fig1 generally shows a pacemaker 10 implanted in a patient 12 . the pacemaker leads 14 and 15 electrically couple the pacemaker 10 to the patient &# 39 ; s heart 11 via a suitable vein 18 . the leads act to both sense polarizations in the heart , and to deliver pacing stimuli the heart . fig2 is a block circuit diagram illustrating a multi - programmable , implantable , dual - chamber , bradycardia pacemaker 10 capable of carrying out the present invention . although the present invention is described in conjunction with a microprocessor - based architecture , it will be understood by those skilled in the art that it could be implemented in other technology such as digital logic - based , custom integrated circuit ( ic ) architecture , if desired . it will also be understood that the present invention may be implemented in cardioverters , defibrillators and the like . lead 14 includes an intracardiac electrode 24 located near its distal end and positioned within the right ventricle 16 . electrode 24 is coupled by a lead conductor 14 through an input capacitor 26 to the node 28 , and to the input / output terminals of an input / output circuit 30 . similarly , the lead 15 has a distally located intracardiac electrode positioned within the right atrium 17 . electrode 22 is coupled by a lead conductor 15 through an input capacitor 75 to a node 76 , and to the input / output terminals of the input / output circuit 30 . input / output circuit 30 contains the operating input and output analog circuits for digital controlling and timing circuits necessary for the detection of electrical signals derived from the heart , such as the cardiac electrogram , output from sensors ( not shown ) connected to the leads 14 and 15 , as well as for the application of stimulating pulses to the heart to control its rate as a function thereof under the control of software - implemented algorithms in a microcomputer circuit 32 . microcomputer circuit 32 comprises an on - board circuit 34 and an off - board circuit 36 . on - board circuit 34 includes a microprocessor 38 , a system clock 40 , and on - board ram 42 and rom 44 . off - board circuit 36 includes an off - board ram / rom unit 46 . microcomputer circuit 32 is coupled by data communication bus 48 to a digital controller / timer circuit 50 . microcomputer circuit 32 may be fabricated of custom ic devices augmented by standard ram / rom components . it will be understood by those skilled in the art that the electrical components represented in fig2 are powered by an appropriate implantable - grade battery power source ( not shown ). an antenna 52 is connected to input / output circuit 30 for purposes of uplink / downlink telemetry through a radio frequency ( rf ) transmitter / receiver circuit ( rf tx / rx ) 54 . telemetering both analog and digital data between antenna 52 and an external device , such as an external programer ( not shown ), is accomplished in the preferred embodiment by means of all data first being digitally encoded and then pulse position modulated on a damped rf carrier , as substantially described in u . s . pat . no . 5 , 127 , 404 , issued on jul . 7 , 1992 , entitled &# 34 ; telemetry format for implantable medical device &# 34 ;, which is held by the same assignee as the present invention and which is incorporated herein by reference . a reed switch 51 is connected to input / output circuit 30 to enable patient follow - up via disabling the sense amplifier 146 and enabling telemetry and programming functions , as is known in the art . a crystal oscillator circuit 56 , typically a 32 , 768 hz crystal - controlled oscillator , provides main timing clock signals to digital controller / timer circuit 50 . a vref / bias circuit 58 generates a stable voltage reference and bias currents for the analog circuits of input / output circuit 30 . an adc / multiplexer circuit ( adc / mux ) 60 digitizes analog signals and voltages to provide telemetry and a replacement time - indicating or end - of - life function ( eol ). a power - on - reset circuit ( por ) 62 functions to initialize the pacemaker 10 with programmed values during power - up , and reset the program values to default states upon the detection of a low battery condition or transiently in the presence of certain undesirable conditions such as unacceptably high electromagnetic interference ( emi ), for example . the operating commands for controlling the timing of the pacemaker depicted in fig2 are coupled by bus 48 to digital controller / timer circuit 50 wherein digital timers set the overall escape interval of the pacemaker , as well as various refractory , blanking and other timing windows for controlling the operation of the peripheral components within input / output circuit 50 . digital controller / timer circuit 50 is coupled to sense amplifiers ( sense ) 64 and 67 , and to electrogram ( egm ) amplifiers 66 and 73 for receiving amplified and processed signals picked up from electrode 24 through lead 14 and capacitor 26 , and for receiving amplified and processed signals picked up from electrode 22 through lead 15 and capacitor 75 , representative of the electrical activity of the patient &# 39 ; s ventricle 16 and atrium 17 , respectively . similarly , sense amplifiers 64 and 67 produce sense event signals for re - setting the escape interval timer within circuit 50 . the electrogram signal developed by egm amplifier 66 is used in those occasions when the implanted device is being interrogated by the external programmer / transceiver ( not shown ) in order to transmit by uplink telemetry a representation of the analog electrogram of the patient &# 39 ; s electrical heart activity as described in u . s . pat . no . 4 , 556 , 063 , issued to thompson et al ., entitled &# 34 ; telemetry system for a medical device &# 34 ;, which is held by the same assignee as the present invention , and which is incorporated herein by reference . output pulse generators 68 and 71 provide the pacing stimuli to the patient &# 39 ; s heart 11 through output capacitors 74 and 77 and leads 14 and 15 in response to paced trigger signals developed by digital controller / timer circuit 50 each time the escape interval times out , or an externally transmitted pacing command has been received , or in response to other stored commands as is well known in the pacing art . in a preferred embodiment of the present invention , pacemaker 10 is capable of operating in various non - rate - responsive modes which include ddd , ddi , vvi , voo and vvt , as well as corresponding rate - responsive modes of dddr , ddir , vvir , voor and vvtr . further , pacemaker 10 can be programmably configured to operate such that it varies its rate only in response to one selected sensor output , or in response to both sensor outputs , if desired . details of the adir / vvir mode of the present invention follow below , with reference to fig3 through 9 . in those figures the following abbreviations are used to indicate the occurrence of cardiac events : as for atrial sense ; ap for atrial pace ; vs for ventricular sense ; and vp for ventricular pace . the pacemaker 10 operates as a combination of a separate aair pacemaker for the atrial channel , and a separate vvir pacemaker for the ventricular channel . atrial blanking follows both atrial and ventricular events , with the blanking period equal to approximately 180 ms when the ventricular event is either paced or premature , and approximately 120 ms at the start of an orthodromically conducted ventricular beat . the blanking periods may be different from the above numbers , according to the needs of the patient , etc . the lower rate of the ventricular pacemaker is lower than the lower rate of the atrial pacemaker so that ventricular pacing occurs only during episodes of av block . in addition to av block , the patient must also experience atrial arrhythmias ( i . e ., flutter , fibrillation ) in order for the ventricular pacemaker to be activated . thus , in cases of av block , but sinus rhythm , the pacemaker 10 switches to a fully automatic mode , which includes such modes as dddr , dddr with , ddir , vvir , etc . table 1 summarizes the operation of the pacemaker 10 under various conditions . table 1______________________________________summary of adir / vvir pacemaker operationcondition result______________________________________1 . av conduction aair activated ; vvir sensing only2 . sinus rhythm with av block pacemaker switches to dddr operation3 . atrial arrhythmia , vvi activated no av block4 . atrial arrythmia with vvir activated av block______________________________________ fig3 is a timing diagram illustrating condition 1 in table 1 , supra . during normal operation with av conduction , the atrial pacemaker is enabled while the ventricular pacemaker is disabled ( i . e ., the only function of the ventricular pacemaker in this case is to monitor ventricular sense events ). an atrial refractory period starts at the beginning of each atrial event . the pacemaker 10 uses atrial - to - atrial ( a - a ) timing to determine the escape interval . at the detection of a ventricular event ( sense in this case ) the atrial pacemaker begins an atrial blanking period ( 120 ms in the preferred embodiment ) followed by an atrial refractory period . fig4 illustrates the operation of the pacemaker 10 after the occurrence of a premature ventricular contraction ( pvc ), a ventricular sense event occurring without an intervening atrial event since the last ventricular event . at the occurrence of a pvc ( the third ventricular sense event shown ) the longer atrial blanking period starts ( 180 ms ), and the atrial pacemaker is reset to pace after the expiration of the current a - a interval minus the intrinsic conduction time of the patient as measured by the pacemaker . this reset delay period is hereby referred to as the &# 34 ; pseudo &# 34 ; v - a interval . fig5 illustrates an alternate approach to a pvc involving rate smoothing to minimize ventricular rate drops . in this approach the pseudo v - a interval used to reset the atrial pacemaker equals the current ventricular - to - ventricular ( v - v ) interval minus the intrinsic conduction time . fig6 illustrates the pacemaker &# 39 ; s adir / vvir operation during the presence of atrial arrhythmia and intermittent av block . as will be appreciated by those skilled in the art , numerous references discuss methods for detecting arrhythmias . one such reference is an article entitled &# 34 ; automatic tachycardia recognition &# 34 ; by robert arzbaecher et al ., pace 7 ( 1984 ) 541 - 547 , hereby incorporated by reference . another reference is u . s . pat . no . 4 , 880 , 005 issued to benjamin d . pless et al . on nov . 14 , 1989 for &# 34 ; pacemaker for detecting and terminating a tachycardia ,&# 34 ; which is also incorporated by reference . during the first two cardiac cycles shown , the atrial paces are conducted normally to the ventricles , thus inhibiting the ventricular pacemaker . in the third and fourth cardiac cycles , however , av block triggers the ventricular pacemaker , causing it to pace the ventricle at the expiration of the ventricular escape interval . recall that atrial blanking and atrial refractory periods follow each ventricular event ( when the ventricular event is paced the blanking period equals 180 ms in the preferred embodiment ). in the above example , a potential exists for retrograde conduction of ventricular pace events , which is undesirable . to eliminate this problem , the atrial pacemaker is reset following a ventricular pace ( at expiration of the ventricular escape interval ) as shown in fig7 . the pseudo v - a interval is chosen to be either the current a - a escape interval minus the intrinsic conduction time as described in conjunction with fig4 or the current ventricular rate minus the av interval as described in conjunction with fig5 . fig8 illustrates the response of the pacemaker 10 to a non - conducted premature atrial contraction ( pac ). after the third cardiac cycle in the illustration , an atrial sense event occurs during the atrial refractory period . in order to minimize the drop in ventricular rate after the occurrence of a non - conducted pac , the atrial escape interval is timed not from the pac , but from the previous atrial event . in order to determine that a pac has not conducted , the pacemaker must wait for the av interval plus a predefined interval unique to the delay in conduction from pacs . if the pac is conducted , the atrial escape interval is timed from the pac . otherwise , the atrial escape interval is timed from the previous atrial event . fig9 is a flowchart summarizing the procedure / program 900 used by the pacemaker 10 to implement the adir / vvir mode . steps 902 through 916 describe the operation of the atrial pacemaker , while steps 920 through 938 describe the operation of the ventricular pacemaker . atrial pace events ( step 904 ) trigger ventricular blanking at step 906 . both atrial sense events ( step 902 ) atrial pace events trigger the atrial blanking and atrial refractory periods at step 908 . at step 910 the atrial escape rate is set equal to the current sensor rate . the pacemaker 10 determines at step 912 whether a ventricular pace is scheduled within the programmed minimum ventricular pace ( vp )- to - atrial pace ( ap ) interval ( the minimum interval which must occur after a vp before an ap can occur ). if the vp is to occur in the minimum vp - ap interval , it is moved up in time to occur at the scheduled ap time minus the minimum vp - ap interval ( step 914 ). at step 916 the atrial refractory period is set to end 300 ms before the next scheduled atrial pace event . step 918 , the last step in the program 900 , places the pacemaker 10 in a monitoring mode to await re - triggering of the program by an atrial or ventricular event ( steps 902 , 904 , 920 or 922 ). a ventricular sense event ( step 920 ) advances the program 400 to step 924 , which determines whether the ventricular event is a pvc . if so , the program advances to step 930 , where a short atrial blanking period is started ( i . e ., about 120 ms , as described supra .). at step 926 , whenever a ventricular pace occurs ( step 922 ), or a pvc , a longer atrial blanking period is started ( i . e ., about 180 ms , as described supra .). following a ventricular pace or a pvc the next atrial pace is scheduled to occur after the atrial escape interval minus the av conduction time ( step 928 ). at step 932 the ventricular blanking and refractory periods are started , as well as the atrial refractory period ( the atrial period is set to end 300 ms before the next scheduled atrial pace , as in step 916 ). the pacemaker 10 determines at step 934 whether the next atrial pace is scheduled within the programmed minimum ventricular pace - to - atrial pace interval ( the minimum interval which must occur after a vp before an ap can occur ). if the ap is to occur in the minimum vp - ap interval , it is moved back in time to occur at the scheduled ap time plus the minimum vp - ap interval ( step 936 ). if the next atrial pace is not scheduled to occur within the minimum vp - ap interval , the next ventricular pace is then scheduled to occur at the current ventricular escape interval ( step 938 ). the program then advances to step 918 so that the pacemaker 10 resumes its monitoring mode , as described supra . variations and modifications to the present invention are possible given the above disclosure . however , such variations and modifications are intended to be within the scope of the invention claimed by this letters patent .
a rate - responsive cardiac pacemaker implements a novel pacing mode , identified as adir / vvir , which is especially effective for patients with sick sinus syndrome and only intermittent atrioventricular block . within the same pacemaker circuitry , an aair pacemaker and a vvi pacemaker are provided with atrial blanking following both atrial and ventricular events . ventricular blanking after atrial pacing is minimized for better detection of r - waves following an atrial paced event .