Berkovits, E.E. (Ing.),? Walter Zuckerman, M.D., and Dwight E. Harken, M.D.

Transcription

1 NOTES An Implantable Demand Pacemaker Lawrence I. Zaroff, M.D.,* Barouh V. Berkovits, E.E. (Ing.),? Walter Zuckerman, M.D., and Dwight E. Harken, M.D. E lectrical stimulation in the treatment of heart block is now commonly accepted. With fixed-rate pacemakers, stimulation is continued following resumption of normal sinus conduction. A variety of recent publications confirm the danger of competitive stimulation associated with fixed-rate pacemakers [2, 4-6, 9, 10, 121. A new instrument for stimulation was introduced in June, 1964 [7, 81. This instrument imitates the natural escape mechanism and initiates only nonparasystolic escape beats. The beats are stimulated only when the interval after the last QRS complex exceeds the repolarization time of the artificial pacemaker. The demand pacemaker thus acts as a natural automatic fiber with controlled repolarization time. The new implantable demand pacemaker provides both sensing and stimulation through the same electrode. The demand pacemaker was first designed for extracorporeal bedside use. Development of an implantable unit was reported on by one of us (B.V.B.) in May, 1965 [ll. Experimental work with this implantable demand pacemaker using intrathoracic direct myocardial electrodes in eight animals is reported here. MATERIALS AND METHODS The implantable unit measures 6 x 5 X 2 cm. A preliminary model was enclosed in a paraffin case initially, but the later modifications have been enclosed From the Departments of Surgery, Harvard Medical School and Peter Bent Brighnm Hospital, Boston, Mass. *Present address: Department of Surgery, Rochester General Hospital, University of Rochester School of Medicine, Rochester, N.Y. tdirector of Research and Development, American Optical Company, Boston, Mass. This work was supported by U.S. Public Health Service Grants No. 5 T1 HE , HE 8G98-02, and ROl-HE Accepted for publication June 14, VOL. 4, NO. 5, NOV.,

2 ZAROFF, BERKOVITS, ZUCKERMAN, AND HARKEN in permanent stainless-steel cases covered by Silastic. The electrodes have been of the Chardack type, implanted close to the apex of the left ventricle. Although myocardial electrodes were used in all experiments, this unit is also adaptable for intravenous use. Four animals without heart block had pacemakers inserted through a thoracotomy incision with implantation of the power unit in the subcutaneous tissues or beneath the latissimus dorsi. In these four animals arrest or bradycardia was produced by administering acetylcholine, thus affording an opportunity for the demand pacemaker to take over the ventricular pacing. In four additional animals, after implantation of the electrodes through a right thoracotomy incision, permanent heart block was produced by suture of the conducting bundle utilizing inflow occlusion [l I]. In these animals isoproterenol-induced ventricular tachycardia was the mechanism used to produce a heart rate sufficient to verify cancellation of the pacemaker stimulus. The generator contains a switch which may be activated by an external magnet to convert from demand pacing to continuous pacing for testing purposes. Thus, testing at increasing time intervals after the QRS complex assessed the ability of the power unit to initiate stimulation and ventricular contraction, whereas the production of ventricular tachycardia established the ability of the same type of unit to turn itself off. Testing was also carried out by switching from a demand to a continuous mode in all animals. RESULTS In the four animals without permanent, surgically-induced heart block, temporary block was induced by acetylcholine administration with testing of the pacemaker immediately after insertion and at weekly intervals. In each instance on the demand mode setting, the pacemaker immediately began to stimulate as soon as asystole developed. A minute dose of acetylcholine causing even one dropped beat was sensed instantly (Fig. 1). In animal T858, operation was performed in April, The pacemaker functioned well for 2% months, at which time the paraffin case was found to be damaged. This was replaced with a stainless-steel encased model that has functioned perfectly since. Whenever the pacemaker was turned to the continuous mode, competition always resulted. Of the four animals with permanent heart block, one animal chewed through a paraffin case that had been placed too far laterally on the chest wall one week postoperatively. A second animal, No. T356, died on the fourth postoperative day of congestive heart failure because he had been left on a demand rate of 80 beats per minute, too slow for many dogs. Figure 2 illustrates the cessation of pacing when a blocked animal is given isoproterenol. In view of the disasters that have been reported in which electrical appliances, automobile starters, and other commonplace devices encountered in our FIG. 1. The administration of a small dose of acetylcholine to an animal in normal sinus rhythm caused a single dropped beat; the pacemaker detected it and deliuered an impulse. 464 THE ANNALS OF THORACIC SURGERY

3 NOTE: Demand Pacemaker FIG. 2. The administration of isoproterenol to an animal in heart block nccelcrated the intrinsic ventricular rate and turned the pacemaker of. FIG. 3. A 60-cycle 25 ma. A.C. current applied to an animal an complete block who was being paced in the demand mode. The top tracing is a continuous arterial pressure; the bottom tracing is the EGG. The blackened area is the period of electrical interference. There is no malfunction of the pacemaker. Animal TABLE 1. ~ ~~ ~~ Without heart block T470 T858 U66 U25 With heart block T238 T356 T302 U516 RESULTS OF DEMAND PACING IN EIGHT DOGS Results Paraffin unit replaced after 2% months with unit enclosed in stainless steel; continued excellent function function until animal destroyed case one week after implant, but animal died on fourth postoperative day of congestive heart failure (rate too slow) initially, but animal died in 1 month of empyema

4 ZAROFF, BERKOVITS, ZUCKERMAN, AND HARKEN daily lives have interfered with pacing, an experiment to test these interfering sources was carried out. Dog U516 was utilized for a special experiment of this type. Heart block was produced, and a demand pacemaker set at a rate of 120 was inserted. The arterial blood pressure was monitored directly through a femoral artery with a straingauge and recorder. The dog was then tested for electrical interference with the pacemaker by an electric shaver, by a standard surgical electric cautery, and then directly by 10 to 25 ma. of A.C. 60-cycle current between the right upper extremity and the left lower extremity and then other areas at random. These explorations for electrical interference were conducted under Nembutal anesthesia. There was absolutely no interference with the sensing or discharging of the pacemaker function under any of these circumstances (Fig. 3). This pacemaker continued to work well for a month, but the dog later died of intercurrent infection. Table 1 summarizes the results in all the animals. DISCUSSION Standard pacemakers function at fixed rates and deliver impulses to the myocardium with no regard to the intrinsic activity of the heart. Thus, artificial impulses may fall into any phase of the cardiac cycle, including the vulnerable period. Recent evidence has indicated that such stimulation can result in ventricular fibrillation. Even in the voltages used, such effects certainly do exist; although the threshold to ventricular fibrillation is not exceeded commonly, it is apparent now that transient lowering of fibrillation threshold can endanger the patient [3, 131. Our experience with the implantable pacemaker unit has been uniformly excellent. Except for instances when the temporary paraffin case has been violated, the unit has functioned perfectly at all times. We have not observed malfunction of the demand mode. Should this occur, continuous pacing would begin at once. Another point worthy of emphasis is that the demand unit functioned accurately despite pronounced electrical interference, as demonstrated in dog U5 16. This accurate function is due to the special circuitry that provides selective recognition of the QRS complex without 60-cycle interference. The laboratory evidence is such that clinical trial is warranted." Such a demand unit would be particularly useful in treating varying and intermittent heart block as well as in treating heart block that may be temporary, such as occurs after myocardial infarction or open-heart surgery. SUMMARY An implantable demand pacemaker has been inserted in eight dogs, with excellent functional results. By monitoring the QRS com- *At present, eight demand pacemakers of this design have been implanted successfully in human beings and will he reported separately. 466 THE ANNALS OF THORACIC SURGERY

5 NOTE: Demand Pacemaker plex, this pacemaker programs the artificial stimulation to avoid competitive beats by imitating the natural escape mechanism of the heart. Environmental electrical activity does not interfere with this unit s function. REFERENCES 1. Berkovits, B. V. The Demand Pacemaker. Presented at the Electrical Control of Cardiac Activity Graduate Symposium, Buffalo, N.Y., May, Bonnabeau, R. C., Jr., Bilgutay, A. M., Stearns, L. P., Wingiore, R., and Lillehei, C. W. Observations on sudden death during pacemaker stimulation in complete atrioventricular block leading to development of P wave pacemaker without atrial leads. Trans. Amer. SOC. Artif. Intern. Organs 9:158, Castellanos, A., Jr., Centurian, M. J., Lemberg, L., and Berkovits, B. V. Concealed digitalis toxicity exposed by pacemaker stimuli. Clin. Res. 14:241, Castellanos, A., Jr., Lemberg, L., Jude, J., and Berkovits, B. Repetitive firing occurring during synchronized electrical stimulation of the heart. J. Thorac. Cardiov. Surg. 51:334, Dressler, W., Jonas, S., and Rubin, R. Observations in patients with implanted cardiac pacemaker: IV. Repetitive responses to electrical stimuli. Amer. J. Cardiol. 15:391, Elmqvist, R., Landegren, J., Pettersson, S. O., Senning, A., and William- Olsson, G. Artificial pacemaker for treatment of Adams-Stokes syndrome and slow heart rate. Amer. Heart J. 65:731, Lemberg, L., Castellanos, A., Jr., and Berkovits, B. V. Pacemaking on demand in AV block. J.A.M.A. 191:12, Lemberg, L., Castellanos, A., Jr., Berkovits, B., and Gosselin, A. The Demand Cardiac Pacemaker: A New Instrument for the Treatment of A-V Conduction Disturbances. Presented at the Interamerican College of Cardiology Meeting, Montreal, Quebec, Canada, June, Sowton, E. Artificial pacemaking and sinus rhythm. Brit. Heart J. 27:311, Taval, M. E., and Fisch, C. Repetitive ventricular arrhythmia resulting from artificial internal pacemaker. Circulation 30:494, Weirich, W. L., Gott, V. L., and Lillehei, C. W. The treatment of complete heart block by the combined use of a myocardial electrode and an artificial pacemaker. Surg. Forum 8:360, Widmann, W. D., Eisenberg, L., Levitsky, S., Mauro, A., and Glenn, W. W. L. Ventricular fibrillation complicating electrical pacemaking. Surg. Forurn 14:260, Wiggers, C. J., Wegria, R., and Pinera, B. Effects of myocardial ischemia on fibrillation threshold: Mechanism of spontaneous ventricular fibrillation following coronary occlusion. Amer. J. Physiol. 131:309, VOL. 4, NO. 5, NOV.,