US 3911897 A
A heart assist device is controlled in a normal mode of operation to copulsate with the heart and produce a blood flow waveform corresponding to the flow waveform of the heart being assisted. A blood pump in the device is connected serially between the discharge of a heart ventricle and the vascular system, and during the normal mode of operation, the pump is operated to maintain a programmed pressure at the ventricle discharge during systolic cardiac pulsation. A pressure transducer detects the pressure at the discharge and controls the pump through a hydraulically powered, closed-loop servomechanism. In the event that the heart beat stops or becomes severely arrhythmic, the device switches to an autonomous mode of operation and a waveform generator in the pump controls provides an ideal blood flow waveform independent of cardiac pulsations.
Description (OCR text may contain errors)
United States Patent [1 1 Leachman, Jr.
OTHER PUBLICATIONS Sato et al., Transactions of the American Society for Artificial Internal Organs, Vol. XV, 1969, pp. 449453.
LEFT VENTRICLE Oct. .14, 1975 ABSIRACT A heart assist device is controlled in a normal mode of operation to copulsate with the heart and produce a blood flow waveform corresponding to the flow waveform of the heart being assisted. A blood pump in the device is connected serially between the discharge of a heart ventricle and the vascular system, and during the normal mode of operation, the pump is operated to maintain a programmed pressure at the ventricle discharge during systolic cardiac pulsation. A pressure transducer detects the pressure at the discharge and controls the pump through a hydraulically powered, closed-loop servomechanism. In the event that the heart beat stops or becomes severely arrhythmic, the device switches to an autonomous mode of operation and a waveform generator in the pump controls provides an ideal blood flow waveform independent of cardiac pulsations.
20 Claims, 2 Drawing Figures U.S. Patent Oct.14,1975 Sheet 1 of2 4o y l 4 Q J E M 26 F 28 26 I h k l4 l2 H l LEFT AQRT[C LIGATION VENTRICLE VALVE AORTA PULSE STRETCHER 6| OUTPUT '1 3 i I P i RESET RAMP SINGLE VOLTAGE 'TO' w FUNCTION FREQUENCY AVEFORM DELAY PULSE CONVERTER GENERATOR 120 I26) lze SAMPLE HOLD WAVEFORM US. Patent Oct. 14, 1975 Sheet 2 of2 3,911,897
FIG. 2 so POSITION PRESSURE TORQUE TRANSDUCER TRANSDUCER MoToR 82 I02 52' I I I PosITIoN sAMPLE a HOLD VOLTAGE AMPLIFIER AMPLIFIER COMPARATOR I as 54 fee SIGNAL CONDITIONER sTART Dg,3 PosITIoN I 56 REFERENcE INPUT I SE 1 SWITCH I 6 PUL ID H I I 3 DIscRIMINAToR I SWITCH I I00 r L jE eIc I08 I t: 62 I 58 ALARM I INPUT H2 PRESSURE 64 REFERENcE S i A ERRoR \J MANUAL R T R DETECTOR DEFIBRILLATOR 6 I f 60 f STIIEIEEER I I l E PACEMAKER I SWITCH I I J I06 [70 FLow $2 51 f I WAVEFORM I J I VOLTAGE F76 Iflz I 74 84 RESET DIFFERENTIAL RAMP SINGLE 78 SUMMING SAMPLE a HOLD JUNCTION HEART ASSIST DEVICE BACKGROUND OF THE INVENTION The present invention relates to prosthetic devices and, more particularly, is concerned with a heart assist device which can operate in copulsation with the heart or establish autonomous control of blood flow from the heart.
Several approaches to the problem of providing artificial assistance to a weak or diseased heart have been proposed or developed. One of the most promising approaches involves the concept of copulsation in which a blood pump in series with the heart is operated in synchronism with ventricular contractions to augment blood flow through the vascular system and reduce the work load on the heart at the same time. Copulsation by itself, however, can be achieved in various manners and does not necessarily result in duplication of the blood pressures and flows that would normally be experienced from a healthy heart. It is desirable that the heart assist device produce a pulsatile flow which the vascular system is accustomed to and in this respect, the blood flow or velocity waveform during each pulsation should be such that there is no breakdown or damage to the blood or the vascular system.
The need for an optimum blood flow waveform is apparent from the physical characteristics of the heart and the vascular system into which it empties. The vascular system properly performs its function only when it transports a needed amount of blood in a given time to the appropriate cells of the body. Examination of blood vessels at different locations in the vascular system reveals that the wall of each vessel has the minimum cellular structure required to withstand the most severe stresses imposed upon it by the circulatory system. Significantly, this minimum structure of the vessels is designed for the particular blood flow waveform produced by the heart. It can be demonstrated that other flow waveforms delivering the same average blood flow during a given ventricular contraction generate greater arterial wall stresses than the natural heart waveform. Greater stresses applied cyclically over long periods of time produce deleterious effects which the body may not be able to withstand.
Furthermore, development of a heart assist device to implement the concept of copulsation has previously been unsuccessful. One state-of-the-art device in this area utilizes a pressure-ratio system in which the pressures of the ventricle discharge and of the blood pump discharge connected to the vascular system are held at a preselected and constant ratio. A closed-loop servosystem controls the pump and relies upon pressure sensors detecting the ventricular discharge pressure and the pump discharge pressure to maintain the fixed ratio. A description of the pressure-ratio copulsation sys tem is given in Volume XV of the Transactions of the American Society for Artifical Internal Organs, 1969, beginning at page 449.
Because the vascular system represents a complex elastic impedance network to blood flow, however, rapid changes in blood pump output can give rise to arterial pressure oscillations unrelated to hear pulsation. These arterial oscillations are sensed by the pressure sensor at the pump discharge in the pressure-ratio system and introduce spurious errors into the feedback loop which cause the pump to depart from rather than amplify the ventricular pressure waveform. Instabilities of this type render the pressure-ratio system ineffectual and unusable.
The more useful interpretation of heart action recognizes the blood flow or velocity waveform rather than the pressure waveform. As indicated above, the pressure waveform is more significantly the result of vascular impedance to heart action than an indication of the heart action itself. It is known also that the heart produces a positive flow output even when no significant pressure pulse is generated. Duplication of the flow waveform rather than the pressure waveform is, accordingly, considered to be a more significant goal to be achieved by a heart assist device.
It is, therefore, a general object of this invention to disclose a heart assist device which provides a blood flow waveform that duplicates closely the natural heart output regardless of pressure fluctuations which may be produced in the vascular system.
SUMMARY OF THE INVENTION The present invention resides in a heart assist device which operates in copulsation with the heart and duplicates the blood flow waveform of the natural heart, that is the blood velocity-time function during systolic contraction, as long as the heart is not severely arrhythmic.
The heart assist device includes blood pumping means connectible between a ventricle of the natural heart and the associated vascular system of the body, which may include the coronary arteries themselves. A closed-loop pumping control means is connected with the heart and the pumping means for regulating the pumping means in copulsation with the heart. Blood is pumped away from the ventricular discharge and into the vascular system with a flow waveform established by maintaining programmed systolic pressure of backpressure at the ventricle discharge.
The pumping control means includes a pressure sensor which is responsive to the discharge pressure of the ventricle to which the pumping means is attached. A closed-loop servomechanism is connected in driving relationship with the pumping means to operate the pumping means in synchronous response to the sensed pressure and maintain the programmed systolic pressure. By controlling the pumping means in this manner, blood passes from the ventricle through the pump and into the vascular system with the same flow waveform as that produced by the ventricle. The vascular system, therefore, experiences the stress that the blood flow waveform from the natural heart produces and the heart is permitted to operate against a reduced pressure load at the same time.
The pumping control means is also capable of autonomous pumping operation to assume complete control of heart pulsations and blood circulation. Switching of the pumping control means between autonomous operation and copulsation operation is performed by a discriminator which initiates autonomous operation in the presence of severely arrhythmic heart pulsations. A pacemaker may also be driven synchronously with the blood pumping means by the control means to stimulate heart contractions simultaneously with the reduction in the heart load by the pumping means.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates in cross-section the blood pump and the manner in which the pump is connected into the vascular system adjacent the heart.
FIG. 2 is a schematic diagram showing the controls which regulate the blood pump operation for both copulsation and autonomous operation.
FIG. 3 is a schematic diagram of a variable time-base, programmed pressure reference.
DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the blood pump, generally designated 10, of a heart assist device in one embodiment of the present invention. The pump is surgically connected to the aorta immediately adjacent the aortic valve in the left ventricle and preferably is installed within the body.
The pump 10 includes a cylindrical inlet chamber 12 which is connected by means of a blood compatible inlet tube 14 to the aorta between the left ventricle discharge and a ligation in the aorta a short distance from the discharge. A cylindrical discharge chamber 16 of the pump is coaxially aligned with the inlet chamber 12 and is connected to the aorta at the opposite side of the ligation from the tube 14 by another blood compatible tube 18. The coronary arteries (not shown) are preferably connected to the aorta at the same side of the ligation as the tube 18 discharging blood from the pump 10. With the surgical installation indicated, all blood leaving the left ventricle must pass serially from the heart through the pump 10 and then into the aorta. The pump is operated as explained in greater detail below to maintain programmed pressure or back pressure at the ventricle discharge which causes the blood flow or velocity waveform entering the aorta from the discharge chamber 16 to correspond with a blood flow waveform produced by the systolic contraction of the ventricle.
Since the major circulatory load of the heart is borne by the left ventricle, the heart assist device of the present invention is most commonly employed with that ventricle. However, the utility of the device is not so limited and it is also possible to connect the pump to the right ventricle to transmit the venous blood to the lungs through the pulmonary arteries. Within the scope of the present invention, the term vascular system" is used generally to refer to the arterial system connected to either the right or left ventricles.
Within the pump 10 a reciprocating piston 20 is provided with piston heads 22 and 24 inside the inlet chamber 12 and discharge chamber 16 respectively. The piston head 22 is sealed at the cylindrical wall of the inlet chamber 12 by means ofa flexible diaphragm 26 which rolls back and forth along the walls of the chamber as the piston 20 is reciprocated. A corresponding diaphragm 28 seals the piston head 24 at the walls of the discharge chamber 16. The piston' 20 is mounted within a cylinder 29 and together they form a hydraulic actuator in a servomechanism including a spool valve 30 and an electrical torque motor 32. In conventional fashion, the torque motor 32 receives electrical signals from a pump control and positions the spool valve to regulate the flow of hydraulic fluid between the hydraulic actuator and an inlet port 34 and discharge port 36.
The diameters of the cylindrical chambers 12 and 16 are the same and since the piston heads 22 and 24 are fixedly connected to the piston 20 for simultaneous movement with the piston, the volumes of the inlet and discharge chambers vary by the same amounts but in opposite senses when the piston is displaced. In other words, when 'a given volume of blood is ingested into the inlet chamber 12 from the left ventricle, a matching volume of blood is expelled from the discharge chamber 16 into the aorta.
Furthermore, the pump 10 includes a flow-checking passageway 40 interconnecting the inlet chamber 12 and the discharge chamber 16, and a check valve 42 in the passageway allows blood flow only from the inlet chamber to the discharge chamber. Therefore, when the piston head 22 moves from left to right as viewed in FIG. 1 to ingest blood into the inlet chamber 12, the piston head 24 must expel blood from the discharge chamber 16 into the tube 18 because the passageway 40 is temporarily closed by the check valve 42. On the other hand, when the piston 20 moves from right to left in FIG. 1, the check valve 42 opens and permits blood then filling the inlet chamber 12 to flow through the passageway 40 into the discharge chamber 16. As described in greater detail below, the reciprocation of the piston 20 is synchronized with the cardiac pulsations or periods so that blood is ingested into the inlet chamber 12 and expelled from the discharge chamber 16 during the systolic period of the left ventricle, and blood is transferred from the inlet chamber to the discharge chamber during the diastolic period.
Hydraulic fluid used to operate the actuator including the piston 20 may be derived from a self-contained and batteryoperated power source installed within the body or carried externally of the body.
FIG. 2 illustrates the controls which regulate the blood pump in FIG. 1 to achieve either copulsation of the pump and left ventricle or autonomous operation of the pump.
A principal operation of the heart assist device is copulsation of the heart ventricle and pump 10 in a manner which causes the blood flow waveform emanating from the pump 10 to duplicate the flow waveflow from the ventricle during systolic contractions. In contrast to the pressure-ratio systems of the prior art having two pressure sensors, a single pressure sensor or transducer 50 supplies electrical signals as feedback in a closed-loop servomechanism including the torque motor 32 located in the pump 10 but illustrated separately in FIG. 2. The transducer 50 must be located in a position to detect the blood pressure at the discharge of the left ventricle and in one form of the invention, the transducer is located in the inlet chamber 12 of the pump 10 as illustrated in FIG. 1.
The copulsation operation of the heart assist device is a cyclic operation synchronized with the heart and it will be assumed that the beginning of each cycle corresponds with the beginning of each heart beat when the ventricle is filled with blood and about to begin a systolic contraction.
As the systolic contraction begins, pressure at the discharge of the ventricle, and correspondingly in the inlet chamber 12, begins to rise, and an electrical signal from the transducer passes to an input amplifier 52 where it is amplified to a maximum level of several volts. The electrical signal produced by the transducer is proportional to the pressure in the chamber and hence, represents an analogue pressure signal. The amplified signal passes through a shaping network or signal conditioner 54, which filters out unwanted noise, and then through an electronic switch 56 into and input signal comparator 58 and a double pole/double throw electronic switch 60. The electronic switch 56 serves as a gate transmitting the analogue pressure signal during the systolic period if the heart is beating properly as explained in greater detail below.
In the signal comparator 58, the analogue pressure signal is compared with an electrical, programmed pressure level signal determined by the setting of the pressure reference 62 which, for example, may be an adjustable potentiometer. The programmed pressure reference signal represents a desired pressure to be maintained at the discharge of the left ventricle and can be adjusted to increase or decrease the load against which the ventricle operates during the systolic period.
If the input pressure detected by the transducer 50 is greater than the programmed reference pressure, the output ofthe comparator 58 is turned on, and when the input pressure again drops below the reference pressure the output is turned off. Hence, the comparator 58 normally produces a control pulse in synchronism with and coextensive with the systolic period of the left ventricle. This control pulse is used as a mode signal for controlling the servomechanism.
To insure that perturbations of the pressure signal detected by the transducer 50 do not prematurely cut off the mode signal, the control pulse is transmitted through a pulse stretcher 61 which effectively delays the trailing edge of the pulse a few milliseconds to insure that the systolic contraction has in fact ended. The output of the pulse stretcher 61 is transmitted to the electronic switch 60 and a single pole/double throw electronic switch 63 to pull in and set these switches in the positions opposite the deactivated positions illustrated.
The analogue pressure signal from the electronic switch 56 and the pressure reference signal from the reference 62 are then transmitted through parallel circuits in the switch 60 to the differential error detector 64 which produces an error signal representative of the difference between the two signals. That error signal is transmitted through the electronic switch 63, a rectifier 66 and a servo-amplifier 68 to drive the torque motor 32 and piston 20.
The rectifier 66 prevents the servo-amplifier 68 and torque motor 32 from responding to negative error signals which are generated when the pressure sensed by the transducer 50 drops below the reference pressure. Such negative error signals occur, for example, at the end of each systole during the several milliseconds that the control pulse from comparator 58 is extended by the pulse stretcher 61. The error signal during this period could rapidly reverse the movement of the pump piston 20 and interfere with the programmed return of the piston described below.
In the pressure mode of operation, therefore, the torque motor 32 is in a closed servo loop which attempts to hold the ventricle discharge pressure at the level of the reference pressure and causes the piston 20 in the pump to move from left to right in FIG. 1 as long as the pressure sensed by the transducer 50, that is the systolic pressure of the left ventricle, does not drop below the programmed pressure established by the reference 62. At the same time, a quantity of blood equal to the quantity expelled from the left ventricle is expelled from the discharge chamber 16 into the aorta, and as long as the servomechanism including the piston 22 follows the systolic pressure closely with small error signals, the flow waveform produced by the piston head 24 will be the same as the flow waveform produced by the ventricle during the systolic period. The vascular system, therefore, receives a quantity of blood matching the ventricle output during each systole and experiences a pressure waveform which should be easily to]- erated without deleterious effects since the waveform is effectively generated by the heart. At the same time, the load or pressure against which the left ventricle operates remains substantially uniform at a reduced level determined by the pressure reference 62. It is contemplated that the pressure reference may be adjusted upwardly during a recuperative period to permit a damaged or diseased heart to eventually reach a functional level approximating that of a healthy heart.
The foregoing description of the servomechanism entails the pressure mode of operation which is coextensive with the systolic period of the ventricle. During this mode, the piston 20 moves from left to right in FIG. 1 to ingest and expel blood as described. During the diastolic period of the ventricle, the piston 20 is returned to its extreme left hand position and to accomplish this return, the servomechanism switches to a position mode of operation.
The return of the piston 20 to its extreme left hand or starting position must be completed during the diastolic period of the ventricle and during this period, blood is transferred from the inlet chamber 12 of the pump 10 to the discharge chamber 16. Since rapid motion of the piston 20 can create turbulance which is injurious to the blood cells, it is desirable to return the piston to its starting position within the longest possible period and at the lowest possible velocity. To do this, the controls generate a linear piston displacement program or signal which has the smallest slope permitting the piston to be returned to the starting position within some portion of the previous diastolic period, for example, percent or percent of that period. Utilizing only a portion of the previous diastole reserves some time to accommodate accelerations between heart beats.
The durations of the systolic period and the diastolic period vary significantly because the beat rate of the heart is not constant and the lengths of the periods relative to one another also vary with beat rate. In order to return the piston 20 to its starting position within the variable length diastolic period, the controls driving the servomechanism measure the duration of each diastolic period and temporarily store the duration of the preceding diastolic period.
To measure the duration of a diastolic period, the control pulse from the pulse stretcher 61 is transmitted through the blocking diode of an isolation rectifier 70 and its trailing edge triggers a ramp function generator 72 into operation. The ramp voltage from the generator decreases linearly with time and reaches a minimum value only at the end of the longest diastole. At the end of a diastole, the leading edge of the next control pulse from the pulse stretcher 61 triggers a single pulse generator 74 and a reset time delay 76 connected with the ramp function generator 72. The generator 72 also responds to the leading edge of the pulse by stopping and holding the ramp voltage. A sample-and-hold circuit 78 responds to the pulse from the pulse generator 74 and stores the voltage of the ramp generator representing the length of the diastolic period. Shortly thereafter, the time delay circuit 76 resets the ramp function generator 72 for the next diastolic period. It should be noted that the stored voltage representing the diastolic period is smaller if the period is longer.
Since the quantity of blood pumped by the ventricle during each systolic period is not constant, the piston will normally not utilize its full stroke which must be large enough to accommodate the maximum quantity of blood ever expelled by the ventricle in a single systole. Therefore, to return the piston to its starting position at the lowest possible velocity in the available time, consideration must be given to the distance to be traveled by the piston and, accordingly, the right-hand or stopping position of the piston at the end of the systolic period must be detected.-
To determine the position at which the piston 20 stops and the distance to be traveled at the end of a systolic period, a position transducer 80 mounted in the pump 10 as shown in FIG. 1 and cooperating with the piston head 24 produces a position signal which is a voltage having the smallest value at the right-hand end of the stroke and largest value at the left-hand end or starting position. The position signal generated by the transducer 80 is held in a position sample-and-hold circuit 82 when that circuit is triggered by the trailing edge of the pulse emanating from the isolation rectifier 70. The sampled position signal in the circuit 82 is transmitted as an input voltage to a differential input multiplier 84 and a start position reference 86 transmits a voltage representative of the starting position to another input of the differential input amplifier 84. The two position input voltages are internally subtracted and the resulting voltage difference represents the return distance. The return distance is also divided in the multiplier 84 by the maximum stroke distance to obtain a fraction representing a normalized return distance or that portion of the maximum stroke which will be traversed to reach the starting position.
The normalized return distance is then multiplied in the differential input multiplier by the voltage stored in the sample-and-hold circuit 78 representing the time of the previous diastolic period. The product of this multiplication represents the desired piston velocity since the product takes into consideration the distance to be traveled and the period of time available. It should be noted that the voltage in the sample-and-hold circuit 78 is smaller if the diastolic periods are longer, therefore, the product or velocity is as it should be, smaller if the periods are longer and larger if the periods are shorter. Also, of course, the product or velocity is greater if the return distance is greater.
The product generated in the multiplier 84 is transmitted to an integrator 90 where it is integrated with respect to time between the trailing edge of the pluse received from the pulse stretcher 61 at the end of the systolic period and the leading edge of the pulse at the beginning of the next systolic period. In other words, the desired velocity signal is integrated during the diastolic period by the integrator 90 to produce a ramp voltage rising linearly with time. This voltage represents a displacement program for returning the piston to the starting position. The slope of the ram voltage is the velocity established by the multiplier 84 and is corrected during each cardiac period to insure return of the piston to its starting position within a time period no greater than the previous diastolic period and at the minimum velocity needed to reach the starting position safely within the period.
The integrator 90 is reset by the leading edge of the control pulse 'of the next systole so that the ramp voltage always starts from a zero voltage level. The position transducer 80, however, produces the zero output at the extreme right-hand position of the piston 20. Since the piston normally begins its return stroke from a position other than the extreme right-hand position, a large negative error signal would be generated in the servomechanism operating in a closed loop with the position transducer if the ramp voltage were applied directly as an input signal at the beginning of the return stroke. To compensate for the initial piston position, the voltage stored in the sample-and-hold circuit 82 representing the piston stopping position is added to the ramp voltage of the integrator at a summing junction 92. The output voltage of the summing junction, therefore, is a voltage rising from some initial value representing the piston position at the beginning of the return stroke.
This output voltage is applied through a voltage limiter 94 to an input of the electronic switch 60. Also, the output of the position transducer 80 representing actual piston position is fed back to another input of the electronic switch 60. The two position signals are transmitted through the switch when the trailing edge of the pulse from pulse stretcher 61 de-energizes the switch and the switch assumes the position illustrated schematically. The error detector 64 compares the position signals and produces a position error signal. At the same time, the electronic switch 63 is de-energized so that the position error signal is applied through the electronic switch 63 to the servoamplifier and torque motor 32 which drives the piston to the starting position.
The voltage limiter 94 is provided to stop the piston motion at the same starting position during each cycle of the pump 10 in copulsation with the heart. The slope of the ramp function generator 72 is selected to insure return of the piston within 80 percent or 90 percent of the previous diastolic period, that is, before the integrator 90 stops integrating; therefore, without the voltage limiter, the piston would be driven past the starting position. The limiter, then, holds the output of summing junction 92 at the limited value until subsequent commands are received.
Accordingly, the servomechanism assumes a position mode operation in response to the trailing edge of the pulse from the pulse stretcher 61 and the piston 20 is returned to its starting position by the servomechanism under closed loop control.
As long as the heart continues to beat in a regular manner, the pump control illustrated in FIG. 2 continues to switch between the pressure and position modes of operation in synchronism with the systolic and diastolic periods of the left ventricle. This synchronism is maintained by utilizing the systolic pressure pulse detected by the transducer 50 as the synchronizing signal.
Since it is the pressure pulse which switches the servomechanism, or more specifically the electronic switches 60 and 62, between the pressure and the position modes of operation, it is possible that during a return stroke of the piston 20 a pressure pulse could be ing as a control gate for the pressure signal remains open during a diastole and is closed only if the piston 20 is in its starting position in FIG. 1. Furthermore the switch remains closed thereafter only if a control pulse from the stretcher 61 is present. In other words, the electronic switch 56 is held closed only if the piston is at its extreme left hand position or if the piston is moving from left to right during a systolic contraction of the left ventricle.
To this end, a logic circuit 100 controls the electronic switch 56 is response to signals received from a position voltage comparator 102 and the pulse stretcher 61. The voltage comparator 102 receives information from the position transducer 80 and the start position reference 86 and produces an output to the logic circuit only when the piston 20 is in the start position. If the logic circuit receives a signal from either the voltage comparator 102 or the pulse stretcher 61, and as long as an additional signal from a pulse discriminator 104 described in greater detail below is absent, the logic circuit 100 closes the electronic switch 56 and analogue pressure signals will pass to the signal comparator 58 and the electronic switch 60.
At the end of a systolic pressure pulse, the logic circuit 100 opens and disables the electronic switch 56 and further pressure pulses sensed by transducer 50 during the return stroke of the piston do not cause the switches 60 or 63 to be positioned in the pressure mode condition.
The pump 10 continues cyclic operation in copulsation with the left ventricle as long as a regular heart beat persists. If, however, the heart beat should cease or become severly arrhythmic, that is either too long in the case of severe bradycardia or too short indicating ventricular tachycardia or fibrillation, the pump controls switch to an autonomous operation in which the pulsation rates are no longer controlled by the heart. The controls for the pump as disclosed include a secondary control system that automatically provides an artificially generated blood flow waveform from the pump to establish blood circulation completely independent of the natural heart.
The secondary control system includes a pulse width discriminator 104 which monitors the delay between the control pulses received from the pulse stretcher 61. The discriminator produces two delay periods in response to the trailing edge of each pulse, the delay periods representing the maximum and minimum delay periods considered acceptable for the heart. If the leading edge of the subsequent control pulse occurs either before the minimum delay period has elapsed or after the maximum delay period has elapsed, the discriminator produces a latched output signal. This discriminator signal is applied to the logic circuit 100 to disable the electronic switch 56 and prevent further signals from passing through it. These signals could trigger the signal comparator 58 and pull the electronic switches 60 and 63 into the pressure mode positions. The discriminator signal is also transmitted to a flow waveform generator 106 which produces a timevoltage waveform representing the piston position signals that would be generated by the transducer 80 during a normal ventricular contraction and dilation. This waveform signal is applied as the input to the electronic switch 60 along with the feedback signal from the position transducer 80 and is repeated continuously as long as the input from the pulse discriminator 104 is present.
During autonomous operation of the pump 10, the servomechanism drives the piston 20 in a closedloop positioning mode of operation in accordance with the waveform generator 106. To prevent the integrator 90 and the associated ramp function generator 74 and dif ferential input multiplier 84 from interfering with the position commands from the waveform generator 106, the signal from the discriminator 104 is applied through a blocking diode of the rectifier in the same manner as the control pulse from the stretcher 61. The integrator is, therefore, reset to zero and the sample-andhold circuit 78 is placed in the sample mode of operation with zero output. The only signals then received at the operative inputs of the electronic switch 60 are the voltage waveform produced by the generator 106 and the position feedback signal from the position transducer 80. Accordingly, closed loop positioning control of the piston 20 continues as long as the discriminator output persists.
The blocking diodes of isolation rectifier 70 prevent the signal from the pulse width discriminator from reaching the electronic switches 60 and 63 during autonomous operation of the pump control and prevent the control pulses from the pulse stretcher 61 from reaching the logic circuit and other components connected with the discriminator 104. Since the discriminator signal and control pulses have contradictory effeet, the isolation provided by the rectifier 70 is needed.
To warn an individual wearing the heart assist device that the pump control is operating autonomously, the signal from the discriminator 104 is also applied to an alarm 108.
The flow waveform generator 106 can also produce a timing pulse to trigger a pacemaker l 10 and cause the heart to receive a mild electrical shock in synchronism with the leading edge of the flow waveform. In this manner, normal contraction may be reinduced in the heart in synchronism with the autonomous operation of the pump 10.
The pulse width discriminator 104 may be manually reset by the user. If the heart has recovered and assumed normal contractions, the alarm 108 remains off and the entire pump control system reverts to copulsatile rather than autonomous operation. If the alarm turns back on immediately after manual resetting of the discriminator 104, the individual will know that normal heart action has not been resumed. At this point he has the option of remaining on autonomous operation or utilizing the manual defibrillator 1 12 which raises a single pacemaker pulse to the level of defibrillator action and administers a severe electrical shock to a fibrillating heart in order to return to normal operation. If he elects to utilize the defibrillator, he can again determine whether it has been successful by resetting the pluse width discriminator 104 and listening for the alarm signal.
Although the programmed systolic pressure or back pressure established by the pressure reference 62 has been described above as being constant throughout the systole, it is contemplated that the pressure reference may also generate a variable, programmed pressure during each ventricular contraction. FIG. 3 discloses in greater detail one form of the pressure reference 62 that is capable of producing a variable, programmed pressure waveform with an adjustable time base. The time base is derived by sampling and storing the previous systolic period in a manner similar to the sampling and storage of the previous diastolic period during the positioning mode of operation which returns the piston 20 to its starting position.
A ramp function generator 120 is triggered at the beginning of a systolic period by the leading edge of the control pulse from pulse stretcher 61 and produces a steadily decreasing ramp voltage. At the end of the systolic period the ramp generator is stopped by the trailing edge of the control pulse and a sample-and-hold circuit 122 responds to a pulse from the pulse generator 124 to store the ramp voltage held by the generator 120. The single-pulse generator 124 also responds to the trailing edge of the control pulse to trigger the sample-and-hold circuit 122. The voltage stored in the circuit 122 is, therefore, representative of the duration of a systol and it will be noted that the longer the systole, the smaller the stored voltage.
During the next systole a voltage-to-frequency converter 126, commonly referred to as a voltage controlled oscillator (VCO), is triggered into operation by the leading edge of the control pulse. The output of the converter 126 is a pulse train having a pulse rate proportional to the input or stored voltage in circuit 122. The pulses serve as clock-pulses for reading a pressure program memory or waveform generator 128. The waveform generator is basically a read-only memory having analogue pressure voltages or signals stored at sequentially addressed memory sites. The clock pulses cause sequential reading of the sites and produce a desired systolic pressure of backpressure waveform that becomes the variable pressure program to be maintained by the blood pump during each systole.
Since the frequency of the clock pulses from the converter 126 determines the rate at which the pressure waveform is produced by the generator and since the frequency depends upon the voltage stored in the sample-and-hold circuit 122, the pressure waveform is produced in a period of time related to the last preceding systolic period. By setting the slope of the ramp produced by the generator 122 in conjunction with the converter 126, the programmed pressure waveform from the generator 128 can be produced in a period of time equal to the last preceding systolic period.
The reset time delay 130 responds to the trailing edge of the control pulse from pulse stretcher 61 to reset the ramp generator 120 during the diastolic period. The waveform generator 128 would be reset at the same time and the converter 126 is also shut off. To provide an input to the signal comparator 58 before the end of a diastolic period, the output of the waveform generator 128 may be latched or automatically stepped back to its initial value during the diastolic period.
Accordingly, the components producing the pulse train input for the waveform generator 128 form an adjustable time-base generator which derives its time base from the previous systolic period. Either the variable pressure program or the constant pressure program produced by a potentiometer may be selected to most advantageously serve the recipient of the heart assist device.
While the present invention has been described in several embodiments, it will be understood that numerous modifications and substitutions can be had without departing from the spirit of the invention. Many of the components described and illustrated in the pump control of FIG. 2 may be replaced by other components performing equivalent functions. The ramp function generator 72, the sample-and-hold circuits 78 and 82, the integrator 90, the flow waveform generator 106 and other components may be digital devices rather than the analogue devices described. The specific blood pump shown and described is not the only design available for copulsation and suitable modifications of the servomechanism driving the disclosed pump or other pumps can be made as long as the blood flow waveform produced by the ventricle is matched at the pump outputfAccordingly, the present invention has been described in a preferred embodiment by way of illustration rather than limitation.
1. A heart assist device comprising:
blood pumping means connectible between a ventricle of the natural heart and the associated vascular system of the body; and
pumping control means connected with heart and the pumping means for regulating the pumping means in synchronism with heart pulsations including:
a pressure sensor detecting blood pressure at the ventricle during systole; and
servo control means connected in driving relationship with the pumping means and having programmed means defining a programmed systolic pressure and a servo control loop responsive to signals from the pressure sensor and the programmed means for driving the pumping means to maintain the programmed systolic pressure at the ventricle discharge during systole and to produce from the pumping means during intervals synchronized with heart pulsations a blood flow waveform which duplicates the ventricular blood flow waveform generated while maintaining the programmed systolic pressure.
2. A heart assist device as defined in claim 1 wherein:
the servo control means is a hydraulically powered servomechanism.
3. A heart assist device as defined in claim 1 wherein:
the blood pumping means comprises a displacement pump having inlet and discharge chambers which vary in volume by corresponding amounts in opposite senses.
4. A heart assist device as in claim 3 wherein the displacement pump also includes a flow-checking passageway ducting blood from the inlet chamber to the discharge chamber.
5. A heart assist device as defined in claim 3 wherein: the inlet chamber of the displacement pump is connected with the heart ventricle; and the pumping control means has the pressure sensor mounted in the inlet chamber of the displacement pump to detect systolic pressure.
6. A heart assist device as defined in claim 1 wherein the blood pumping means comprises a piston pump having coaxially aligned cylindrical inlet and discharge pumping chambers of the same diameter and a reciprocating piston in one chamber fixedly connected with a reciprocating piston of the other chamber.
7. A heart assist device as in claim 6 wherein the piston pump further includes a passageway leading between the inlet and discharge chambers and a check valve in the passageway preventing flow from the discharge chamber into the inlet chamber.
8. A heart assist device as defined in claim 1 wherein:
the blood pumping means includes an inlet chamber connectible with the heart ventricle, a discharge chamber connectible with the vascular system and a cooperating reciprocating member which moves from a starting position in one direction to ingest blood from the ventricle into the inlet chamber and expel blood from the discharge chamber and moves in the opposite direction to transfer ingested blood from the inlet chamber to the discharge chamber; and
the servo control means of the pumping control means has means for reversing the direction of movement of the reciprocating member in synchronism with the respective systolic and diastolic cardiac periods.
9. A heart assist device as defined in claim 8 wherein:
the pumping control means comprises copulsation pumping control means providing a pressure mode during the systolic cardiac period causing the servo control means to respond to the systolic pressure signals detected by the sensor to stroke the reciprocating member in said one direction and maintain said programmed systolic pressure, and providing a positioning mode of operation during the diastolic cardiac period causing the servo control means to stroke the reciprocating member in said opposite direction to the starting position.
10. A heart assist device as defined in claim 9 wherein the copulsation pumping control means includes:
a switch connected with and controlled by the pressure sensor, timing and sampling means controlled by the switch and sensor for measuring the diastolic period and means connected to the servomechanism for returning the reciprocating member to the starting position after the systolic period and within a time period less than the preceding diastolic period measured.
11. A heart assist device as defined in claim 10 wherein the copulsation pumping means further includes means for returning the reciprocating member to the starting position within said time period at a minimal velocity.
12. A heart assist device as defined in claim 1 further including in the pumping control means:
an autonomous pump control having a waveform generator providing a predetermined fiow waveform for regulating the pumping means independently of the heart pulsations; and
discriminator means responsive to systolic pressure for energizing the autonomous pump control in the presence of severely arrhythmic heart pulsations.
13. A heart assist device as defined in claim 1 wherein:
the programmed means defines the programmed systolic pressure as a constant pressure level throughout the systolic period.
14. A heart assist device as defined in claim 1 wherein:
the programmed means defines the programmed systolic pressure as a variable pressure waveform during the systolic period.
15. A heart assist device as defined in claim 14 wherein:
the programmed means includes an adjustable timebase generator responsive to a heart function for producing the variable pressure waveform in periods of time dependent upon the heart function.
16. A heart assist device for operation in synchronism with ventricular heart contractions comprising:
blood pumping means serially connectible between the heart and the vascular system of the body for producing a pulsatile blood flow; and
control means connected to the pumping means including a pressure sensor connected with a heart ventricle and detecting blood pressure pulsations produced by the contractions of the ventricle, and means responsive to the pressure sensor for driving the pumping means to maintain a programmed reference pressure at the heart ventricle during systolic contractions and to reproduce the blood flow waveform from the ventricular contractions obtained when the programmed pressure reference is maintained.
17. A heart assist device as defined in claim 16 wherein:
the blood pumping means is a piston pumping having an inlet connectible with a heart ventricle; and
the pressure sensor is mounted in the pump adjacent the inlet to sense ventricular discharge pressure.
18. A heart assist device comprising in combination:
a pacemaker connected to the natural heart for electrically stimulating regular heart contractions;
a cyclically driven blood pump connectible to the vascular system of the body and producing pulsatile flow when driven; and
blood flow waveform generating means connected with both the pacemaker and the blood pump for driving the pump and pacemaker in synchronized relationship to produce pulsatile flow from the pump in synchronism with the heart stimulated by the pacemaker.
19. A heart assist device as in claim 18 wherein the blood flow waveform generating means is connected to control the cyclically driven blood pump and to trigger the pacemaker.
20. A heart assist device as in claim 18 wherein:
the blood pump is connectible with the discharge of a heart ventricle and the vascular system; and
pump control means responsive to the waveform generating are provided for cyclically driving the pump in copulsation with the contractions of the ventricle connected with the pump.
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