Electronic Analysis for Pacemaker Failure

Transcription

1 Electronic Analysis for Pacemaker Failure Seymour Furman, M.D., Doris J. W. Escher, M.D., Bryan Parker, and Norman Solomon, M.D. T he physician or surgeon who implants a cardiac pacemaker incurs a continuing responsibility for the patient s welfare. The prosthetic device may be erratic in its behavior, and because of the consumable batteries it undergoes progressive decay and eventual cessation of function. No accurate or reliable figures exist that will be a guide to removal of all pacemaker units before clinical failure. One approach to reduction of sudden failure is the removal of all functioning pacemakers when the average failure interval for 10% has been reached [2]. This approach cannot be recommended because of the unnecessary premature surgery and inefficiency of equipment usage and because of the 10% sudden failure rate, which is still allowed to occur. Removal at an average failure time is inadequate, because such an interval is composed of a wide scatter of figures about the average and would still allow a large number of sudden failures, while some units would be removed when they are still capable of significant function. To limit the number of sudden failures with possible disastrous consequences and to achieve maximum use of equipment, both the patient and his implanted prosthesis must be monitored frequently enough to reduce unpredicted failures to a minimum level. PRINCIPALS OF TESTING Just as a patient is periodically observed for proper cardiac function, so the pacemaker should be observed, during scheduled patient visits, for proper operation of its various functions. Analysis of a standard electrocardiogram, analysis of triggered function in such a unit, and careful measurement of the parameters of pacemaker output with an interval timer and by display of the pacemaker artifact on an oscilloscope screen allow determination of the state of function of the pacemaker circuit [l]. The important considerations are: (1) the battery From the Division of Surgery and Medicine, Montefiore Hospital and Medical Center, Bronx, N.Y. Supported in part by US. Public Health Service Grant No. HE Presented at the Fifth Annual Meeting of The Society of Thoracic Surgeons, San Diego, Calif., Jan , 1969.

2 FURMAN ET AL. voltage must be maintained; (2) the other electrical characteristics of the pacer output, slope of the pulse, pulse duration and shape, must be constant [5]; (3) the mode of pacemaker function must remain constant at all times, and the triggered function must remain appropriate; and (4) the pacemaker rate, or for a triggered pacemaker the rate produced by a magnetic switch or external stimulation, must be constant. EQUIPMENT A pacemaker clinic must include equipment for electrocardiography, for testing of pacemaker function, and for accurate recording and retrieval of data on each patient and pacemaker. OSCILLOSCOPE An oscilloscope appropriate for such use should have a storage screen with a differential vertical amplifier and a variable time base, with a range to encompass pacemaker impulses (0.5 msec. per centimeter) and ECG s (0.1 sec. per centimeter). Such a scope might be a Tektronix 564 with a 2A61 differential amplifier and a Type 2B67 time base. A Polaroid camera should be available for recording the pacemaker output wave form as it appears on the screen, so that the illustration can be used for future reference and for recording data on paper or for computer analysis. Unipolar electrodes provide large artifacts at the limbs (50 to 300 mv.) and are more readily interpreted than bipolar artifacts, which are smaller (1 to 30 mv.). Both can be measured and compared to previously determined standards, and studies can be performed on the same patients. INTERVAL TIMER The interval timer (such as the Hewlett-Packard Electronic Counter No. 5223L) should be capable of reading the interval between pacemaker impulses to within one tenth of a millisecond. The interval is then converted to a rate accurate to one one-hundredth of a beat per minute. A variation in asynchronous rate by one-half beat per minute during successive beat intervals confirms instability of circuit function. A general slowing or speeding of the rate is indicative of circuit changes of a gradual nature, whereas variation of interval from beat to beat is an indication of impending pacemaker failure. Almost all pacemakers have a fixed rate that is a function of the pacemaker circuit and can be measured. Many triggered units, such as the American Optical Co. Cardiocare and the Cordis Ectocor, have a fixed rate mode activated by a magnetic switch for measurement of rate. 58 THE ANNALS OF THORACIC SURGERY

3 Electronic Analysis for Pacemaker Failure ELECTROCARDIOGRAPHIC ANALYSIS Interpretation of the electrocardiogram will indicate whether pacemaker artifacts produce a ventricular response and whether the pacemaker is responding appropriately to spontaneous cardiac function when it exists. Each triggered pacemaker should be tested for appropriateness of its triggered function. This is readily accomplished by stimulating the implanted pacemaker through the intact skin with an external pacemaker. The ventricular synchronous pacemaker will follow the imposed rate without delay between the imposed pacemaker artifact and the implanted artifact [3]. The atrial synchronous pacemaker responds similarly, but with the A-V delay, which is part of the circuit (Fig. 1). The demand unit will be disabled by the external pacemaker artifacts and will not emit its own stimuli during external stimulation (Fig. 2). ELECTRONIC ANALYSIS OF PULSE GENERATOR IMPULSES Several parameters should be checked. Stimulus amplitude is the maximum amplitude achieved by an impulse, although a measurement at 0.5 msec. following the beginning of the impulse may be used. A I I t I FIG. 1. Ventricular synchronous unit triggered for test purposes. Left: pacing at the escape rate. Right: triggered pacing by transcutaneous stimulation of the pulse generator. The upward-directed line is the external impulse; the downward line is the implanted pacemaker impulse. VOL. 8, NO. 1, JULY,

4 FURMAN ET AL. M,Nrl!D #I( ".*.A FIG. 2. Demand pacing with application of a 5 ma. external impulse over the pacer.. The low-energy artifacts produce no cardiac response and suppress the implanted pacemaker impulses. decrease is caused either by a reduction of output current (usually because of reduction in battery voltage) or by a change in the dipole vector of the impulse. Respiratory movement yields more pronounced vector changes for bipolar than for unipolar electrodes, but large vector changes suggest either electrode break or displacement. Consistent measurement in a single lead, usually lead I for a unipolar electrode and lead I1 for a bipolar electrode, is adequate to yield reproducible results (Fig. 3). Stimulus shape is a valuable indicator of the pulse generator and electrode system. It is linearly related to the current flowing between PACE MAKER MFG : Cordis TYPE: 1298 ELECTRODE MFG: Cordis TYPE: Unipolar 4mm Final pacemaker current into 500 ohms = 6mA Final pacemaker battery voltage =4.7 volts rnsec Amplitude (at t=0.5msec) Pulse duration (msec) I.7 2.I Months after generator implant Rate (beats per minute) Plateau time (msec) to 65.6 (erratic) 0.7 FIG. 3. Two groups of data deriued six months apart. The unipolar impulse shows a decline of 40%, lengthening of the pulse duration, and slowing of the pacemaker rate with development of an erratic pulse-to-pulse interval. Pulse generator replacement was indicated. 60 THE ANNALS OF THORACIC SURGERY

5 Electronic Analysis for Pacemaker Failure PACEMAKER MFG: Cordis TYPE: Ventrlcor 1118 ELECTRODE MFG: Cord16 TYPE: Unipolar 4mm Final pacemaker current into 500 ohms -2.5 ma Flnal pacemaker battery voltage = 23 volts 0 -m M mlk I Amplitude (at 1=0.5msec) Pulse duration (msec) Months after generator implant Rate (beats per minute) Plateau time (msee) FIG. 4. Data derived at two-month intervals. The rate has declined by approximately two beats, and both the pulse duration and the plateau time have become proionged. Pulse generator replacement was indicated. the electrodes, and it changes with electrode defects and deterioration of the pulse generator output. Each pulse generator and its electrode has a characteristic impulse shape. Three main categories exist: voltage limited, current limited, and current and voltage limited. In the voltage-limited shape, current flow increases sharply to its maximum and immediately decreases continuously and exponentially throughout the impulse. The Cordis Atricor 102F, Medtronic, and American Optical Co. Cardiocare units have different varieties of this output shape. In the current-limited shape, current flow increases sharply and remains constant throughout the impulse, producing a relatively rectangular shape. The Cordis 1 11E and the Ector 129F are of this variety. In the current- and voltage-limited shape, the impulse shape initially resembles the current-limited output, with a short plateau followed by a voltage-limited exponential decay. All Cordis Ventricor, Atricor, and Ectocor models not listed above are of this variety. As this type of unit fails, the plateau is prolonged and the exponential decay shortens, tending toward a current-limited appearance [6] (Fig. 4). PR 0 CED URE Each patient is seen at three- to six-month intervals, and more frequently as the second anniversary after implantation is reached. During each visit the patient s cardiac status and pacemaker status are reviewed. A 12-lead electrocardiogram, a long rhythm record of lead 11, and the electronic analysis of the pacemaker output are performed. Each triggered pacemaker is tested by triggering to observe whether its response is constant. VOL. 8, NO. 1, JULY,

6 FURMAN ET AL. Based on our observations of pacemaker longevity, we have established a routine pulse generator replacement interval. This interval is the time beyond which a pulse generator is not allowed to remain, even if all its electronic parameters seem satisfactory. In 1967, only 12% of pulse generator replacements reached this period; in 1968, 14% did. The various types of units performed as follows: asynchronous, 30 months; demand, 30 months; atrial synchronous, 24 months; and ventricular synchronous, 24 months, although few units have yet reached this period. PULSE GENERATOR REPLACEMENT When all data are available, a pulse generator is replaced if any one of the following exists: (1) a rate change of 10% or more has occurred; (2) the electrocardiogram and triggered studies indicate that the pulse generator no longer functions in the mode for which it was designed; (3) the heart is no longer consistently stimulated; (4) the stimulus artifact has fallen by 40%; (5) stimulus duration has changed by 25% or has become grossly distorted; or (6) the routine replacement interval has passed [4]. RESULTS PACEMAKER LONGEVITY Accumulated data have demonstrated two major characteristics of pacemaker longevity: (1) 80% of asynchronous pulse generator failures occur over a one-year interval during the second year of pacemaker function; and (2) monthly analysis of pulse generator failure indicates a wide scatter, rather than a grouping (Fig. 5). These characteristics, therefore, do not allow a statistical prediction of individual pulse generator failure and do not allow the choice of a single period that will adequately avoid the bulk of failures while not substantially shortening the functional life of the pacemaker. CLINICAL EFFECTIVENESS The final tests of the adequacy of any follow-up technique will be based on the following characteristics: (1) as low as possible a death rate because of sudden pacemaker failure; (2) as low as possible an emergency pulse generator replacement rate because of unpredicted pacemaker failure; (3) maximum utilization of each pulse generator; (4) maximum number of pulse generator replacements done on a predicted basis while they approach exhaustion but are still functioning; and (5) as few pulse generators as possible removed while still capable of a significant duration of function. 62 THE ANNALS OF THORACIC SURGERY

7 Electronic Analysis for Pacemaker Failure CORDIS 127A Total sample - 33 Total calculated-22 Dead or pacemaker removed while functioning- 5 Remainina-6 Average -longevity months c E ; : 14 O n 10 E Number of months after implant E 10 : 12 L o 14 E E 20 L Number of failures FIG. 5. The behavior of a single model of cardiac pacemaker with units remaining operatiue (left) and individual failures (right) as a function of time. In this series, 44 replacements were done during 1967, a period in which pacemakers were closely observed but neither electronic analysis nor interval timing was used. During this period, 45% of all pulse generator replacements were emergencies and 39% were elective replacements. Sixteen percent were emergency pulse generator replacements done pari passu with repair of an electrode malfunction. In 1968, a transitional period, 89 replacements were done, and the technique described was used in the last seven months of the year. Pulse TABLE 1. PULSE GENERATOR REPLACEMENT Reason for Replacement " - Emergency 27 (61%) 25 (28%) Unpredicted pulse generator failure 20 (45%) 18 (20%) Cessation of pacing 12 (27%) 15 (17%) Gross change (continued pacing) 8 (18%) 3( 3%) Electrode failure 7 (16%) 7( 8%) Elective 17 (39%) 64 (72%) Predicted pulse generator failure 11 (25%) 47 (53%) Mode change 1 ( 2%) 6( 7%) Routine replacement 5 (12%) 11 (12%) Total "Percentages are of total replacements for the year. VOL. 8, NO. 1, JULY,

8 FURMAN ET AL. generator replacement for unpredicted failure was done in 20%; 8% of pulse generator replacements were done during repair of an electrode malfunction. Seventy-two percent of all replacements were elective (Table 1). Of these pulse generators, one that was replaced after 30 months showed significant residual life. One pacemaker was removed at 27 months on a false determination of battery exhaustion. All other pulse generators that were removed were confirmed in their actual or impending failure upon return to the manufacturers. One patient died of a clearly malfunctioning pacemaker, although she had been checked and the unit had been satisfactory one month earlier. SUMMARY Careful serial observations of implanted cardiac pacemakers are needed to reduce unforeseen pacemaker failures and their consequences. Analysis of triggered pacemaker function, constancy of pacemaker rate, and electronic analysis of pacemaker output allow accurate prediction of impending pacemaker failure or exhaustion before they are clinically manifest. REFERENCES 1. Knuckey, L., McDonald, R., and Sloman, G. A method of testing implanted cardiac pacemakers. Brit. Heart J. 27:483, Pacemaker Follow-up Reliability Report. Medtronic, Inc., Oct. 16, Smyth, N. P. D., Bacas, J. M., and Keller, J. W. Experimental and clinical use of a variable parameter cardiac pacer. Dis. Chest 53:93, Sowton, E. Detection of impending pacemaker failure. Israel J. Med. Sci. 3:260, Sprawls, P., Miller, W. B., and Logan, W. D. Observation of electromagnetic signals from implantable pacemakers. J. Thorac. Cardiouasc. Surg. 49:748, Van Den Berg, J. W., Rodrigo, F. A., Thalen, H. J., and Koops, J. Photoanalysis of the condition of implanted pacemakers and electrode circuits. Proc. Kon. Nederl. Akad. Wet. [Biol. Med.1 70:419, DISCUSSION DR. HAROLD STERN (New Haven, Conn.): I have had the privilege of reviewing Dr. Furman s excellent paper and was delighted to learn about his scientific approach to the latest plague of the chest surgeon, acute pacemaker failure. Analysis of the stimulus wave form and his very accurate measurement of stimulus intervals will serve to teach us about the failure characteristics of the devices we are using. His work, as you know, was done with the Cordis product, and I would like to see him carry it over to other products, perhaps in other institutions. For example, a rate change of 10% with the unit I use may be much more ominous than a rate change of 10% with a Cordis. Perhaps he can answer that. 64 THE ANNALS OF THORACIC SURGERY

9 Electronic Analysis for Pacemaker Failure What we really want to know is the time during which the pacemaker is safe, even if it means taking out some units prematurely. Electronic component failure should approach zero in this space age, when we are told that devices circling the globe are good for four, five, or six years. We have been changing our units routinely at 18 months whether they need it or not, and the hospitalization has been very brief. The patients are more confident and worry less about battery failure with this method. Our experience is much more limited than Dr. Furman s, but it seems to me that 25 emergency replacements in one year is excessive. What we need is a regular, detailed analysis by the manufacturers of these products which would include not only the longevity curve but also what happens when the pacemaker fails: slowing, speeding up, sudden standstill, battery failure, component failure. In that way we could learn more about the apparatus we use, and perhaps even change to a better product. DR. SAMUEL W. HUNTER (St. Paul, Minn.): It has been ten years now since we first put a pacemaker in a human being. I would like to remind the Society that there are approximately 15,000 new cases of total heart block per year in the United States. Moreover, pacemakers for arrhythmias other than total heart block have been developed to the point that if we multiply that 15,000 by 4 or 5, we would come closer to the annual number of pacemakers that will be used in this country within the next several years. The problem arises when you get a call at three o clock in the morning from a patient whose pacemaker has failed. Perhaps this is not going to happen after the sophisticated methods described by Dr. Furman have been introduced, but I am not so sure that it won t. Pacemakers fail for a multiplicity of reasons, such as the electrode eroding through the right ventricle, as just described by Dr. Peter Allen of Vancouver. I am exploring with Medtronic the idea of establishing a computerized record system. If such a system were established, we could dial into a number at the computer center and obtain the necessary information quickly and accurately. Some such system is necessary because of the increasing number of pacemaker patients. Not only do we need this information ourselves, but we must be prepared to send it quickly to surgeons at other centers. DR. FURMAN: Dr. Stern, this method of analysis is suitable for any of the pacemakers presently available. It is somewhat more difficult to get a satisfactory analysis with a bipolar unit because of the smaller size of the electrocardiographic wave form; nevertheless, it is perfectly possible to do. I completely agree that one should have immediate access to all the pacemaker data on any particular patient. It is incumbent upon us to follow our patients very carefully. Despite the best efforts of the manufacturers, and they are improving their efforts, it is really up to us, once we know what the pacemaker will do, to see the patients sufficiently frequently so we will pick up failures. Of course, it is up to the manufacturers to let us know what the pacemaker will do and, very honestly, when they can anticipate that it may fail. VOL. 8, NO. 1, JULY,