US 3206768 A
Description (OCR text may contain errors)
Sept. 21, 1965 R. J. PRESTON 3,206,768
ELECTROMAGNETIC ARTIFICIAL HEART HAVING CONTROL MEANS RESPONSIVE TO CHANGES IN BLOOD PRESSURE AND BODY RESPIRATION RATE Filed June 1, 1962 2 Sheets-Sheet 1 PUL MOIV/7/Q Y A o IIIIIII/IIIIIII 1/54/00; PEEJJK/E6 EKG/WW6 W/A/0/A/6 N VE N TOR RICHARD J. PRESTON AGE-NT R. J. PRESTON 3,206,768 ELECTROMAGNETIC ARTIFICIAL HEART HAVING CONTROL MEANS RESPONSIVE TO CHANGES IN BLOOD PRESSURE AND BODY RESPIRATION RATE 2 Sheets-Sheet 2 INVENTOR RICHARD J. PRES TON BY (9040M a: BMW
AGENT Sept. 21, 1965 Filed June 1, 1962 United States Patent This invention relates to an artificial heart, and in particular to the use of a pump as a replacement for or as an aid to the heart in transporting blood through the body.
In recent years great advances have been made in the medical field, particularly in work with the heart. Openheart surgery has now become common, and medical and .electronic aids are now extensively used to add years of life to damaged or defective hearts. Artificial organs are now well known, as are newly developed heart-lung machines which perform the functions of these organs during surgery. Another advance has been the use of electronic equipment implanted within the human body to perform functions such as supplying electronic im ,pulses to the heart, thereby regulating the rate of heart beat. Many of these advances have been brought about through the development of subminiature electronics and electronic equipment, miniature power supplies and advances in materials. The primary advance, however, has been the increase in knowledge of the workings of the ,human body through research and experimentation.
The present invention involves the application of the recent advances in both engineering and medicine to thereby allow the complete replacement of a human heart by the substitution of an electromechanical pump which will perform the same function in the human body as the heart performs. Recent developments in electromagnetic pumps have shown that such pumps can presently be designed with a size and efficiency which will allow their use as a permanent replacement for the heart.
It may also be desired to utilize such pumps to assist the heart in its function of pumping blood through the body. Small pumps may be inserted in the circulatory system of man to supply more blood to desired portions of the body when it is found that the biological system is deficient.
An artificial heart blood-pumping device may also prove invaluable in augmenting the heart during operations or during the extremes of space or planetary travel.
It is, therefore, an object of this invention to provide a system and apparatus for replacing or aiding the heart by completely or partially taking over the operation of pumping blood through the circulatory system.
Another object of this invention is the use of an electromagnetic pump to replace or augment the heart.
A further object of this invention is a system and apparatus for delivering additional blood to particular parts of the human body.
Another object of this inventionis a system for regulating the pumping action and the supply of blood through the human body when an artificial heart or auxiliary blood pump is used.
A further object of this invention is an apparatus and system used outside the body to profuse portions of the body during experiments in partial or profound hy- .pothermia.
These and other features and advantages will be apparent from the specification and claims, and from the accompanying drawings which illustrate an embodiment of the invention.
FIGURE 1 is a schematic representation of the human circulatory system with a pump substituted for the heart; and
FIGURE 2 is a schematic drawing of a typical electromagnetic pump; and
FIGURE 3 is a functional block diagram of an electronic system for operating an artificial heart.
The human circulatory system supplies the muscles, nerves and tissues with blood, the blood carrying necessary elements to the tissues and withdrawing waste products. The digestive system supplies the blood with nourishment and the respiratory system provides the blood with oxygen and allows certain waste to pass from the body. FIGURE 1 shows schematically a portion, of the circulatory system. The blood flows along a closed system of tubes from the heart to the arteries, capillaries and veins. The center of the circulatory system is the heart, which lies in the chest between the two lungs and above the sheet of muscle known as the diaphragm. The heart is really a portion of the circulatory system tubing, with greatly enlarged channels and thickened walls. The heart receives the blood from veins, sends it to the lungs, receives it back from the lungs and then pumps it through the arteries.
The circuits through which the blood flows in the body may be divided into two broad groups comprising first, the arteries which transport the blood to the remote portions of the body into the capillaries, and second, the veins which transport the blood back from the capillaries to the heart.
Referring particularly to FIGURE 1, a pump 10 has been substituted for the heart in the circulatory system. The blood is received from the various extremities: of the body through two large veins called the superior vena cava and inferior vena cava which feed the used blood to the pump 10. As will be explained later, the pump may be controlled to provide a pressure to the blood at a rate equivalent to that of the human heart. The blood then proceeds out of pump it) through the pulmonary artery to the lungs where an exchange of oxygen and carbon dioxide takes place. The oxygenated blood which leaves the lungs through the pulmonary vein is shown connected directly to the aorta which thereupon transports the blood to the other portions of the body. The pump 10 may also be inserted into the circulatory system after the lungs rather than before the lungs. This cycle takes approximately one minute. In the human heart, the blood flow is'from the veins to the right side of the heart, out of the heart and through the pulmonary artery to the lungs, out of the lungs to the left side of the heart, and then is pumped out of the heart and through the aorta to the body.
While it is apparent that a single pump can supply sufficient pressure to push the blood through the lungs and also through the complete circulatory system, it appears preferable to return the blood from the lungs to either an addition-a1 pump or "o a dual channel of pump 10. The additional pumping action may be necessary to prevent the rupturing of the lungs by the higher pressure required if only one pump is used, and the use of two pumping stages appears desirable. If necessary, the second pumping stage would be inserted after the lungs to pump the oxygenated blood into the aorta, and thence to the arteries.
The pump required as a replacement for the heart must meet several requirements. First, the pump must be leakless. Second, the pump must have an exceptionally vhigh degree of dependability such as might be obtained with a device having no moving parts to wear out. Third, the pump must be reasonably eflicient and must be capable of fitting into a small space. Fourth, the pump must be capable of operating with low power dissipation.
The electromagnetic pump meets all the requirements enumerated above. FIGURE 2 shows schematically a typical electromagnetic pump. All electromagnetic pumps utilize the motor principle, that is, a conductor in a magnetic field, carrying a current which flows at right angles to the direction of the field, having a force exerted on it which is mutually perpendicular to both the field and the current. In these pumps, the fluid, blood in this case, is the conductor. The force, suitably directed in the fluid, manifests itself as a pressure if the fluid is properly contained. The field and the current can be produced in different ways and the force may be utilized in different ways. There are a number of different types of electromagnetic pumps, all of them using this principle.
The blood has been found to be somewhat conductive in its natural state. Conductivity can be added to the blood by introducing an iron complex such as ferric chloride. This may take the form of capsules or injections. If too much iron is added to the blood, the excess will be excreted.
The most elementary electromagnetic pump has been called the Faraday type shown in FIGURE 2. In this pump, the fluid is contained in a thin-walled duct. A constant magnetic field is passed through the fluid on one axis perpendicular to the direction of flow. The field is developed by a winding, D.C. excited, arranged on a suitable magnetic core which provides both pole faces and a magnetic return path. Current is forced through the fluid by impressing a voltage across the axis of the duct mutually perpendicular to both the field and the direction of flow. The operation of the pump is similar to a DC. shunt motor. The separately excited field magnetizes the gap. Flow of the current in the fluid in the gap is similar to current in a DC. motor armature. An 1 R loss appears in the fluid. As the fluid flows a back voltage or is generated by the fluid moving in the field, opposing the flow of current in the fluid. The product of the back voltage and the effective current in the fluid represents the pumping power developed.
The Faraday pump can be made to work on alternating current when the field is excited by A.C. properly phased with the voltage applied to the armature. These A.C. pumps are similar to the pump shown in FIGURE 2. However, the A.C. version of the Faraday pump has a lower efliciency than the DC. pump since the fluid acts as a shorted turn in the transformer.
A helical flow induction pump requires guide veins in the ducts, but these pumps are well suited for high pressure, low-flow applications where space is at a premium and are attractive for use as a replacement for the heart.
The linear induction pump is a modification of the helical pump and provides large flow at moderate heads for limited space and power supply and is also attractive for heart application.
Engineering Magazine, April 27, 1956, contains an article entitled Electromagnetic Pumps, at page 264 which describes in detail the different types of electromagnetic pumps, the theory of operation and the manner in which losses and efliciencies may be computed.
Recent studies have shown that any of the above types of electromagnetic pumps can be developed small and light enough to replace the heart in the human body. The efliciency of such a pump would be aproximately 40%.
The weight of the average human heart is between 1000 and 1500 grams (2.2 lbs. to 3.7 lbs.). An artificial heart of five lbs. could be handled easily by a human being. The maximum chest cavity space available for a heart replacement, if the diaphragm is partially cut away,
is six inches in diameter by twelve inches long. The back pressure of the biological circulatory system as felt by the heart varies between minus five to plus fifteen centimeters of H 0. The average pressure in the human heart is between 100 to 180 millimeters of mercury. The
average rate of flow of blood in the human circulatory system averages between four to fifteen liters per minute, and may go as high as eighteen liters per minute for an untrained person under stress and twenty-five liters per minute at a severe stress peak for a trained athlete.
Development work on electromagnetic pumps has shown that pumps of the size, weight, efliciency and capacity required to replace the heart can be produced. The channel of the pump should preferably be flat and wide for optimizing the design and efficiency of the pump.
The specific resistance of whole blood at 37 centigrade has been computed as a function of frequency. The chart below shows the results of the experimentation.
Specific resistance Frequency: (OhITl/CHl-g) c.p.s 166 1 kc. -180 1 me 100 me. 80-100 1000 me. 64-80 10,000 mc. 93-11 pump 10 for the purpose of illustrating that the pump 10 may either completely replace the heart or serve as an aid to the heart. The pump may be placed in parallel with the heart or a small pump may be located any place within the arterial system to pump additional blood to whatever part of the body is in need of more blood.
FIGURE 3 also shows, in block diagram form, a typical electronic control system for regulating the pumping action of the pump 10.
A transducer 20 is connected to a vein to sense venous pressure rate. It has been found that pressure waves are created in each beat of the heart, and this pressure rate may be sensed in either the arteries or the veins. It appears Preferable to sense this pressure at a vein primarily because of the danger involved with arterial penetrations. FIGURE 1 shows the venous pressure transducer 20 connected to a vein leading into the inferior vena cava. Any vein may be used, but a vein adjacent the heart appears to be preferable since it would be more convenient at that point. An optimized location giving easy access to a vein is where the leg joins the bottom of the trunk.
The venous pressure rate transducer 20 and the other electronic circuits to be described will preferably be implantable molecular circuits. Recent advances in molecular circuitry has shown that such circuits may be constructed of very small size and low power dissipation. The venous pressure rate sensor itself will P eferably be a tunnel diode crystal with venous pressure acting upon the junction to shift the frequency of oscillation of the crystal as a function of the pressure. Another oscillator may be used for the venous pressure rate set 22. This oscillator 22 will produce an A.C. signal which is a function of the desired pumping rate for pump 10. The oscillator signals from both the pressure rate transducer 20 and the pressure rate set 22 are fed to a summing network 24 which may be a beat frequency generator network where an A.C. error signal is generated having a frequency proportional to the difference between the pressure set and the actual venous pressure rate. This error signal is amplified by amplifier 26 and fed to a frequency convertor 28 which transforms the AC. error signal into an AC. signal proportional to the error signal.
The output of the frequency convertor 28 is fed to a control circuit 30 which controls the application of the magnetic field from magnetic power supply 32 to pump 10. A current power supply 34 provides current to pump 10. As shown in FIGURE 2, the magnetic field and current are at right angles to each other and orthogonal to the blood flow. Since current through the blood may have a tendency to ionize or dissociate the blood, it is desirable to keep the pump current at a minimum value. It has been found that ampere per cm. through the blood will not cause any harmful effects. If the conductivity of the blood and the pump current are kept constant, variations in the magnetic field intensity may be used to vary the pumping rate of pump 10. Thus, it is preferable to use the signal from control circuit 30 to vary the magnetic field strength supplied by magnetic power supply 32 to pump 10. The power supply 34 will supply constant current to pump 10. It is clear that, depending on the type of pump used, a DC. system or control signal may be preferred. If block 28 were to be replaced by a discriminator, the DC. could be easily supplied.
Pump 10 may also be operated by keeping the magnetic field constant and varying the current as a function of the error signal. Likewise, both magnetic field and current may be varied. The particular pump used will influence the choice of the parameter to be varied and also the particular circuits used in the control system.
The electronic control system thus far described is typical of many servo systems and is adequate to keep pump 10 operating at a fixed rate to thereby supply a relatively constant volume of blood through the circulatory system. However, the heart does not pump a constant volume of blood through the circulatory system, but the volume in fact varies continuously depending on the needs of the body. Thus, under extreme stress conditions a much greater volume of blood is required to supply oxygen to the body organs. A respiration rate transducer 36, which is responsive to respiration rate, can be used to change the set point of the venous pressure set 22. Respiration rate has been found to vary in direct proportion to metabolic activity. Now when more oxygen is required by the various parts of the body and the lungs are required to work harder, the human inhales more oxygen, and rate transducer 36 will sense this increased respiration rate and vary the set point to create a larger error signal and force pump 10 to pump more blood throughout the body. The transducer 36 may consist of two electrodes inserted into the muscular tissue of the chest wall adjacent the lungs, and the impedance change due to lung action will give a measure of respiration rate. Amplification of this signal may be necessary. A tank-type circuit may also be used.
Although the disclosed system uses heart or pump rate for control, the blood pressure must also be considered. Blood pressure increases with rate, so that a higher rate will give a higher pressure. The veins and arteries expand somewhat with pressure, and can readily withstand any pressure increases within the rates which occur in the pump. Recent work with animals has shown that a constant blood pressure is not harmful, and indicates that a constant pressure blood supply may be used by man.
FIGURE 1 shows a block 38 labeled electronics. This block contains all the electronic circuitry as shown in FIGURE 3 including power supplies 32 and 34. FIG- URE 1 also shows that venous pressure transducer and respiratory rate transducer 36 provide control signals to block 22. These control signals may be telemetry signals, which eliminates the need for wires within the body. Block 38 is connected directly to pump 10.
Because of the fact that the veins are located at a distance through the circulatory system from the heart, the pressure wave sensed in any vein is necessarily delayed from the beat of the 'heart which produced the pressure wave. A time delay may be introduced into the system by providing either an anticipation circuit or a lag cir- 'cuit in the summing network or pressure transducer to compensate for the time delay. The power supply for pump 10 may be a small battery such as used in the Pacemaker, but the battery may need to be larger since more power is needed to operate the pump then is needed to stimulate the heart. Other types of power supplies may be used, for example, those which use the body heat to power thermoelectric supplies, chemical power supplies which utilize the body chemistry, or biological fuel cells using the stomach as a generator, or muscle power in which muscle movement is used for power. A particular type of power supply under development is the transponder which radiates by means of RF energy power through an antenna located internally which thus provides suitable power to the pump.
The heart contains at least two major valves which close after each beat to prevent the blood pumped by the heart from pushing back into the heart chamber from which it was pumped. Pump 10 does not need this type of check valve. A bias may be provided to the magnetic field or current supply of the pump so that a minimum forward pressure is always produced.
The control circuitry of FIGURE 3 has been described as A.C. circuitry, but it is obvious that DLC. circuitry may also be used. The electronics and the control system are well known and the actual circuits used will obviously depend upon the pump and the power supply. However, alternating current operation of the pump is preferred primarily because the specific resistance of the blood decreases with the frequency so that it is advantageous to use as high a frequency as possible.
Any heat which is generated in either the pump or the electronics may be transferred through the blood to the skin or neutralized by means of thermoelectric cooling unctions.
It is apparent that since a pump can be used to replace the heart, the same type of pump may be inserted anywhere within the body to assist the heart or the arteries for supplying blood to the organs. For example, it would be advantageous to profound hypothermia to pump blood and profuse the brain through the carotid. artery and ugular vein while the rest of the body was in a state of profound hypothermia and suspended metabolic activity.
The electromagnetic artificial heart can be useful in both implant and extra-corporeal capacities, the latter primarily during operations and during space travel. While this description is primarily directed to implant uses for such an artificial heart, extra-corporeal uses, with similar control techniques, may also be desirable.
Numerous changes may be made in the components and circuitry, as for example using different types of pumps, without departing from the scope of this invention.
1. Apparatus for delivering blood to at least a portion of the circulatory system of a living body comprising an electromagnetic pump, means for connecting said pump in a blood conducting passage of the circulatory system of a living body, actuating means for said pump, mean for producing a signal indicative of the actual blood pressure in said system, means for producing a reference signal indicative of desired blood pressure, means for comparing said actual blood pressure signal with said reference signal to produce an error signal proportional to the ditference between said desired blood pressure and said actual blood pressure, means conducting said error signal to said pump actuating means to vary the rate of said pump and eliminate said error signal, and means for varying said reference signal in response to changes in the respiration rate of said body.
2. Apparatus as in claim 1 in which said means for producing a, signal indicative of actual blood pressure comprises an electronic sensor adapted to be connected to a vein for said body and responsive to pressure variations in said vein.
3. Apparatus as in claim 2 in which said means for varying said reference signal in response to changes in the respiration rate of said body comprises a transducer having a pair of electrodes adapted to be inserted into the muscular tissue of the chest wall of said body adjacent the lungs.
4. Apparatus as in claim 3 in which said pump comprises a duct adapted for fluid flow therethrough, a magnetic core having windings thereon to provide a magnetic field perpendicular to the direction of fluid flow,
8 and means to produce a voltage across said duct mutually perpendicular to said fluid flow and to said magnetic field, said error signal being applied to said magnetic core windings to vary the magnetic field strength and thereby vary the rate of said pump.
5. Apparatus as in claim 4 in which a constant bias signal is generated in said pump magneticcore field windings to thereby produce a minimum forward pressure to said fluid and prevent reverse fluid flow through said pump.
References Cited by the Examiner UNITED STATES PATENTS 2,917,751 12/59 Fry 3-1 2,925,814 2/60 Vibber 128214 3,066,607 12/62 Cole 1031 RICHARD A. GAUDET, Primary Examiner.