependent on the temporal relationship between the atrial sense event and its preceding ventricular event. This permits the benefit of atrially based physiologic stimulation to be brought to more patients. Conceptually T1 may be regarded as a post-ventricular atrial refractory period which is adjusted automatically based on the pacer&#39;s rate. However, unlike traditional refractory periods, the atrial sense amplifier is &#34;on&#34; and is sensing during this time period. Note that this T1 interval is long at low rates and short at higher rates. The physician can set a large T1 period via SF programming to prohibit or prevent a pacemaker mediated tachycardia without comprising the ability of the pacer to track the faster atrial rates. 
     The alert reader will recognize that the benefit conferred by SF, which is the ability of the patient&#39;s atria to dictate whether the pacer will follow the DDD or DVI regime, could extract a penalty in terms of rate variability. If one recalls that DVI pacing occur at a fixed lower rate and that DDD pacing occurs at the instantaneous atrial rate, then the ability to move from DDD to DVI and back again could result in large jumps in ventricular stimulation rates. ASBU, the atrial stimulation back-up rate, addresses this problem. This feature is described in excruciating detail later. For now it should be considered as a &#34;flywheel&#34; or the pacer&#39;s escape interval. The computation of the pacer&#39;s atrial escape interval contains &#34;inertial&#34; terms which ensure that the rate accelerates in a physiological sensible manner. 
     We now move to the behavior of the pacer at high rates. Once again the matter is described later in unrelenting detail, however, the IPTUR feature should be examined briefly to understand its relation to the schema. If a sequence of atrial sense events have fallen in T2, then the pacer&#39;s ASBU has been shortened to approximate the naturally observed Q-P or V-A time. When the pacer&#39;s rate and the heart&#39;s rate are similar, one runs the risk of competitive pacing. Competitive pacing occurs when a natural cardiac depolarization and a pacing stimulus occur together close in time. Let us assume that ASBU has scheduled an atrial stimulus 150 ms hence, and at present we sense a naturally occurring atrial event which has occurred in T1. IPTUR dictates that we cancel the scheduled atrial pace event even though it has landed in T1 and would at lower rates be ignored (DVI-like behavior). IPTUR, in essence, sets a window of inhibition which overrides the SF dictated response at higher rates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a timing diagram showing the relationships between intervals used to describe the invention. 
     FIG. 2 is a state diagram of a prior art DDD pacer. 
     FIGS. 3, 4 and 5 are hypothetical schematic timing diagrams depicting the pacer timing intervals and the simultaneous electrocardiographic recordings which show the interaction between the pacer and the heart. 
     FIG. 6 is a state diagram depicting the pacer of the present invention. 
     FIG. 7 is a functional block diagram of the pacemaker of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     Throughout the description, reference will be made to terms defined as follows: 
     VE is a ventricular event, either sense or pace; 
     VP is a ventricular pace event generated by the ventricular pulse generator. 
     VS is a venytricular sense event generated by the ventricular sense amplifier. 
     AP is an atrial pace event generated by the atrial pulse generator. 
     AS is an atrial sense event generated by the atrial sense amplifier. 
     [VE→AP] n  is the time interval from a ventricular event to the next scheduled AP event for the nth pacing cycle. This corresponds to the V-A escape interval of the pacer. 
     ASBU is the atrial stimulation back-up rate which corresponds to the V-A escape interval of the pacer. It comprises the arithmetic summation of two time intervals and is computed by the pacer in real time. [PVARP+AOT]. 
     PVARP is the post ventricular atrial refractory period. The value is programmable. The total atrial refractory period is equal to the arithmetic sum of PVARP and the A-V delay interval of the pacer. 
     AOT is the atrial observation time. This period extends from the end of the PVARP to the end of the ASBU. [ASBU-PVARP]=AOT. 
     T1 is the first portion of the AOT. 
     T2 is the second portion of the AOT. 
     SF is the sense factor of the pacer and is the ratio of T2/T1. The value of SF is programmable and should vary from 10.0&gt;SF&gt;0.1. 
     K1 is a programmable constant having a nominal value of 12 ms which is added to the nth value of [VE→AP] to generate the n+1 value of [VE→AP] or ASBU when an AS event occurs in T2. 
     K2 is a programmable constant having a nominal value of 50 ms which is added to the nth value of [VE→AP] to generate the n+1 value of [VE→AP] or ASBU when no AS events occur in the AOT. 
     K3 is a programmable constant having a nominal value of 150 ms. This time interval overlaps the last protion of T2 and ends with the AP event. Any AS event within the K3 portion of T2 is used to compute the next ASBU and is used to inhibit the impeding AP event. This provides protection from competitive pacing in the atrium. 
     As shown in connection with FIG. 1, the &#34;V-A&#34; or atrial escape interval of the pacemaker may be varied on a beat-to-beat basis. The atrial escape or VA interval is defined as the time from a ventricular event (VE) labeled 1 to the next atrial pace event (AP) olabeled 2 and may be expressed [VE→AP)] n . This time interval [VE→AP] may be shortened for a subsequent pacing cycle [VE→AP] n+1  if an atrial event occurs within this time interval during a current pacing cycle. Similarly, the subsequent VA, atrial escape interval [VE→AP] n+1  of the pacemaker may be extended if no atrial activity is detected within the current interval. The atrial escape interval [VE→AP] n  is referred to as the atrial stimulation back-up interval. It may be recomputed on each beat, and it is abbreviated ASBU and labeled 3 in the figure. ASBU will vary between a preset maximum value and a preset minimum value. ASBU reflects the interval from a ventricular event to the next scheduled atrial pace event for any given pacing cycle. 
     In a DDD mode pacemaker, the atrial refractory period is extended beyond ventricular events. Consequently, the pacer exhibits a post ventricular atrial refractory period (PVARP) labeled 4 in the figure, which renders the pacer refractory or insensitive to atrial depolarizations occurring within this fixed time after the ventricular event, depicted by atrial sense event 8 in the figure. In most DDD pacers, this PVARP is a programmed parameter. In a conventional DDD pacer, the conclusion of the PVARP starts a time window during which the pacer can sense atrial events. For our purposes, we define the time period starting with the end of the atrial refractory period to the time out of the ASBU interval as the atrial observation time, abbreviated AOT and labeled 5 in the figure. 
     In the present invention, the AOT is bifurcated into a first sensing period T1 labeled 6 and a second sensing period T2 labeled 7 where AOT=T1+T2. The ratio of time intervals T2/T1 is named SF for sense factor. The sense factor may range from 0.1 10.0 and is a physician programmable parameter, 10.0&gt;SF&gt;0.1. 
     The maximum value of ASBU is defined by physician selection of the lower pacing rate for the pacemaker while the minimum value for ASBU is comparable to the duration of the post ventricular atrial refractory period which is defined by the physician when he selects the atrial refractory period. 
     In operation, ASBU is shortened when an atrial sense event occurs in T2 depicted by atrial sense event 9 in FIG. 1. In this instance, the pacer shortens the atrial escape interval so that it just exceeds the observed atrial rate by a small amount. In this instance, the value of ASBU is set equal to the time period from the ventricular event ot the atrial sense event with the addition of a small increment of positive hysteresis K1, ASBU n+1  =[VE→AS] n+  K1, where the value of K1 is on the order of 12 ms and is programmable. 
     Consequently, an atrial sense event in the T2 portion of the atrial observation time (AOT) can accelerate the pacer through the ASBU calculation. Additionally this atrial sense event during T2 resynchronizes the pacer by starting the A-V delay timer and also inhibits the otherwise scheduled atrial pace event. 
     Clearly atrial sense events can also occur during the T1 portion of the atrial observation time as depicted in the figure by atrial sense event 10. The pacer does not use atrial sense events during T1 to update the ASBU value. Atrial events sensed during T1 may or may not be used to inhibit the scheduled atrial pace event depending on the temporal proximity between the atrial sense and the scheduled atrial pace events. If the time from the As event to the AP event is less than a constant K3, then the scheduled AP even is cancelled [AS→AP]&lt;K3, and the pacer exhibits an atrial inhibited response. If, however, the time from the As event to the AP exceeds K3, then the pacer will not cancel the AP event and will not exhibit an atrial inhibited behavior. 
     In the alternative, a cardiac cycle may transpire wherein no atrial sense event occurs. In this case, the value of ASBU is lengthened gradually. In this instance the n+1 value of ASBU is set equal to the preceeding n value with the addition ofa fallback interval K2; ASBU n+1  =ASBU n  +K2. The value of K2 is in the order of 50 ms and is programmable. 
     In conclusion, it should be observed that ASBU may be shortened or accelerated by rapidly recurring atrial activity, and ASBU will be lengthened by either the absence of atrial activity or the occurrence of atrial activity early in the cardiac cycle. 
     FIGS. 3, 4 and 5 shown schematic representations for a pacemaker according to the present invention. In each figure, a schematic EKG trace is shown in conjunction with the various pacemaker time intervals to depict the interaction of the pacemaker with the heart. 
     Turning to FIG. 3, cardiac pacing cycle A shows the sequential stimulation of both chambers of the patient&#39;s heart at the programmed lower rate limit (LR). The electrogram begins with a spontaneously occurring R-wave at 49 which gives rise to a ventricular sense event at 50, initiating the post ventricular atrial refractory period of the pacemaker shown at 52. At the conclusion of the atrial refractory period, the pacemaker enters the atrial observation time. The first portion of the atrial observation time is labeled T1 in the diagram and begins at 54. At the conclusion of the first portion of the observation time, the second portion of the observation time begins at 56. As shown in FIG. 3, no atrial sense events occur within the atrial observation time. Therefore, at the conclusion of this time period, the pacemaker emits an atrial pacing pulse at 58, depicted as the pacing artifact 59 preceding atrial complex or P-wave 61. 
     This time interval from a ventricular event 50 to the subsequent generation of an atrial pacing event 58 is referred to as the atrial escape interval of the pacemaker and is referred to as the atrial stimulation back-up interval or ASBU. Complex A shows a pacemaker which has a programmed post ventricular-atrial refractory period of 200 ms with a V-A excape interval programmed to 600 ms. The sense factor of this pacemaker has been set at 1 thus bifurcating the atrial observation time which is defined as the time period from the conclusion of the atrial refractory period to the generation of the atrial pacing event 58 into two equal time intervals T1 and T2. Complex A represents the maximum value for ASBU for the programmed settings enumerated above. 
     After the generation of the atrial pacing event 58, the A-V delay timer of the pacemaker begins timing out the A-V interval shown as 60 in FIG. 3. At the conclusion of this interval, a ventricular pacing event 62 is generated since no ventricular sensed activity was detected by the pacemaker within the A-V interval. The ventricular pace event 62 gives rise to the pacing artifact 63 shown on the schematic EKG prior to the stimulated R-wave 64. 
     The ventricular event 62 once again starts the post ventricular atrial refractory period timer at 66. At the conclusion of the atrial refractory period, the atrial observation time begins at 68. During the first portion T1 of the atrial observation time, an atrial sense event occurs at 70. This is shown in the schematic EKG as P-wave 72. This P-wave, since it falls within T1, does not give rise to a new value for ASBU nor does not inhibit the scheduled atrial pace event for the cardiac cycle shown in B in FIG. 3, nor does it initiate the A-V timer. In this particular instance, the pacemaker treats atrial event 70 as if it fell within an atrial refractory period. 
     Once again, at the conclusion of the atrial observation time at 72. the pacemaker generates an atrial pace event 74 giving rise to a pacing artifact shown on the schematic EKG at 73 preceding P-wave 75. 
     The atrial pace event at 74 initiates the A-V delay timer resulting in the generation of a ventricular pacing pulse 76 at the conclusion of the A-V delay time and shown on the schematic EKG as pacing artifact 77 preceding the stimulated R-wave 78. At this point, the pacemaker enters the cardiac cycle C in FIG. 3. The computed atrial stimulation back-up rate for this cardiac cycle is 600 ms based upon the information shown within the preceding cardiac cycles A and B. During this cardiac cycle, however, an atrial sense event 80 occurs during the latter portion of the atrial observation time falling within T2. The atrial sense event 80 truncates the remaining portion of T2 and starts the A-V delay timer at 84. Since no naturally conducted R-wave follows P-wave 86, the A-V delay timer times out at 88 generating the ventricular pacing artifact 87 preceding ventricular complex 89. 
     In pacing cycle D, the occurrence of the atrial sense event 80 during the atrial observation time has accelerated the pacemaker by shortening the atrial escape interval. 
     ASBU for cardiac cycle D is 510 ms. This value is computed by adding a small hysteresis increment of 10 ms to the observed [VP→AS] time interval of 500 ms of cardiac cycle C. The sense factor (SF) remains constant at 1 thus bifurcating the 310 ms atrial observation time into equal 155 ms sense windows T1, T2. 
     During cardiac cycle D no atrial events are detected. Therefore, the pacemaker provides the atrial pacing event 90 at the conclusion of the atrial observation time giving rise to the pacing artifact 91 preceding the provoked P-wave 92 on the EKG. At the conclusion of the A-V delay, the pacemaker generates a ventricula4 pace event 94 giving rise to the pacing artifact 95 preceding stimulated R-wave 97. 
     Since there has been no detected atrial activity during cardiac cycle D, the value of ASBU is incremented for cardiac cycle E. A fallback increment K2 is added to the value of ASBU. The fallback increment K2 may vary between 10 and 100 ms and has a nominal value of 50 ms giving rise to the ASBU for cardiac cycle E of 560 ms. 
     Although the fallback increment may be made a constant, it is also contemplated that this value may be logically coupled to the SF value selected by the physician. In general, large values of SF would correspond to large values of K2, and low SF values would map to small values of K2. 
     With respect to FIG. 3, the computation of the atrial stimulation back-up interval has been shown. The pacemaker&#39;s atrial escape interval was accelerated by atrial event 80 and was extended in the absence of atrial activity as shown in connection with cardiac cycle E. 
     In operation, the value of ASBU may vary from a maximum imposed by the lower rate programmed by the physician as depicted in cardiac cycles A and B up to a minimum value dictated by the duration of the PVARP. 
     The operation of the pacemaker at high atrial rates and at various sense factors is shown in connection with FIGS. 4 and 5. 
     Turning to FIG. 4, the lower rate of the pacemaker has remained programmed to the 600 ms value, and the PVARP has remained at 200 ms. In the figure, however, the sense factor has been changed from 1 to 0.25, yielding a larger T1 portion of the atrial observation time. 
     In the figure, pacing cycle J begins with spontaneous R-wave 400 which gives rise to ventricular sense event 402 initiating the post ventricular atrial refractory period at 404. At the conclusion of the PVARP, the atrial observation time begins. In this example, the SF is 0.33. 
     The absence of atrial activity during the AOT results in the scheduled stimulation of the atria at 410, the conclusion of the atrial observation time is depicted by pacing artifact 412 preceding P-wave 414. This atrial pace event 410 starts the A-V delay time of the pacer at 418. At the conclusion of the A-V delay time, a ventricular pace event 420 is generated as illustrated by pacing artifact 422 preceding R-wave 424. 
     The events of pacing cycle M are identical to pacing cycle J. Pacing cycles K and L are similar to pacing cycles J and M except that atrial activity occurs during cycles K and L. 
     In pacing cycle K, the atrial sense event occurs at 426 within the T1 portion 428 of the atrial observation time. The pacer computes the time interval remaining until the next scheduled atrial pace event 430. This time interval [AS→AP] is compared with a physician programmable constant K3. Then the pacer effectively ignores the atrial sense event in the sense that the machine remains in the atrial observation state. When the computed interval is shorter than K3, the pacer will inhibit the scheduled atrial pace event as more fully described in connection with pacing cycle H. 
     Note that ASBU for each pacing cycle in FIG. 4 has remained constant. This occurs in the example of FIG. 4 because atrial events in T1 do not accelerate the pacer by shortening ASBU, and the increment K2 which may be added to ASBU after each cycle cannot extend the value of ASBU below the lower programmed state. 
     FIG. 5 is similar to FIG. 4 in that all programmable values are the same save sense factor which has a value of 0.25 in FIG. 5. Also, the occurrence of atrial event being the same between FIGS. 4 and 5. 
     Turning to FIG. 5, note that due to the change in SF, the atrial sense event 500 has occurred during the first few milliseconds of the T2 portion 502 of the atrial observation time. 
     In response to As 500, the pacer enters the A-V delay state wherein the programmed A-V delay is timed out. At the conclusion of this A-V delay 504, a ventricular pace event 506 is generated completing pacing cycle F. 
     ASBU for pacing cycle G is calculated based on the occurrence of AS 500. The pacer sets ASBU for pacing cycle G equal to the observed time interval from the ventricular event 501 to the atrial sense event 500 of 310 ms with the addition of an increment K1 of 10 ms. Therefore, the value of ASBU for pacing cycle G is 320 ms with an AOT value of 120 ms. Since no atrial sense events occur during pacing cycle G, the ASBU for pacing cycle H is incremented by K2=50 ms yielding an ASBU of 370 ms; AOT=170 ms; T1=42.5 ms and T2=127.5 ms. 
     During T1 of cycle H, an atrial sense event occurs 508. The pacer once again computes the time to the next atrial pace event which in this case is approximately 120 ms. [AS→AP]&gt;130 ms. In the example, we assume a value of K3 of 150 ms and note that [AS→AP] 150 ms. This illustrates that while operating near the upper rate limit the duration of T1 and T2 may be quite short which reduces the likelihood that all P-waves will be sensed within T2. In the illustration, the naturally occurring atrial event 508 occurs so close to the scheduled atrial pace event that there is a likelihood that the scheduled AP event would fall within the repolarization or vulnerable time of the atrial and thus compete with the natural atrial rhythm. To avoid this, possibly the pacer inhibits atrial pace events which would occur less than a preset time, i.e. K3 prior to the atrial stimulation time. 
     The state machine depicted in FIG. 6 describes the differences between the pacer according to the present invention and the pacer disclosed in U.S. Pat. No. 4,312,355. 
     At the conclusion of the post ventricular atrial refractory, the pacer enters the atrial observation state 200 via state transition atrial refractory. The pacer enters the atrial observation state 200 via state transition 202. The atrial observation time (AOT) is bifurcated by the sense factor (SF), and the total duration of the T1 and T2 segments depends on the SF as well as the observed atrial sensing rate. If an atrial sense event occurs during T1 and the computed time period from the atrial sense event AS to the end of the AOT exceeds the value of K3, then the pacer remains in AOT state depicted by loop 204. In this situation, the machine effectively ignores the detected atrial event. If, however, the AS event in T1 gives rise to a value of [AS→AP] which is smaller than K3, then the pacer will inhibit the otherwise scheduled atrial pace event AP and move to the A-V delay state 206 (AVD) at the conclusion of the atrial observation time (AOT). This is shown by state transition 205. 
     Another path for leaving the AOT state 200 is shown by state transition 208. In this instance, there has been no atrial sense event AS, and the atrial observation time has expired at the end of T2. The machine then enters the atrial pace state 209 where the pacer generates an atrial stimulus on the atrial lead. 
     As illustrated in FIG. 7, the machine herein described has been reduced to practice through the use of a commercially available Commodore PET personal computer 600 coupled to conventional pacer atrial sense amplifier 602, ventricular sense amplifier 604, atrial output stimulus generator 606, and ventricular output stimulus generator 608. All timing functions are performed by computer 600. An annotated software listing written in 6502 ASSEMBLER CODE is reproduced as follows: 
     
         __________________________________________________________________________
100                                                                       
   rem *** ddd-pacemaker with gliding atrial                              
   refractory ***                                                         
105                                                                       
   rem *** vsap and intelligent p wave treatment                          
   (ipt) ***                                                              
110                                                                       
   rem *** basref15+masref15 h.d.funke aug. 14,                           
   1983 ***                                                               
115                                                                       
   rem *** ipt-time 20494,x * ipt off 20529,0 * ipt on                    
   20529,251 ***                                                          
120                                                                       
   rem *** d00 20020,0 * 20029,0 * 20070,0 * 20079,0 *                    
   20202,0 ***                                                            
125                                                                       
   poke59468,14:poke59459,67                                              
130                                                                       
   print&#34;   Please wait, I&#39;m loading&#34;:                                    
   print:print                                                            
135                                                                       
   print&#34;   the machine program!&#34;                                         
140                                                                       
   fori=39645to40197:pokei-19645,peek(i):nexti                            
145                                                                       
   1r$=&#34;70&#34;:av$=&#34;180&#34;:hy$=&#34;25&#34;:sf$=&#34;.3&#34;:hr$=&#34;120&#34;:                        
   wa$=&#34;.5&#34;:wv$=&#34;.5&#34;:mo$=&#34;1&#34;                                              
150                                                                       
   print&#34;   *** DDD-Autoref-Pacemaker ***&#34;:print:                         
   print:goto160                                                          
155                                                                       
   print &#34;  &#34;                                                             
160                                                                       
   bl=40:re=150:cali=115:w7=30                                            
165                                                                       
   print&#34;  Lo Rate ( 50-120 PPM) ?&#34;;tab(32);1r$;                          
170                                                                       
   ifr=Othenprint&#34; &#34;:goto180                                              
175                                                                       
   print&#34;  &#34;;tab(30);:inputlr$:print                                      
180                                                                       
   1r=val (1r$):iflr&lt;50orlr&gt;120thenprint&#34;  &#34;:gotto165                     
185                                                                       
   print&#34;  AV Time ( 20-250 ms) ?&#34;;tab(32);av$;                           
190                                                                       
   ifr=Othenprint&#34;   &#34;:goto200                                            
195                                                                       
   print&#34;  &#34;:tab(30);:inputav$:print                                      
200                                                                       
   av=val (av$):ifav&lt;20orav&gt;250thenprint&#34;  &#34;:goto165                      
205                                                                       
   print&#34; AV Hyst ( 00-100 ms) ?&#34;;tab(32);hy$;                            
210                                                                       
   ifr=Othenprint&#34;  &#34;:goto220                                             
215                                                                       
   print&#34;  &#34;;tab(30);:inputhy$:print                                      
220                                                                       
   hy=val(hy$):ifhy&gt;av-20orhy&gt;100thenprint&#34;  &#34;:goto205                    
225                                                                       
   print&#34; Sense Fact ( 0.1-1.0 ) ?&#34;;tab(32);sf$;                          
230                                                                       
   ifr=Othenprint&#34;  &#34;:goto240                                             
235                                                                       
   print&#34;  &#34;;tab(30);:inputsf$:print                                      
240                                                                       
   sf=val(sf$):ifsf&lt;.1orsf&gt;1thenprint&#34;  &#34;:goto225                         
245                                                                       
   mr=int(60000/(av+re+20))                                               
250                                                                       
   print&#34;  Hi Rate (&#34;;int(1r+1);:if1r+&gt;99.9thenprint                      
   &#34;  &#34;;                                                                  
255                                                                       
   ifint(1r+1)&lt;100thenpoke33745,32                                        
260                                                                       
   print&#34;- &#34;;int(mr);tab(23);&#34;PPM) ?&#34;;tab(32);hr$;                        
265                                                                       
   ifr=Othenprint&#34;  &#34;:goto275                                             
270                                                                       
   print&#34;  &#34;;tab(30);:inputhr$:print                                      
275                                                                       
   hr=val(hr$):ifhr&lt;1r+1orhr&gt;mrthenprint&#34;  &#34;:goto250                      
280                                                                       
   print&#34;  PWA  ( 00-2.0 ms) ?&#34;;tab(32);wa$;                              
285                                                                       
   ifr=Othenprint&#34;  &#34;:goto295                                             
290                                                                       
   print&#34;  &#34;;tab(30);:inputwa$:print                                      
295                                                                       
   wa=val(wa$):ifwa&gt;2thenprint&#34;  &#34;:goto280                                
300                                                                       
   print&#34;  PWV  ( 00-2.0 ms) ?&#34;;tab(32);wv$;                              
305                                                                       
   ifr=Othenprint&#34;  &#34;:goto315                                             
310                                                                       
   print&#34;  &#34;;tab(30);:inputwv$:print                                      
315                                                                       
   wv=val(wv$):ifwv&gt;2thenprint&#34;  &#34;:goto300                                
320                                                                       
   print&#34;  DDD(1), DVI(2), VVI(3) ?&#34;;tab(32);mo$;                         
325                                                                       
   ifr=Othenprint&#34;  &#34;:goto335                                             
330                                                                       
   print&#34;  &#34;;tab(30);:inputmo$:print                                      
335                                                                       
   mo=val(mo$):ifmo&lt;&gt;1andmo&lt;&gt;2andmo&lt;&gt;3thenprint&#34;  &#34;:                      
   goto320                                                                
340                                                                       
   print:print&#34;  New entries: press any key.&#34;                             
345                                                                       
   print&#34;  New start: run 145&#34;:r=1                                        
350                                                                       
   rem *** 1r, av, hy, sf, hr, bl, re, wa, wv, cali,                      
   w7 ***                                                                 
355                                                                       
   av=int(av/4):hy=int(hy/4):bl=int(bl/4):re=int(re/4):                   
   w7=int(w7/4)                                                           
360                                                                       
   wa=int(wa*115):wv=int(wv*115)                                          
365                                                                       
   poke20128,bl:poke20251,re:poke20106,wa:poke20229,wv                    
370                                                                       
   poke20010,ca:poke20060,ca:poke20133,ca                                 
375                                                                       
   poke20183,ca:poke20256,ca:poke20306,ca                                 
380                                                                       
   poke20104,254:ifwa=Othenpoke20104,255:poke20106,1                      
385                                                                       
   poke20227,253:ifwv=Othenpoke20227,255:poke20229,1                      
390                                                                       
   ifmo=1thenpoke20070,251:poke20104,254:ifwa=Othenpoke                   
   20104,255                                                              
395                                                                       
   ifmo=2thenpoke20070,0:poke20104,254:ifwa=Othenpoke                     
   20104,255                                                              
400                                                                       
   ifmo=3thenpoke20070,0:poke20104,255                                    
405                                                                       
   w4=av-bl-w7:w6=av-hy-w7:ws=int(15000/1r)-av-re:rem                     
   ws=w1+w2                                                               
410                                                                       
   w1=int(ws/(sf+1)):w2=ws-w1:tm=int(15000/hr)-re-w6-w7                   
415                                                                       
   rem *** w1, w2, b1, w4, re, w6, tm, w7 ***                             
420                                                                       
   ifw1&lt;1thenw1=1                                                         
425                                                                       
   ifw2&lt;1thenw2=1                                                         
430                                                                       
   ifw4&lt;1thenw4=1                                                         
435                                                                       
   ifw6&lt;1thenw6=1                                                         
440                                                                       
   iftm&lt;1thentm=1                                                         
445                                                                       
   poke20370,tm:poke20390,tm:poke20410,tm:poke20429,w7                    
450                                                                       
   poke20353,w1:poke20005,tm:poke20055,w2:poke20178,w4:                   
   poke 20301,w6                                                          
455                                                                       
   sys20004:geta$:ifa$&lt;&gt;&#34;&#34;thengoto155                                     
460                                                                       
   poke20055,int(peek(20005)*sf):ifpeek(20005)&lt;                           
   tmthenpoke20005,tm                                                     
465                                                                       
   goto455                                                                
ready.                                                                    
20000                                                                     
    rts                                                                   
20001                                                                     
    nop                                                                   
20002                                                                     
    brk                                                                   
20003                                                                     
    brk                                                                   
20004                                                                     
    1da                                                                   
       50                                                                 
20006                                                                     
    stam                                                                  
       20002                                                              
20009                                                                     
    1da                                                                   
       115                                                                
20011                                                                     
    stam                                                                  
       20003        WAIT P.R.                                             
20014                                                                     
    1dam                                                                  
       59471                                                              
20017                                                                     
    ora                                                                   
       251                                                                
20019                                                                     
    cmp                                                                   
       251                                                                
20021                                                                     
    beq                                                                   
       20045                                                              
20023                                                                     
    1dam                                                                  
       59471                                                              
20026                                                                     
    ora                                                                   
       247                                                                
20028                                                                     
    cmp                                                                   
       247                                                                
20030                                                                     
    beq                                                                   
       20048                                                              
20032                                                                     
    decm                                                                  
       20003                                                              
20035                                                                     
    bne                                                                   
       20014                                                              
20037                                                                     
    decm                                                                  
       20002                                                              
20040                                                                     
    bne                                                                   
       20009                                                              
20042                                                                     
    jmp                                                                   
       20528                                                              
20045                                                                     
    jmp                                                                   
       20476                                                              
20048                                                                     
    jmp                                                                   
       20250                                                              
refract.                                                                  
20051                                                                     
    nop                                                                   
20052                                                                     
    orap                                                                  
       87                                                                 
20054                                                                     
    1da                                                                   
       14                                                                 
20056                                                                     
    stam                                                                  
       20052                                                              
20059                                                                     
    1da                                                                   
       115                                                                
20061                                                                     
    stam                                                                  
       20053                                                              
20064                                                                     
    1dam                                                                  
       59471                                                              
20067                                                                     
    ora                                                                   
       251                                                                
20069                                                                     
    cmp                                                                   
       251          WAIT P.R.                                             
20071                                                                     
    beq                                                                   
       20095                                                              
20073                                                                     
    1dam                                                                  
       59471                                                              
20076                                                                     
    ora                                                                   
       247                                                                
20078                                                                     
    cmp                                                                   
       247                                                                
20080                                                                     
    beq                                                                   
       20098                                                              
20082                                                                     
    decm                                                                  
       20053                                                              
20085                                                                     
    bne                                                                   
       20064                                                              
20087                                                                     
    decm                                                                  
       20052                                                              
20090                                                                     
    bne                                                                   
       20059                                                              
20092                                                                     
    jmp                                                                   
       20102                                                              
20095                                                                     
    jmp                                                                   
       20365                                                              
atrial stim. rate back-up (ASBU)                                          
20098                                                                     
    jmp                                                                   
       20250                                                              
refract                                                                   
20101                                                                     
    nop                                                                   
20102                                                                     
    sei                                                                   
20103                                                                     
    1da                                                                   
       254                                                                
20105                                                                     
    1dx                                                                   
       57                                                                 
20107                                                                     
    stam                                                                  
       59471                                                              
20110                                                                     
    dex                                                                   
20111                                                                     
    nop                                                                   
20112                                                                     
    nop    STIM A                                                         
20113                                                                     
    nbe                                                                   
       20110                                                              
20115                                                                     
    1dx                                                                   
       255                                                                
20117                                                                     
    stym                                                                  
       59471                                                              
20120                                                                     
    c1i                                                                   
20121                                                                     
    jmp                                                                   
       20348                                                              
SLOW DOWN                                                                 
20124                                                                     
    nop                                                                   
20125                                                                     
    brk                                                                   
20126                                                                     
    brk                                                                   
20127                                                                     
    1da                                                                   
       10                                                                 
20129                                                                     
    stam                                                                  
       20125                                                              
20132                                                                     
    1da                                                                   
       115                                                                
20134                                                                     
    stam                                                                  
       20126                                                              
20137                                                                     
    1dam                                                                  
       59471                                                              
20140                                                                     
    ora                                                                   
       251          WAIT BLANKING                                         
20142                                                                     
    cmp                                                                   
       0                                                                  
20144                                                                     
    beq                                                                   
       20168                                                              
20146                                                                     
    1dam                                                                  
       59471                                                              
20149                                                                     
    ora                                                                   
       247                                                                
20151                                                                     
    cmp                                                                   
       0                                                                  
20153                                                                     
    beq                                                                   
       20171                                                              
20155                                                                     
    decm                                                                  
       20126                                                              
20158                                                                     
    bne                                                                   
       20137                                                              
20160                                                                     
    decm                                                                  
       20125                                                              
20163                                                                     
    bne                                                                   
       20132                                                              
20165                                                                     
    jmp                                                                   
       20177                                                              
20168                                                                     
    jmp                                                                   
       0                                                                  
20171                                                                     
    jmp                                                                   
       0                                                                  
20174                                                                     
    nop                                                                   
20175                                                                     
    brk                                                                   
20176                                                                     
    brk                                                                   
20177                                                                     
    1da                                                                   
       28                                                                 
20179                                                                     
    stam                                                                  
       20175                                                              
20182                                                                     
    1da                                                                   
       115                                                                
20184                                                                     
    stam                                                                  
       20176                                                              
20187                                                                     
    1dam                                                                  
       59471                                                              
20190                                                                     
    ora                                                                   
       251                                                                
20192                                                                     
    cmp                                                                   
       0            WAIT                                                  
20194                                                                     
    beq                                                                   
       20218        REFRACT.                                              
20196                                                                     
    1dam                                                                  
       59471                                                              
20199                                                                     
    ora                                                                   
       247                                                                
20201                                                                     
    cmp                                                                   
       247                                                                
20203                                                                     
    beq                                                                   
       20221                                                              
20205                                                                     
    decm                                                                  
       20176                                                              
20208                                                                     
    bne                                                                   
       20187                                                              
20210                                                                     
    decm                                                                  
       20175                                                              
20213                                                                     
    bne                                                                   
       20182                                                              
20215                                                                     
    jmp                                                                   
       20428                                                              
Wait R for R-inhib??? without VSAP                                        
20218                                                                     
    jmp                                                                   
       0                                                                  
20221                                                                     
    jmp                                                                   
       20385                                                              
VSAP                                                                      
20224                                                                     
    nop                                                                   
20225                                                                     
    sei                                                                   
20226                                                                     
    1da                                                                   
       253                                                                
20228                                                                     
    1dx                                                                   
       57                                                                 
20230                                                                     
    stam                                                                  
       59471        STIM. V                                               
20233                                                                     
    dex                                                                   
20234                                                                     
    nop                                                                   
20235                                                                     
    nop                                                                   
20236                                                                     
    bne                                                                   
       20233                                                              
20238                                                                     
    1dx                                                                   
       255                                                                
20240                                                                     
    stxm                                                                  
       59471                                                              
20243                                                                     
    cli                                                                   
20244                                                                     
    jmp                                                                   
       20250                                                              
Refract.                                                                  
20247                                                                     
    nop                                                                   
20248                                                                     
    brk                                                                   
20249                                                                     
    brk                                                                   
20250                                                                     
    1da                                                                   
       37                                                                 
20252                                                                     
    stam                                                                  
       20248                                                              
20255                                                                     
    1da                                                                   
       115                                                                
20257                                                                     
    stam                                                                  
       20249                                                              
20260                                                                     
    1dam                                                                  
       59471        WAIT REFRACT.                                         
20263                                                                     
    ora                                                                   
       251                                                                
20265                                                                     
    cmp                                                                   
       0                                                                  
20267                                                                     
    beq                                                                   
       20291                                                              
20269                                                                     
    1dam                                                                  
       59471                                                              
20272                                                                     
    ora                                                                   
       247                                                                
20274                                                                     
    cmp                                                                   
       0                                                                  
20276                                                                     
    beq                                                                   
       20294                                                              
20278                                                                     
    decm                                                                  
       20249                                                              
20281                                                                     
    bne                                                                   
       20260                                                              
20283                                                                     
    decm                                                                  
       20248                                                              
20286                                                                     
    bne                                                                   
       20255                                                              
20288                                                                     
    jmp                                                                   
       20538                                                              
20291                                                                     
    jmp                                                                   
       0                                                                  
20294                                                                     
    jmp                                                                   
       0                                                                  
20297                                                                     
    nop                                                                   
20298                                                                     
    brk                                                                   
20299                                                                     
    brk                                                                   
20300                                                                     
    1da                                                                   
       32                                                                 
20302                                                                     
    stam                                                                  
       20298                                                              
20305                                                                     
    1da                                                                   
       115                                                                
20307                                                                     
    stam                                                                  
       20299                                                              
20310                                                                     
    1dam                                                                  
       59471                                                              
20313                                                                     
    ora                                                                   
       251 WAIT R                                                         
20315                                                                     
    cmp                                                                   
       0   (after atrial sense)                                           
20317                                                                     
    beq                                                                   
       20341                                                              
           VSAP - DECIMAL?                                                
20319                                                                     
    1dam                                                                  
       59471                                                              
20322                                                                     
    ora                                                                   
       247                                                                
20324                                                                     
    cmp                                                                   
       247                                                                
20326                                                                     
    beq                                                                   
       20344                                                              
20328                                                                     
    decm                                                                  
       20299                                                              
20331                                                                     
    bne                                                                   
       20310                                                              
20333                                                                     
    decm                                                                  
       20298                                                              
20336                                                                     
    bne                                                                   
       20305                                                              
20338                                                                     
    jmp                                                                   
       20428                                                              
Wait R for R-in???? witout VSAP                                           
20341                                                                     
    jmp                                                                   
       0                                                                  
20344                                                                     
    jmp                                                                   
       20405                                                              
VSAP                                                                      
20347                                                                     
    nop                                                                   
20348                                                                     
    clc                                                                   
20349                                                                     
    1dam                                                                  
       20005                                                              
20352                                                                     
    cmp                                                                   
       101                                                                
Lowest rate needed         SLOW DOWN                                      
20354                                                                     
    bcs                                                                   
       20361                                                              
20356                                                                     
    adc                                                                   
       2                                                                  
Slow down by 2×4=8ms                                                
    AFTER                                                                 
20358                                                                     
    stam                                                                  
       20005                                                              
20361                                                                     
    jmp                                                                   
       20127                                                              
Blanking                                                                  
    ATRIAL STIM                                                           
20364                                                                     
    nop                                                                   
20365                                                                     
    sec                                                                   
20366                                                                     
    1dam                                                                  
       20005                                                              
20369                                                                     
    cmp                                                                   
       49                                                                 
20371                                                                     
    bcc                                                                   
       20381                                                              
20373                                                                     
    sbcm                                                                  
       20052                                                              
This time from wait P.R.                                                  
           elapse                                                         
20376                                                                     
    adc                                                                   
       5                                                                  
Soothing rate changes                                                     
    ASBU                                                                  
20378                                                                     
    stam                                                                  
       20005                                                              
Store new time back                                                       
20381                                                                     
    jmp                                                                   
       20300                                                              
Wait R                                                                    
20384                                                                     
    tax                                                                   
20385                                                                     
    sec                                                                   
20386                                                                     
    1dam                                                                  
       20005                                                              
20389                                                                     
    cmp                                                                   
       49                                                                 
Control for already existing                                              
           maximal rate                                                   
20391                                                                     
    bcc                                                                   
       20401               VSAP                                           
20393                                                                     
    sbcm                                                                  
       20175                                                              
This time from wait R could                                               
    AFTER                                                                 
           not elapse because of early                                    
                           ATRIAL                                         
           R.              STIM                                           
20396                                                                     
    sbc                                                                   
       4                                                                  
20398                                                                     
    stam                                                                  
       20005                                                              
Time for acceleration stored back                                         
20401                                                                     
    jmp                                                                   
       20250                                                              
Refract.                                                                  
20404                                                                     
    tax                                                                   
20405                                                                     
    sec                                                                   
20406                                                                     
    1dam                                                                  
       20005                                                              
20409                                                                     
    cmp                                                                   
       49                                                                 
Max rate checking required?                                               
20411                                                                     
    bcc                                                                   
       20421               VSAP                                           
20413                                                                     
    sbcm                                                                  
       20298                                                              
This time could not elapse                                                
    AFTER                                                                 
           because of early R.                                            
                           ATRIAL                                         
20416                                                                     
    sbc                                                                   
       4                   SENSE                                          
20418                                                                     
    stam                                                                  
       20005                                                              
Later time stored back                                                    
20421                                                                     
    jmp                                                                   
       20250                                                              
Refract.                                                                  
20424                                                                     
    tax                                                                   
20425                                                                     
    nop                                                                   
20426                                                                     
    brk                                                                   
20427                                                                     
    brk                                                                   
20428                                                                     
    1da                                                                   
       7                                                                  
20430                                                                     
    stam                                                                  
       20426                                                              
20433                                                                     
    1da                                                                   
       125 WAIT R                                                         
20435                                                                     
    stam                                                                  
       20427                                                              
           (for R-inhibition)                                             
20438                                                                     
    1dam                                                                  
       59471                                                              
           (if R, no VSAP)                                                
20441                                                                     
    ora                                                                   
       251                                                                
20443                                                                     
    cmp                                                                   
       0                                                                  
20445                                                                     
    beq                                                                   
       20469                                                              
20447                                                                     
    1dam                                                                  
       59471                                                              
20450                                                                     
    ora                                                                   
       247                                                                
20452                                                                     
    cmp                                                                   
       247                                                                
20454                                                                     
    beq                                                                   
       20472                                                              
20456                                                                     
    decm                                                                  
       20427                                                              
20459                                                                     
    bne                                                                   
       20438                                                              
20461                                                                     
    decm                                                                  
       20426                                                              
20464                                                                     
    bne                                                                   
       20433                                                              
20466                                                                     
    jmp                                                                   
       20225                                                              
Stim R.                                                                   
20469                                                                     
    jmp                                                                   
       0                                                                  
20472                                                                     
    jmp                                                                   
       20250                                                              
Refractory                                                                
20475                                                                     
    tax                                                                   
20476                                                                     
    tax                                                                   
20477                                                                     
    tax                                                                   
20478                                                                     
    tax                                                                   
20479                                                                     
    tax                                                                   
20480                                                                     
    1da                                                                   
20482                                                                     
    stam                                                                  
       20020                                                              
20485                                                                     
    nop                                                                   
20486                                                                     
    1dam                                                                  
       20002                                                              
20489                                                                     
    adcm                                                                  
       20055                                                              
20492                                                                     
    nop                                                                   
20493                                                                     
    cmp                                                                   
       60                                                                 
1PT-Time                                                                  
20495                                                                     
    bcs                                                                   
       20513                                                              
20497                                                                     
    nop                                                                   
20498                                                                     
    1da                                                                   
       79                                                                 
20500                                                                     
    stam                                                                  
       20094                                                              
20503                                                                     
    1da                                                                   
       124                                                                
20505                                                                     
    stam                                                                  
       20093                                                              
20508                                                                     
    jmp                                                                   
       20023                                                              
20511                                                                     
    nop                                                                   
20512                                                                     
    nop                                                                   
20513                                                                     
    1da                                                                   
       78                                                                 
20515                                                                     
    stam                                                                  
       20094                                                              
20518                                                                     
    1da                                                                   
       134                                                                
20520                                                                     
    stam                                                                  
       20093                                                              
20523                                                                     
    jmp                                                                   
       20023                                                              
20526                                                                     
    tax                                                                   
20527                                                                     
    tax                                                                   
20528                                                                     
    1da                                                                   
       251                                                                
20530                                                                     
    stam                                                                  
       20020                                                              
20533                                                                     
    jmp                                                                   
       20054                                                              
20536                                                                     
    tax                                                                   
20537                                                                     
    tax                                                                   
20538                                                                     
    ida                                                                   
       78                                                                 
20540                                                                     
    stam                                                                  
       20094                                                              
20543                                                                     
    1da                                                                   
       134                                                                
20545                                                                     
    stam                                                                  
       20093                                                              
20548                                                                     
    jmp                                                                   
       20000                                                              
20551                                                                     
    tax                                                                   
20552                                                                     
    tax                                                                   
__________________________________________________________________________