Hermetically Sealed Cardiac Pacer System and Recharging System Therefor

The present invention relates to cardiac pacers and particularly involves an hermetically-sealed pacer containing a single rechargeable cell.

Present commercially available non-rechargeable mercury-zinc battery powered implantable cardiac pacemakers have a mean life of approximately 24 months and eventually fail from battery exhaustion or electronic component malfunction usually due to gradual absorption of body fluids through the pacemaker housing. More recently developed longer life units such as rechargeable and nuclear cell models have not as yet been fully proven by long clinical experience and even if functionally successful, could prove to be relatively expensive.

As a result there is a need for a safe, relatively inexpensive pacemaker which is hermetically sealed against moisture intrusion and which is capable of an extended long life. Such a device should also be light in weight, have a shape and surface condition compatible with its biological environment, be immune to radio frequency fields and have components which are readily avaialable and of proven reliability. The presnt invention is intended to fulfill all of these requirements.

The present invention embodies an hermetically-sealed cardiac pacemaker which may be operated on a single, magnetic-induction-rechargeable mercury-silver cell that is sealed along with an outgas alleviating means in a separate container within a suitable metallic outer case, which case also houses the system electronics. Both the container and the case are hermetically sealed in an improved manner and the case acts along with the internal electronics as a charging current regulator system which maintains a substantially constant power transfer from the charging transmitter to the internal receiver within clinically applicable variations in their proximate spacing.

FIG. 1 is a functional block diagram illustrating a cardiac pacemaker system comprising external charging and monitoring apparatus and an implanted pacemaker unit in accordance with the present invention.

FIG. 2 is a perspective view of a preferred embodiment of an implantable pacemaker unit in accordance with the present invention. FIG. 2a illustrates an alternate form of end cap for use with the pacemaker unit of FIG. 2.

FIG. 3 is an alternate form of implantable pacemaker unit having a different configuration from that of FIG. 2.

FIg. 4 is a further alternate form of implantable pacemaker unit illustrating another suitable configuration in accordance with the present invention.

FIG. 5 illustrates a preferred embodiment of recharging circuitry for use in a cardiac pacer system in accordance with the present invention.

FIG. 6 is a plot showing a comparison of electromagnetic energy transfer with and without the shorted turn regulator in the circuitry of FIG. 5.

FIg. 7 is a plot showing a comparison of power cell charging current regulation with and without the shorted turn regulator formed by the metallic interface in the circuitry of FIG. 5.

FIG. 8 is a plot illustrating the variation in cell charging current with implant depth at a cell voltage of 1.2 volts using the circuitry of FIG. 5.

FIG. 9 is a plot of charging current for the pacemaker cell using the techniques of the present invention wherein the curves are drawn for cell voltages from 0 to 2.0 volts and for implant separations of 0, 0.5, and 1.0 inches.

A functional block diagram illustrating a cardiac pacemaker system in accordance with the present invention is shown in FIG. 1.

The implanted pacemaker unit 1 comprises a single rechargeable cell 2 which powers a suitable pacing pulse generator 3 for supplying an output via a signal catheter to a ventricular heart electrode, and a receiver coil 4 with charging electronics 5 for recharging the cell along with a telemetry oscillator 6 that provides remote readout of the cell voltage and pacing pulse rate. A preferred pacing pulse generator and telemetry oscillator are disclosed in our respective co-pending applications Ser. Nos. 590,409 and 590,414, filed concurrently herewith.

The external charging and monitoring unit comprises a charger signal source 7 and power amplifier 8, which provide an electromagnetic signal to a tuned circuit including condenser C and charger or transmitting coil 9. During the charging operation the charger coil 9 transmits electromagnetic energy, through the intact skin of the person in whom the pacemaker has been implanted, to the receiver coil 4. The charger coil 9 also may be used as a receiving coil during precharge and postcharge monitoring of the cell voltage and pacing pulse rate by sensing the signal from the telemetry oscillator 6 which is fed through the receiver coil 4. The received telemetry signal from the pacemaker is processed through a telemetry monitor amplifier 10 and may be converted by a frequency discriminator 11 into a reading on an appropriate meter 12, as well as being fed to an audio monitor 13 which converts it to an audio signal. The audio signal is also compatible in frequency to telephone transmission so that the telemetry data may be transmitted through a telephone transmitter 14 to a telephone receiver 15 and read out at a remote data center 16. An implant depth meter 17 may also be connected to the tuned circuit.

The pacemaker power cell 2 is preferably a rechargeable version of the primary silver modified zinc-mercury battery presently used in over 95 percent of all pacemakers. This rechargeable version, which is obtainable commercially, for example, as Mallory Model RMCC-1420S, based on present testing is estimated to be capable of a total functional life with recharging of approximately twenty years. However, such electrochemical cells suffer from the problem of outgassing, that is, hydrogen gases are exuded from the cell continuously at low rates. This is a particular problem if it is desired to hermetically seal the case, as in the present invention, since outgassing will result in a pressure buildup inside the container and may attack the electronic components also housed therein. For the particular cell mentioned above, outgassing rates as high as 20 cc per year have been reported along with average rates of 6 to 7 cc per year. In the past, non-hermetic metal coverings over epoxy encased units, such as disclosed in U.S. Pat. No. 3,690,325, have been tried to permit outgassing while preventing the intrusion of body fluids, but have led to serious complications.

To overcome the outgassing problem, the present invention utilizes a hydrogen "getter" in the form, for example, of palladium sponge or lanthanum pentanickel. Measurements of the hydrogen absorption capabilities of palladium indicate that 85 cc of hydrogen can be collected for each gram of sponge used so that three grams of palladium sponge will provide a "getter life" in excess of 10 years when used with a single cell as contemplated by the present invention. Further, lanthanum pentanickel would appear to offer even better results as the getter material.

More particularly, a preferred embodiment of implantable pacemaker unit 1 in accordance with the present invention is shown in greater detail in FIG. 2. It will be seen that in order to prevent attack of the gas on the electronic components, the electrochemical cell 21 and the getter material 22 are disposed in a separate hermetically sealed container 23 within the main outer casing 20 of the pacemaker, and appropriate terminals or leads 24 extending through the container 23 are connected to the remainder of the electronics 3', 5', and 6', which in turn are connected to receiver coil 4'. The hermetic sealing of this interior container 23 may be accomplished by any known technique consistent with avoiding damage to the contents as it is protected within the sealed outer casing 20.

However, particular problems are encountered in providing an hermetic seal for the entire implantable pacemaker. Such sealing while using a sealing process that does not generate excess heat so as to prevent damage to the electronic components must also be accomplished using a method for final sealing of the unit that is body-compatible. In the past, the use of metallic outer casings usually required welded seams to be of the same metal as the case to avoid "local action" and adverse chemical reactions with body fluids. The present invention overcomes this and other problems by employing an integral metallic outer case, such as of deep drawn stainless steel in a rectangular configuration (FIG. 2), having an end opening to permit the insertion of the components. A lid or cover member in the form of a plate 26 is disposed within the opening in a recessed manner and its periphery is finally sealed to the inner walls of the opening by soldering with suitable low temperature solder. As particularly seen in FIG. 2, this lid configuration forms a recessed end or receptable 27 in the case 20 which is then covered with body-acceptable epoxy or other suitable biocompatible sealing material which forms a sealing member 28. The final epoxy sealing member over the solder seam 29 protects the seam from body fluids and also forms the enclosure for the pacemaker signal catheter receptacle 30.

As a result of this arrangement and sealing technique, the need for high temperature welds is obviated and the casing seam is of minimum length and isolated from direct contact with body tissue or fluids. Alternate forms of case configuration are shown in FIGS. 3 and 4 where like numerals with respective distinguishing letter suffixes are used to identify components corresponding to those shown in FIG. 2. It will be seen that although casing 20x in FIG. 3 is cylindrical in form and cover member 26y in FIG. 4 is not a flat plate, all the configurations allow the final hermetic seal to be isolated with a biocompatible interface protective material. The interface barrier allows the use of low temperature solders and nonidentical case and weld materials. Use of an epoxy or other non-metallic biocompatible material such as segmented polyurethane for the sealing member 28 over the soldered or welded seam 29 not only prevents any serious attack of body fluids on the seam but also prevents any attack of the seam material on human tissue adjacent to the implant. Low melting temperature (430.degree. F) tin-silver solders are commercially available that are considered to be relatively nontoxic, and although they would be unacceptable for exposed joints in long-term implants due to chemical reaction, with the epoxy cover protection they can make suitable soldered seams for the present invention.

Also as a result of the present sealing technique, the receptacle 30, which is used for entrance or exit of biologic or device generated signals from the pacemaker interior, is mechanically secured and protected by the epoxy member 28. The member 28 also acts to house and insulate the point of connection of the signal catheter and cardiac electrode for the pacemaker function to the receptacle 30. The receptacle leads 30a passing through the plate 26 to the receptacle 30 are sealed via a glass-to-metal seal electrical feedthrough 30b.

A preferred method of assembling pacemakers of the forms such as shown in FIGS. 2 through 4 will now be described in greater detail. The first step is the provision of an extruded metallic can that is seamless, except for a single open end. The can may be of a suitable metal, such as stainless steel or titanium, and of any suitable form for containing the electronics including the three configurations shown. After the proper insertion of the electronics, the can is sealed by the insertion of a lid or cap member, which may be in the form of a plate, in the open end in a recessed position so that a "well" is formed in the case for use as a receptacle for the biocompatible non-metallic barrier.

The plate is preferably sealed to the can interior by known low temperature techniques, such as soldering or electron beam welding, to avoid heating of the internal components. If the can and cap member are selected to be of stainless steel, the interior of the can where the plate is located, the edge of the plate, and the feedthrough area should be pretinned with a caustic flux such as Welco 500 or the equivalent, to achieve "wetting" and adherance to the casing, which preferably is stainless steel of the type 316L suitable for implant use. Tin-silver solders such as Welco No. 5 or the equivalent may be used to form the seam. After tinning, thorough cleaning of all parts with industrial solvents is recommended to ensure removal of the highly corrosive and conductive flux residues. Sonic cleaning in an acetone bath followed by a second sonic cleaning with isoproponol has also been used with success in flux removal.

For final assembly and joining of the plate to the inside of the main case, after installation and potting of the electronics and energy source, non-corrosive fluxes that form non-tacky and nonconductive residues should be employed. Ersin liquid flux type 4370 or equivalent is a satisfactory final assembly flux. Cleaning of the well area of flux residues after the joint is soldered using tin-silver solders, such as Welco No. 5 or equivalent, is then accomplished by swabbing with acetone and isoproponol. At this stage, sonic cleaning is not recommended since the electronics and battery would be subjected to the acoustic energy. The well area after cleaning is then sand-blasted to improve adherance of the epoxy or other barrier to the steel. Various chemical treatments, such as chemical etching, to improve adherance may also be found suitable. Molding fixtures for forming the output receptacle and catheter entrance cavities should be coated with a release agent such as Miller Stevenson MS-122 or the equivalent, prior to pouring the epoxy barrier. Care should be exercised to prevent release materials from contaminating the well area.

The final barrier is formed with a biocompatible epoxy such as Hysol R-2038 resin and H2-3404 hardener or equivalents, so that the only materials in contact with tissue at the implant site are the stainless steel case, the epoxy barrier and the insulation used on the signal catheter which is typically a silicone rubber.

An alternate form of cover member 26 is shown in FIG. 2a wherein the cover plate is formed with an upstanding peripheral lip 26x around whose uppermost edge a sealing weld 29x is formed. This configuration is particularly suitable for use with welds, such as would probably be required with a titanium case, since the lip extends the lip extends the length of the thermal path from the weld joint to the internal electronics package to minimize thermal transmission thereto.

While the metallic case is extremely suitable for accomplishing hermetic sealing in the manner described when a non-rechargeable power cell, such as a lithium cell, is used, it could pose a problem with regard to the transfer of the electromagnetic energy from the external charger coil 9 to the internal receiver coil 4, when a rechargeable cell is used, since the metallic interface can cause losses with attendant heating problems during the charging process as a consequence of induced eddy currents. However, in accordance with the present invention this heat energy loss is turned to advantage through a special tuned transmitting system that not only aids in regulating the cell charging current, but also acts to regulate the transmitted power so that case heating is minimized as implant depth is reduced. The system, shown schematically in FIG. 5, utilizes a closed link or shorted turn 40 which may be provided by the wall of the metallic case. The receiver coil 4 is placed in proximity to the shorted turn 40 and the transmitting circuit (LC) is tuned to resonance at a frequency which is chosen so that losses in the metallic case are reflected to the transmitting circuit and thereby regulate the input power as a function of the spacing d between the transmitting and receiver coils. The regulating function of the shorted turn 40 at the receiver coil 4 is accomplished by reflecting a variable loss with variable spacing to the transmitting coil 9 to thereby modify the transmitting circuit Q with distance. As the distance d between the transmitter and shorted turn is reduced, the Q is reduced, raising the transmitting circuit (LC) impedance and thereby reducing the power input to the transmitting coil 9. The net effect is a more constant transfer of power into the receiver coil 4 as d is varied. The advantage of the shorted turn 40 is illustrated in FIG. 6 which contains a plot of the output voltage of the receiver coil with and without the shorted turn 40 as the distance d between the two coils is varied. It will be seen that the variation in the output voltage is considerably reduced with the shorted turn in place. FIG. 7 is a plot of battery charging current as a function of variable spacing with and without the metallic (stainless steel) interface as the shorted turn and further illustrates that the charging current is well regulated and therefore known when the interface load is in place. Serious overcharging or undercharging of the battery is thus prevented in the event of an unknown implant depth within the limits shown plotted. This simple arrangement obviates the need for more complex and less reliable electronic regulation, but additional electronic regulation may be employed at a much reduced stress in conjunction with the shorted turn regulator.

A suitable electronic circuit for such regulation is included in FIG. 5. Whereas the metallic case effectively functions as the shorted turn 40 and through the action of eddy current losses loads the tuned transmitting induction coil 9 in the just-described charge current regulating system, additional regulation and rectification is provided by a network of diodes D.sub.1 -D.sub.6 and the silver-mercury cell 2 which load the receiver coil 4 located behind the metallic interface. A frequency f is chosen such that the combination of the metallic interface (acting as an energy absorber) and the loaded receiver coil 4 regulates the current into the battery 2 as the distance d varies. When charging the cell, electromagnetic energy is transmitted from the transmitting coil 9 to the receiver coil 4 resulting in an alternating voltage at the output of the receiver coil. On the negative halfswing of the receiver coil voltage, current is supplied to the cell 2 via diode D.sub.6 and also to the diode string D.sub.3, D.sub.4 and D.sub.5. The relative magnitudes of the current supplied to each depends on the cell voltage and the particular diodes chosen. The effect is to prevent overcharging of the cell 2 by shunting the current away from the cell as the cell voltage rises. The cell and diode current are reflected as a load to the transmitting coil 9, which modifies the Q in the same manner as the Q loading by the metallic case. The diode string D.sub.1, D.sub.2 is employed to provide nonlinear loading on the positive halfswing of the receiver coil voltage to aid in the regulation of the transfer of energy as a function of distance between the two coils. Diode D.sub.1 is a Zener diode with a breakdown potential chosen sufficiently high so that at a large distance d the received coil voltage is less than the breakdown voltage. The transmitted energy is unaffected since no current flows into diode D.sub.1. As the distance d is reduced, however, the diode string of D.sub.1 and D.sub.2 begins to conduct on the positive halfswing and thereby improves the control of energy transfer through reflection of additional losses from the receiver coil 4 to the transmitting coil 9. Diode D.sub.2 prevents conduction through diode D.sub.1 on the negative halfswing.

The advantageous feature of the overall combination is that the Zener breakdown voltage of diode D.sub.1 can be chosen optimally to trim the regulation desired for a particular physical arrangement and for the required charging currents for the cell. A plot is provided in FIG. 8 illustrating the effect on charge current regulation of the various techniques discussed. For each set of data the maximum charge current was adjusted to about 28 milliamps. Curve I shows the charging current for an untuned system. The effect of tuning is shown in curve II in which the effect of the charge current reflected to the transmitting coil provides the regulation. Curve III shows the additional improvement by introduction of a stainless steel interface and curve IV shows the additional improvement employing the Zener diode load. The change in current over a 1.0 inch distance change for the untuned system is 22 milliamps. With the addition of the case and diode loading, the variable is reduced to 4 milliamps over the same variation in implant depth. An important feature of this regulation scheme is the automatic reduction of the transmitted power as the separation distance is reduced. Metallic case heating is thereby much lower for small separation distances than would be encountered in a system employing constant transmit power. FIG. 9 is a plot of charging current for the pacemaker cell using the techniques of the present invention. The curves are drawn for cell voltages from 0 to 2.0 volts and for implant separations of 0, 0.5, and 1.0 inches.

The implant depth may be detected by the meter 17 (FIG. 1) which incorporates a built-in rectifier that senses the voltage change on transmitting coil 9 as a result of the change in Q.

It will thus be seen that the present invention provides a safe relatively inexpensive pacemaker, which embodies an outer case, preferably of metal, hermetically sealed against moisture intrusion, and an inner container sealed against cell outgassing that might damage the electronics. A single rechargeable cell may be used to achieve extended long life, in which event the metallic case may be made to act along with the internal electronics as a charging current regulator system maintaining a substantially constant power transfer from the transmitter to the receiver despite variations in their proximate spacing.