Abstract:
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.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to impedance measuring, and more particularly relates to impedance measuring in an implantable medical device, especially in the presence of electromagnetic interference. 
   BACKGROUND OF THE INVENTION 
   Implantable medical devices (IMDs) have many functions including the delivery of therapies to cardiac patients, neuro-stimulators, muscular stimulators, and others. For purposes of this application reference will be made only to implantable cardiac devices, it being understood that the principles herein may have applicability to other implantable medical devices as well. 
   An implantable cardiac device (ICD) may be a device commonly referred to as a pacemaker, which is used to stimulate the heart into a contraction if the sinus node of the heart is not properly timing, or pacing, the contractions of the heart. Modern cardiac devices also perform many other functions beyond that of pacing. For example, some cardiac devices may also perform therapies such as defibrillation and cardioversion as well as providing several different pacing therapies, depending upon the needs of the user and the physiologic condition of the user&#39;s heart. For convenience, all types of implantable cardiac devices will be referred to herein as ICDs, it being understood that the term, unless otherwise indicated, is inclusive of an implantable device capable of administering any of a number of therapies to the heart of the user. 
   In typical use, an ICD is implanted in a convenient location usually under the skin of the user and in the vicinity of the one or more major arteries or veins. One or more electrical leads connected to the pacemaker are inserted into or on the heart of the user, usually through a convenient vein or artery. The ends of the leads are placed in contact with the walls or surface of one or more chambers of the heart, depending upon the particular therapies deemed appropriate for the user. 
   One or more of the leads is adapted to carry a current from the pacemaker to the heart tissue to stimulate the heart in one of several ways, again depending upon the particular therapy being delivered. The leads are simultaneously used for sensing the physiologic signals provided by the heart to determine when to deliver a therapeutic pulse to the heart, and the nature of the pulse, e.g., a pacing pulse or a defibrillation shock. 
   The sensing of the physiologic signal from the heart requires a very sensitive sensing method since the signals sensed are of quite low amplitude. The presence of external, or non-physiologic, electromagnetic interference (EMI), if the field is large enough, can compromise the cardiac sensing function such that the pacemaker may fail to deliver a needed therapy or may deliver an unwanted therapy. Some types of non-physiologic EMI, such as continuous wave at high frequencies, can easily be distinguished from physiologic signals and can thus be ignored or rejected by the pacemaker circuitry. Other forms of non-physiologic EMI, however, are not easily distinguishable from physiologic signals and therefore can block or override the desired physiologic signals. 
   Many state-of-the-art ICDs are capable of performing either bipolar or unipolar sensing and pacing in either chamber of the heart. Unipolar pacing requires an elongated lead having only one insulated conductor therein and only one generally distal electrode disposed thereon. In most unipolar configurations the protective canister of the ICD is conductive and functions as an electrode in pacing or sensing. For bipolar pacing and/or sensing a lead having two mutually insulated conductors disposed thereon is required. Typically, one electrode is disposed at the distal end of the lead and is referred to as the “tip” electrode, while the second electrode is spaced somewhat back from the distal end of the lead and is referred to as the “ring” electrode. The current path for bipolar pacing extends from the pulse generator in the ICD, along a first of the two lead conductors to the tip electrode, through the cardiac tissue to the ring electrode and back to the ICD along the second of the two conductors. 
   Most modern ICDs may be programmed to pace and sense in either the bipolar or unipolar mode. This gives the implanting physician considerable flexibility in configuring an ICD system to suit the particular needs of a given patient or user. Additionally, if one of the two leads in a bipolar ICD were to fail for some reason, (e.g., breakage of a conductor due to metal fatigue, an open outer coil, an ineffective ring set screw connection, poor connections, tissue degradation at the electrode site, oxidation, etc.) it would be necessary to reprogram the ICD into unipolar pacing and sensing mode in order for the ICD to continue to perform properly. 
   In order to detect the failure of a lead in a bipolar ICD unit the impedance of the leads is monitored continuously and, in the event an impedance is detected that is outside a specified range, the ICD is automatically switched to unipolar pacing and sensing until the problem can be rectified. The switch can take several tens of seconds, however, because the impedance measurement must be confirmed by a series of readings before the switch is made. During this time, no pulses are provided, which results in less than optimal therapy. The delay is necessary because a high impedance reading may be caused by electromagnetic interference (EMI). In such cases an out-of-range impedance may be detected when no lead failure has occurred. 
   Accordingly, it is desirable provide a mechanism and method such that therapy can continue in the event of an out-of-range impedance measurement 
   Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
   BRIEF SUMMARY OF THE INVENTION 
   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 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 exceeds a predetermined value, 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. 
   Also provided is, in an implantable cardiac device having a pulse generator for normally producing bipolar pulses that is capable of switching to producing unipolar pacing pulses, a bipolar electrical lead coupling the device to a heart, means for measuring the impedance of the bipolar lead while producing bipolar pacing pulses, and a detector for detecting the presence or absence of electromagnetic interference, whereby, if the impedance of the bipolar lead exceeds a predetermined value, the pulse generator produces the next pulse as a unipolar pacing pulse and if electromagnetic interference is present the pulse generator returns to production of bipolar pacing pulses. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
       FIG. 1  is an illustration of an implantable cardiac device having been implanted in a conventional manner in a patient; 
       FIG. 2  is a block diagram of an implantable cardiac device usable in the instant invention; and 
       FIG. 3  is a flow chart describing the operation of the implantable cardiac device of the instant invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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. 
     FIG. 1  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. 
     FIG. 2  is a block diagram of an implantable cardiac device  10  usable in the instant invention. While the device of  FIG. 2  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  FIG. 2  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  FIG. 2 ). 
   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  FIG. 2 , 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  FIG. 2 , pace/control circuit  20  is coupled to lead recognition circuit  22  by means of multiple signal lines, designated collectively as  38  in  FIG. 2 . 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  FIG. 2 , 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. 
     FIG. 3  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  ( FIG. 2 ) is provided. Counter  62  is incremented each time a unipolar pacing pulse is detected. Since unipolar pulses are inhibited ( 70  in  FIG. 3 ) 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.