Heart pacer
United States Patent 3866616

A nuclear heart pacer having a heat-to-electricity converter including a solid-state thermoelectric unit embedded in rubber which is compressed to impress hydrostatic precompression on the unit. The converter and the radioactive heat source are enclosed in a container which includes the electrical circuit components for producing and controlling the pulses; the converter and components being embedded in rubber. The portions of the rubber in the converter and in the container through which heat flows between the radioactive primary source and the hot junction and between the cold junction and the wall of the container are of thermally conducting silicone rubber. The primary radioactive source material Pu238 is encapsuled in a refractory casing of WC-222 which in turn is encapsuled in a corrosion-resistant casing of platinum rhodium, a diffusion barrier separating the WC-222 and Pt-Rh casings. The Pt-Rh casing is in a closed basket of tantalum. The tantalum protects Pt-Rh from reacting with other materials during cremation of the host, if any. The casings and basket suppress the transmission of hard X-rays generated by the alpha particles from the Pu238. The outside casing of the pacer is typically of titanium but its surface is covered by an electrically insulating coating, typically EPOXY resin, except over a relatively limited area for effective electrical grounding to the body of the host. It is contemplated that the pacer will be inserted in the host with the exposed titanium engaging a non-muscular region of the body.

Purdy, David L. (Indiana, PA)
Magovern, George J. (Pittsburgh, PA)
Smyth, Nicholas (Bethesda, MD)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
607/37, 976/DIG.416
International Classes:
A61N1/375; A61N1/378; G21H1/10; (IPC1-7): A61N1/36
Field of Search:
View Patent Images:
US Patent References:
3598128LEAD-STORING PACER1971-08-10Chardack
3253595Cardiac pacer electrode system1966-05-31Murphy, Jr. et al.

Other References:

Myatl, "Biomedical Engineering," Vol. 6, No. 5, May, 1971, p. 192-196..
Primary Examiner:
Kamm, William E.
1. A heart pacer for a host including a primary source of heat energy, a converter connected to said source for converting said energy into electrical energy, means, connected to said converter, for deriving electrical pulses from said electrical energy, output-conductor means including a catheter, connected to said deriving means and to be connected to the heart of said host, for impressing said pulses on said heart, and a container for said source, converter and deriving means, the said container being composed of electrically conducting material and being electrically connected to said deriving means to serve as ground therefor, the outer region of said container being coated with electrically insulating material except over a relatively small limited area thereof on one side only, the conducting material of said limited area to be connected electrically to non-muscular body parts of said host connecting

2. The heart pacer of claim 1 wherein the limited area is in the form of a

3. The heart pacer of claim 2 wherein the insulating coating is flush with

4. The heart pacer of claim 1 wherein the deriving means includes electrical circuit-component means for converting the electrical energy from the converter into pulses, and the output circuit-conductor means includes output assembly means, said output-assembly means including a feed-through assembly means, said output assembly being connected through the feed-through assembly means to the deriving means within the container and extending out of the container and being connected outside of the

5. The heart pacer of claim 1 wherein the container is of generally ellipsoidal form and the small limited area is a dimple of generally oval form.


This application relates to an application to David L. Purdy for Electrical Generator filed July 12, 1973 and assigned to CORATOMIC INC.


This invention relates to the generation of electricity by thermoelectric conversion of heat from a local primary source and has particular relationship to nuclear heart pacers or pacemakers. To the extent that this invention has other uses than in heart pacers it is understood that such uses are within the scope of this application.

A nuclear heart pacer includes a primary source of radioactive material, a thermoelectric converter which converts the heat from the source into electricity and an electrical circuit powered by the converter which converts the output of the thermoelectric converter into pulsations and controls the flow of the pulsations to the heart.

This prior-art heart pacer is encased in a conducting material such as titanium. In the use of this pacer muscle twitching has on occasions been experienced. In addition, the operation of this pacer has at times been irregular.

It is an object of this invention to overcome these disadvantages and to provide a heart pacer which shall not produce muscle twitching and shall operate with stability.


This invention arises from the realization that the prior-art pacer, having a conducting casing throughout, makes electrical contact with muscles of the host in areas of the casing. The current flow through the surface of the casing then actuates these contacted muscles to twitch. In addition, these contacted muscles, during the normal activity of the host, inject electrical pulsations into the prior-art pacer which confuse its operation and render this pacer at times unreliable.

The heart pacer in accordance with this invention is encased in a metallic container which is coated with an insulator, typically EPOXY resin, except over a limited area. The pacer is inserted into the body of the host with the conducting area out of contact with any muscular tissue of the heart.


For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a view in perspective of a heart pacer in accordance with this invention with parts of the outer container broken away to show the interior;

FIG. 2 is a view in end elevation with parts sectioned of the heart pacer shown in FIG. 1;

FIG. 3 is a view in section taken along lines III--III of FIG. 2;

FIG. 4 is a view in longitudinal section of the battery of the heart pacer shown in FIGS. 1-3;

FIG. 5 is a plan view of the strap assembly which applies pressure to compress the medium in which the thermoelectric unit is embedded;

FIG. 6 is a view in end elevation of the cylindrical bracket for positioning the radioactive block of the capsule shown in FIG. 4 as viewed from one end;

FIG. 7 is a view in end elevation of this bracket as viewed from the opposite end;

FIG. 8 is a view in section taken along line VIII--VIII of FIG. 7;

FIG. 9 is a view in section taken along line IX--IX of the portion of FIG. 4 showing the thermoelectric unit;

FIG. 10 is a view in end elevation of the thermoelectric unit as viewed from the hot-junction end;

FIG. 11 is a view in end elevation of the thermoelectric unit as viewed from the cold-junction end;

FIG. 12 is a diagrammatic view showing the manner in which the individual thermoelectric elements of the thermoelectric unit are interconnected;

FIG. 13 is a block diagram of a typical electrical circuit of a heart pacer in accordance with this invention;

FIG. 14 is a view in perspective showing one way in which the heart pacer in accordance with this invention is installed in the host; and

FIG. 15 is a view in perspective showing another way in which the heart pacer in accordance with this invention is installed in the host.


The apparatus shown in the drawings is a heart pacer 21 (FIGS. 1-3) including a battery 23, printed circuit boards 25 and 27 (FIG. 2), a solid-state electronics package 29, a storage capacitor 31, a magnetic switch 33, a transformer 35 and an output assembly 37 for connecting the output of the transformer 35 to the catheter or heart lead 39 which is placed on the heart muscle. The boards 25 and 27 serve as a cradle for the battery 23. The battery 23 and the circuit components 25-35 are embedded in a potting compound 41 of a resilient material in a container 43 (FIG. 3) typically of titanium. The potting compound 41 is typically predominately 2CN, a silicone rubber which is thermally insulating and which responds to pressure like a fluid, transmitting pressure uniformly in all directions. The 2CN is sold by Emerson-Cummings Corp. of Pittsburgh, Pa. However, a portion 47 of the compound between the cold-junction end 49 of the battery 23 and the container 43 is composed of a thermally highly-conducting material, typically ECCOSIL 4952, a silicone rubber. ECCOSIL 4952 is sold by Microtechtronics Corp., Buffalo, N.Y. The container 43 is encased in a coating 50 of EPOXY resin except for a window 51. The container 43 serves as ground for the electrical circuit 23-35 and as a radio-frequency shield for the pacer and the window 51 serves to connect the ground to the body of the host. The window 51 projects outwardly from the remainder of the container 43 and is flush with the EPOXY coating 50 as shown in FIG. 2. Typically, the heart pacer 21 has an overall length of 2.45 inches, a width of 1.88 inches and a depth of 0.80 inch. Typically, the window or dimple 51 has a heighth of about 0.050 inch and is of oval shape about 1.8 inches by 1.1 inches. While the outwardly extending dimple is preferred, the conducting area could also be in a recession.

The battery 23 (FIG. 4) includes a primary source 61 and a solid-state thermoelectric converter 63. The source 61 is enclosed in a highly evacuated container 65 encompassed by a heat shield 67.

The source 61 includes a ceramic block 71 formed of a powder of plutonium-oxide, the plutonium being predominately Pu238. Typically the source includes about 0.272 grams of Pu238 O2. This fuel is sufficient to operate the pacer according to this invention for 20 years without renewal of the source. This source delivers 117 milliwatts initially and 100 milliwatts at the end of 20 years. The block 71 is mounted in positioning bracket 73 in inner capsule 75, a hollow sphere. The bracket 73 (FIGS. 7, 8, 9) is formed of a hollow cylinder 77 having tabs 79, spaced about 120°, projecting outwardly from one end and tabs 81, interposed between tabs 79 and spaced about 120°, and projecting inwardly from both ends. The block 71 is supported within the bracket 73 by the tabs 81. The spacing between the tabs 81 is set to accommodate the length of the block 71 between the tabs 81. Typically, tabs 81 at one end are first projected inwardly; the block 71 is then inserted in cylinder 77 engaging tabs 81; thereafter, tabs 81 at the other end are bent to engage and hold the block 71. The tabs 79 engage the inner surface of sphere 75 and help to secure the bracket 73 in the sphere 75. Tabs 79 are bent inwardly from the 90° position about 15° as shown in FIG. 4.

The inner capsule 75 is formed of hollow hemispheres 83 and 85, the rims of the hemispheres being provided with cooperative projections 87 and 89 respectively which are coextensively engaged to form the sphere 75. The hemispheres 83 and 85 are closely dimensioned to about 0.001 inch. Typically, the sphere is composed of a tantalum alloy, WC-222, has an inside radius of 0.125 inch and an outside radius of 0.155 inch. WC-222 has the following chemical composition: W 9.6-11.2%, HF 2.2-2.8%, C 0.008-0.0175%, Ta balance. The bracket 73 typically has a length of about 0.186 inch and a diameter of about 0.125 inch. The inner capsule 75 is fire resistance and has high strength so that the block 77 remains locked in the inner capsule regardless of what impacts the heart pacer may suffer and also if the host should be cremated. The tantalum alloy also absorbs hard X-rays. The inner capsule 75 is enclosed in an outer capsule 91 including a hood 93 formed of a hemisphere from which a cylinder extends; a spherical dish-shaped member 95 is sealed to the rim of the cylinder. Typically, the hermisphere and cylinder of the hood 93 have an inner diameter of 0.316 inch and a thickness of about 0.010 inch. The member 95 has the same thickness and is correspondingly dimensioned to seal to the hood 93. The outer capsule 91 is compoxsed of platinum-rhodium alloy Pt10Rh. The outer capsule 91 protects the source 61 against corrosion and oxidation. A diffusion barrier 97, aluminum oxide, typically of about 0.001 inch thickness is disposed between the inner capsule 75 and the outer capsule 91. This barrier prevents the alloying of the inner and outer capsules 75 and 91.

During assembly the block 71 is secured in the bracket 73 with the tabs 81 holding the block. The bracket 73 and block 71 are then inserted between the hemispheres 83, 85 and the hemispheres are joined at the steps 87 and 89 into a sphere. The sphere is then welded in an inert-gas (argon) atmosphere. The sphere 75 is then inserted between the members 93 and 95 of the outer capsule 91 and this capsule is welded at the joint 99 between these members.

The unit 71-75-91 is disposed in a basket 101 typically of tantalum. The basket 101 is in the form of a hollow hemisphere terminating at its rim in a hollow cylinder. Typically, the basket 101 is about 0.005 inch in thickness. Near its outward end the basket 101 is welded to the flange 103 of a flanged sleeve 105, typically of tantalum. The stem 107 of the sleeve 105 merges into the flange 103, at their inner surfaces, the transition surface being a spherical annular surface of the same radius as the dish-shaped member 95. The dish-shaped member 95 is seated in this surface. The stem 107 is welded to a disc 109, typically of tantalum. The inner surface of the disc 109 is spherical and coextensive with the spherical annular surface of the flange 105 and of the same radius and the member 95 is seated in this surface. The disc 109 has an outwardly projecting rim 111.

A flanged disc 113 is brazed to the rim 111. This disc 113 is composed of the titanium alloy Ti6A14V. A cylinder 115, also typically of Ti6A14V, which contains the converter 63 is welded to the flange 117 of the disc 113. The flange 117 is trepanned to limit the flow of heat from the weld. The alloy Ti6A14V has high strength and low thermal conductivity and is used for this reason.

The evacuated container 65 is defined by outer sheel 121 and the cylinder 115. These members 115 and 121 are joined by a ring 123 typically of Ti6A14V. The shell 121 is typically composed of titanium and includes a hollow hemispherical end 125 from the rim of which a cylinder extends. A hollow frusto-conical shell 127 extends from the rim of the cylinder. The cylindrical rim of the end 125 and the shell 127 are joined by a weld. A backing annulus 128, typically of titanium, is provided behind the welded joint. The end 125 and the shell 127, throughout the major portion of lengths, have a thickness typically of 0.010 inch. But the shell 127 flares out at its constricted end 129, to a thickness of about 0.090 inch. The thickened end 129 is chamfered on the outside and flattened on the inside and is welded to the ring 123 around the flattened area. The ring 123 is internally welded to the cylinder 115. A reinforcing ring 124 internally of the cylinder 115 forms a part of a welded joint between the cylinder 115 and the ring 123. The ring is trepanned to suppress the flow of heat from the weld.

The vacuum within container 65 is maintained by a getter 131 typically CERRALOY 400. The getter 131 is mounted in an annular space provided between the sleeve 107, the flange 103, and a cylinder 133 which extends from the rim of the basket 101 and is bent inwardly at teeth 135 extending from its end. The getter 131 is held by a ring 137, typically of var-glass tubing.

The heat shield 67 has a hemispherical section 141, a cylindrical section 143 and a conical section 145. The hemispherical section 141 includes a plurality of hemispheres 147, typically of MONEL metal, concentric with sphere 75 and extending between the spherical part of the basket 101 and the inner surface of the end 125. The hemispheres have dimples 149 so that their spacing is maintained. The cylindrical section 143 includes a tape of alternate layers of MONEL metal foil 151, typically 0.125 inch wide by 0.001 inch thick, and "E" glass insulation 153, typically, 0.125 inch wide by 0.005 inch thick. The tape is wrapped about a center formed of the cylindrical parts of the hood 93 and the basket 101 and the strip 133 which holds the getter 131. The cylindrical section 143 firmly engages the basket 101 and the strip 133 and supports the asembly 71-75-91, which is relatively heavy, radially and prevents its displacement radially. Radial displacement of the assembly 71-75-91 exerts a torque on the joint 115-123-124 and may rupture this joint. So that the section 143 firmly engages the assembly 71-75-91, dimples 144 are pressed into the shell 125. The conical section 145 includes a plurality of hollow frusto-conical shells 161 coaxial with the cylinder 115, typically composed of MONEL metal, each extending from the cylindrical shield 143 to a position above the ring 123. The shells 161 are bent over perpendicularly to the cylinder 115 near the ring 123 and are engaged by an annular spacer 163, typically of Ti6A14V.

The head 125 and the conical section 127 are initially separated pieces with the backing ring 128 tack-welded to the hemispherical head 125. In assembling the container 121 the ring 123 is welded to the end 129 of the conical section 127 of the container 65. The strip (typically 0.005 inch thick) which forms the holder 133 for the getter 131 is wrapped around the cylindrical end of the basket 101. The getter 131 and the glass tubing 137 are inserted and the teeth 135 of the strip are bent so as to hold the getter 131 and the tubing. The tape 151-153 is wrapped around the assembly 133-101-93. The hemispherical shells 147 are positioned about the hemispherical portion of the basket 101 and the head 125 is positioned over the outermost shell 147. Th rims of the shells 147 engage the cylindrical shield 143. The frusto-conical shells 161 are stacked in the frusto-conical section 127 between the spacer 163 and the end of the section 127. Several of the larger shells 161a are of 0.005 inch thickness titanium; the other shells 161b are of 0.004 inch thick MONEL. A tube 171 of E-glass insulation is wound about each shell 161 supporting the adjacent shell. The sections 125 and 127 are then abutted with backing ring 128 extending between the rims of these members (125, 127). The assembly is then placed in a chamber which is evacuated to a pressure of 10-6 Torr and the joint between the rims of the members 125 and 127 is welded and treated out so that the low pressure is maintained.

The thermoelectric converter 63 is disposed in cylinder 115. The converter 63 includes a solid-state thermoelectric unit 181 embedded or potted in a medium 183 which responds to compression like a fluid, hydrostatically, transmitting the compression in all directions.

The thermoelectric unit 181 includes a plurality of positively and negatively doped strips 185 and 187, typically of bismuth telluride, embedded in polymeric insulation 189 such as EPOXY. So embedded this block is itself stress resistant and protects the strips 185 and 187 from rupture. Typically, there may be 81 of such strips 185 and 187 arrayed as shown in FIG. 10. Successive negative and positive strips are interconnected by solder strips 191 (FIGS. 11, 12) at the hot-junction of the unit 181 and alternate pairs of positive and negative strips are interconnected by solder strips 193 (FIGS. 10, 12) at the cold-junction. Typically, positive strip 185a and negative strip 187a are interconnected by solder strip 191a at the hot-junction and negative strip 187a and positive strip 185b are interconnected by solder strip 193b at the cold-junction. Diagonally positioned positive and negative end strips 185c (FIG. 11) and 187c are connected to output conductors 201 and 203. The pairs of strips 185 and 187 of the array of strips form thermocouples connected in series between conductor 201 and conductor 203.

The potting 183 includes a disc 211 (FIG. 4) of thermally conducting resilient material, typically silicone rubber ECCOSIL 4952, interposed between the source 61 and the thermoelectric unit 181, a hollow cylinder 213 of thermally insulating material, typically rubber, SYLGARD 184, encircling the unit 181, and a disc 215 of ECCOSIL 4952 having an internally generally frusto-conical rearward projection. SYLGARD 184 is sold by Techtronic Corporation of Buffalo, N.Y. The potting compound 211-213-215 serves as axial support for the cylinder 115 and prevents the assembly 71-75-91-141-143 from buckling the cylinder 115.

The converter 63 is assembled in the cylinder 115. First, the cylinder 211 is deposited on the trepanned disc 113. Next, the thermoelectric unit 181 is positioned centrally on the disc 211. Then the cylinder 213 is deposited around the unit 181. A washer, 216, typically of titanium, is then positioned to cover the trepan groove 217 of the disc 123. A split ring 219, typically of Ti6A14V alloy is placed coaxially with the cylinder 115 on the washer 216. The ring 219 is resilient but has a high restoring force. A cylindrical mass of thermally conducting material, typically ECCOSIL 4952, is then deposited in the cylindrical space defined by the ring 219, the washer 216 and the disc 123. Before this mass is cured a plug 231 is positioned at the outer edge of the mass.

The plug 231 is typically composed of electrolytic grade copper and has a head 237 split or slotted at the center and a body 239 which terminates in a generally frusto-conical portion. The body 239 has a central opening which Communicates with an opening 241 in the head. The central opening has a ceramic bushing 243.

The conductors 201 and 203 are brought out centrally before the cylindrical mass is deposited within the inner ring 219, the washer 216 and the disc 123. After the mass is deposited, but before it cures, the conductors 201 and 203 are strung through the ceramic bushing 243 of the plug 231 and brought out through the slot between the parts of the head 237. The plug 231 is then set at the outer end of the mass of thermally conducting material, while being maintained coaxial with the cylinder 115. As the mass is cured the ring 219 prevents the mass from expanding radially. Once the mass is cured the plug 231 is pressed into the mass building up hydrostatic pressure within the potting material 183. The split ring 219 confines the mass radially but the gaps in this spring open to a predetermined spacing which measures the compression of the mass. The force exerted on the mass of ECCOSIL 4952 may be as high as 150 pounds; however, where the thermoelectric unit 181 is stress-resistant, the force may be as low as 15 pounds. When the gap has the desired spacing the mass is secured by straps 233 of spring 235. The spring 235 (FIG. 5) is composed of sheet titanium alloy, typically about 0.005 inch in thickness. This spring has an annular center 251 from which the straps 233 extend radially uniformly spaced around its periphery. In securing the plug 231, the center 251 is spot welded to the sloping shoulder 253 of the head 237 and the straps 233 are bent around the head and spot welded to the thickened portion 129 of the shell 127.

Initially the container 43 (FIG. 3) which is composed of commercially pure titanium is in two parts. One part has an opening 261 for the output conductor 263 from the transformer 35. Initially, the battery 23, the printed circuit boards 25 and 27, block 29, containing the circuit components (integrated circuit and separate transistors not shown) potted in EPOXY resin, the storage capacitor 31, the magnetic switch 33 and the transformer 35 are assembled and connected outside of the container 43.

The output conductor 263 is also initially connected into a feed-through assembly 264 (FIG. 3). This assembly includes a ferrite radio-frequency filter 265 which suppresses electromagnetic disturbances and which encircles the conductor 263. The assembly also inlcudes a ceramic insulating sleeve 267 throgh which the conductor 263 is sealed gas-tight. The sleeve 267 is sealed into a flanged sleeve 269, typically of titanium. The ferrite trap 265 is secured to the inner side of the flanged sleeve 269 by a spring washer 271. The conductor 263 is connected near its external end to a flexible connector 272.

Once the components are interconnected the battery 23 and the parts 25-35 are mounted in one of the parts of the container 43 with the battery cradled between the boards 25 and 27. The boards 25 and 27 are contoured to rest on the inner surface of the container 43. The ground conductors (FIG. 13) are connected to the container 43. The assembly 264 is disposed adjacent the opening 261 with the flanges 273 of sleeve 269 appropriately positioned adjacent the hole 261. The two parts of the container 43 are then welded to form the container. The ECCOSIL 4952 mass 47 between the cold-junction end 49 of the battery and the container 43 is injected with a syringe inserted through opening 261. The 2CN potting rubber is then injected through opening 261 and encompasses the parts 23 through 35. The container 43 and its content are then placed in a chamber (not shown) which is evacuated and filled with an inert gas (argon) at about one atmospheric pressure. The inert gas permeates the converter 63 through the opening 231 and the bushing 243. Within the chamber, in the inert-gas atmosphere, the flange 273 is welded gas-tight to the rim of the hold 261 sealing the hole. The conncector 272 is now connected to a terminal block 275.

The terminal block 275 is in the form of a rectangular parallelapiped having a cylindrical opening through which the inner end 277 of the catheter 39 passes. Laterally a set screw 279 (FIG. 2) is provided in the block 275. Over the head of the set screw 279 there is a silastic plug 281.

The catheter 39 is inserted in heart pacer 21 by the doctor who installs it in the host's heart. During the construction of the pacer 21 the catheter 39 is replaced by a pin (not shown) of the diameter of the catheter 39. This pin is encircled by a suture boot 283. The pacer as now assembled is mounted in a mold (not shown) with the dimple 51 which forms the window facing downwardly and masked. The EPOXY resin 50 is then molded about the titanium casing except at the dimple 51 and the EPOXY excapsulation 285 for the conductor 263, the connector 272, the block 275, the bottom 283 and the pin (not shown) is formed. The pin is then removed.

After installation in the heart of the host, the catheter 39 is inserted in the opening formed by the pin and the end 277 is secured by the set screw 279 in the terminal block. The head of the set screw is closed by the plug 281 and the plug 281 can be sealed by silastic insulation.

The electrical circuit (FIG. 13) used in the practice of this invention is of the solid-state type and includes, in addition to the magnetic switch 33, a DC to DC converter 301, and amplifier 303, a monostable 305, a noise-rate turn-on 307, a multivibrator reset 309, a multivibrator 311, and an output 313.

The output of the battery 23 is about 0.6 volt open circuit; the DC-DC converter 301 derives about -5.4 volts from this output for operating the remainder of the circuit.

The amplifier 303 amplifies any input signal impressed on its input terminal 305 which is greater than 1.5 millivolts to 2.0 millivolts. This stage amplifies R-waves from the heart, pacer pulses and any noise which might be present at the input 315. The gain of this amplifier 303 falls off rapidly once the frequency of the incoming noise is out of the amplifier's band pass.

The monostable 305 performs two different functions. First, when it is triggered by a signal from the amplifier 303, it puts out typically a 0.275 second duration square pulse; and the monostable 305 cannot be triggered on again until it times out. This pulsing interval serves as the pacer's refractory period. The pacer's mode cannot be affected during this time. The amplifier 303 ramains sensitive, but the multivibrator 311 cannot be reset until the monstable 305 completes its pulse of 0.275 second. The amplifier 301 remains operative to drive the noise rate turn-on 307. Second, the leading edge of the monostable pulse resets the multivibrator 311.

The multivibrator 311 controls the rate and width of the output pulses of the pacer 21. The multivibrator 311 can operate in two different modes. One mode is the normal rate which is 70±2 beats per minute. The other is the noise rate which is 85±5 beats per minute. In both modes the pulse duration remains at 1.1±.01 milliseconds.

The output 313 is the stage which produces the negative pulse which actually paces the heart.

The multivibrator reset 309 stops the multivibrator 311 from putting out a pulse (heart pacing pulse). If a monostable pulse (duration 0.275) occurs because of an R-wave fed back from the heart or a pacer pulse, this stage 309 resets the multivibrator rate capacitor (not shown) to near zero volts. Every time the multivibrator 311 is reset it waits for 850 milliseconds (70.6 beats per minute) before it puts out another pacer pulse. Every time the pacer 21 itself puts out a heart pacing pulse it resets itself.

The noise-rate turn-on 307 constantly monitors the output of the amplifier 303. If the noise rate circuit senses that the amplifier's output pusles are at a specific frequency (15 hertz ±5 hertz, the noise turn-one frequency) it disables the monostable stage 305. This in turn prevents the multivibrator reset stage 309 from discharging the rate capacitor (not shown) in the multivibrator 311. If the rate capacitor in the multivibrator is not discharged, which happens every time the monostable stage 305 puts out its 0.275 second pulse, the multivibrator 311 continues to run, but the capacitor does not discharge to zero on every pulse and the rate increases to 85±5 beats per minute (the noise rate). When the magnetic reed switch 33 is closed by a magnet, external of the body, the pacer 21 becomes a fixed rate unit at the noise rate.

In the heart pacer under the control of the circuit shown in FIG. 13 R-waves, when they occur, disable the multivibrator 311 from delivering heart-pacer pulses to the heart. Pulses are only delivered in the absence of R-waves. The operation of such a heart pacer is referred to as an R-wave-inhibited demand pacer.

Typical parameters of the heart pacer in accordance with this invention are:

1. Pulse Duration -- 1.1 milliseconds, nominal -- 1.0-1.2 milliseconds, range. A sufficiently long pulse duratino to always insure heart capture is provided. A minimum pulse duration is desired to conserve electrical power output. If the pulse duration were reduced substantially below the 1.1 milliseconds, the amplitude requirement may become excessive, thereby causing fibrillation.

2. Pulse Amplitude -- 8 milliamps, nominal -- 7.5 to 8.5 milliamps, range. This amplitude is selected to always provide effective capture after the rise of threshold levels after electrode endothelialization. A higher amplitude, possibly 10 milliamps, may be considered, since a nuclear powered system is not battery energy sensitive, but the possibility of fibrillation might arise with such high levels. A lower amplitude in some are cases could prevent heart capture.

3. Basic Rate -- 70 beats per minute, nominal --68 to 72 beats per minute range. This rate appears to be the most common required by patients.

4. Noise Rate -- 85 beats per minute, nominal -- 80 to 100 beats per minute, range. This is the rate at which the pacemaker operates when noise interference is so great that it masks the normal Q-R-S heart complex. A rate higher than the basic rate is selected to minimize the possibility of competition with the normal heart rate should it be in normal rhythm.

5. Noise Rate Turn-On -- Approximately 20 Hertz. This rate is selected to prevent interference from all conceivable noise modes, microwave ovens and the like, with the exception of a sporadic impulse which would not be fatal. It is sufficiently low to rule out 60 Hertz rates which are the most probable, particularly with the expanding use of microwave ovens.

6. Magnetic Switch Rate -- 85 beats per minute, nominal -- 80 to 100 beats minute, range. This rate is a fixed rate, at which the pacer operates when energized by a magnet placed near the patient's magnetic reed switch. This turnon feature allows checking of the pacemaker performance since in the R-wave inhibited mode with normal Q-R-S complexes, the physican cannot tell whether the pacemaker is operating.

7. R-Wave Sensitivity -- ± 1.75 millivolts, nominal -- 1.5 to 2.0 millivolts, range. This sensitivity is selected to assure sensing of the R-wave, and is sufficiently high to prevent interference from abnormal P waves.

8. Refractory Period -- 275 milliseconds, nominal -- 250 to 300 milliseconds, range. This electronic refractory period begins with either a normal R-wave or the pacemaker stimulus. It is sufficiently long to prevent stimuli during the T-wave onset or decay--this period being the critical period for possible pacemaker induced fibrillation, i.e., no stimulus occurs during the T-wave of normal heart repolarization.

In installing the pacer according to this invention in the body of the host, the surgeon makes an incision 401 in the body of the host 402 preferably in the chest above the heart. The catheter 39 is then passed through a vein to the heart muscle. The catheter is then inserted in the pacer 21 and the pacer is inserted in the opening with the insulating coating 50 in contact with the muscles 403, FIG. 14, or 404, FIG. 15, and the window or dimple 51 away from the muscles 303 and in engagement with a non-muscular part of the chest, either the rib cage 405, FIG. 14, or the skin 406, FIG. 15.

While a preferred embodiment of this invention has been disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.