Externally Rechargeable Cardiac Pacemaker

Transcription

1 Externally Rechargeable Cardiac Pacemaker Arthur VV. Silver, M.D., George Root, M.S., Francis X. Byron, M.D., and Harry Sandberg, M.S. T otally implantable cardiac pacemakers currently available are powered by self-contained mercury batteries having a relatively short life. The estimated period of useful function is not inore than five years, and experience has shown that actual lifetime may be much less than this [3, 7, 151. This means at least another operative procedure to replace the battery, with the attendant risk of infection and, probably, recurrent cardiac arrhythmia such as heart block or tachycardia. Older patients can therefore anticipate several replacements, and the child with surgically produced heart block could expect more than ten replacements in the course of a normal human life span. With the intent of extending the period between required replacements or of completely eliminating the necessity of replacement, an experimental model of a constant-rate pacemaker using rechargeable nickel-cadmium batteries has been developed. This type of battery can be completely sealed with no danger of leakage and may be repeatedly recharged without damage. Power to recharge the batteries is supplied by induction from an externally produced magnetic field. This magnetic field can pass through tissue and clothing unattenuated [l, 81. This principle was used earlier by Elmqvist and Senning in 1959 to construct a rechargeable unit [5] but problems arose in its clinical application [ll, 121. Current technology and improved methods of pacemaker manufacture may now overcome the obstacles encountered previously. The pacemaker package contains the additional circuitry necessary to convert the magnetic field into useful charging current and to give From the City of Hope Medical Center, Duarte, Calif., and Hycon Manufacturing Company, Monrovia. Calif. This study was supported in part by thc Kational Institutes of Health General Research Grant FR and the General Rescarcli Funds of Hycon Manufacturing Company. Presented at the First Annual Meeting of The Society of Thoracic Surgeons, St. Louis, Mo., Jan , THE ANNALS OF THORACIC SURGERY

2 Externally Rechargeable Pacemaker FIG. 1. Experimental model of the rechargeable pulse generator (without electrodes). The coil, which picks up the charging current from the external electromagnet, is wrapped around the other components. FIG. 2. Compact electromagnet which FIG. 3. Power supply for recharging is held over the subcutaneously im- the pacemaker. The meter indicates planted pulse generator during period that charging is taking place and when of recharging. the implanted batteries are charged. an external indication of when the batteries are fully recharged, at the same time automatically preventing overcharging. In case of certain types of battery failure, power to operate the pacemaker could be supplied directly by the charging circuit. The experimental model is shown in Figure 1. It is approximately the same size and weight as the pacemakers presently in clinical use [4, 9, 151. The components are embedded in epoxy resin. The small, hand-held electromagnet and power supply for recharging are illustrated in Figures 2 and 3. VOL. 1, NO. 4, JULY,

3 SILVER, ROOT, BYRON, AND SANDHERG SPECIFICATIOhTS Ah'D CIRCUITRY The nickel-cadmium cells used are approximately the same size as the mercury batteries found in clinical pacemakers. This type of cell is presently used in industrial applications requiring high current drains where 50% or more of the total battery capacity is used on each discharge. Under these rather strenuous conditions, cycle-lives of 1,000 to 2,000 cycles with total lifetimes of several years are achieved [6, 131. However, the cardiac pacemaker requires only a very low current drain and not more than 1% of the battery capacity is used between recharges. Accelerated tests have shown that under these very light load conditions, cycle lives of 10,000 to 12,000 cycles may be expected [13]. To our knowledge no test data are presently available on total lifetime in years, but if the number of cycles were the only determinant of battery life, a rechargeable pacemaker might be expected to last 25 to 30 years before need for surgical replacement. It is possible that over this greatly extended lifetime, other failure mechanisms may come into play, limiting lifetime to something less than this projected figure. Figure 4 shows the schematic diagram of the external circuitry used to provide the magnetic field for charging. This device consists of a phase-shift oscillator operating at approximately 5 kc. and a pushpull power amplifier driving the electromagnet, L1, which thus produces an alternating magnetic field. A built-in power supply converts line voltage to about 15 volts D.C., although batteries could be used to make a completely self-contained unit. When power is extracted from the field created by the electromagnet, as when eddy currents in a proximate metal structure occur or as when charging current is induced in a pickup coil, the current required by L1 increases to supply these additional ~ V o i o c P O l l E R SUPPLY -- 1', POWER AYPLIFIE FIG. 4. Schematic diirgrartz of thc pxtcrnal circititry. This provides the magnetic field for charging and indicates when charging zs taking place and when it is completrd. 382 THE ANNALS OF THORACIC SURGERY

4 Externally Rechargeable Pacemaker G-G I r _. I 1 I S C R r n I I FIG. 5. Schematic diagram of the implantable pacemaker including charging and Pacemaking circuitry. The pacemaking circuit is compatible with the charging magnetic field. losses. This increase in current is monitored by meter, M, which thus serves to indicate when and how quickly the internal batteries are charging. Total power consumption is approximately 4 watts. Figure 5 shows the schematic diagram of the implantable pacemaker containing charging circuitry and a constant-rate pacemaking circuit which is compatible with the magnetic field used. As the external electromagnet is brought near the implanted pacemaker, charging current is induced in the tuned coil, Le, and flows through transistor, Q, and diode, D, into the batteries. Meanwhile, the external meter reading has increased, indicating that charging is being accomplished. As current continues to flow into the batteries, battery voltage rises slowly. When this voltage reaches a preset level, the silicon-controlled rectifier, SCR, fires and prevents transistor, Q, from passing any additional current. Charging current immediately drops to zero, thereby protecting the batteries from overcharging. This sudden drop in power drawn from the magnetic field is signaled by a corresponding drop in the current through the external meter, which provides a positive indication that the batteries have been fully charged. Diode D now prevents battery drain into the charging circuit. With the model presently used, adequate charging current is obtained with the magnet 1 to 1.5 cm. from the implanted pacemaker. Daily recharging requires about three minutes at this distance. It had originally been intended to use a pacemaking circuit very similar to that in one of the commercially available units which utilizes a transformer to produce the pacemaking pulses [4]. It was found that this circuit was severely affected by the magnetic field used in charging, to the extent that it ceased operation entirely. The transformer could not easily be adequately shielded from this effect, and so a new circuit was developed which contains no magnetic components. This circuit VOL. I, NO. 4, JULY,

5 SILVER, ROOT, BYRON, AND SANDBERC produces pulses of 4 v. amplitude and 2 msec. duration at a fixed rate of about 68 per minute. The pulses are biphasic, which is important in preventing ion diffusion and subsequent polarization of the heart electrodes. This is essential for very long-term pacing in order to prevent electrode erosion due to electrolysis [4, 101. EXPERIMENTAL STUDIES During recharging it is inevitable that a certain amount of ripple will be produced in the output. This ripple takes the form of a smallamplitude sine wave at the charging frequency (5 kc.) superimposed upon the normal pulsed output. Capacitors C1 and Ca are included in the circuit to reduce this ripple to a negligible level. Output ripple during charging is therefore less than 20 mv. root mean square. To determine what value of ripple could safely be tolerated, the voltage necessary to produce arrhythmia in a canine heart was measured as a function of frequency. Electrodes were implanted in the right ventricle of a dog, simulating conventional pacemaker placement, although heart block was not induced and normal sinus rhythm continued. The voltage necessary to produce the first sign of arrhythmia was measured at various frequencies. The results are shown in Figure 6. It is seen that although the heart is very sensitive to stimulation at low frequencies, sensitivity drops off very rapidly as the frequency increases beyond about 1,000 cycles per second (cps). At the charging frequency of 5,000 cps, over 1.5 v. is required to produce arrhythmia. This is much greater than the maximum output ripple of 20 mv. root mean square. Thus no arrhythmias were anticipated or encountered during battery recharging. FREQUENCY (CPS)- 1 KC 10 KC FIG. 6. Voltage required at mriotts frequencies to proditce arrhythmia in the canine heart. Electrodes implanted in right ventricular iuall. Sinits rhythm. 384 THE ANNALS OF THORACIC SURGERY

6 Externally Rechargeable Pacemaker Although it was not anticipated that the electromagnetic field used for recharging would have any adverse effects on the skin and subcutaneous tissues, a pacemaker working into a dummy load simulating the approximate load imposed by the heart was implanted in the subcutaneous tissue of a pig especially bred for experimental work (Hormel Institute). This species was selected because it has a thick pannus of subcutaneous fat and dermal responses similar to those of the human. A layer of skin and tissue approximately 1.5 cm. thick covered the pacemaker. The battery pack was recharged daily, requiring an average of one minute per day. Approximately three months after implantation the pacemaker ceased to function and was removed. It was found that body fluids had leaked into an extension containing test leads. These fluids had caused a short circuit which completely discharged the batteries. However, it was found that the circuitry was still in operating condition. At the time the pacemaker was removed, biopsies were taken from the pacemaker site and from a control site. These revealed no abnormalities in the tissue from over the pacemaker other than those directly attributable to the presence of a foreign body. Heart block was created in a dog by the method of Starzl and Gaertner [14], and an experimental model of the rechargeable pacemaker with helical-coil cardiac electrodes [Z] was connected to the right ventricle. The pacemaker pulse generator was implanted subcutaneously in the abdomen. Daily recharging was carried out by holding the small electromagnet over the implanted pacemaker (Fig. 7). This caused no apparent discomfort. The charging times ranged from three to four minutes. Figure 8 illustrates the electrocardiogram during a FIG. 7. Dog with surgically produced heart block and implanted pacemaker being recharged. The electromagnet is held over the pulse generator implanted subcutaneously in the abdomen. The meter monitors the charging process. VOL. 1, NO. 4, JULY,

7 SILVER, ROOT, BYRON, AND SANDBERG RECHARGING STARTED 4 RECHARGING COMPLETED 4 FIG. 8. Electrocardiogram during rccharging of batteries in a dog with heart block and an implanted pacemaker. Small pacemaker impulse precedes each R WQ7JE. Charging does not alter electrocardiogram. period of charging. Note that the charging does not interfere with the pacemaker output, and any ripple from the charging circuit as demonstrated previously is not sufficient to produce cardiac arrhythmia during the period of charging. DISCUSSION The life tests that have been performed indicate that battery lifetime is inversely proportional to the percentage of total capacity removed before recharging. This implies that frequent recharges are advisable to preserve the battery life. Since daily recharging limits current drain to less than 1% of total cell capacity in the case of this pacemaker, we have utilized daily recharging in our current studies. This has the additional advantage of reducing the time required for each recharge to a few minutes a day. The manufacture of a small sealed cell presents technical problems. The configurations necessary to produce a cell with high current-delivering capability are not necessarily those which one would choose if one were concerned only with achieving maximum reliability. It is very possible that a cell designed for low current drains would have a greater life expectancy than that of cells now commercially available. The mercury batteries currently used in the presently available pacemakers and the nickel-cadmium batteries used in our present experimental models were not specifically developed for biological purposes. The need for rechargeable batteries capable of supplying energy reliably 386 THE ANNA1.S 01: THORACIC SURGERY

8 Ex ternally R echargea b le Pacemaker over a period of decades strongly suggests the need for revision of current battery technology to meet these specific biological requirements. The electrical capacity of the nickel-cadmium cells used by us is not as great as that of the nonrechargeable mercury cells. Although they theoretically have the capacity to run the pacemaker for about six months without recharging, the self-discharge or shelf life at 100 F. indicates that the cells lose 50% of capacity in about 150 days [6, 131. This means that the battery pack would continue to operate satisfactorily between two and three months without being recharged. During this period the electrical characteristics of these cells would be almost identical to those of mercury batteries, and so the same indications of impending battery exhaustion, e.g., a change in pulse rate, would be obtained as with clinical pacemakers. Since the external charging circuit gives immediate notice of failure of the internal circuit to accept the charge, the patient will have adequate time to seek medical help. The rechargeable features of this experimental pacemaker in addition to reducing the necessity for surgical removal of exhausted batteries will permit the use of smaller pacemakers containing perhaps only one rechargea,ble cell. This will permit the incorporation of such other features into the pacemakers as synchronous pacing or variable rate and intensity features controlled by external circuitry without sacrificing size limitations. These modifications will be investigated in the laboratory and the optimum model developed for patient use. SUMMARY An experimental pacemaker has been developed which can be recharged by the external application of electromagnetic induction currents. The present experimental model is fashioned after the clinically available pacemakers, but future experimental models are planned which will take fuller advantage of the rechargeable features of the pacemaker. Experiments to date indicate that external recharging of the battery is feasible and can be carried out without danger of producing cardiac arrhythmias. REFERENCES 1. Abrams, L. D., Hudson, W. A., and Lightwood, R. A surgical approach to the management of heart-block using an induction coupled artificial cardiac pacemaker. Lancet 1: 1372, Chardock, W. M. A myocardial electrode for long-term pacing. Ann. N.Y. Acad. Sci :893, Chardack, W. M., and Greatbatch, W. Failure Rate Report, Chardack- Greatbatch Implantable Pacemakers. Minneapolis: Medtronic, Inc., Chardack, W. M., Gage, A. A., and Greatbatch, W. A transistorized, selfcontained implantable pacemaker for the long-term correction of complete heart block. Surgery 48:643, VOL. 1, NO. 4, JULY,

9 SILVER, ROOT, BYRON, AND SANDBERG 5. Elmqvist, R., and Senning, A. An implantable pacemaker for the heart. In Medical Electronics, Proceedings of the Second International Conference on Medical Electronics. London: Iliffe, 1960, p Francis, H. T. Space batteries. In Technology Handbook (N.A.S.A. SP- 5004). Washington, D.C.: G.P.O., Gadboys, Howard L., et al. Surgical treatment of complete heart block: An analysis of 36 cases. J.A.M.A. 189: 123, Holswade, G. R., and Linardos, C. Induction pacemaker for control of complete heart block. J. Thorac. Cardiov. Surg. 44:246, Kantrowitz, A. The treatment of Stokes-Adams syndrome in heart block. Progr. Cardiov. Dis. 6:190, Rowley, B. A. Electrolysis: A factor in cardiac pacemaker electrode failure. I.E.E.E. Trans. Biomed. Electronics BME-10: 176, Siddons, A. H. M. Long-term artificial cardiac pacing: Experience in adults with heart block. Ann. Roy. Coll. Surg. Eng. 32:22, Siddons, A. H. M., and Humphries, 0. N. Complete heart block and Stokes- Adams attacks treated by indwelling pacemaker. Proc. Roy. SOC. Med. 54:237, Stafford, W. T. Electrochemical energy sources: Nickel-cadmium batteries. Electro-Technology 67:82, Starzl, T. E., and Gaertner, R. A. Chronic heart block in dogs: A method for producing experimental heart failure. Circulation 12:259, Zoll, P. M., Frank, H. A., and Linenthal, A. J. Four-year experience with an implanted cardiac pacemaker. Ann. Surg. 160:351, L)ISCUSSION DR. BYRON: If, in fact, the rechargeable pacemaker proves with further experience to be completely feasible, we can depend on an unlimited source of power over a long span of time; this creates a whole sphere of possibilities. For instance, our engineer colleagues, Mr. Sandberg and Mr. Root, tell us that the pacemaker could be programmed to increase the output gradually so that if we find that electrode resistance is one of the factors in limitation of pacemaker life this could be overcome. Also, in children where there is need for frequent increases in the pacemaker s rate such as during play, febrile illnesses, etc., it would be possible to modify the rate considerably without worrying about battery life or implanting a pacemaker which is too bulky to be well tolerated. 388 THE ANNALS OF THORACIC SURGERY