the following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention . furthermore , there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention . fig1 is an illustration showing generally where a implantable cardiac device ( icd ) 10 is placed in a conventional manner in a patient 12 . icd 10 is conventionally housed within a hermetically sealed , biologically inert outer canister , which itself may be of a conductive material and serve as an electrode in the icds pacing / sensing circuit . one or more leads , collectively identified as 14 are electrically coupled to icd 10 in a conventional manner , extending into the patient &# 39 ; s heart 16 via a vein 18 . disposed generally near the distal end of lead 14 are one or more exposed conductive electrodes for receiving electrical cardiac signals and / or for delivering electrical stimuli or other therapies to heart 16 . lead 14 may be implanted with its distal end in either the atrium or the ventricle of heart 16 . lead 14 is preferably a bipolar lead such that lead 14 actually has two separate and mutually insulated leads , the first having a terminal at the distal end of lead 14 and the second having a terminal near , but set back from the distal end . such leads are well known in the art . an implantable cardiac device may have a pulse generator for normally producing bipolar pulses that is capable of switching to producing unipolar pacing pulses . a bipolar electrical lead couples the device to a heart . there are provided means for measuring the impedance of the bipolar lead while producing bipolar pacing pulses , whereby , if the icd is programmed to pace or sense in a bipolar configuration , and the impedance of the bipolar lead is out of a normal , predetermined , range , the pulse generator provides unipolar pacing pulses . when the bipolar impedance returns to within a predetermined value , the pulse generator will return to bipolar pacing pulses . fig2 is a block diagram of an implantable cardiac device 10 usable in the instant invention . while the device of fig2 is shown as a pacemaker , it is understood that other icds or imds could also be used , including devices such as defibrillators , cardioversion devices , neuro - stimulators , and the like . icd 10 comprises a primary pacing / control circuit 20 and a lead recognition circuit 22 . of course lead recognition circuit 22 may be associated with other circuitry for performing other cardiac functions such as minute ventilation sensing . much of the circuitry associated with pacing control circuit 20 may be of conventional design in accordance with u . s . pat . no . 5 , 534 , 018 , assigned to the assignee of the instant invention , and which is incorporated by reference herein in its entirety , including those documents incorporated into that patent by reference . to the extent that certain components of icd 10 are conventional , they will not be described in great detail here , since it is believed that the design and implementation of such components would be a matter of routine to those of ordinary skill in the art . for example , pacing / control circuit 20 includes a sense amplifier circuit 24 , pacing output circuit 26 , a crystal clock 28 , a random - access memory and read only memory ( ram / rom ) unit 30 , a central processing unit ( cpu ) 32 and a telemetry circuit 34 , all of which are well known in the art . icd 10 preferably includes an internal telemetry circuit 34 so that it is capable of being programmed or reprogrammed externally . programmers and telemetry circuit are well known in the art . coil 36 is a pick - up coil or antenna that allows communication between the telemetry circuit 34 and the external programmer ( not shown ). icd 10 is coupled to leads 14 which , when implanted , extend transvenously between the implant site of icd 10 and the patient &# 39 ; s heart . for clarity , the connection between leads 14 and the various components of icd 10 are not shown in fig2 although it will be apparent to those of ordinary skill in the art that , for example , leads 14 will necessarily be coupled , either directly or indirectly to sense amplifier 24 and pacing output circuit 26 , in accordance with common practice , such that cardiac electric signals may be conveyed to sensing circuitry 24 and pacing pulses may be delivered to cardiac tissue , via leads 14 . in the present embodiment two bipolar leads are employed , an atrial lead 14 a having atrial tip and ring electrodes ( atip and aring ), and a ventricular lead 14 v having ventricular tip and ring electrodes ( vtip and vring ). those of ordinary skill in the art will appreciate that a separate , electrically insulated conductor extending along the length of leads 14 a and 14 v is associated with each of the electrodes atip , aring , vtip , and vring . that is , electrical signals applied , for example to the vring electrode are conducted along lead 14 v on a first conductor , whereas signals applied to the vtip electrode are conducted along a second , separate conductor in lead 14 v . in addition , as noted above , the conductive , hermetically sealed canister of icd 10 serves as an indifferent electrode ( case in fig2 ). as previously noted , central processing unit 32 may be an off - the - shelf microprocessor or microcontroller . although specific connections between cpu 32 and the other components of icd 20 are not shown in fig2 , it will be apparent to those skilled in the art that cpu 32 functions to control the timed operations of pacing output circuit 26 and sense amplifier circuit 24 under control of programming stored in ram / rom 30 . crystal oscillator circuit 28 provides the main timing clock signals to pace / control circuit 20 and to lead recognition circuit 22 . it is also understood that the circuitry of icd 10 is powered by a battery inside the hermetically sealed case of icd 10 in accordance with common practice in the art . for the sake of clarity , the battery and the connections between the battery and the various circuit elements are not shown . pacing output circuit 26 , which functions to generate pacing stimuli under control of signals issued by cpu 32 , may be , for example , of the type disclosed in u . s . pat . no . 4 , 476 , 868 to thompson , entitled “ body stimulator output circuit ,” which patent is hereby incorporated herein by reference in its entirety . again , however , it is believed that those of ordinary skill in the art could select from among many various types of prior art pacing output circuits which would be suitable for the purposes of practicing the present invention . as shown in fig2 , pace / control circuit 20 is coupled to lead recognition circuit 22 by means of multiple signal lines , designated collectively as 38 in fig2 . an i / o interface 40 in pace / control circuit 20 and a corresponding i / o interface 42 in lead recognition circuit 22 functions to coordinate the transmission of signals between the two units 20 and 22 . with continued reference to fig2 , lead recognition circuit 22 includes a lead interface circuit 44 , which essentially functions as a multiplexer to selectively couple the lead conductors associated with the atip , aring , vtip , and vring electrodes of leads 14 a and 14 v to the remaining components of lead recognition circuitry 22 . in the preferred embodiment , the selection of particular conductors can be accomplished by interface circuit 44 under control of control signals originating from pace / control circuit 20 and communicated to lead interface circuit 44 via lines 38 . coupled to lead interface circuit 44 in lead recognition circuit 22 is an excitation and sample circuit 50 which functions to generate biphasic excitation pulses which are conveyed along leads 14 a and 14 v for the purposes of measuring impedance between various combinations of electrodes atip , aring , vtip , and vring , as determined by the multiplexing function of lead interface circuit 44 . in addition , excitation and sample circuit 50 performs a sampling function on electrical signals present on the conductors of leads 14 a and 14 v . as noted above , the hermetically sealed case of icd 10 may be used as well . this is accomplished through delivery of sub - threshold biphasic voltage pulses on the possible pacing / sensing paths ( atrial unipolar and bipolar , ventricular unipolar and bipolar , and the case ), such that the impedances observed along those paths can be evaluated . to this end , excitation and sample circuit 50 includes circuitry for generating the small sub - threshold biphasic voltage pulses , which , through lead interface circuit 44 are periodically and sequentially issued along each of the possible pacing paths . by sub - threshold is meant that the voltage pulses are well below the level of voltage and duration that would be applied by a pacing pulse . the sample values obtained by excitation and sample circuit 50 are provided to a logarithmic analog - to - digital converter (“ logadc ”) circuit 52 . as its name suggests , logadc circuit 52 performs a logarithmic analog - to - digital conversion function on the sample values obtained by sample and excitation circuit 50 , resulting in the derivation of values corresponding to the current and voltage on the conductors of leads 14 a and 14 v and the case . these values , in turn , are used to derive an impedance value reflecting the impedance associated with a given pacing path defined by the conductors of leads 14 a and 14 v . this impedance value is determined in a digital interface circuit 54 which also functions to coordinate the transfer of digital information between lead recognition circuit 22 and pace / control circuit 20 on lines 38 . finally , a lead recognition voltage regulator (“ irreg ”) circuit 56 is provided to define a reference voltage used by excitation and sample circuit 50 . the impedance values obtained are provided to the cpu 32 for comparison with other received impedance values and for comparison with impedance values that represent impedances within the acceptable range for bipolar pacing / sensing . in prior art icds , the receipt of an out - of - range impedance value would cause the icd to continue to deliver pacing pulses even if they were ineffective due to the high impedance for a period of time , sometimes several tens of seconds , to determine whether the impedance value is correct or is the result of an anomaly such as the presence of an emi field in the vicinity of the icd that may have caused the out - of - range impedance measurement . subsequently , the icd will either resume bipolar pacing or switch to unipolar pacing in which only one of the conductive pathways is necessary to provide a pacing pulse , the other terminal being provided by the canister of the icd . during this time , the patient may not receive optimal therapy . the instant invention overcomes this situation by providing an alternative solution . lead recognition circuit 22 has associated therewith an emi detector 60 that is coupled to a counter 62 . the emi detector 60 is capable of detecting the presence of electromagnetic interference above a predetermined threshold level that may be disruptive to the operation of the icd . the counter 62 is coupled to the cpu 32 to receive signals indicating that the impedance measurement for the lead delivering pacing pulses is out - of - range . the counter is incremented upon receipt of an indication that the impedance of a lead is out - of - range and that a unipolar pulse has been issued , and after a predetermined number of counts , the counter causes the icd 20 to be reprogrammed to a unipolar pacing / sensing mode . fig3 is a flow chart describing the operation of the implantable cardiac device of the instant invention . a determination is first made as to whether the lead polarity is programmed in a bipolar pace / sense mode 64 . if so , the sub - threshold lead impedance is sensed with each pacing cycle 66 , and a determination is made as to whether the emi sensed is out - of - range 68 . if the emi is out - of - range , that is , an emi signal is detected , the impedance measurement is ignored 70 and normal bipolar pacing / sensing continues . if the emi is not out - of - range , the bipolar impedance is checked 72 . if the impedance is out - of - range , the polarity of the next pacing pulse is switched to unipolar , so that the patient does not miss a scheduled therapy . the process is repeated with each successive pacing / sensing cycle . in order to make a final determination that there is a problem with lead impedance rather than merely an emi problem , a counter 62 ( fig2 ) is provided . counter 62 is incremented each time a unipolar pacing pulse is detected . since unipolar pulses are inhibited ( 70 in fig3 ) if the emi level is out - of - range , unipolar pulses are counted only when the emi is in normal range . after a predetermined number of unipolar pulses are provided over a predetermined period , i . e ., the counter could be reset by known means to zero if a series of good measurements occurred over a period of time , an assumption is made that the lead is indeed faulty , and the counter 62 issues a signal to the cpu 32 to reprogram the icd to unipolar mode . by this mechanism the patient continues to receive pacing pulses even though the impedance measurement is initially out - of - range , and will receive pacing pulses until the problem may be resolved with lead replacement or by some other means . impedance measurements may be used for a number of other diagnostic or therapeutic tasks . for example , impedance measurement can be used to determine minute ventilation in order to adjust the pacing rate of an icd in those instances when the user is engaged in strenuous activities which require a higher heart rate than normal , or simply to confirm the polarity programming of the icd . the use of an emi measurement in conjunction with any of a number of impedance measurement diagnostics can ensure that the impedance measurements are not unduly affected by the presence of disruptive emi . while at least one exemplary embodiment has been presented in the foregoing detailed description of the invention , it should be appreciated that a vast number of variations exist . it should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples , and are not intended to limit the scope , applicability , or configuration of the invention in any way . rather , the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention . it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims .
in an implantable medical device having an electrical lead coupled to tissue of a user and a circuit for measuring the impedance of the lead , a method and apparatus for responding to impedance variations in the lead which includes measuring the impedance of the lead while monitoring physiologic parameters of the user , detecting the presence or absence of electromagnetic interference , and if the impedance of the lead is out - of - range , determining whether the electromagnetic interference exceeds a predetermined value , and if the electromagnetic interference exceeds a predetermined value , administering a therapy to the tissue of the user .