EP0246302A1 - External pulsatile cardiac assist device - Google Patents

Abstract

The device described below is intended to withdraw venous blood from the vena cava or from the heart to a point located outside the patient, to apply a pulsatile pressure to the outside of the patient's body, in addition to oxygenation and filtration if necessary, and to return the pulsed blood to the aorta, in order to increase the pressure and the duration of the diastole. In surgical mode, the heart is completely avoided by the blood collected and reintroduced near the blocked heart. In percutaneous mode, entry is through the cannulas into the artery and the femoral vein, the venous cannula being extended to the right atrium or preferably the left atrium and the arterial cannula being extended to the aorta .

Description

EXTERNAL PULSATILE CARDIAC ASSIST DEVICE Technical Field This invention relates to new and useful improvements in external pulsatile cardiac assist devices and more particularly seeks to provide a unitary system that can be readily available for temporary or emergency procedures in either a surgical or percutaneous mode. Other than cannulae connections to the vena cava or heart and a femoral artery or aorta, the unit is external to the patient. This avoids the use of miniaturization of design and permits more efficient procedures, including filtration of the blood, development of pulsatile drive, automatic control of and measurement of blood flow and easy visibility of and access to all the working parts.

The primary field of this invention is medicine as it relates to assisting or taking the place of cardiac output of a weakened or diseased heart and particularly during heart surgery, trauma periods accompanying infarction or other heart damage, various other procedures, such as angioplasty, or waiting for a suitably matched transplant. In addition, electronics is a secondary field utilized to communicate the patient's condition and to control the parameters of the cardiac assistance.

Background Art Since the advent of the heart-lung machine, which permits long periods of open-heart surgery, many new surgical techniques have been developed, including internal heart and heart valve repair, coronary artery bypass, natural and artificial heart transplant, etc. During surgical procedures, after heart attacks or to supplement chronically weakened hearts, procedures have been developed to assist or take over entirely the circulation of the blood through the patient's body. Some of these have provided placing a small pulsatile balloon in the aorta whereby the work load of the heart is reduced; but only relieve up to about 20% of the heart's load. Several artificial hearts and left ventricular assist devices have been developed. Because of space limitations in the human body, lack of filtration and problems of blood compatibility of materials in long- term use these have not been very successful. For temporary problems such as surgery, evolving infarction treatment, or holding a potential transplant patient until a natural heart is available, there is a need for pulsatile pumps that can be housed on a hospital emergency cart, quickly connected to the patient, and which can provide service that is better than that available in the miniaturized units wherein the working units are positioned within the patient's body.

Moreover, when a patient is being treated for an evolving myocardial infarction, there is a need to relieve the heart of its work load to the maximum possible extent and also provide substantial pulsatile pressures to supply blood to all the organs of the body. At this time there is a need to open collateral circulation routes in the arterial network of the myocardium. This is most effectively accomplished by maintaining pressure for a longer duration during diastole. In this emergency situation, surgical intervention may not be necessary if it is possible to provide a large amount of unloading of the heart and maintaining a pulsatile pressure pattern that assists the perfusion of blood into the myocardial arterial network.

Disclosure of the Invention The invention consists of a method and device for pulsatilely assisting or replacing of the natural heart beat during emergency periods with the only patient connection being cannulae to the venous and arterial sides of the circulatory system. This permits greater efficiency in maintaining a natural type of pulsatile pressure, filtration of the blood supply, more rapid and appropriate response to patient's condition and automatic control of the blood flow in response to sensing the electrocardiogram, pressure waveform, and actual blood flow of the patient. The unit is readily adaptable to be mounted on a portable hospital cart and permits quick replacement of a malfunctioning unit without jeopardy to the patient.

If there is an occlusion in the blood circuit, means are provided to automatically sense that event, activate an alarm, and reduce the pumping pressure to avoid harm to the patient and to various components of the pumping system. When the rate of the heart becomes too fast, the unit automatically augments only alternate beats and beats possessing a predefined regularity of rhythm by utilization of a specific settable refractory period to thus maintain better stroke efficiency.

In case the heart stops beating unexpectedly, the microprocessor will after a specific settable period of time, cause the pump to deliver pulses of blood at a specific settable rate and with a volume per stroke that will automatically deliver the quantity of blood per unit time that the perfusionist or physician has set on the unit's front panel. When the heart starts to beat again the microprocessor instantly responds and returns the unit to the mode of operation in which the pulsatile delivery of blood is synchronized with the patient's heart beat so as to augment it at the end of cardiac systole. This is the Escape operation.

The balloon and pulsatile unit is driven by a hydraulic system containing a solution of sterile isotonic saline so that possible leaks cause no harm to the patient. Furthermore, the hydraulic system employs a sealed bellows so that there is no leakage as might occur around a piston drive.

The unit is entirely controlled by a microprocessor with a software program which responds to measuring the and EKG. The augmentative pulsatile delivery timing and flow rate may be adjusted by the perfusionist or physician who observes the screen. The EKG is measured from the patient's skin directly or through an available hospital monitor. The electronic circuitry filters the EKG signal to recognize the R-wave and to ignore pacing stimuli. Multiple successive R-waves are averaged continually. The device, when synchronized with the heart beat, is generally adjusted to augment regular flow by starting its pulsatile delivery on the descending T- wave and ending just as the next systole is about to commence, thereby providing better flow during cardiac diastole. Further to the summary of this invention, the specific nature of which will be more apparent, the invention will be more fully understood by reference to the drawings, the accompanying detailed description and the amended claims.

Brief Description of the Drawings Fig. 1 is a diagrammatic functional view of the components of this invention and a patient;

Fig. 2 is a perspective view of the hospital cart on which the unit is mounted;

Fig. 3 is a prior art human cardiac cycle chart to which has been added an aortic pressure line resulting from this invention;

Fig. 4 is a diagrammatic view of the pulsating device;

Fig. 5 is a perspective view of a valve within the pulsating device;

Fig. 6 is a diagrammatic view of a portion of the servomechanism; Fig. 7 is a block diagram of the servomechanism;

Fig. 8 is a cross section of the mechanical drive assembly including the bellows;

Fig. 9 is a cross section of the diaphragm housing;

Fig. 10 is a diagrammatic view of the electronic control and electrical system;

Best Mode for Carrying Out the Invention As shown in Fig. 1, venous blood is removed from patient 16 and returned to the arterial side in either a surgical or percutaneous mode. The surgical mode is similar to open heart surgery where the heart is exposed through an open chest. Blood is removed through a cannula in the vena cava by gravity or by suction and passed to the oxygenator 17.

In the percutaneous mode, a cannula is inserted in the right femoral vein and passed to the right or left atrium to take blood. If taken from the left atrium, the oxygenator may be excluded or bypassed, as the lungs will be functioning. A roller pump 18 with approximately 80-100 mm Hg suction is used to remove the blood in this mode.

The blood is passed to (or bypasses) oxygenator 17 and then goes to (or bypasses) pulsatile pump chamber 19 which is activated by bellows 20 from mechanical drive 21 optionally through isolation diaphragm 22. The blood then passes through (or bypasses) blood filter 23 and goes back into the arterial side. In the surgical mode, the blood return is to the aorta. In the percutaneous mode it is returned to the left or right femoral artery or the cannula may be extended past the bifurcation into the abdominal aorta.

A portable hospital cart 24 whereon the unit is mounted with a viewing screen 26 and operating controls is shown in Fig. 2. Although normally driven by AC, an emergency battery is provided for automatic change-over, the mode being shown at lights 27 and 28. The extent of battery charge is shown at 29 and once the unit returns to AC, the batteries will automatically recharge. Batteries have sufficient charge to provide a 90 minute minimum operation.

The screen can display EKG and blood pressure waveforms, and pump strokes, which are used as a guide in setting optimal timing of the pulsatile blood delivery. This delivery may be displayed by intensifying the corresponding portion of the EKG line on the screen. Light and/or bell alarms 31 are shown for EKG pre-set ranges, occlusion, battery charge and the blood flow meter. Loudness of alarms may be increased by rotary switch 32.

The unit may be set by switch 36 on panel 33 to synchronize to the natural heart rate or to replace the normal heart by switch 35 on panel 34. The delivery strokes of the pump may be set for alternate beats of the heart at 37, which is particularly useful in weaning a heart from the machine. The start and duration of the delivered stroke can be varied at switches 38, 39. The mean flow in volume/time is displayed at 41 and varied by control 43. Transfusions can be made during zero flow in 20 ml does at 42 in 1 to 5 sequences. Fig. 3 is a prior art textbook chart of the human cardiac cycle except for the added dot-dash supplemental aortic pressure line 44 that shows the effect of the present pulsatile unit during diastole in the percutaneous mode, i.e. with the heart still beating. With the external pulse being timed to begin on the descending T wave, the aortic pressure is increased during diastole. This creates higher perfusion pressure throughout the body's vascular system at a time when arterial blood is needed and particularly to the coronary arteries at a time when collateral circulation is needed to limit the final size of the infarct.

In the surgical mode, where the heart circulation is being completely bypassed, a pulsatile blood pressure is being applied to the entire vascular tree instead of the constant pressure provided heretofore by heart-lung machines having roller pumps. Pulsatile pressure is natural to the vascular system and causes better perfusion and less resistance. The pulsatile pump chamber 19 is shown in Fig. 4, having structural components of polycarbonate or other rigid medical grade material and flexible components of silicone, silastiσ and/or urethane or other very flexible medical grade material. Inlet 45 and outlet 46 blood lines are connected to the primary blood flow line. One-way tricuspid inlet 47 and outlet 48 valves are placed at each end of bladder 49 which is supported within and spaced from rigid cylinder 51. The cylinder is provided with a hydraulic fluid inlet 52 and a stopcock 53. Detailed view of a tricuspid valve having a circular base 54 and three cusps 56 is shown in Fig. 5. The hydraulic inlet 52 is connected optionally to isolation diaphragm 22 shown in Fig. 9 or directly to bellows 20 (Fig. 8) housing. The housing 57 is fastened together incorporating a silicone sheet 58 as a diaphragm and gasket. Openings on both sides of the diaphragm would allow the hydraulic system to be quickly filled and stopcocks 59 allows air to be removed. The diaphragm housing is provided on both sides with a hydraulic line fitting 61, 61, one leading to upstream chamber 62 and the other to downstream chamber 63.

The motor generator combination is mounted adjacent to the bellows 20. Each utilizes a separate armature winding with the tachometer generator winding being temperature compensated. Commutation to both of the armatures is accomplished via metal graphite brushes interfaced with polished copper segments. Shafts are supported in two sets of shielded ball bearings. The nominal output of the motor is 1/2 hp with a maximum speed of 4000 rpm and a maximum torque rating of 160 oz - in. The generator output has a voltage gradient of 21 volts per 1000 rpm. The power amplifier is a linear amplification system that functions as a controllable power source for the DC motor. This unit interfaces with the controller, which produces an approximation of an analog voltage that is proportional to the desired motor velocity in amplitude and direction. The circuitry utilizes all solid state components and incorporates protection which guards against overloading either the amplifier or the motor. The peak current available at the motor terminals is 22 amps and the maximum voltage is 40 volts positive and negative. Peak power output is limited to 530 watts.

The purpose of the mechanical drive is the conversion of the signals from he controller into corresponding hydraulic displacements.

The servosystem accepts signals from the controller in the form of voltage waveforms and converts them into angular displacements at the motor shaft. The major components of this system include a permanent magnet motor 66 with a close coupled tachometer generator 67, a position sensor, e.g. a feedback potentiometer or equivalent 68; and a power amplifier. These are arranged as a servomechanism with the tachometer generator forming the inner loop effecting output-rate damping and the potentiometer forming the outer loop with position feedback.

The rotational motion of the servomotor is translated into rectilinear motion at the bellows assembly with the use of an oscillating crank 69. This crank is an integral part of the gear segment with an effective linear displacement of two inches produced by a radius of two inches swinging through an angle of 60 degrees. The output of the crank is taken through an arm via a trunnion 72 to a linear bearing 73 which limits the motion to a straight line. The arm pivots with the aid of two 1/2 diameter needle bearing 74 that ride on hardened pins 76 at each end. The lower pin is held in the gear segment and the upper pin is tied to the 1/2 inch shaft of the linear bearing via a yoke. The linear bearing, consisting of two recirculating ball bearings retained in a housing, guides the hardened shaft 77 to deliver the straight line mechanical displacement to the bellows.

To produce an effective torque multiplication between the servomotor and the oscillating crank, a gear set is utilized. This set consists of a segment with 38 teeth, on a 12 inch pitch circle of 16 diametral pitch in mesh with a 14 tooth pinion coincident with the motor shaft. The drive ratio thus produced gives a torque multiplication of 12 to 1. The gear segment is locked with a set screw to a 7/8 inch diameter shaft that is supported in two needle bearings and is trapped between two solid thrust washers to constrain its motion. The pinion is locked onto the motor shaft, deriving support from the integral bearings in the housing along with a single needle bearing acting as a pilot.

The mechanical motion of the gear segment is limited by solid stops at each end of the travel to a maximum of 72 degrees to prevent damage to the mechanism.

Position is obtained from the rotary potentiometer 68 driven by the gear segment. The potentiometer is linear over a 350 degree rotation through its 1000 ohm element. The element is energized from the negative 10 volt regulated supply on the servo amplifier. The wiper divides this supply voltage with respect to ground and returns it to the auxiliary input of the amplifier to close the position portion of the control loop. The 1/4 inch shaft of the potentiometer carries a 36 tooth 64 pitch spur gear pinion locked in place with a set screw. This gear is in mesh with a 180 tooth 64 pitch bull gear attached to the gear segment and constrained to follow the oscillations about the 7/8 inch diameter shaft with a 1/8 inch diameter diamond shaped pin axially penetrating the 1/8 inch bull gear and pressed into an aluminum spacer that is locked to the shaft. The gear set produces a step up ratio of 1 to 5 and therefore produces an input to the potentiometer of 300 degrees for 60 degrees of rotation on the larger gear segment. This results in a one-to-one correspondence between voltage and position for a voltage between the ground reference and 10 volts and the angular position of the gear segment and its attached crank. The pitch line of the potentiometer gear set closely approximates the thrust line of the crank arm to optimize the ratio of resolved motion to actual output by minimizing the effect of backlash in the mesh of the gears. The potentiometer housing style is a servo mount that is locked to the transmission via three servo feet with screws. Fine adjustment of the position offset in the control loop is obtained by rotary adjustments to this servo mount. Coarse adjustments to the offset are made by changing the relative position of the teeth in mesh with the gear set. Mechanical motion is transformed into hydraulic displacement via the bellows 20, which consists of a urethane tube having convolutions 64. These are reinforced with wire rings to limit the reaction of the tube to flexing between the adjacent convolutions on both the push and pull strokes. A set of parallel rods lies axially along the outside periphery of the convolutions to constrain the tube in line with the linear bearing.

The tube end closest to the bearing is capped with a blind plug that is attached to the mechanical output. Sealing against fluid loss is maintained by an o-ring in the plug. The block that surrounds the output end serves to transmit all the thrust from the bellows motion into the drive frame thereby allowing only hydraulic displacement to be transmitted into the output tube 78.

The hydraulic displacements produced by the bellows are fed into diaphragm housing 22 that provides a sterile barrier between pump chamber assembly 19 and the mechanical drive 21. Each of the priming ports has a stopcock 59 that is used to introduce the fluid and extract air on each side of the assembly to balance the position of the diaphragm. Once the system is primed with fluid and balanced, the stopcocks are closed to maintain the condition of the system during operation.

The entire diaphragm assembly is sterilized prior to use in the system to assure that the hydraulic medium in the pump chamber assembly remains sterile. The purpose of the pump chamber assembly is to convert the hydraulic displacement from the drive unit into pulsatile blood flow to the patient.

The hydraulic portion consists of chamber 55 in Fig. 4, formed by a rigid plastic tube that is capped on each end, and supports a concentric silicone rubber tube held within.

The bladder 49 forms an interface between the hydraulic medium and the extracorporeal blood path.

The atraumatic trileaflet check valves 56 permit blood flow only in the forward direction.

With hydraulic fluid flowing out of the chamber 55, the inlet valve is opened as the outlet valve is closed causing blood to enter the bladder. With hydraulic fluid flowing into the chamber 55 assembly, the outlet valve is opened as the inlet valve is closed, causing blood to exit the bladder and advance towards the patient 16. The function of the electronics is to accept inputs at the user and EKG interfaces and send a signal to the motor interface to effect the required pump stroke profile. Computation capability is provided by a Z80 microprocessor.

The microprocessor polls the input switch and knob positions. It interprets them either directly from their binary state, or a digitization of their position. Flow through the blood circuit is measured and polled by the microprocessor. Flow and rate values are indicated on 7-segment LED displays to provide visual feedback for the user. The EKG interface provides the capability of accepting either a preamplified EKG signal from a monitor or alternatively it can read EKG potentials directly from the skin. A medical-grade isolation amplifier with leakage current less than 10 microamperes protects against patient shock hazard. The EKG signal is processed then fed into an interrupt line to the microprocessor.

The microprocessor employs a real-time clock to determine the heart beat rate and computes effective heart rate as the moving average of a plurality of beats (for example 4) . The range of both the start and duration controls is based on percentages of the time between beats indicated by the effective heart rate.

ALT is defined as the ratio of the number of natural heart beats to the number of delivered strokes.

AL= Number of natural heart beats

Number of delivered pump strokes

The ALT ratio is set by pushing the appropriate selector button on the front panel. ALT ratios other than 1:1 are used to wean a patient off the pump when the heart recuperates.

The microprocessor also simultaneously generates an output waveform which, after amplification, controls the motor; which in turn drives the pump.

Fig. 7 shows the servomechanism block diagram which provides accurate tracking of pump position to the position signal.

FIG. 10 shows the details of the electronic block diagram with appropriate notations having been made on the diagram.

The herein described system produces truly pulsatile blood flow which can either be synchronized with a beating heart or assume the entire circulatory burden in case of asystole. In the synchronized mode of operation the pump delivery is timed to provide cardiac augmentation throughout the diastolic period. It can be adjusted continuously to assume between zero and 100% of the pumping burden, and thereby provide any desired amount of afterload reduction. In asystole the pumping rate can be set between 40 and 120 beats per minute. A microprocessor causes the pump to adjust automatically to synchronous or asynchronous operation in accordance with the heart's requirements, although the automatic operation can be superseded by manual controls when desired. The pump can deliver eight liters per minute in the anticipated range of physiological parameters.

The blood propelling element is a sac which is alternatively compressed and expanded to produce the appropriate stroke volume. A tricuspid check valve at each end of the sac forces the blood to move in the proper direction. The assemblage of sac and check valves have been designed to avoid blood trauma, and tests have shown it to be less hemolytic than conventional roller pumps operating in the usual continuous flow mode. The compression and expansion of the sac is caused by a sealed hydraulic system utilizing, sterile isotonic saline as the hydraulic fluid. The hydraulic system provides positive displacement and accurate control, in contrast to pneumatic systems in which precise control is prevented because of the compressibility of the gas. Various sensors are built into the system to detect unexpected or undesirable interactions with the patient and automatically go into a protective mode of operation and sound an alarm; however, in all clinical cases performed to date these precautions have not yet been called into action. The microprocessor is contained herein as the controller 81 in Fig. 10 and has been programmed to accept all the parameters noted in the figure, the specific program being set forth hereinafter.

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Claims

Claims

1. A cardiac assist device comprising means for removing pre-aortic blood to a point outside the body, means outside the body to apply a pulsating flow to said removed blood, and means to introduce said pulsated blood to the aorta while the aortic valve is closed.

2. The device of Claim 1 wherein said removal completely bypasses said heart.

3. The device of Claim 1 wherein said removal means is a cannula adapted for insertion in the femoral vein with no surgical intervention to said heart.

4. The device of Claim 1 wherein said reintroduction means of said blood is a cannula adapted for insertion in the femoral artery with no surgical intervention to said heart.

5. The cardiac assist device of Claim 1 comprising a cannula adapted to be inserted in the femoral vein to remove pre-aortic blood the left atrium from a patient, a cannula adapted to be inserted in the femoral artery to reintroduce said removed blood to said patient at a point from the femoral artery to the ascending aorta, a circulatory system for said blood between said cannulae, a suction pump in said system outside of said patient and downstream to said removing cannula, an optional oxygenator in said system outside of said patient and downstream to said removing cannula, a hydraulic pulsatile pump chamber in said system downstream to said oxygenator, a bellows hydraulic drive unit outside of said patient and said system operatively connected to said pulsatile pump chamber, an optional blood filter positioned out of said patient and downstream of said pulsatile pump chamber, and means to reciprocate said bellows drive unit whereby said reintroduced blood will have a pulsatile flow and pressure characteristic that will relieve at least part of the heart workload and create higher diastolic blood flow for longer duration in said patient.

6. The cardiac device of Claim 5 wherein said pulse is timed to start during the declining heart T-wave.

7. The cardiac assist device of Claim 1 comprising means to remove all venous blood by gravity from a patient, means to reintroduce said removed blood to the patient's aorta, a circulatory system for said blood between the two means that excludes the natural heart, an oxygenator in said system outside of said patient and downstream to said removal means, a hydraulic pulsatile pump chamber in said system downstream to said oxygenator, a bellows hydraulic drive unit outside of said patient and said system operatively connected to said pulsatile pump chamber, an optional blood filter positioned out of said patient and downstream of said pulsatile pump chamber, and means to reciprocate said bellows drive unit whereby said reintroduced blood will have a pulsatile flow characteristic that will create pulsatile and higher blood flow for longer duration in said patient.

8. The device of Claim 10 comprising an elongated flexible bladder having entry and exit rigid ends, a rigid casing spaced from and surrounding at least part of said bladder to form a chamber therebetween, an inlet check valve mounted in said inlet end, an exit check valve mounted in said exit end, and an hydraulic egress into said chamber, whereby periodic movement of liquid into and out of said chamber through said egress will create a pulsatile flow of blood passing through said bladder.

9. The device of Claim 8 wherein said bladder and casing have cylindrical shapes.

10. A blood pulsation device comprising a flexible bladder, a rigid casing spaced from and surrounding said bladder to form a chamber therebetween, an inlet check valve mounted in said bladder, an exit check valve mounted in said bladder at a point spaced from said inlet valve, and an hydraulic egress point to said chamber whereby periodic movement of fluid into and out of said chamber will create a pulsatile flow of blood passed through said bladder.

11. In a method for producing pulsatiling blood flow outside the body, the improvement comprising detecting the EKG R-wave and triggering subsequent pulsating means with said R-wave.

12. The method of Claim 11 wherein said triggering is timed to cause said pulsation to start during the declining curve of the T-wave.

13. A method for treating an infarσted heart comprising augmenting aortic pressure after closing of the aortic valve, whereby the coronary artery is exposed to a higher blood pressure for a longer duration during diastole.

14. A method for treating an infarcted heart comprising augmenting the pulsatile heart beats from outside the body whereby the muscle of said heart is relieved of stress during said augmenting.

15. A method for treating an infarcted heart comprising removing blood from an atrium to a point outside said body, applying a pulsating flow to said removed blood outside said body, and reintroducing said pulsating blood to the aorta while the aortic valve is closed whereby the coronary artery is exposed to a higher blood pressure for a longer duration during diastole and the muscle of said heart is relieved of stress.

16. The method of Claim 15 wherein said pulsating blood is reintroduσed during the declining curve of the EKG T-wave.

1 . The method of Claim 16 wherein the EKG R-wave is sought, and when found, synchronizes said

external pulsating pressure.