Lead impedance scanning system for pacemakers.

0001This invention relates to body implantable systems for electrical stimulation of physiologic function and, more particularly, to electronic circuitry associated with pacemakers for monitoring implanted pacemaker operation and providing indications of detected departures from standard levels of selected parameters.

BACKGROUND OF THE INVENTION

0002The technology of cardiac pacemakers has developed to a high level of sophistication of system performance. The current generation of cardiac pacemakers incorporate microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. These pacemakers may be programmed to control the heart in correcting or compensating for various heart abnormalities which may be encountered in individual patients. A detailed description of modern cardiac pacemaker technology is set forth in International Application Number WO-A 86-05698, entitled STIMULATED HEART INTERVAL MEASUREMENT, ADAPTIVE PACER AND METHOD OF OPERATION, assigned to the assignee hereof. The disclosure of that application is incorporated herein by reference.

0003In order to efficiently perform its function as a pump, the heart must maintain a natural AV synchrony. The term "AV synchrony" relates to the sequential timing relationship that exists between the contractions of the atria and the ventricles. In a given heart cycle or beat, these contractions are typically manifest or measure by sensing electrical signals or waves that are attendant with the depolarization of heart tissue, which depolarization immediately precedes (and for most purposes can be considered concurrent with) the contraction of the cardiac tissue. These signals or waves can be viewed on an electrocardiogram and include a P-wave, representing the depolarization of the atria; the QRS wave (sometimes referred to as an R-wave, the predominant wave of the group), representing the depolarization of the ventricles; and the T-wave, representing the repolarization of the ventricles. (It is noted that the atria also are repolarized, but this atrial repolarization occurs at approximately the same time as the depolarization of the ventricles; and any electrical signal generated by atrial repolarization is generally minute and is masked out by the much larger QRS-wave on the electrocardiogram.)

0004Thus, it is the P-QRS-T cycle of waves that represents the natural AV synchrony of the heart. These waves, including the time relationships that exist therebetween, are carefully studied and monitored through conventional ECG techniques whenever the operation of the heart is being examined.

0005Initiation of the cardiac cycle normally begins with depolarization of the sinoatrial (SA) node. This specialized structure is located in the upper portion of the right atrium wall. The SA node depolarizes spontaneously at an intrinsic rate of a little better than once each second (typically about 72 beats per minute). The rate of depolarization and, therefore, the heart rate are influenced by various physical factors, which may produce tachycardia or bradycardia depending upon the particular patient condition.

0006Optimally, in a normal cardiac cycle and in response to the initiating SA depolarization, the atrium contracts and forces the blood that has accumulated therein into the ventricle. A short time later (a time sufficient to allow the bulk of the blood in the atrium to flow through the one-way valve into the ventricle), the ventricle contracts, forcing the blood out of the ventricle to body tissue. A typical time interval between contraction of the atrium and contraction of the ventricle might be 60 ms; a typical time interval between contraction of the ventricle and the next contraction of the atrium might be 800 ms. Thus, it is an atrial contraction (A), followed a relatively short time thereafter by a ventricle contraction (V), followed a relatively long time thereafter by the next atrial contraction, that produces the desired AV synchrony. Where AV synchrony exists, the heart functions very efficiently as a pump in delivering life--sustaining blood to body tissue; where AV synchrony is absent, the heart functions as an inefficient pump (largely because the ventricle is contracting when it is not filled with blood).

0007Multiple-mode, demand-type, cardiac pacemakers are designed, insofar as is possible, to maintain an AV synchrony for damaged or diseased hearts that are unable to do so on their own. A demand-type pacemaker is one that provides a stimulation pulse only when the heart fails to produce a natural depolarization on its own within a prescribed escape interval. In a dual chamber pacemaker, this is realized by placing electrodes in both the right atrium and right ventricle of the heart. These electrodes are coupled through intravenous and/or epicardial leads to sense amplifiers housed in an implanted pacemaker. Electrical activity occurring in these chambers can thus be sensed. When electrical activity is sensed, the pacemaker assumes that a depolarization or contraction of the indicated chamber has occurred. If no electrical activity is sensed within a prescribed time interval, typically referred to as an atrial or ventricular escape interval, then a pulse generator, also housed within the pacemaker housing, generates a stimulation pulse that is delivered to the indicated chamber, usually via the same lead or electrode as is used for sensing. This stimulation pulse causes or forces the desired depolarization and contraction of the indicated chamber to occur. Hence, by first sensing whether a natural depolarization occurs in each chamber, and by second stimulating at controlled time intervals each chamber with an external stimulation pulse in the absence of a natural depolarization, the AV synchrony of the heart can be maintained. Thus, with a demand pacer, the heart will either beat on its own (without stimulation from the pacemaker) at a rate that is at least just slightly faster than the stimulation rate defined by the escape interval, or the heart will be stimulated by the pacer at a rate controlled by the escape interval. The stimulation rate provided by the pacemaker is typically referred to as the "programmed rate."

0008Pacemakers have come to be classified according to type and functional mode of operation. For several years, a three-letter code has been used to describe the various types of pacemakers. In accordance with this code, the letter in the first position has been used to indicate the chamber being paced. Thus, V represents the ventricle, A represents the atrium, and D indicates that both the atrium and the ventricle are being paced. The letter in the second position for this code indicates the chamber being sensed. As with the chamber being paced, V, A and D when in the second position of the code indicate the chamber being sensed is the ventricle, the atrium or both the atrium and ventricle, respectively. In addition, O represents the lack of sensing. The letter in the third position in this three-letter code signifies the mode of response of the pacemaker. In this position, T represents triggered response, I represents inhibited response, D represents double response (atrial triggered and ventricle inhibited or atrial triggered/inhibited and ventricle inhibited). O indicates none and R indicates reversed (where the pacemaker is activated by a fast heart rate but does not respond to a slow heart rate--in other words, a specialized antiarrhythmic pacemaker).

0009As noted, most pacemakers include a sensor circuit that looks for electrical signals from spontaneous heart activity. On detection of such activity, the pacemaker stimulation action is modified, depending upon the functional mode or type of pacemaker. For example, in the VVI mode (ventricle paced and sensed, response inhibited mode), sensing of heart activity under certain time restrictions is interpreted as normal heart activity such that the stimulating action is inhibited.

0010The discussion thus far has followed the assumption that a pacemaker and its associated circuitry operate without malfunction. By the very nature of man-made devices, such is not always the case. Whereas electronic circuitry can be, and is, incorporated within the pacemaker itself for exercising or testing various circuit components, the status of battery power sources, and the effectiveness of various amplifiers, waveform shaping stages and the like, it is often more difficult to test the integrity of the leads and implanted electrodes to which the pacemaker is coupled for pacing operation.

0011At the implanting of the pacemaker and electrode system, minor damage is sometimes incurred which may affect the system's electrical insulation. This type of damage may go undetected and be without present effect on the implanted system, but the condition may manifest itself after extended time in service. When a breakdown or significant degradation of the pacemaker lead insulation occurs, it can have serious or even disastrous results, depending upon whether or not the breakdown is of a catastrophic nature. Various types of lead damage may produce different types of failure or degradation. For example, an isolation defect on the stimulation electrode shunts the energy intended for the heart to some other point. A lead breakage can in some environmental situations temporarily or permanently reduce the stimulation output, sometimes drastically. Another type of detectable error relates to the failure of the electrode tip to be in proper contact with the heart wall.

0012While the lead defects which have been mentioned thus far are more in the nature of catastrophic failures, there may also occur failures or degradation of a less drastic nature which may be intermittent of which may build up over time. In a common situation, errors often start as temporary or intermittent errors. These can be virtually impossible to discover with commonly used techniques. It is not permissible to test on a random or periodic basis for lead faults in the manner in which power lines or telephone lines, for example, may be tested as, for example, by applying an over-voltage to a suspected circuit, simply because the breakdown of insulation in pacemaker leads under such condition may produce catastrophic results in the patient. It would be desirable to be able to use the signals encountered in the normal operation of an implanted pacemaker in the process analysis to determine impending failure or serious degradation from the temporary or intermittent errors which may be detected. A system for performing such a function would be expected to monitor standard heart operation and to use detected deviations or departures from signal norm to indicate the occurrence of such.

0013There may be various approaches to the regular monitoring of heart signals for the detection of abnormalities, some of which may relate to the circuitry employed in the implanted pacemaker system. One asserted pacemaker function analyzer for automatic evaluation and indication of the quality of performance of cardiac pacing systems is the subject of United States patent 4,527,567 of Fischler et al. The analyzer of that patent is said to provide a comprehensive examination of asynchronous, demand and demand-hysteresis pacemakers of all makes, including the state of the pacemaker's battery, the intactness of the electronic circuitry and of the electrodes, and the proper location of the electrodes in the heart. The manner in which this analysis is performed by the circuitry of that patent is entirely different from the operation of the lead impedance scanning system of the present invention.

0014In FR-A-2,369,836 a failure of the pacemaker leads is detected by monitoring the lead impedance and establishing if the lead impedance is higher than a first reference level (broken lead) or lower than a second reference level (short). The lead impedance is monitored by sensing the voltage across a resistor through which the stimulation current flows and comparing this voltage to a reference voltage. When such a failure is detected, the patient's tissue is stimulated via an auxiliary lead so that the state of failure may be sensed by the patient.

0015In GB-A-2,026,870 a failure of one of the pacemaker leads is detected by measuring a high lead impedance. To measure the impedance, the time required to recharge the output capacitor after a stimulation to a predetermined voltage level is ascertained. Upon sensing a failure of one lead, a second lead is connected via select switches from the pacemaker to the patient's heart.

SUMMARY OF THE INVENTION

0016The invention is defined in claim 1 and 13.

0017In brief, arrangement in accordance with the present invention comprise electrical circuitry for monitoring signals transmitted via the pacemaker system leads and noting excessive variations in measured impedance in relation to a moving average of impedance measurements. The monitoring provides a measurement of lead impedance with the occurrence of every heart stimulation pulse. Thus, not only are permanent lead abnormalities, such as insulation breakdowns, lead breaks and the like, detected by arrangements in accordance with the present invention, but so also are lead impedance anomalies of a temporary or intermittent nature as well as significant but gradual variations in lead impedance which may be symptomatic of impending lead failure.

0018In one particular arrangement in accordance with the present inventgion, voltage level on a storage capacitor which is the source of heart stimulation pulses is sampled both before and after a stimulation pulse. The difference in voltage level between two such samples, due to the partial discharge of the storage capacitor resulting from the delivery of the stimulation pulse, is used to determine the instantaneous lead impedance which is a function of this voltage difference. The derived value is compared with a moving average of stored lead impedance measurements and an error counter is incremented if the current value differs from the moving average by a predetermined amount. Similarly, the output form the circuit of the pacemaker which is provided for sensing heart activity is used to monitor changes in sensed signals which may related to problems with leads involved in sensing. Any detected abnormality in the sensing lead results in the incrementing of a second error counter. The lead failure in this case is indicated as abnormal (unphysiological) sensed signal slew rates. For example, a lead fracture will momentarily provide step voltages or transient pulses with existing normal potentials between the body and conducting materials.

0019During a patient checkup, the count levels stored in the first and second error counters may be transferred to an appropriate test readout in order that attention may be directed to the fact that a possible lead problem exists.

BRIEF DESCRIPTION OF THE DRAWINGS

0020The above and other objects, features and advantages of the present invention will be more apparent from the following more particular description thereof presented in conjunction with the accompanying drawings, wherein: <ul id="ul0001" list-style="none"><li>Fig. 1 is a schematic representation of a dual chamber cardiac pacemaker shown implanted in association with a heart for pacing;</li><li>Fig. 2 is a schematic block diagram of one particular arrangement in accordance with the invention for incorporation in a pacemaker like that shown in Fig. 1; and</li><li>Fig. 3 is a schematic circuit diagram showing further details of the arrangement of Fig. 2.</li></ul>

DETAILED DESCRIPTION OF THE INVENTION

0021The following description is of the best presently contemplated mode of carrying out the invention. This description is not to be taken in a limiting sense but is made for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the appended claims.

0022Referring now to Fig. 1, there is shown a simplified representation of one way that an implanted pacemaker 10 may make electrical contact with the heart. Fig. 1 depicts the use of two bipolar leads 12 and 14, each being directed into a separate chamber of the right heart. A bipolar lead comprises a single filar that includes two electrically insulated conductors. For example, the lead 14 includes a first conductor 16 that is electrically connected to a distal tip 18 of the lead. This distal tip is typically placed in a cavity of the right atrium 19 referred to as the atrial appendage 20. A known distance from the distal tip 18 an electrode ring 22 is electrically connected to the other conductor 24 of the bipolar lead 14. Similarly, a distal tip 26 and a conductive ring 28 are associated with the bipolar lead 12 that is placed in the apex of the right ventricle 30. The manner in which the leads 12 and 14 are inserted into the heart, as well as the manner in which the pacemaker 10 is implanted in the body of a patient, are well known in the art.

0023The diagram of Fig. 1 may be considered to represent a rate-responsive pacer operating in the VVI mode if the bipolar lead 14 with its associated distal tip 18 and electrode ring 22 is eliminated from the figure so that only the bipolar lead 12 is left with its tip and ring 26, 28 inserted in the right ventricle 30, as shown in Fig. 1.

0024A block diagram of a circuit in accordance with the present invention is shown in Fig. 2. The circuit 40 is shown comprising a stimulation timing circuit 42 which contains the normal pacemaker timing and logic circuitry. The circuit 40 is coupled to control the actuation of a switch S1 upon the occurrence of a stimulation signal STIM. The source of the pacing output at terminal 46 is a capacitor C1 which is coupled to be charged by a charging circuit 44 and which delivers the pacing pulse to the output 46 through a series capacitor C4 when the switch S1 is closed. Resistor R2 is provided to complete the circuit to capacitor C4 when switch S1 is open.

0025A sampling stage 50 is coupled to sample capacitor C1 before and after delivery of the packing pulse. Sampled voltages from the sampling stage 50 are delivered to an analog-to-digital (A/D) converter 52, the output of which in digital form is applied to a stimulating impedance discriminator 54 which contains the circuitry for evaluating changes in lead impedance as a function of the voltage difference between the two levels sampled before and after delivery of a stimulation pulse corresponding to the following equation: <maths id="math0001" num=""><math display="inline"><mrow><mtext>R = -Tp/Cl 1n(1-dV/Vo))   (1)</mtext></mrow></math><img file="EP0338364B1_D0001.tif" /></maths>    where <dl id="dl0001"><dt>R</dt><dd>represents a lead impedance,</dd><dt>Tp</dt><dd>is the stimulation pulse duration,</dd><dt>Cl</dt><dd>is the source capacitor for the stimulation pulse,</dd><dt>Vo</dt><dd>is the source voltage, and</dd><dt>dV</dt><dd>is the voltage difference between the two sampled values.</dd></dl>

0026This equation is derived from the well-known relation with respect to time (t) for the voltage (v) across a discharging capacitor with initial voltage (V_o): <maths id="math0002" num=""><math display="inline"><mrow><msub><mrow><mtext>v = V</mtext></mrow><mrow><mtext>o</mtext></mrow></msub><mtext> exp(-t/RC)   (2)</mtext></mrow></math><img file="EP0338364B1_D0002.tif" /></maths>    The stimulating impedance discriminator 54 maintains a moving average of lead impedance measurements according to Equation (1) and compares each new measurement with that average. If the measurement of lead impedance differs from the moving average by a predetermined value, an associated counter 56 is incremented to count the event as the occurrence of an error.

0027A polarity detector 55 is shown connected to the stimulating impedance discriminator 54 to provide an indication for the readout logic stage 70 of the direction of change for any differences in measurement of lead impedance which are detected by the stimulating impedance discriminator 54. Thus, for example, the polarity detector 55 provides an output indication of whether detected changes in lead impedance measurements involve an increase or decrease in impedance. This corresponds to an indication that lead degradation is in the direction of an open circuit (corresponding to measured increases in lead impedance) or in the direction of a short circuit (corresponding to a measured reduction in lead impedance).

0028A sensing detector 60 is coupled to the terminal 46 to respond to sensed heart activity. The output of the sensing detector 60 is applied to a sensing impedance discriminator 62 which receives a delayed signal DS from a monostable delay circuit 64 that is triggered by the STIM output of the stimulation timing circuit 42. This serves to prevent indication of high slew rate signals from stimulation output origin. The output of the sensing impedance discriminator 62 is applied to a second counter 66. The sensing impedance discriminator 62 responds to the slope (slew) of the signal from the sensing detector 60 such that when a rate of change greater than 10 volts per second is detected, the count in the counter 66 is incremented. A second polarity detector 63 is shown coupled to the sensing impedance discriminator 62 with an output being directed to the readout logic stage 70, in much the same fashion and for the same purpose as the polarity detector 55 which is connected to the stimulating impedance discriminator 54. The detector 63 provides an output indication of increasing impedance where the slope of the signal from the sensing detector 60 is positive and an output indication of decreasing impedance where the slope of that signal is negative. Thus, additional diagnostic information is provided at the readout logic stage 70 in response to polarity detectors 55, 63 over and above the mere indication of a detected change in lead impedance.

0029A readout logic stage 70 is coupled to the outputs of both counter stages 56 and 66 to provide an indication of the number of errors detected by the respective portions (stimulating signal and sensing signal) of the lead impedance analyzing circuit 40. The inputs designated by the letter C indicate connections from the various stages to a system clock (not shown).

0030Fig. 3 represents the circuit 40 of Fig. 2 in more detailed schematic circuit form. In this figure, like elements have been given corresponding designations to those shown in Fig. 2.

0031In Fig. 3, the charging circuit 44 is represented as comprising a switch S2 and a capacitor C2, the latter having a small capacitance by comparison with C1. Thus, when capacitor C2 is connected to capacitor C1, as indicated with the condition of switch S2 shown in Fig. 3, C2 quickly assumes the voltage level of C1 without producing noticeable change therein. The circuit of R1 and C3 serves to create a moving average of the voltage level of C1, since C3 has a much greater capacitance than C2. Switches S3 and S4 are connected to provide alternating charge and discharge pulses to one input terminal of a first comparator stage 80 which is connected to receive at its other input the moving average of C1 voltage level. The output of the comparator 80 is coupled to the first stage 82 of a three-bit shift register 88, having additional stages 84 and 86. The outputs of the individual stage 82, 84, 86 are applied to three-terminal NAND gates 90, 92 while the outputs of the first stage 82 are additionally applied to two-input NAND gates 94, 96 which control switches S3 and S4. The outputs of the NAND gates 90, 92 are applied to a further NAND gate 98, the output of which is applied to one input of AND gate 100. An active output from gate 98 signifies the occurrence of an event corresponding to the detection of three successive errors or anomalous impedance measurements. The output of AND gate 100 is coupled to a programmable down counter 102, which in turn is coupled to an up counter 104. The programmable down counter 102 is connected to be set to a predetermined value delivered over the data bus 110, the latter being connected to receive the count in the up counter 104.

0032The monostable delay circuit 64 of Fig. 2 comprises a flip flop 120 coupled to receive the output of an NOR gate 122 and to provide an output through a buffer amplifier 124 to another NOR gate 126. This serves to provide a delayed signal DS following a stimulation pulse from the stimulation timing circuit 42 of Fig. 2 in phase with a clock signal.

0033The sensing portion of the circuit comprises a sense amplifier 130 coupled to receive heart signals for terminal 46 and apply them through a high pass filter 132 to one input of a second comparator stage 134, the other input of which is coupled to a voltage reference Vref. The output of comparator stage 134 is applied to a flip flop 136, the output of which is applied, with the stimulation signal STIM, to an AND gate 138 which in turn is coupled to drive an up counter 140. The counter 140 is connected to provide data to the data bus 110.

0034In the operation of the circuit of Fig. 3, following a stimulating pulse from the stimulating timing circuit 42 (Fig.2), the delayed DS signal is developed. During this time, capacitor C2 is connected to capacitor C1 via switch S2, and the same voltage level is developed on both capacitors C1 and C2. Alternation of the switch S2 transfers charge samples in bucket and dipper fashion from capacitor C1 to capacitor C3 in the moving average circuit to provide a voltage at one input of the comparator 80. As described in connection with Fig. 2, the stimulation pulse activates switch S1 to provide a pacing signal at terminal 46 from the capacitor C1. A momentary change of lead impedance in the lead connected to output terminal 46 creates a difference of potentials in capacitors C2 and C3 which is sensed by the comparator 80 and applied to the shift register 88 at input stage 82. If three consecutive deviation signals are applied to the shift register 88, gates 90, 92 and 98 are activated to develop a condition at the input to gate 100 signifying an event. This in phase with a clock pulse activates AND gate 100 to decrement the down counter 102. The counter 102 is set to a predetermined count value on receipt of a STIM signal. When the voltage difference between capacitor C2 and capacitor C3 exceeds a certain threshold value, the output of gate 98 is active long enough to develop the output from the programmable down counter 102, thereby incrementing the up counter 104.

0035A rapidly changing input signal to the sense amplifier 130 produces a voltage level at the comparator 134 which is higher than Vref. As a result, the signal form the comparator 134 sets the flip flop 136 if the DS signal is low. When this occurs at least once during a stimulating pulse interval, the next stimulating pulse from the stage 42 (Fig. 2) will increment the second counter 140. At pacemaker follow-up, the two counters 104 and 140 are read via the data bus 110. From the numbers read out of the counters 104 and 140, determinations of system performance, relative to the impedance condition of the pacer leads may be made.

0036As shown and described hereinabove, arrangements in accordance with the present invention monitor a prescribed pacemaker parameter related to the integrity of the implanted leads and keep a count according to the occurrences of a predetermined number of detected anomalies in succession. An anomaly is determined to be a deviation from the norm for that parameter by some predetermined amount. What constitutes the norm is determined from the operation of the system over time. Isolated anomalies corresponding to a single occurrence are disregarded, but if the particular anomaly persists over three heart beats, the occurrence of the event is recorded in a counter. Thus, at some later tine, such as during a routing patient checkup, for example, the contents of the counter may be noted so that a decision may be made with respect to whether or not the possible problem corresponding to the noted events needs to be corrected.

0037The preferred embodiment of the invention advantageously provides two distinct related systems for noting anomalies with respect to the testing of lead impedance. One system is based upon measurement of output energy delivered to the stimulation circuit during pacing; the other system involves the measurement of lead impedance from the monitoring of sensed heart signals. An analysis of the count readouts in the independent counters of both systems may further enhance the process of determining the particular lead problem which is indicated, particularly from a correlation of the separate count readouts as in the case of a pacemaker system which uses the same lead for sensing and pacing.