Current Status of Pacemaker Power Sources

CURRENT REVIEW Current Status of Pacemaker Power Sources G. Frank 0. Tyers, M.D., and Robert R. Brownlee, M.S.E.E. ABSTRACT After years during which pacers of very similar design and capabilities were provided by a small number of manufacturers, many different lithium, halogen, rechargeable, and nuclear power sources are now available. The variety of chemistries, methods of construction, and sealing techniques used in the batteries of the different manufacturers is almost unlimited. This has made it necessary for physicians who implant and follow pacers to acquire a general knowledge of the field if they are to make an informed choice of pacemaker power source for implantation and if they are to manage recalls with a minimum of patient and physician trauma. More experience is required before it can be definitely determined which of the new pacer power sources will prove superior, but when coupled with well-designed, hermetically sealed pulse generators, all are capable of providing continuous pacing for at least 5 years and the 10-year pacemaker is now a probability. The first totally implantable cardiac pacemaker, a rechargeable device powered by nickel cadmium cells, was developed and used clinically by Senning in 1958, but even with recharging, the functional life was measured in months [60]. From 1960 to 1969, essentially all implantable pacemakers were powered by multiple Mallory RM-1 zinc-mercuric oxide cells in nonhermetic epoxy enclosures with only minor functional differences observed between the units of different manufacturers [39]. The average 2-year life of these pacers necessitated frequent operations for replacement and stimulated the search for improved pacemaker power sources. The importance of these initial contributions should, however, not be under- From the Department of Surgery, Division of Cardiovascular and Thoracic Surgery, The University of Texas Medical Branch, Galveston, TX. Supported by US Public Health Service Grant HL 13988. We gratefully acknowledge the assistance of Miss Valerie Wagaman and Mrs. Yvonne Hricak in the preparation of this manuscript. Address reprint requests to Dr. Tyers, Division of Cardiothoracic Surgery, Department of Surgery, The University of Texas Medical Branch, Galveston, TX 77550. valued. One patient who received a rechargeable unit is still alive after multiple primary pacer changes, and several hundred thousand individuals have been returned to a productive and essentially normal life by permanent cardiac pacing [191. Three general classes of pacemaker power sources have been developed. These are external, mixed, and internal. Only the internal or totally implantable devices are commonly used at present. Therefore, external and mixed pacing are considered only briefly. External Ventricular Pacing The modern era of artificial electrical cardiac stimulation began in the early 1950s with the outstanding work of Zoll and associates [601. Using a 60-cycle alternating-current-powered Grass stimulator and electrode plates held firmly to the chest wall by an encircling rubber strap, they resuscitated the hearts of terminal patients with complete heart block and repeated asystole and confirmed the one-to-one relationship of the electrical stimulus and the patient s pulse. Although strong electrical stimuli were applied (on occasion to subcutaneous needles), patients were able to eat, sleep, and continue some activities in spite of the limitations imposed by the external stimulator and connecting wires [601. With adequate monitoring and grounding, this system was and remains both simple and useful for emergency resuscitation of cardiac standstill but has no applicability to even temporarily extended pacing. The great advantage of an external system is the ease with which any malfunctioning component can be replaced or repaired. Mixed Ventricular and Atrial Pacing Like external pacing, the mixed system has an external and easily replaceable power source, but the electrical stimulus is delivered directly to the heart by a conductive element usually called an electrode [60]. Both temporary and permanent mixed systems are currently used, 571 0003-4975/78/0025-0617$1.50 @ 1978 by G. Frank 0. Tyers

572 The Annals of Thoracic Surgery Vol 25 No 6 June 1978 and while applicable to both atrial and ventricular pacing, the former is rarely used. Temporary Mixed Pacing Systems External rechargeable or primary batterypowered stimulators, calibrated for rate, current or voltage, and occasionally duration, are available from many pacemaker manufacturers and may be used relatively safely with percutaneous electrodes that pass transvenously to the apex of the right ventricle or directly through the chest wall into the myocardium. Temporary mixed transvenous electrode pacing is often used to support patients with complete heart block prior to insertion of a permanent pacer and patients with postmyocardial infarction heart block who usually revert to sinus rhythm if they survive. Mixed transthoracic systems are most commonly used for postoperative control of arrhythmia with temporary electrodes sutured to the heart at the time of cardiac operation. They are also used occasionally for emergency applications similar to those discussed for the external systems; special electrodes are placed blindly into the area of the cardiac apex through a needle. With all of the percutaneous systems, the external stimulator must be kept dry and all connections must be carefully insulated if ventricular fibrillation is to be avoided [601. Permanent Mixed Pacing Systems Inductive transmission of radiofrequency energy is not greatly attenuated by biological tissue, permitting the development of permanent pacing systems that combine an external transmitter and power source (for example, standard D flashlight battery) with a totally implanted receiving or secondary coil connected to the heart by a conductive element. Some 100 patients have been chronically paced with inductively coupled energy [60, 691, more than 50 of them for a period exceeding 5 years without a secondary surgical procedure and a few for more than 10 years without reoperation. In spite of this rather remarkable record, the problems associated with maintaining close proximity between the implanted coil and the external power source have precluded widespread acceptance of this type of pacer. With current technology, however, units that do not require tight coupling to the external power source are feasible. Internal Atrial and Ventricular Pacing Pacemaker is now used as a generic term for a totally implantable system capable of chronic electrical stimulation of the heart. Currently, more than 100,000 patients with a wide variety of arrhythmias receive permanent internal pacing systems yearly [19]. Electrodes connecting the subcutaneous pulsed power source to the heart (atrium or ventricle) may be placed directly into the myocardium or indirectly against the endocardium through a peripheral vein, preferably the cephalic vein. Chemical Batteries DEFINITIONS. To facilitate understanding of the variety of chemical cells now used in implantable cardiac pacemakers, several definitions must be considered. Cell and battery are often employed more or less interchangeably in the medical literature. A power cell is composed of an anode (e.g., zinc, lithium, or cadmium), a cathode (e.g., mercuric oxide, iodine polymer, or nickel oxide), and an electrolyte (e.g., potassium hydroxide or lithium iodide) and produces electricity as a result of a chemical reaction or a combination of reactions. Cells are generally named for the materials used in the anode and cathode, and in this review, the anode is always listed first. Cells are referred to as wet, dry, or solid state depending on the nature of the electrolyte, the chemical substance that conducts ions and electrons between the anode and the cathode within the cell. A wet cell has a large quantity of fluid electrolyte between the anode and the' cathode (e.g., an automobile lead-acid battery). A dry cell has a pastelike, nonspillable, conductive electrolyte (e.g., the Mallory zinc-mercuric oxide pacemaker cell and the ARCO, SAFT, and Cordis lithium cells). In contrast, a solid-state cell has an absolutely dry and solid conductive crystalline electrolyte (e.g., lithium iodide or rubidium silver iodide) between the anode and the cathode [21]. Examples are the Catalyst Research, Wilson Greatbatch, and Mallory lithium cells currently used in pacemakers.

573 Current Review: Tyers and Brownlee: Pacemaker Power Sources To produce an electrical current the anode of a cell ionizes, resulting in the migration of positively charged metallic ions through the electrolyte toward the cathode. Electrons are left behind on the anode, which becomes negatively charged relative to the cathode. When the anode and cathode are connected by a conductive pathway, an electric current (flow of electrons) passes from the anode to the cathode. If the resistance in the conductor is relatively high, as in a pacemaker circuit, the flow of electrons will be slow and the chemical energy in the cell will last for a relatively long period of time. If the resistance of the conductor is low, as in a wet and short-circuited nonhermetic (epoxy) pacemaker, the chemical energy of the cell will be exhausted in a comparatively short period [S]. If the cell is primary, it must be discarded once the chemical energy has been exhausted and the voltage between anodal and cathodal terminals has dropped too low to run the circuitry. If the cell is secondary or rechargeable, application of an electrical current across its terminals will reverse the chemical reactions of discharge and restore its chemical energy. A battery is a group of connected cells, either in parallel (anode to anode, cathode to cathode) to increase capacity or in series (anode to cathode) to multiply the single-cell voltage by the number of cells in the battery. For the first 10 years following the introduction of internal pacers, the battery was usually 4 to 6 Mallory RM-1 zinc-mercuric oxide cells connected in series or in a series and parallel combination. When a voltmeter is placed in the circuit between the anode and the cathode of a cell, the voltage measured is a function of the chemical reactions occurring within the cell. For example, the beginning-of-life voltage for a zincmercury cell is 1.35 volts compared with 2.8 volts for a lithium-polymer iodine cell (Catalyst Research, Wilson Greatbatch) and 3.1 volts for a lithium-silver chromate cell (SAFT). The ampere-hour rating of a cell is a function of its size. Thus, for any given anode, cathode, and electrolyte chemical combination, doubling the cell size approximately doubles the amperehour rating. The standard Mallory RM-1 Group I1 zinc-mercuric oxide cell is rated at 1 am- pere-hour; that is, prior to exhaustion of its chemical energy, it can produce a current of 1 ampere for 1 hour, or 1 milliampere for 1,000 hours, or 20 microamperes for 50,000 hours or 5.7 years. It has the potential to drive a pacer for more than 5 years if the pacer circuit will run on an electromotive force (EMF) as low as 1 volt. Historically, pacemaker circuits have been relatively inefficient because they required much higher voltages than this for operation. This explains why all of the earlier primary epoxypotted pacers needed multiple zinc-mercuric oxide cells connected in series, that is, a battery. However, we have developed discrete component pacemaker circuitry that will run on as little EMF as 1 volt [651, and newer technologies such as complementary metal oxide semiconductor (CMOS) and integrated injection logic (I*L) will function at 2 volts (a single lithium cell) and 1 volt, respectively. The ampere-hour capacity is by itself not a totally reliable means of estimating the usable energy capacity of a cell since it does not describe other cell variables such as impedance and voltage decay with time of the different chemical systems. The watt-hour capacity is a function of cell voltage and cell size (amperehours) and allows for a somewhat more meaningful comparison of different pacemaker power sources. For example, the 1 ampere-hour Mallory zinc-mercuric oxide cell has a capacity of approximately 1.3 watt-hours (1 ampere-hour X 1.33 volts) while the 0.6 ampere-hour SAFT lithium-silver chromate cell has a somewhat higher energy capacity of approximately 1.8 watt-hours (3 volts X 0.6 ampere-hour) and the 1.2 ampere-hour Catalyst Research 804 lithium-polymer iodine cell has a capacity that is greater than 3 watt-hours, more than double the pacing energy in the 1 ampere-hour RM-1 cell. By dividing the watt-hour capacity of a cell by its weight or its volume, the watt-hours per gram or watt-hours per cubic centimeter, respectively, can be determined to facilitate comparison of the energy densities of the wide variety of pacer power sources now in use. Assuming that all of the energy is available at the current drains and impedances involved, then the higher the energy density, the longer the theoretical pacemaker life that can be contained

574 The Annals of Thoracic Surgery Vol 25 No 6 June 1978 in a given volume. Perhaps the most important comparative specification is the pacemaker lifetime to volume ratio because the majority of the volume of current pacers is taken up by the power source. The energy density of the familiar Mallory RM-1 Group I1 zinc-mercuric oxide cell is more than 0.5 watt-hour per cubic centimeter, and the energy density of the rechargeable zinc-mercury cell is essentially the same during its primary discharge cycle. After 7 years of continuous cycling, each rechargeable zinc-mercuric oxide cell currently under test has delivered more than 9 watt-hours per cubic centimeter [67]. The highest primary energy density of currently available pacer power sources is 0.9 watt-hour per cubic centimeter for the ARC0 lithium-carbon cell, and the lowest is 0.05 watt-hour per cubic centimeter for the Pacesetter BF 20 cadmium-nickel oxide rechargeable cell. As with the zinc-mercuric oxide cell, watt-hour capacity can be increased by recharging, but its effectiveness is diminished by the high self-discharge. The self-discharge rate is the rate at which a cell loses energy spontaneously during storage or in addition to the energy being removed by the circuitry following pacemaker manufacture. Temperature, humidity, and rate of discharge affect the measurement of the capacity lost to other than useful work, but approximate figures for presently used systems range from a 1% loss per year (0.02% per week) with the Mallory LSA 900-6 lithium-iodine, lead salt cell (used in some Coratomic and Intermedics pacers) and the lithium-iodine cells, to 15% per week with the Pacesetter cadmium-nickel oxide rechargeable cell. With proper storage, the self-discharge rate of all the other pacer power sources considered in this report ranges from 2 to 5% per year. It becomes obvious that the lower the self-discharge rate of the cell, the longer the potential primary life of the implantable pacer or the lower the requirements for recharging. A nominal estimate of the drain of single-cell pacemaker circuits approximates 25 microamperes. Multiple cells or higher-voltage systems draw somewhat less current if properly designed. By rating pacemaker cells in units of 25 microampere-years (arbitrarily designated as pacemaker years) rather than in milliampere- hours, the detrimental effects of self-discharge can be more easily assessed. For example, the new 1 ampere-hour cadmium-nickel oxide cell becomes a less than 1 pacemaker-year cell while the 1 ampere-hour zinc-mercuric oxide cell remains a 4 pacemaker-year cell. The impedance of a cell also affects its range of usefulness for biological applications. Impedance may be defined as the degree of nonconductivity of a material to an electric current. All chemical power sources have an inherent internal impedance that varies with the chemical and mechanical constituents of the cell and with the degree of cell discharge. For example, a rapid rise in impedance severely limited the usefulness of the early lithium-polymer iodine cells developed by Catalyst Research Corporation. Even though chemical energy remained in these cells in terms of ampere-hours, it was essentially unavailable in terms of the 25 microampere-year units of pacer drain just defined. The Catalyst ResearcWWilson Greatbatch 702E cell used by Cardiac Pacemakers, Inc., in the original lithium pacer, the Maxilith, was a success because design changes slowed the rate of rise of the internal impedance during discharge. Newer cells of the lithium-polymer iodine type such as the Catalyst Research 8031 804 series exhibit even a slower rise in impedance, while other types of lithium systems may show a so-called bathtub curve (early drop, late rise) or only a late rise in impedance. The impedance, volume, and voltage changes of the various lithium systems are included in Tables 1 and 3 through 6. To illustrate the marked difference between the zinc-mercuric oxide and the lithium-polymer iodine systems, Ruben reported that the impedance of the Mallory RM-1 cell rises from approximately 0.6 to 0.8 ohm and then drops back toward 0.6 ohm as its chemical energy is exhausted [58]. In contrast, a typical lithium-polymer iodine cell (Catalyst Research 802-35 mm) begins life at a low impedance and steadily rises to an impedance in the range of 20,000 to 30,000 ohms prior to end of pacemaker life [35, 451. Although this high internal impedance, which limits the rate at which energy can be drawn from the lithium-iodine cell, is not a severe problem for most pacemaker functions, it does reduce the applicability of this type of cell

575 Current Review: Tyers and Brownlee: Pacemaker Power Sources Fig 1. Metal-encased but nonhermetically sealed pacer. Note the open seam in the edge view (right). to other systems requiring relatively large energy drains over shorter periods (e.g., neural stimulators and artificial limbs). In contrast, the rechargeable zinc-mercuric oxide cell is ideally suited to these high-drain applications. A final definition relates to pacemaker encapsulation. Prior to 1970, essentially all implantable cardiac pacers were coated in epoxy or silicone rubber or both materials, which only moderately slowed but did not prevent the eventual entry of moisture into the pacemaker electronic components [711. The inability of the early Mallory battery-powered pacers to achieve their theoretical longevities was in part related to the dissolution of conductive electrolytes in the absorbed fluid, resulting in the development of low-impedance (i.e., highly conductive) circuits which rapidly discharged the batteries [8, 13,43,47]. Figure 1 illustrates a metal-sheathed but not hermetically sealed pacer. In contrast, Figure 2 illustrates the unnecessarily long circumferential weld which is exposed directly to body fluids when a unit of round or oval design is implanted. As reported by MacGregor and associates [39], the majority of the 40,000 recent pacemaker recalls were related to moisture intrusion. Figure 3 shows the disrupted metal envelope of a Biotronik pacer caused by swelling of the contained epoxy following implantation. In one series, Mindt [43] noted that the epoxy was improperly cured. This caused a degree of swelling and internal shorting that resulted in the explosion of some implanted units (2 of the authors patients, including the unit on the right in Fig 3). Chirife and co-workers [13] reported that it can also cause runaway to the point of induced ventricular fibrillation. Fluid absorption underlies a variety of recent problems variously termed dendritic growth or metalization by the manufacturers [171 and explains many of the disappointingly short longevities reported in recent pacemaker series [18,39,42,43,47,49, 511. To eliminate many of the moisture-related problems, an increasing number of companies (Figs 2, 4) have begun to hermetically seal implantable pacers in sealed (usually welded) metallic containers 181. Literally, a hermetically sealed unit is airtight but the different pacer manufacturers define an acceptable helium leak

576 The Annals of Thoracic Surgery Vol 25 No 6 June 1978 Fig 2. Hermetically sealed pacer powered by multiple RM-1 Group 11 Mallory zinc-mercuric oxide cells. Note the welded circumferential seam in the edge view (right). rate for their product as being hermetic. For example, 4 X lop8 standard cclsec means that the "hermetic" unit may leak approximately 0.0035 cc of helium gas per 24 hours under standard conditions of temperature and external pressure. Many years will be required for the development of significant vapor pressure levels in these units. While corrosion problems have been encountered by both Cordis and Edwards [39, 481 and although hermetic sealing by itself cannot guarantee against a future recall by any of the manufacturers [lo, 39, 561, the problems with hermetic units to date have been minor relative to the overall number of units in the field. The elimination of all nonhermetic pacers from the market will finally allow pacemaker functional life to approximate engineering predictions, based on properly defined cell capacity, because the electronics and power source will no longer be forced to work in a warm, moist environment (Fig 5). With regard to hermeticity, rectangular pac- ers have at least three important theoretical advantages over rounded pacers [65]: (1) The length of the hermetic seal is shortened; (2) the weld seam can be protected from direct attack from body fluid by recessing the seam at the small end of the rectangle and then covering it with an epoxy or other biocompatible material connector (see Fig 4, right, vs Fig 2); and (3) internal heating of components during welding is minimized in short-seam designs. Round pacers of necessity must have a longer circumferential exposed weld or seal. Thus, the current manufacturers of round and oval pacers have at risk more than 3 km of exposed weld for every 10,000 pacemakers implanted compared with less than 0.8 km of epoxy-protected seal for every 10,000 rectangular pacers of our design [65] (see Fig 4, right). Rectangular units may also be somewhat more resistant to rotation and the capstan effect occasionally seen with round pacers. Aesthetically, rounded and oval units have been promoted as more natural, but in fact they simulate a subcutaneous tumor, which a rectangular unit does not. In most currently available hermetic pacers, the battery is hermetically sealed and major

577 Current Review: Tyers and Brownlee: Pacemaker Power Sources Fig 3. Metal-encased nonhermetic units showing spreading of the seam secondary to fluid absorption and swelling of the epoxy. Two years after implantation, the unit on the right suddenly expelled several cubic centimeters of a corrosive gas-containing mixture into the patient s tissues. portions of the circuit (hybrids) are hermetically sealed within a third or outer hermetic envelope, the exterior of the pacer. The interstices within the pacer are filled with either inert gas, epoxy, or silicone rubber. The filled units offer a slightly greater margin of safety should the external hermetic enclosure fail, but they are somewhat heavier. It is doubtful if the extra weight has any biological importance, but there is a risk that the enclosed epoxy will contract and crush the electronic components. Haynes Alloy No. 25, 316L stainless steel, and titanium are currently all used for the exterior hermetic enclosure, and at present none has a clear advantage over another [2, 29, 30, 48, 50, 521. Ceramic, glass, and polymer hermetic enclosures have also been studied 132, 681. PRIMARY ZINC-MERCURIC OXIDE CELLS. The original Mallory pacer cell, the RM-1, was modified in 1969 by the addition of silver to the mercuric oxide in the cathode and again in 1973 by the addition of an improved barrier between the anode and the cathode [21, 551. This improved cell, RM-1 Group 11, can theoretically power a pacer for from 5 to 7 years depending on the pacer load (electrode), circuit drain, and number of cells utilized [44]. The hermetically sealed Edwards Model 8116, used in more than 2,000 clinical implants since 1974, utilizes these cells [25]. The hermetically sealed Cordis Kappa unit also uses RM-1 Group I1 cells. It was withdrawn from the market in April, 1976, because of connector, sealing, and getter problems [641 but it has since been reintroduced. Although a number of pacers that are not totally hermetically sealed are available with the RM-1 Group I1 cell, they cannot be recommended. LITHIUM CELLS. The solid electrolyte lith- ium-polymer iodine cell was developed in 1968. As can be seen from Table 1, various anode configurations (single versus double, central versus peripheral), various cathode iodine polymer mixtures (16:l to lo:l, currently, 15:1), and various construction techniques have been used [61]. The high ratio of iodine to polymer in the earliest cells proved caustic to connectors and cases and led to early cell failures. With the development of the double peripheral lithium anode to separate the central caustic iodine cathode from the metal case, higher iodine ratios have again been used [35]. The advantage of the higher iodine content is that it increases the energy capacity of each cell per unit volume, thus allowing smaller pacers or considerably increased lifetime to volume ratios. Approximately 200,000 lithium-iodine powered pacers have been implanted to date without a single proven battery failure [l, 4, 10, 14, 15, 24, 28-30, 35, 52, 561. The Catalyst Research-Wilson Greatbatch 702E cell, used in the Cardiac Pacemakers, Inc., Maxilith (1972), the Telectronics series 120 (1974), and the Biotronik IDP-144 (1975), is unequivocally capable of powering a pacemaker for 6 years, and Greatbatch has shown that there is a 90% probability of this cell achieving a functional life of 10 years [24]. However, the 702E is a very large cell, and a whole series of smaller daughter cells has evolved from it (Tables 1, 2). For example, the Catalyst Research series 801 cells (introduced in mid-1975) and 802 cells (introduced in

578 The Annals of Thoracic Surgery Vol 25 No 6 June 1978 Fig 4. From left to right: a multiple zinc-mercuric oxide cell, nonhermetic, epoxy-encased pacer (Medtronic Xytron); a hermetically sealed, fhinly epoxy covered seam, lithium-iodine pacer (Intermedics Interlith); and a hermetically sealed, short, deeply epoxy protected seam, rechargeable silver zincsilver mercuric oxide pacer (Brownlee-Tyers). Note in the unit to the right that the hermetic seal is recessed within the can so that it is protected by a 2 cm epoxy cap which also functions as the connector. January, 1976) are theoretically superior and have been used in the Intermedics Interlith for more than 10,000 implants with no cell failures [29]. The similar Wilson Greatbatch 752 [461 powers more than 3,000 of the Miniliths (Cardiac Pacemakers, Inc.) [3] while an equal number are powered by 2 of the entirely different nonhermetic SAFT (Table 3) lithium-silver chromate cells [2], which swell 10% during discharge [23]. In addition to a number of lithiumiodine cells, Telectronics (more than 8,000 lithium implants worldwide) has utilized Wilson Greatbatch lithium-bromine and SAFT nonhermetic lithium-silver chromate energy sources [52]. In their first 477 lithium units, there was only one failure (not cell related) after Fig 5. Rings of fluid on the batteries of a nonhermetic pacer that failed early after implantation. a mean of 15 months of pacing (longest followup, 27 months). Mallory (Table 4) has developed a high-capacity (4.8 watt-hours), redundant, hermetically sealed lithium-iodine cell which undergoes slight internal volume changes during discharge and requires an

579 Current Review: Tyers and Brownlee: Pacemaker Power Sources Table 1. Catalyst Research Hermetically Sealed Lithium Anode P2VP Iodine Cathode Solid-state Lithium-Iodide Electrolyte Cells Ampere- Vol- Com- Model Year Hours Anode UP BOLV V. nb umec Disc. NM Comment PanY 702A 1970 7028, 1970 702C 702P 1971 702E 1972 801-23 mm 1973/756 801-35 mm 802-23 mm 1973i76" 802-35 mm 803-23 mm 1973176~ 80330 mm 804-23 mm 197376'' 1.5 <3.5 3.5 3.5 1.5 3.1 1.8 3.8 1.4 2.0 1.2 SIC SIC SIC D/C DIP DIP WP wp 16:l 16:l 1O:l 1O:l 1O:l 15:l 15:l 15:l 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 Yes Yes Yes No so No No No NA No PC E or cs PC No No No Low current, low capacity Leakage, corrosion Less drop in voltage Large, sharp corners; iodine vapor Smaller; less voltage drop Increased capacity Proprietaty changes; dimension changes; 9 mm thick Like 803 None SE CPI, EA CPI, TEL, BIO INT, AT, SE. V, L, D INT, AT, SE, V, L, D. PAC INT.The downward slope of the arrow illustrates cell voltage decrease with time. The slopes of the different curves reflect the true variety of discharge performance seen with the different cells. bthe upward slope of the arrow illustrates impedance rise with time. 'Volume changes are illustrated by arrows, as in V and n. dthe year before the solidus indicates when the prototypes were built; the year after, when production began. PZW = poly-2-vinylpyridine; UP = iodine to polymer ratio; BOLV = beginning-of-life voltage; V = cell voltage with time; n = impedance with time; Disc. = discontinued; NM = new material; S = single; C = central; NA = not applicable; SE = Siemens-Elema; PC = polyester cement sealant; CPI = Cardiac Pacemakers; EA = Electronique Applique; D = double; E = epoxy; TEL = Telectronics; BIO = Biotronik; P = peripheral; SO = special order; CS = ceramic spacer; INT = Intennedics; AT = American Technology; V = Vitatron; L = Elettro Medicali; D = Devices Implants Ltd; PAC = Pacesetter. Table 2. Wilson Greatbatch Hermetically Sealed Lithium Anode Solid-state Electrolyte Batteriesa Ampere- Cath- CR Model Hours ode BOLV Unit Comment Company 702A-P 3.5 P.1, 2.8 Same 3 under test since 1970; 702E projected 90% at 10 yr 741 1.0 P.1, 2.8 ~801 "Pediatric" double peripheral anode 752 1.4 P.I* 2.8 402 Full production 1975; 741 with increased IIP ratio 755 3.0 P.1, 2.8... Introduced 1976; central anode, I2 vapor 742 1.1 P.1, 5.6... 2 lithium-iodine cells in series; "sqroundb;" needs internal spacers and insulators; not redundant 766 3.5 P.Bq 3.5... lntroduced 1976 See Table 1 CPI TEL CPl TEL MED TE L Voltage, impedance, and volume data are similar to those in Table 1. Significant differences in design and manufacturing techniques compared with Catalyst Research units after 1975. Other models include 743 and 743A4.0 ampere-hours. bterm coined by Medtronic meaning partly square and partly round. P. = polymer; CR = Catalyst Research Corporation; MED = Medtronic; other abbreviations same as for Table 1.

~~~~~ 580 The Annals of Thoracic Surgery Vol 25 No 6 June 1978 Table 3. SAFT Polypropylene Seal Lithium Anode Silver Chromate Cathode, Liquid Lithium Perchlorate Propylene Carbonate Electrolyte Cells Ampere- Model Year Hours BOLV V R Volume Comment Company Li355 NA 2.4 3.1 > u r Not used None in pacers Li210 1973 0.6 3.1 > u r Usually 2 redun- CPI, TEL, dant cells in parallel E, ESB *ESB is phasing out this cell. E = Edwards; ESB = Medcor; other abbreviations same as for Table 1 Table 4. Malloy Hermetically Sealed Lithium Anode Lead Sulfide-Lead Iodide Cathode, Solid-state Lithium lodide Electrolyte Batteries Ampere- Model Year Hours BOLV V R Vol Comment Company LSA 250-6 1971 0.25 5.7 \ P -* Discontinued Evaluated by MED LSA 900-6 1972 0.9 5.7 \ P -t 21 spring INT, COR loaded, 1.9 volt cells in 7 parallel (redundant) sets of 3 cells connected in series addition to lithium iodide, as with Catalyst Research and Wilson Greatbatch cells, lead sulfide and lead (both electronically conductive) are reaction products. The lithium iodide separator may not be self-healing. COR = Coratomic; other abbreviations same as for Table 1. Table 5. General Telephone and Electronics Hermetically Sealed Lithium Anode, Porous Carbon Cylinder Cathode, Liquid Thionyl Chloride-Lithium Aluminum Chloride Electrolyte Cells Ampere- Model Year Hours BOLV V R Volume Comment Company LI-2 1974 1.8 3.7 I f I 2 redundant parallel cells ARCO LI-3 or4 1976 0.9 3.7 I f I Like LI-2 but ARCO smaller size and capacity athese cells have the highest energy density of the available lithium cells. ARCO Medical has exclusive agreement to manufacture them. Abbreviations same as for Table 1. internal spring construction [63]. It has been used in more than 3,000 CMOS-I Intermedics Model 221 units with no power cell failures [30]. In addition to Cardiac Pacemakers, Inc., Intermedics, and Telectronics, a fourth company, ARCO Medical, has a noteworthy long-term experience in the production and sale of lithium-powered pacers [33, 361. ARCO s LI-2 (4,000 implants), LI-3, and LI-4 units (approximately 1,600 implants each) all use a unique lithium-thionyl chloride battery (Table 5) developed by General Telephone. While hermeti-

~~ ~ ~~ 581 Current Review: Tyers and Brownlee: Pacemaker Power Sources Table 6. Newer Lithium Anode Cells Ampere- Manufacturer Hours BOLV V C! HS SS Comment Company Dupont 1.8 2.1 7 No No Cupric sulfide Cordis cathode exclusive agreement Mallory 1.o 2.8 3 s No No Carbon cathode, None sulfur dioxide electrolyte Power Conversion 1.O 2.8 3 2 No No Similar None to Mallory Electrochemica 2.0 2.8 3 2 No No Similar None to Mallory HS = hermetically sealed; SS = solid-state electrolyte; other abbreviations same as for Table 1. cally sealed, the lithium-thionyl chloride cell (like the nonhermetic SAFT cell) is not solid state; that is, the electrolyte is not dry, and it shrinks slightly during discharge. The larger LI-2 has been available for clinical implantation for more than three years. A number of newer lithium-powered pacemakers are also available. Medcor and Edwards use the SAFT cell, Coratomic uses a Mallory lithium cell, and American Technology uses Catalyst Research 800 series cells. The Intermedics Thin Lith series is powered by the thinner (9 mm thick) Catalyst Research series 803/804 lithium-iodine cells, while the Cardiac Pacemakers, Inc., Microlith is powered by the similarly new Wilson Greatbatch 755, which reverts to the central lithium anode configuration. This results in the potential for iodine vapor and iodine polymer (10 times more corrosive) contact with the 304 stainless-steel battery case [7].* Both Medtronic and Cordis entered the lithium field late [18, 193, but by March, 1977, more than 3,000 Xyrels [73] and 1,700 Lambdas [6, 501 had been distributed. The roundish, high-voltage Medtronic battery (Wilson Greatbatch 742; see Table 2) consists of 2 separate lithium-iodine cells in series, which makes its production somewhat more challenging than previous lithium batteries [19]. The lithium-copper sulfide battery developed by Dupont and licensed exclusively to Cordis [19] is one of a series of newer nonhermetic, non-solid-state lithium anode cells [40] now available (Table 6). As with the Schneider AA: Personal communication, 1977. Table 7. Comparison of Lithium Cells Manufac- Clinical turer HS ss Experience CWG Yes Yes >200,000 SAFT No No >10,000 Mallory Yes Yes >5,000 GTE Yes No >7,500 Dupont No No >3,000 HS = hermetically sealed; SS = solid state; CR = Catalyst Research; WG = Wilson Greatbatch; GTE = General Telephone and Electronics. Medtronic lithium battery, multiple cells (3 in the case of Cordis) are required in series to achieve a high enough EMF to power the relatively high voltage pacer circuitry, and each component cell swells 5% during discharge. Both Cordis and Medtronic have chosen to use relatively large (more than 15 cc displacement) batteries that require multiple internal separators and connectors, rather than to develop lower voltage circuitry. The trade-off is a new, more complicated battery for the ability to continue to use known circuitry that had been essentially developed to operate from a multiple RM-1 cell battery. From the preceding, it can be readily appreciated that a great variety of lithium cells is available and in use (Table 7). With rare exceptions (Biotronik IDP-144, American Pacer Bifocal 8175B), all of the currently available pacers using lithium anode power sources are hermetically sealed [4, 281, but the lithium cells they contain are not all hermetic or solid state, many are associated with iodine vapor pressure

582 The Annals of Thoracic Surgery Vol 25 No 6 June 1978 which is corrosive, and several change dimension during discharge. More experience is required to determine how many of the newer lithium cells will live up to the outstanding record of the 702E, especially as most of the newer lithium pacers contain lower-capacity cells. The double anode cells, which utilize a lithium barrier to prevent contact between the corrosive halogen polymer in the cathode and the metal cell case, and the high-energy density lithiumcarbon cell appear to offer the best probability of providing reliable long-term cardiac pacing. The various nonhermetic lithium cells (e.g., SAFT, Dupont-Cordis) that swell during discharge, cells in which the barrier between the anode and the cathode may not be self-healing (e.g., LSA 900-6), lithium batteries involving the increased complexity of connecting multiple cells in series (e.g., Wilson Greatbatch 742 and Dupont-Cordis), and cells in which corrosive high halogen (iodine or bromine) cathodal mixtures are not separated from the metallic cell case by the lithium anode (Wilson Greatbatch 755 and 766) possess theoretical disadvantages that render them somewhat less attractive than the lithium envelope, solid-state cells. A bromine-polymer mixture (Wilson Greatbatch 766) is 10 times more corrosive than an iodinepolymer mixture, and even the latter may etch the 0.375 mm steel cell case to a depth of 0.025 to 0.05 mm per year [71.* SECONDARY CHEMICAL CELLS. These cells are of two kinds, cadmium-nickel oxide and silver zinc-silver mercuric oxide. The role of the nickel-cadmium cell in cardiac pacing has been long and discouraging, but two recent rechargeable pacemakers are of clinical interest. After a series of modifications were made in hermetically sealed cadmiumnickel cells developed for the space program, a markedly improved rechargeable pacemaker powered by a 0.2 ampere-hour cell was introduced clinically in 1973 and has now been used in more than 3,000 patients [37, 381. Patient acceptance of this unit has been remarkably good [12]. Reported malfunctions have related to a circuit component other than the battery and to turning over of the units with temporarily un- Schneider AA: Personal communication, 1977. recognized loss of charging effectiveness [57]. Initially, 90 minutes per week was required for recharging, and the cell has the capacity to power the pacer for five to six weeks without recharging. The weekly recharging requirement has now been reduced to 60 minutes, and several hours of recharging once every four weeks have been successfully accomplished in a few patients. The major disadvantage of this pacemaker is its short functional life if charging is discontinued for any reason. Larger, 1 ampere-hour cells of similar manufacture are being investigated, but the previously discussed high self-discharge rate of the cadmium-nickel oxide system still limits its maximum duration of function as a primary unit to much less than a year. In addition, very high recharging energies (600 milliamperes) are under discussion. The other secondary chemical cell is the silver zinc-silver mercuric oxide cell. Although a rechargeable zinc-mercury cell has been available for a number of years [58], it was not until 1969 that a modified high-reliability pacemaker cell with improved barriers and silver added to both the cathode and the anode was developed by Fagan [20]. All cells in continuous, modestly accelerated body temperature tests (dogs, biological simulator) begun in 1969 continue to function in their ninth year [66, 701. These cells have each delivered more than 20 ampere-hours of energy at a potential of 1.2 to 1.6 volts. Hermetically sealed units have paced animals with complete heart block since 1973, and there have been no failures to date [27]; a clinical test of 10 units was begun in October, 1974, and at the end of the fourth year all pacers continue to function normally [65]. Recharging is accomplished by placing a small, lowenergy coil on the skin over the pacemaker but can also be achieved by an automatic loosely coupled system that requires no patient cooperation. The unique advantages of the zincmercuric oxide rechargeable unit include its proven ability to function for more than 8 years, continuous pacing for up to 4 years without recharging, and total rechargeability after complete discharge of the cell. After 8 to 10 hours recharging once every six months or after 50 hours recharging once every 3 to 4 years, the

583 Current Review: Tyers and Brownlee: Pacemaker Power Sources pacemaker is capable of several years of continuous pacing should charging be discontinued for any reason [65]. This is the first system that allows the patient and physician the freedom and flexibility of a primary pacer combined with the obvious advantages of rechargeability. Recharging has to date been carried out at only 20 milliamperes, but preliminary studies indicate that recharging requirements can be reduced to less than half by increasing the recharging energy to 40 to 50 milliamperes. Nuclear Power Sources After initial development costs of five million dollars [31], nuclear pacemakers powered by plutonium 238 thermopiles were first tested in dogs in 1969 and patients in 1970 [691. Details of clinical use in the United States were reported by Parsonnet in 1972 [53]. All units are hermetically sealed. More than 2,000 clinical units (Alcatel-Medtronic Model 9000, ARC0 Medical NU-5, Cordis Nu, and Coratomic ClOO and C101) have been implanted, and all known clinical failures are due to hermetic seal failure, circuit component malfunction, or a sensing circuit design error in the ClOO [62, 691. Only the Model 9000 has accumulated enough experience to prove statistically that its random failure rate is no greater than that of earlier nonhermetic zinc-mercuric oxide pacers [69]. In fact, independent 5-year in vitro and in vivo testing of currently available nuclear pacers has unmasked previously unidentified inherent or induced defects in models that had previously passed tests under conditions specified by the Atomic Energy Commission [26], and the longterm effects of ionizing radiation on the electronic components of nuclear pacers have yet to be determined [53]. A continuing argument between the proponents and opponents of nuclear pacers revolves around the question of radiation safety [66]. The levels of "safe" exposure have been continually reduced since the discovery of radiological diagnosis and treatment [59]. Currently allowable levels of whole-body exposure are 5 rems per year for nuclear workers, 0.5 rem per year for the general public, and 0.1 rem per year for students [69]. A rem (roentgenequivalent-man) is the dose of ionizing radia- tion that will produce a biological effect approximately equal to that caused by one roentgen of x-ray or gamma-ray radiation. An annoying habit of nuclear proponents is to report radiation exposure per hour. Thus the surface dose from a plutonium 238 pacer containing 250 mg of isotope is 5 to 15 millirems per hour and the radiation level at the surface of the patient (and in the marrow of the ribs) is 1 to 2 millirems per hour. These exposures can be determined to be equal to 130 and 17.5 rems per year (milliremsl hourx24 x365+1,000), respectively. When one considers the known 20-year effect of radiation in the induction of thyroid cancer [113 and the known susceptibility of bone marrow to the induction of leukemia by radiation [69], a small but real increased risk of cancer must be borne by the younger patient receiving a nuclear pacer. This does not take into consideration the increased amount of plutonium in some units. In addition, the allowable contamination of plutonium 238 with up to 0.6 part per million of plutonium 236 (2.3 times the gamma ray dose of plutonium 238) results in a progressive increase in radiation exposure for up to 16 years after implantation, due to the buildup of more active daughter isotopes [69], while the allowable contamination with longer-lived plutonium 239 poses a potential late environmental hazard. Thus the use of nuclear pacers in the very young is not recommended by Parsonnet [54]. The small but real risk of environmental contamination with plutonium is discussed in great detail in the Nuclear Regulatory Commission (NRC) final generic statement on the routine use of nuclear cardiac pacers [69] and is not reviewed here. However, it should be noted that helium gas results from the decay of plutonium 238 so that as the pacemaker is exposed to the caustic biological environment over the years, pressure. is gradually building up within. Further, the NRC admits that control and accountability for every unit cannot be assured, and some have already been lost to follow-up. Ninety-eight percent of the original 3,100 microcuries of plutonium 239 would still be present after a period exceeding 800 years iit a unit lost in sea water [69]. Although it is highly likely but not proved that plutonium 238 nuclear pacers will consistently provide 5 to 10

584 The Annals of Thoracic Surgery Vol 25 No 6 June 1978 years of pacing function, the multiple disadvantages of (1) not insignificant local radiation to the younger patient, (2) larger size or even higher radiation exposure with smaller and therefore less well shielded units, (3) higher cost, and (4) more attractive nonnuclear sources of equally proven longevity make it unlikely that plutonium 238 nuclear pacers will exceed 1% of the market. It is generally accepted that no exposure to ionizing radiation should be permitted without the expectation of commensurate benefit, and it should be recalled that up to 6,000 cancer deaths per year result from current levels of diagnostic radiation [591. The promethium 147 Betacel has been discontinued by the manufacturers as it is not theoretically superior to current chemical power sources [69]. A consideration of its advantages and disadvantages is therefore not germane. Biological Power Sources While a number of systems have been developed that successfully convert the mechanical (e.g., piezoelectric crystals) and chemical (biogalvanic cell, bioautofuel cell) energy of the recipient into electrical energy adequate to power a pacemaker, none of these systems has to date achieved the reliability of even the original RM-1 primary zinc-mercury cell. Research continues in this area [5,22,41,53,72] but there are presently no developments of clinical relevance. Comment After years during which the pacers of the different manufacturers were more similar than different and offered very few real choices to the surgeon performing implants, a wide variety of potentially greatly improved pacer power sources is now available. The newer primary, rechargeable, and nuclear power sources all provide the potential for up to 10 years of continuous pacing from a single implant, and, when encased in a hermetic enclosure, each has demonstrated the ability consistently to stimulate the heart for 5 years. The late but now general recognition that epoxy-coated pacers function unreliably in a warm-water environment and that hermetic sealing of all pacer compo- nents, including the battery, is essential for reliability [81 played as important a role in this achievement as the development of improved power sources. While details of the chemistry and construction of the wide variety of available lithium cells may seem unimportant to the physician primarily involved in pacemaker implantation, follow-up, or both facets, this information will be essential in the management of future recalls and advisories on lithium-powered pacers. These will almost certainly occur. The physician dealing with such pacers should understand that there are different cathodal materials (e.g., polymer iodine versus bromine versus silver chromate), different cell construction methods (hermetically sealed versus nonhermetically sealed, cells versus more complicated batteries, cathode case contact versus lithium envelope construction), and different cell electrolytes (solid state versus dry cell and a wide variety of materials). Armed with this knowledge and the additional information that even certain identical appearing pacers of a given manufacturer may contain different cells [2, 31, the responsible physician can quickly exclude and inform patients who do not have any of the suspect units. Conversely, the physician can increase surveillance if suspect units of a different manufacturer contain the same power cell as used in some of his patients pacers (the Cardiac Pacemaker, Inc., Miniliths 502 and 602 contain the Wilson Greatbatch 752 solid-state lithiumiodine cell while the grossly identical Miniliths 0501 and 0601 contain 2 parallel SAFT nonhermetic, non-solid-state lithium-silver chromate cells). Certainly, not all lithium anode cells will prove to be of equal reliability, and additional time is necessary to determine their relative merits. Investigation of newer pacemaker power sources is also continuing [21, 541. Some of the primary cells under consideration include a sodium amalgam-bromine cell with a solid ceramic electrolyte that is reported to have a 10-year capability (General Electric, ARC0 Medical), two different configurations of a small 6 watt-hour Mallory lithium cell (Fig 6), and a high-voltage primary but potentially rechargeable zinc-silver oxide cell (VARTA).

585 Current Review: Tyers and Brownlee: Pacemaker Power Sources maker patient and the implanting and following physician. The 5-year pacer is now a realj.ty and the 10-year pacer, a probability. Fig 6. Prototype high energy density circular LSAC 1000-6 (left) and rectangular LSAR 1050-6 (right) Mallory lithium cells. Newer rechargeable cells that may have biological applications include Electrochemica nongasing cadmium-mercuric oxide cells [161 and Gates sealed lead-acid cells [34], and a new demand pacer utilizing the rechargeable silver zinc-silver mercury cell is currently being tested by Intermedics. Another technique, which we are investigating, involves a combination of primary lithium and rechargeable external energy techniques. A low-energy, largevolume, alternating-current magnetic field powers the pacemaker while the patient is at home and at work if the work area is limited. Energy is drawn from the lithium cell only during lengthy excursions outside the fields provided in the immediate home or work environment [91. The search for newer pacemaker power sources is not just academic. Lithium not only is used in the production of tritium, which along with deuterium fuels fusion reactors, but is also a key ingredient in a newly developed storage battery for utilities and electric automobiles. Thus, the demand for lithium is expected to outstrip the supply by a factor of two before the turn of the century [701. Only time will tell which of the many available lithium, halogen, rechargeable, and nuclear power sources will prove superior, but when used in well-designed, hermetically sealed pulse generators, all have the potential to greatly improve the outlook for both the pace- References 1. Adducci AJ: Letter regarding CPI lithium-iodine pacemakers (product information). January 28, 1977 2. Anderson JA: Minilith pulse generators models 0501,0601 (SAFT). Cardiac Pacemakers, Inc, technical note no. 5, May 1975 3. Anderson JA: Model 502, 602, Minilith pulse generator (WGL). Cardiac Pacemakers, Inc, technical note no. 8, May 1975 4. Anderson SG: Letter regarding Biotronik lithium pacemakers (product information). Aug 15,1975 5. Armour JA, Roy OZ, Firor WB, et al: A batteryless biological cardiovascular pacemaker. Surg Forum 17:164, 1966 6. Beers AB: Announcement of the general availability of the new generation of noninvasively programmable pacers, the lithium powered Omni- Stanicor. Cordis Corporation, Mar 1, 1977 7. Brown WR: An examination of Wilson Greatbatch Ltd type 755 cell cases after load testing. Calspan Corporation, Oct 1976 8. Brownlee RR, Tyers GFO: Pacemaker recall. IEEE Spectrum 13:32, 1976 9. Brownlee RR, Tyers GFO: Cardiac pacer energy conservation. US patent application no. 741,491. Nov 12, 1976 10. Burr LH: The lithium iodide-powered cardiac pacemaker. J Thorac Cardiovasc Surg 73:421,, 1977 11. Carroll RG: Irradiation related thyroid cancer, patient management guidelines. Pa Med 80:24, 1977 12. Castle LW: A study of compliance behavior and attitudes of patients with rechargeable cardiac pacemakers. Circulation 52:Suppl2:13, 1975 13. Chirife R, Frank1 WS, Mendizabal R, et al: Ventricular fibrillation induced by a defective demand pacemaker. Chest 69:247, 1976 14. Clark DW: Letter regarding ESB Medcor lithium pacemakers (product information). July 1977 15. Creitz WW, Schneider AA: Characterizing lithium-iodine pacemaker cells with a microprocessor-controlled testing system. Med Instrum 103, 1976 16. Eisenberg M: The new mercuric oxide-cadmium battery system for medical and implantation applications. Presented at the American Institute of Chemical Engineers Intersociety Energy Conversion Engineering Conference, Washington, DC, Sept 23-26, 1969 17. Emmitt RB: Medtronic Inc. Cyrus J Lawrence, Dec 30, 1975, p 1

586 The Annals of Thoracic Surgery Vol 25 No 6 June 1978 18. Emmitt RB: The Xytron recall. Cyrus J Lawrence, Mar 3, 1976, p 3 19. Emmit RB: The pacemaker market. Cyrus J Lawrence, July 12, 1976, p 3 20. Fagan FG: Heart pacer rechargeable cell and protective control system. US patent no. 3,824,129, Mar 14, 1973 21. Fester K, Doty RL: Solid-state batteries for cardiac pacemakers. Med Instrum 7:172, 1973 22. FontenierG, Mourot M, Freschard R: Long termin vivo behavior of a platinum endoauricularmagnesium hybrid battery. Med Instrum 9:171, 1975 23. Gerbier G, Lehmann G, Lenfant P, et al: Reliability of lithium-silver chromate batteries for cardiac pacing and the related quality control systems. SAFT dry battery division, Jan 1976 24. Greatbatch W: The statistical reliability of lithium powered implantable cardiac pacemakers. Presented at the International Symposium on Advances in Pacemaker Technology, Friedrich- Alexander Universitat, Erlangen, West Germany, Sept 28, 1974 25. Grunkemeier GL, Dobbs JL, Starr A: A hermetically sealed mercury cell pacemaker. J Thorac Cardiovasc Surg 72:562, 1976 26. Hauser RG, Giuffre VW: Life-function analysis of nuclear-powered cardiac pacemakers. Med Instrum 109, 1976 27. Hughes HC Jr, Brownlee RR, Tyers GFO: Two to three years of failure-free testing of a rechargeable pacemaker in experimental complete heart block. Circulation 54:263, 1976 28. IDP-144 R-wave inhibited pacemaker (demand type) with lithium iodide battery. Biotronik, Inc, 1975 29. Intermedics C-MOS Interlith unipolar ventricular inhibited pulse generator model number 223. Intermedics, Inc, Feb 1977 30. Intermedics C-MOS-1 unipolar ventricular inhibited pulse generators. Intermedics, Inc, July 1976 31. Is U.S. atomic pacemaker overrated? Med World News, June 22, 1973, p 25 32. Kenny J: Polypropylene as an encapsulation medium for active implants. Med Instrum 10:77, 1976 33. Kolenik SA, Kahn 0, Rzeznik L, et al: A second generation long-life lithium battery powered pacemaker. Med Instrum 9:45, 1975 34. Lindsley EF: Leakproof lead-acid cells. Pop Sci, Nov 1975, p 47 35. The lithium anode cell. Catalyst Research Corporation, Feb 1976 36. Lithium thionyl chloride power cells. ARC0 Medical Products Company, May 1976 37. Love JW, Jahnke EJ: The rechargeable cardiac pacemaker, a clinical evaluation in 25 patients. Arch Surg 110:1186, 1975 38. Love JW, Lewis KB, Fischell RE, et al: Experimen- tal testing of a permanent rechargeable cardiac pacemaker. Ann Thorac Surg 17:152, 1974 39. MacGregor DC, Noble EJ, Morrow JD, et al: Management of a pacemaker recall. J Thorac Cardiovasc Surg 74:657, 1977 40. Mason JF: The lithium battery: it just might revolutionize portable power. Electr Design 10:44, 1973 41. Massie H, Racine P, Pasker R, et al: Study of power generating implantable electrodes. Med Biol Eng 6:503, 1968 42. McGuire LB, O'Brien WM, Nolan SP: Patient survival and instrument performance with permanent cardiac pacing. JAMA 237:558, 1977 43. Mindt W: Letter regarding gradual up-take of water by Biotronik pacers (product information). Nov 20, 1974 44. Mosharrafa M: Comparison between Mallory RM-1-Group I1 and Wilson Greatbatch limited lithium-iodine cells. Cardiac Pacemakers, Inc, technical note no. 2, Sept 1974 45. Mosharrafa M: A new technique for determining the end of life characteristics of lithium-iodine batteries. Cardiac Pacemakers, Inc, technical note no. 7, May 1975 46. Mosharrafa M: The WGL-752 cell. Cardiac Pacemakers, Inc, technical note no. 9, Oct 1975 47. Murphy WP: Letter regarding systematic, moisture-related problem with the Cordis Omnicor (product information). Aug 23, 1976 48. Notice regarding reliability of the K series pacers. Cordis Corporation, Dec 1976 49. Olseth DR: Letter regarding the extension of the Aug 2, 1976, Xytron advisory to an additional group of Xytron units (Medtronic product information). Nov 23, 1976 50. Omnicor Lambda reliability predictions. Cordis Corporation, Dec 1976 51. Omnicor pacer reliability. Cordis Corporation, Aug 1976 52. Pacemaker reliability report. Telectronics Ltd, June 1976 53. Parsonnet V: Power sources for implantable cardiac pacemakers. Chest 61:165, 1972 54. Parsonnet V: What's new in pacemakers. Impulse, Feb 1976, p 1 55. Power source report. Medtronic News 523, 1975 56. Prauer Von HW, Lampadium M, Wirtzfeld A, et al: Cardiac pacemakers with lithium batteries. Impulse, April 1977, p 10 57. Rohlfing BM, Hutchinson JC, Webb WR: The flipped pacemaker: radiographic diagnosis of a cause of malfunction of rechargeable pacemakers. Chest 71:287, 1977 58. Ruben S: The mercuric oxide-zinc cell, in The Primary Battery. Edited by GW Heise, NC Cahoon. New York, Wiley, 1971, pp 207-222 59. Safer X-radiation. Argonaut Mactech Bull, Feb 1974