Self-contained artificial heart

Abstract

A small, self-contained blood pump includes a physiologically-responsive beat rate control system and a pulmonary edema protection system.

Parent Case Text

RELATED APPLICATIONS

This is a division of U.S. Pat. No. 3,828,371, derived from Ser. No. 99,635 filed Dec. 18, 1970.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a single unit mechanical device capable of circulating blood in the human body in place of a human heart and capable of surgical implantation in the thoracic cavity in a single operation.

2. Description of the Prior Art

Previous nuclear powered mechanical heart devices such as the one shown in U.S. Pat. No. 3,379,191 of Apr. 23, 1968, were comprised of two separate units, one to be implanted in the thoracic cavity and a separate power source unit to be implanted in the abdominal cavity with a connecting line to pass the steam generated at the boiler unit of the power source and return the used steam to a condenser and then back to the power source. Another disadvantage of the prior art devices was that the artificial ventricles were mechanically coupled to the reciprocating piston and when the piston drew blood into the ventricles, the atrial system tended to collapse under the negative pressure induced therein. Adequate precautions were not taken against pulmonary edema which resulted from continuous pumping of blood to the lungs beyond capacity of the blood vessels therein. Although prior art devices could be set at a selected beat rate, no one has previously conceived of means for varying the beat rate or blood pump output of an implantable artificial heart according to the body's need for blood as a natural human heart does.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a single unit artificial heart device having an integral energy source and controls adapted to be implanted in the pericardial sac in place of a human heart.

It is a further object of this invention to provide an artificial heart having a thermoelectric module allowing close simulation of the functions of a human heart by electrical control of pumping rate and pulmonary to systemic pumping ratio in response to physiological need.

Another object is to provide an artificial heart which is more efficient and longer lasting than previous devices.

Another object is to provide an artificial heart which is lighter than previous devices.

Other objects of this invention will become apparent from the description of the invention which follows.

Broadly, the invention is a self-contained artificial heart including an electronic control circuit adapted to simulate a natural heart's action by varying the pulse and flow rate in response to a physiological variable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal section taken approximately on the line 1--1 of FIG. 2 of a self-contained artificial heart constructed in accordance with one embodiment of the invention;

FIG. 2 is a cross-section taken on the line 2--2 of FIG. 1;

FIG. 3 is a fragmentary plan view of the left hand end portion of FIG. 1;

FIG. 4 is a diagrammatic one half plan view taken on the line 4--4 of FIG. 2;

FIG. 5 is a part sectional and part elevational view illustrating the piston and cylinder and associated right and left artificial ventricles in compressed position;

FIG. 6 is a circuit diagram of the beat rate and stroke length control system;

FIG. 7 is a circuit diagram of the right ventricle blood inlet valve control system;

FIG. 8 is a circuit diagram of the feedliquid pump control system; and

FIG. 9 is a schematic of the electrical and working fluid flow system.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

Referring to the drawings which show an illustrative embodiment of the blood pump device of the invention, the device 10 contains a source of thermal energy 11 preferably a radioisotope and most preferably a compound of plutonium.sup.238. A thermoelectric converter 12 is in proximity to the thermal energy source 11 and functions to convert a first portion of the thermal energy from Pu compound 11 to electrical energy. Also in close proximity, preferably surrounding the source of thermal energy and most preferably surrounding the converter 12, is a monotube boiler 13 which functions to vaporize and superheat the working fluid. The working fluid may be any one suitable for use in a Rankine cycle engine such as hydrocarbon, halogenated hydrocarbons such as that family of compounds known as the "Freons," or water. The working fluid is changed from its liquid to its gaseous state in the boiler 13 by a second portion of the thermal energy and is passed to an expansion zone 14 established by stationary piston 15 and reciprocally movable cylinder head assembly 16 through a solenoid controlled inlet valve 17 in said stationary piston 15. The stationary piston 15 also contains a solenoid-operated exhaust valve 18. The cylinder head assembly 16 moves first away from the piston 15 when the inlet valve 17 is opened and the exhaust valve 18 is closed, and second toward the piston 15 when the exhaust valve 18 is opened and the inlet valve 17 is closed. When the cylinder head assembly 16 is moving away from the stationary piston 15, it compresses an artificial right ventricle 19 and an artificial left ventricle 20 simultaneously to expel blood therefrom by means of a pusher plate member 21--21 which is attached to the cylinder head 16 but is preferably not physically attached to the ventricles 19 and 20. When the cylinder head 16 is moving back toward the piston 15, it causes the pusher plate 21--21 to move away from the ventricles 19 and 20, allowing them to expand and allow blood to enter naturally as indicated by arrows 23 and 22 (respectively for ventricles 19 and 20) in FIG. 3; that is, as a result of the pressure in the atrial system and not as a result of the negative pressure which would be caused if the pusher plate 21 were attached directly to the ventricles 19 and 20 and forced them open. When the ventricles 19 and 20 are compressed as shown in FIG. 5 as a result of the force of cylinder head assembly 16, blood is caused to be expelled as indicated by arrows 24 and 25 (respectively for ventricles 19 and 20) in FIG. 3 into both the systemic and the pulmonary systems of the mammal in which the blood pump 10 is implanted.

By transmitting the engine cylinder force directly to the blood, a pulse shape very similar to that of the human heart can be obtained, resulting from the high initial force as gas initially enters the cylinder and rapidly decreasing as the engine cylinder moves. Blood, indicated by the arrow 25 from the left artificial ventricle 20, is passed to a connection 26 to the patient's aorta while blood 24 from the right artificial ventricle 19 is passed to a specially designed pulmonary artery connection 27 containing an outlet valve 28. The inlet 29 to this right artificial ventricle 19 preferably contains an electromagnetically controlled valve 30 to be described in more detail later in the specification. The aorta connection 26 also contains a valve 31 to prevent backflow of blood 25 in the opposite direction. The left ventricle 20 also contains a blood inlet valve 32 which allows blood 22 only to enter the left ventricle 20.

The aorta connection 26, pulmonary artery connection 27, left and right artificial ventricles 20 and 19 are constructed of any suitable medical grade material which is compatable with blood and body tissues. Natural and synthetic polymer materials such as polyurethane, dacron, hepranized silastic, and silicon polymers are merely exemplary; the particular material selected does not form a part of the present invention.

A capillary condenser tube 33 is connected to the piston exhaust valve 18 and preferably has an inner surface lined with a fibrous metal wick 34. For example, such a wick-type of capillary lining may be formed from sintered titanium-aluminum-vanadium alloy. Exhaust vapor entering the condenser tube 33 at 36 (FIG. 9) condenses on the porous wick 34 and fills the wick pores. The condensate flows through the wick 34 to the exit 35 (FIG. 9) of the condenser and then through a subcooler 37 (FIG. 4) to the inlet 38 of the feedliquid pump 39. The condensate is retained in the wick by capillarity regardless of the position or attitude of the heart device. The capillary condenser 33 has a large surface area and rejects heat to an interstitial fluid 43 (FIG. 1) which in turn rejects its heat to the blood in the ventricles 19 and 20 and maximizes the distribution of heat while minimizing the blood temperature rise. The feedliquid pump 39 is preferably driven by an electrically operated solenoid 40 driven by an electronic control system shown in FIG. 8 at a constant rate. From the feedliquid pump 39 the working fluid is passed back to the boiler 13 where it is vaporized again.

The availability of the electric power from the thermoelectric module or converter provides for electronic control of various functions of this artificial heart which make it superior to any prior art artificial heart. The thermoelectric module 12 is preferably a series of silicon-germanium semi-conductor thermocouples. The thermoelectric converter 12 provides electricity to electrical control circuits (shown in FIG. 6) which control the electromagnetic gas inlet valve 17 and the electromagnetic exhaust valve 18 in the piston 15 and which function to control precisely either or both the stroke length and the cylce rate of the movable cylinder head assembly 16. In the preferred embodiment, the electrical control circuit (FIG. 6) is adapted to vary the pulse rate and the stroke length in response to a physiological variable such as blood pressure, variations in which are detected by means of a sensor such as a pressure sensitive transistor thereby varying the rate of pumping of blood in response to the physiological variable, closely simulating the behavior of a natural human heart.

The converter 12 also provides electricity for another electrical control circuit (shown in FIG. 7) which is preferably provided to control an electromagnetic valve 30 at the blood inlet 29 to the right artificial ventricle 19 and is adapted to control the amount of blood indicated by arrow 23 (FIG. 3) entering the right ventricle 19. A sensor (not shown) of pulmonary blood pressure is provided which signals the control circuit (FIG. 7) when the pulmonary blood pressure rises above a predetermined level and causes the electromagnetic valve 30 to be closed until the pulmonary blood pressure drops to an acceptable level. Pulmonary edema is thereby effectively prevented because of the artificial heart's adaptability to change the systemic system to pulmonary system pumping rate ratio.

Since the rate of blood pumping and hence flow of working fluid through the cylinder may be varied, and the feedliquid pump 40 preferably operates at a constant rate, there will be excess vapor during the major portion of normal activity and during periods of low blood flow power demands. A relief valve 45 (FIG. 9) is provided to release excess vapor from the boiler superheater 46 into the condenser 33 at 36 at a predetermined pressure.

The source of thermal energy 11 previously mentioned is preferably a compound of Pu.sup.238 isotope and is shielded preferably by a platinum capsule 41 having an absolute filtered vent 42 (FIG. 1) to duct helium generated by said isotope 11 during the course of this decay directly into the interstitial fluid 43. The helium permeates the silastic covering 44 (FIG. 1) into the bloodstream. A microsphere fuel form is coated first with thoria and then with platinum-rhodium to form a ductile mass which withstands any credible impact as well as fire or any credible accident.

An internal cylindrical bellows 47 and an external cylindrical bellows 48 (FIGS. 1 and 2) are provided, one end of each being welded to the movable cylinder head 16 and the other end of each being welded to a stationary member 49 which is affixedly attached to the piston 15. The internal bellows 47 forms a hermetic seal to prevent any possible working fluid escape and to vent any vapor leakage past the cylinder 14 directly to the condenser 33. In the preferred embodiments, there is provided a xenon-filled annular chamber defined by bellows 47 and 48. Much of such annular chamber is occupied by thermally insulating, slidably interfitting Min-K cylindrical cans 50 and 51 which act as thermal insulators. Surrounding this annulus is the external bellows 48 which provides a hermetic seal for xenon 52 containment. Xenon 52 provides the dual function of lowering the thermal conduction of the annulus and generating a null force on the piston 15 when the cylinder 14 is at its smallest volume. With the movement of the cylinder head assembly 16 away from the piston 15, the two bellows 47 and 48 are lengthened and the xenon pressure reduces, thereby increasing the propensity of the gas pressure forces other than the working fluid pressure to urge the cylinder head assembly 16 toward the position providing the smallest volume for expansion zone 14. As the cylinder head assembly 16 advances, it asserts a direct force onto the two artificial ventricles 19 and 20, thereby expelling the blood. At the completion of the pump stroke the pressure in the cylinder will have dropped and the exhaust valve 18 will open for at least a portion of the return stroke.

DESCRIPTION OF CIRCUITRY

The artificial heart is controlled by circuitry responsive to some physiological function such as the average blood pressure, filtered to eliminate the variations from beat to beat. In FIG. 6, dotted lines enclose certain electronic functions, which have inter-relationships shown by the schematic diagram. A variable voltage signal indicative of the variations in blood pressure is regulated by the "Right Atrial Pressure Sensor" unit, there being a suitable transducer in the right atrium. The human body is thus a significant participant of the feedback loop.

Particular attention is directed to the feature whereby this average blood pressure signal varies both the stroke and the rate of the artificial heart, thus simulating the normal heart's capacity for increasing both the volume per beat and the rate of the pumping action. That portion of FIG. 6 within the dotted lines identified as "Rate Control" is a multivibrator, the rate of which is controlled by the average blood pressure. The "Gas Inlet Control" is an emitter coupled monostable vibrator, the pulse width of which is varied by the average blood pressure. The train of thus regulated pulses actuates a driven circuit identified as a "Gas Inlet Switching Network" whereby the gas inlet valve 17 solenoid is energized to control both the stroke and the rate of the reciprocating cylinder head assembly 16. The exhaust valve 18 is regulated by a solenoid energized through a "Gas Exhaust Switching Network" driven by the signal from the "Gas Exhaust Control," the frequency of the pulses being the complimentary output of the "Rate Control" regulated by the feedback signal from the human body through the "Right Atrial Pressure Sensor."

The current from the thermoelectric converter 12 energizes the circuits of FIGS. 6, 7, and 8. Within each set of dotted lines, any transistor described as a second transistor is the one on the right, the leftward transistor being called the first transistor.

The reciprocation rate of the cylinder head assembly 16 is controlled by the rate at which the inlet valve 17 is opened. A less than maximum stroke length is achieved by shortening the duration of the opening of the inlet valve 17. The exhaust valve 18 is electrically actuated toward the open position during at least some portion of the cycle when the intake valve is not so actuated.

The gas inlet valve solenoid is controlled by a "Gas Inlet Switching Network." A "Gas Inlet Control" is an emitter-coupled monostable vibrator whose pulse is triggered by the train of pulses from the "Rate Control," the pulse width being varied by the signal from the "Right Atrial Pressure Sensor." Thus, when the body sends back more blood toward the heart, the pumping capacity is increased in part as a result of increasing the stroke by increasing the pulse width.

The "Gas Inlet Switching Network" is a driver circuit comprising a first and second transistor. A positive current is directed through a diode, a resistor, the solenoid coil for the inlet valve 17 and the collector and emitter of the second transistor. The base of the second transistor is controlled by the emitter signal from the first transistor developed across a resistor voltage divider. The emitter of the second transistor is also connected by two parallel capacitors to the collector through the solenoid winding in the valve 17 for the gas inlet. A diode in parallel with the solenoid winding for the valve 17 of the gas inlet serves to prevent any adverse effects from the intermittent potential in the solenoid. In the operation of the "Gas Inlet Switching Network", the low power signal from the "Gas Inlet Control" regulates both the frequency and duration of valve opening, and the switching network unit controls the flow of the higher power current to the solenoid of the valve 17.

The "Gas Inlet Control" is an emitter-coupled monostable vibrator and includes two common emitter connected transistors, the emitters being maintained above ground by a resistor to ground. The base of a first transistor of the "Gas Inlet Control" is connected through a resistor to the collector of the second transistor, which provides an output signal from the "Gas Inlet Control." The collectors of the first and second transistors of the gas inlet control are connected through their respective resistors to a positive potential. The signal on the collector of the first transistor is coupled to the base of the second transistor through a capacitor. The pulse width is increased when the heart rate is increased because the "Gas Inlet Control" is modulated by the output signal of the right atrial pressure sensor. For example, such atrial signal can be connected through a resistor to the base of the second transistor.

The "Rate Control" unit is an R-C coupled common emitter multivibrator. The base of each transistor is connected by a capacitor to the collector of the other transistor and by a resistor to the positive signal from the "Right Atrial Pressure Sensor." Similarly, the collector of each transistor is connected through a resistor to such positive signal from the "Right Atrial Pressure Sensor".

The operation of the "Rate Control" unit can be clarified by noting that a positive signal from the "Right Atrial Pressure Sensor" is converted by the multivibrator to two trains of pulses at a rate of the general magnitude of a natural heart beat, the pulses from one side of the multivibrator being directed to the "Gas Exhaust Control" and the other train of pulses from the other side of the multivibrator being directed to the "Gas Inlet Control." The frequency of the multivibrator increases when the body needs more blood circulation as communicated by the positive signal from the "Right Atrial Pressure Sensor."

The "Right Atrial Pressure Sensor" includes a transducer responsive to the pressure in the right atrium, such transducer being associated with the base of the transistor in such unit. A voltage divider between the positive terminal of the source of electrical potential and ground provides a controlled voltage to the base. The emitter of the transistor is connected by a resistor to ground. The collector is connected by a resistor to the positive terminal of the source of electrical potential. The output signal at the collector is coupled to supply the positive potential for the multivibrator by an isolating diode in series with a resistor. A capacitor is connected between such resistor and ground. Such association of the electronic components in the unit designated as the "Right Atrial Pressure Sensor" provides a filtered signal (the beat to beat variations being rejected) which is a smoothly varying positive signal indicative of average atrial pressure, and this signal regulates the variations of both the "Rate Control" unit and the "Gas Inlet Control" for controlling both the rate and stroke of the reciprocations of the cylinder head assembly 16. In this manner, the entire system closely simulates the response of a natural heart to right atrial pressure.

The "Gas Exhaust Control" is similar in configuration and operation to the "Gas Inlet Control." A signal consisting of a train of pulses from the multivibrator is applied to the base of the first transistor. The bias, however, is derived directly from the positive potential source, rather than from the positive output of the "Right Atrial Pressure Sensor" because the working fluid can flow through an open exhaust valve during the same fraction of a cycle without regard to whether the inlet valve was long open for a full stroke or a shorter time for a partial stroke.

The "Gas Exhaust Switching Network" includes a transistor, the base of which is connected through a resistor to ground. The solenoid coil operating the gas exhaust valve is connected in series with the collector to the positive potential source. A diode bypasses the solenoid coil of the gas exhaust valve 18 to protect the transistor from a voltage spike when the solenoid current is interrupted.

In the operation of the circuitry of FIG. 6, the pressure sensor at the right atrium provides a positive signal upon which is impressed the collector voltage developed by the base current generated by the right atrial pressure. The pulse rate of the "Rate Control" is controlled by the voltage of the "Right Atrial Pressure Sensor" to provide a train of pulses to the "Gas Inlet Control" and the "Gas Exhaust Control." The respective gas inlet and exhaust switching networks are operated by their controls so that the valves are actuated at rates determined by the pressure in the right atrium, and the inlet switching network is additionally controlled to narrow the proportion of maximum pulse width sent to the gas inlet valve solenoid coil except when a predetermined high pressure of the right atrium is exceeded. Thus, as the body sends back blood to the heart at a greater rate, the heart's pumping capacity is increased by increasing both the stroke of the cylinder head assembly 16 and the frequency or number of strokes per minute.

FIG. 7 depicts a "Pulmonary Edema Protector" and is effectively a blood switching system which prevents the flow of blood to the lungs during those periods when the average pulmonary blood pressure is excessive. When the pulmonary, arterial, average pressure rises above a predetermined value, the Schmidt trigger changes state and closes blood inlet valve 30 by which the blood would flow to the lungs through the right ventricle. The "Pulmonary Edema Protector" includes two transistors, the emitters of which are connected together and maintained above ground potential by a resistor. Each collector is connected through a resistor to the positive terminal, and the bases are each biased by a resistor to ground. The input signal is coupled through a resistor from the "Pulmonary Arterial Pressure Sensor" to the base of the first transistor, and the output signal from the collector of the first transistor is coupled by a resistor to the base of the second transistor.

The signal at the collector of the second transistor of the "Pulmonary Edema Protector" is coupled through a resistor to the base of the single transistor in the "Blood Inlet Switching Network," a circuit similar to the "Gas Exhaust Switching Network" described above with respect to FIG. 6.

In operation, the opening or closing of the right ventricle blood inlet valve 30 is controlled by the "Blood Inlet Switching Network." Such valve is closed in response to any signal from the "Pulmonary Edema Protector," which signal is sent only during the brief moments when the pulmonary arterial pressure exceeds the predetermined limit. The "Pulmonary Edema Protector" protects the system so that the lungs are protected from blood circulation rates greater than the lungs can at that moment satisfactorily process, even when the flow of blood back to the heart might suggest a greater circulation rate. Thus, the inlet valve 30 for the blood for the right ventricle is closed during those moments appropriate for responding to the ability of the lungs to process such rate of blood circulation, but is open much of the time.

In FIG. 8, the voltage source is conducted through a zener diode voltage stabilizer to a feedliquid "Pump Rate/Duration Control," a multivibrator comprising two transistors of opposite conductivity type. The emitter of the first transistor is connected directly to the positive potential. The collector of the second transistor is connected by a resistor to such positive potential. The collector of the first transistor is connected through a resistor, and the emitter of the second transistor is connected directly to ground. The bases of the two transistors are interconnected by a symmetrical network, each including a base-to-collector resistor and a capacitor and resistor in series between the base of one transistor and the collector of the other. The feedliquid "Pump Switching Network" is essentially of the same configuration and operation as the "Gas Inlet Switching Network" of FIG. 6.

In the operation of the circuitry of FIG. 8, the oscillator of the rate control unit provides a train of pulses of controlled duration which actuate the heavy duty switching of the switching network unit to energize the solenoid 40 to operate the pump 39 at a predetermined constant rate. By appropriate choice of the resistors in the symmetrical base-collector network of the feedliquid pump rate/duration control, the relative time between adjacent pulses as well as the pulse duration can be readily controlled.

The embodiments of this invention described in great detail are merely illustrative of the invention. Certain obvious modifications might eventually be apparent to those skilled in this art without departing from the spirit and scope of the invention as set forth in the claims.

Claims

I claim:

1. A self-contained implantable artificial heart comprising:

a source of electric power;

a reciprocating device, said device having a reciprocally movable portion and a stationary portion;

artificial left and right artificial ventricles arranged so that when said reciprocally movable portion moves in one direction, said ventricles are simultaneously compressed to expel blood therefrom;

electric control circuits comprising sensing means adapted to generate an electrical signal indicative of blood pressure, electronic means converting the blood pressure signal to a filtered smoothly varying potential indicative of average blood pressure, said average blood pressure potential being applied simultaneously to a multivibrator to regulate the frequency of reciprocation of the reciprocating device throughout a range of rate of stroke within the range of healthy pulse rate, and to a control to affect a similar regulation of the stroke of the reciprocating device, whereby the range of pumping capacity of the ventricles is significantly greater than the range of stroke rate;

a housing enclosing said source reciprocating device, control circuits, and ventricles, said housing being of a size to fit within the chest cavity after removal of a major portion of the natural heart.

2. The device of claim 1 in which the electrical control means includes sensing means to determine average pulmonary blood pressure, an electromagnetic valve at the blood inlet to the right artificial ventricle adapted to control the time when blood may enter said right ventricle, said control means being adapted to hold said valve closed when said average pulmonary blood pressure is sensed to be above a predetermined level.