US3734087A

US3734087A - External pressure circulatory assist

Info

Publication number: US3734087A
Application number: US00197824A
Authority: US
Inventors: H Sauer; N Hagopian; J Colman
Original assignee: MEDICAL INNOVATIONS Inc
Current assignee: Intermedics Cardiassist Corp; MEDICAL INNOVATIONS Inc
Priority date: 1971-11-11
Filing date: 1971-11-11
Publication date: 1973-05-22
Anticipated expiration: 1990-05-22
Also published as: JPS5576657A; JPS595303B2; JPS5822221B2; JPS596662B2; JPS595302B2; FR2165872A1; JPS4862282A; IT975705B; JPS5576655A; JPS5576656A; FR2165872B3; JPS5586466A; CA1031643A

Abstract

Improved external pressure circulatory assist device of the type which utilizes an electrocardiogram signal to facilitate synchronization of its pressure pulse with the heartbeat. The improvements include a more efficient and clinically suitable mechanical means to induce pressure variations in the legs of a patient, and an automatic control means to maintain the proper pressurization even during physiological changes in the limbs of the patient being treated, and control means to monitor and adjust the pressure wave to achieve the proper phasing of the pressure wave with the cardiac cycle during treatment of the patient.

Description

United States Patent [1 1 Sauer et al.

[4 1 May 22, 1973 [54] EXTERNAL PRESSURE CIRCULATORY ASSIST [75] Inventors: Harold S. Sauer, Carlisle; Nubar D.

Hagopian, Bedford; John M. Colman, Lexington, all of Mass.

[73] Assignee: Medical Innovations, Inc., Waltham,

Mass.

[22] Filed: Nov. 11, 1971 [21] Appl. No.: 197,824

[52] US. Cl ..128/64 [51] Int. Cl. ..A6lh 7/00 [58] Field of Search ..128/24, 30.2, 64

[56] References Cited UNITED STATES PATENTS 3,303,841 2/1967 Dennis ..128/24 R 3,654,919 4/1972 Birtwell ..l28/64 10/1968 MacLeod ..l28/64UX 5/1972 Vail ..l28/64 Primary ExaminerLawrence W Trapp V Attorney-Robert A. Cesari, John F. McKenna and Martin J. ODonnell et al.

[57] ABSTRACT Improved external pressure circulatory assist device of the type which utilizes an electrocardiogram signal to facilitate synchronization of its pressure pulse with the heartbeat. The improvements include a more efficient and clinically suitable mechanical means to induce pressure variations in the legs of a patient, and an automatic control means to maintain the proper pressurization even during physiological changes in the limbs of the patient being treated, and control means to monitor and adjust the pressure wave to achieve the proper phasing of the pressure wave with the cardiac cycle during treatment of the patient.

43 Claims, 7 Drawing Figures PATENTEB H. 12 21975 SHEET 2 BF 5 DELAY TIME TO LEG PRESSURIZATION DIASTOLE (EKG) DIASTOLE (WAIST) TIME (MILLISECONDS) PATENTEDHf-NZZIHH 3134,08.

SHEET 3 OF 5 DESIRED PRESSURE FROM "88" WAVEFORM WAVEFORM PRESSURE 90 AT LEGS SENSOR BLADDER 35 WAVEFORM 'M- GENERATOR ERRoR SIGNAL 8O 2 STROBESiGNAL SERVO AMPLIFIER v S FROM "89" 9 98 OIL RESERVOIR v ACCUMULATOR 52 72 OH. RESERVOIR 74 FILL EIMPTY 46 PUMP PUMP 56 69 50 OFF FILL EMPTY 48 ?o f L FROMH68H Fig. 4a.

Fig. 4b.

PATENTED W 2 3 3. 734, 087

SHEET 4} OF 5 l WATER EMPTY /62 4 68 FROWZZ" EMPTY PUMP PRESSURE CONTROL sENsoR LEvEL TRIGGER IL IL PRESSURE STROBE GENERATQR DELAY DURATION SGNAL I GENERATOR GENERATOR DUAL TRACE oscILLosc0PE A T [7T EKG TRACE 103 104103 I ARTERIAL I06 PRESSURE IN INTEN5|TY |NTENS|TY CONTROL CONTROL TRACE I TRACE2 /108 9OMS I DELAY 110 TO 56 8OMS DELAY EXTERNAL PRESSURE CIRCULATORY ASSIST BACKGROUND or THE INVENTION 1. Field of the Inverrtion This invention relates to a novel apparatus for assist ing blood flow through the circulatory system by synchronizing the pulsing of an external-assist means with the heartbeat. The invention also relates to a novel con trol processes achieved by use of the apparauts.

2. Background of the Invention Methods for atraurnatically assisting blood circulation of patients have been described in the art. In U.S. Pat. No. 3,303,841 to Dennis, a process is described whereby an external compressing of the lower part of the body expresses a volume of blood larger than the volume of blood pumped in a single stroke of the heart. The blood so expressed is forced back into the aorta and greater arterial vessels and thereby allows a reduction in ventricular workload while maintaining a satisfactory perfusion of blood during ventricular diastole. This general type of preocess has been elaborated upon in an article entitled Synchronous Assisted Circulation by Birtwell et al., appearing in The Canadian Medical Association Journal (Vol. 95, pages 652-664) on Sept. 24, 1966. In general, synchronous external pressure assist processes are distinguishable and advantageous over pre-existing counter-pulsation processes because the latter kind of precedure involves the cannulation of a major artery, use of an extra-corporeal blood handling device, the use of stringently sterile techniques and the necessity of administering anticoagulants to the patient. Moreover, the blood trauma or hemolysis produced by extra-corporeal pumping devices limits premissible duration of the assist procedure and compromises the condition of the patient. Finally, these trauma-requiring procedures are not only time consuming, they can present a very significant hazard to many patients, and they increase the risk factor in treating all patients.

Specific apparatus usable in a synchronous assist process is also disclosed in US. Pat. No. 3,403,673 to MacLeod. In general, that apparatus consists of a legencasing arrangement with means for cycling the pressure on the legs within the casing. The invention described in US. Pat. No. 3,403,673 has a number of problems associated with its use. For example, the constrictive-type seals thereof are believed to interfere with optimum blood circulation. Moreover, the inertia of the pressurizing system which includes elastic-type expansible linings is believed to interfere with obtaining optimum pressure-time profile on the legs. The apparatus suggested in such prior art has a number of drawbacks in that it is cumbersome to use; i.e., the patient is inserted into and removed from the device with excessive effort; and it is excessively difficult to achieve the proper pressure-pulsation profile and impossible to achieve what the inventors believe is the more advantageous mode of operation, i.e., at an ambient pressure at or below atmospheric pressure.

In US. Ser. No. 92,606, filed by William C. Birtwell on Nov. 25, 1970 and now US. Pat. No. 3,654,919 and commonly owned with the instant application, an im-' the therapeutic potential of externally assisted circulation. Some results of this work are reported in a paper entitled The l-lemodynamic Evaluation of External Counterpulsation in Man by T. J. Ryan et al., and presented at the American Heart Association Scientific Sessions held in Atlantic City, N.J., duing Nov. 1970.

At the present time the external counterpulsation method, e.g., as generally described in the Birtwell application, is the only controlled method of assist which requires no surgical intervention or sterile procedures, no use of anticoagulants or anesthesia, and produces no significant trauma. These factors are most important because they usually allow a much earlier use of the apparatus on a patient in, say, a state of circulatory shock than could be justified if another type of assist apparatus had to be used.

The selective application of positive and negative pressures to the portion of the vascular bed in the legs serves to control directly the volume of blood within that portion of the vascular bed and, hence, the pressures within the arterial system, particularly the pressures of the aorta. It is these controlled changes in aortic pressure that lead directly to a reduction in cardiac work, to increased perfusion of the systemic circulation, and to preferential perfusion of the myocardium.

During the past year, a considerable amount of research work has been carried out which confirms the important contributions which this approach can make in cardiac therapy. This success has stimulated further inventive effort which has resulted in improvements in the device which increase its physiological effectiveness, the versatility in the clinical environment, and its aesthetic appeal to the medical user.

SUMMARY OF THE INVENTION Therefore, it is an object of the invention to provide an improved external pressure circulatory assist device which is readily and safely applied and authorized by hospital employees of ordinary skill and limited experience with such a device.

Another object of the invention is to allow such a person of ordinary skill to easily achieve a precise phasing of a system with the patients heartbeat.

A further object of the invention is to provide apparatus which is sufficiently quiet to permit its convenient use within a hospital environment.

A further object is to provide apparatus which can operate with a minimum AC power supply requirement.

It is also an object to provide apparatus that is readily adaptable to patients having a relatively large range of leg sizes.

Still another object is to provide automatic control of the pressure waveform applied to a particular patients legs in a circulatory assist process despite a large range of patient leg sizes, geometries, and firmness.

Another object is to provide apparatus which is light in weight and capable of being simply and rapidly applied to a patient.

Other objects of the invention will be obvious to those skilled in the art on reading the instant application.

The above objects have been achieved by constructing an external pressure circulatory assist system comprising one or more of the following advances in the art:

A. A Water-Fill System The Water-Fill System is used to fill a leg-enclosing bladder with the correct volume of waterfor the particular patient. By correct volume is meant the particular volume that is suitable for the particular size and geometry of an individual patients legs. It has been found also that because of patient movement into or out of the machine, a patients legs can change their effective size during treatment. Thus the water-fill system if utilized not only to properly fill and automatically empty water from the leg encasing bladder; it is also used to add or remove water during treatment, thereby to maintain an efficient hydraulic pressure coupling between a mechanical pressure-inducing means to be described below and the patients legs.

Water-Fill System and Pressure-Maintenance System During the filling operation, a pressure sensor which is positioned between the shell which encloses the bladder and the bladder itself senses the pressure in the bladder. When the fill-pressure reaches a predetermined magnitude, usually about millimeters mercury, the filling action is terminated by shutting off the fill pump. This filling operation is normally carried out with the platen in its upward position; i.e., in a bladdercompressing position.

Once the operation of the apparatus is initiated, a displacement sensor which is coordinated with the moving platen determines the position of the platen at the topmost position of each upward stroke. If this position is improper, water is pumped into or out of the bladder until the platen again reaches its proper range.

B. The Mechanical Pressurizing Means In order to achieve adequate counterpulsation, it is necessary that the leg-enclosing bladder be capable of exerting a positive pressure on the legs during cardiac diastole and removing the pressure or producing a negative pressure on the leg (say, to minus 50 mm of mercury) during cardiac systole. The positive pressure applied is normally between about 150 to 250 millimeters of mercury.

It has been found most advantageous to achieve this pressure differential by applying a moving platen to a fairly large surface of the leg bladder containing the liquid. This platen is mounted beneath the leg bladder to provide a reciprocal vertical motion which controls the magnitude of the positive pressure. The pressure within the bladder increases as the platent rises against it and decreases to a minimum, usually about 20 millimeters of Hg, (or lower if negative pressures are used) as the platen falls to its lower level. If the negative pressure capability of the invention is to be used, the platen, bladder and the patients body are enclosed from the waist down in a bag which is partially evacuated, say to a negative pressure of 70 millimeters of mercury below atmospheric pressure. The bag, although conveniently of flexible material, is held away from the legs by the rest of the apparatus so that a low pressure zone is maintained in which negative pressure can be realized. The bag or suction envelope is suitably sealed at the waist by means of a snug fit of its waistband with the body of the patient. The apparatus as described above has a normal operating cycle ranging within from about minus 50 millimeters of mercury to plus 250 millimeters of mercury. In operation, the pressure will be cycled with the lower pressure of the cycle corresponding with the systolic phase of the hearts action and the higher pressure corresponding with the diastolic phase.

The bladder which is advantageously a singlechambered blanket, is placed around the patients legs and a rigid casing is used to enclose both the legs and the mechanical pressure-transmitting platent. The bladder is then filled with water until the entire region around the legs and above the platen contains water and so that the bladder is in contact with substantially the entire surface of the legs is fully expanded at the ankle and waist ends. From this point on, the vertical reciprocating action of the platen will provide the desired positive/negative-pressure cycling. C. Pressurization Waveform Control To achieve counterpulsation, the pressure on the patients legs should fluctuate according to a desired waveform. The apparatus of the invention advantageously comprises a feature whereby the pressure at the legs is monitored, and made the source of a feedback signal which is directly compared to a signal which is representative of the desired waveform for the particular patient. The difference between the signal derived from the actual waveform and the signal representative of the desired waveform is called the error signal. This error signal is amplified and used to control an electrically operated servo-valve which controls the flow of hydraulic fluid to a hydraulic cyclinder which drives the vertically-reciprocating platen. This flow is continuously adjusted in proportion to the error signal causing the pressure waveform at the transducer to be a suitable facsimile of the desired waveform.

The monitoring of the pressure at the legs is conveniently done by a water bladder, approximately 4 by 4 by inches in dimensions, which bladder is placed between the bladder-enclosing shell and the bladder itself. Water in this sensor bladder is connected through a short tube to a pressure transducer which provides the required electrical signal of the actual pressure waveform. A particular advantage of this waveform control system is that it ensures the important characteristics of the desired waveform (e.g., duration, rise, and fall time) as applied to the patients legs are not distorted. This is accomplished in spite of variations in hydraulic coupling between the platen and the legs resulting from such things as differences in leg geometry, size, and firmness from patient to patient, and any dynamic stretching of the bladder or distortions in the leg casing during operation.

Phasing the Pressure Cycle with the Cardiac Cycle It can be shown that a deleterious, physiological effect may be suffered if the heart is taxed by an improper timing of the start of the external assist pressure wave with respect to the cycle of the heartbeat itself. Moreover, though this phenomenon may be readily understood by an operator, it is desirable to avoid chance of operator error and to simplify the operation of the device by providing a simple, accurate, automatic phasing means.

The inventors have provided a suitable visually monitored phasing control system whereby an arterial waveform is displayed on a multi-channel oscilloscope or other suitable display device the conjunction with an ECG waveform. The arterial pressure waveform is measured at any convenient place on the body. Normally, the wrist will provide the most convenient placement of the arterial wave pressure sensing device. The ECG signal will, as is normal, but obtained by placeintroduced in the femoral arteries with the apparatus of the invention, to reach the root of the aorta. The subsequent transmission from the root of the aorta to the wrist, where the arterial waveform is typically obtained, takes another 90 ms. It is important that the artificially induced positive pressure waves be produced in the legs sufficiently before diastole (80 ms) and that the pressure wave reaches the root of the aorta at the beginning of diastole.

The start of the augmented diastolic waveform, as seen at the wrist, which results from use of the applicants apparatus, therefore will take place a total of 170 ms after the pressure if applied to the legs of the patient. Thus it is desirable to have the induced pressure wave initiated by a command signal timed to start 170 ms before the arterial wave, as seen at the wrist indicates the beginning of diastole and 80 ms before the ECG signal indicates the beginning of diastole. The beginning of diastole is indicated in the arterial wave by the so-called dicrotic notch in good approximation by the end of the so-called T-wave which is known to be positive indication of the relaxation of the left ventricle and the end of systole.

In the subject phasing system, the leg pressurization signal is used to intensify the ECG and arterial trace on the oscilloscope after this pressurization signal has been delayed for 80 and 170 ms respectively. The duration of intensification is the duration of each positive pressurization cycle of the leg. With this system, an operator can adjust the delay time until the intensified signal on the ECG or arterial wave form is located at the beginning of diastole. At that point leg pressurization can be triggered in correct synchronization with the cardiac cycle.

D. Leg Encasing Unit A novel leg-encasing unit is employed which, by virwe of its light weight and geometry, provides an efficient casing within which to apply pressurization to the legs, yet is convenient and rapid to apply to a patient. E. Platen Driving Mechanism A novel mechanical linkage arrangement is provided for coupling the platen to the driving hydraulic cylinder whereby a suitable pressure-displacement characteristie is obtained at the platen for pressurizing the bladder while at the same time minimizing the input power requirements of the apparatus.

In this application and accompanying drawings there is shown and described a preferred embodiment of the invention and suggested various alternatives and modifications thereof, but it is to be understood that these are not intended to be exhaustive and that other changes and modifications can be made within the scope of the invention. These suggestions are selected and included for purposes of illustration in order that others skilled in the art will more fully understand the invention and the principles thereof and will be able to modify it and embody it in a variety of forms, each as may be best suited in the condition of a particular case.

In the drawings:

FIG; 1 is a schematic cross sectional view of that ap paratus of the invention into which the legs of the patient fit.

FIG. 2 is a plan view of the leg-enclosing apparatus broken away to show the position of a reciprocating platen.

FIG. 3 is a chart indicating the relative timing of physiological events relative to the description of the invention.

FIG. 4a and 4b, taken together, form a schematic diagram of a unique control system for use in external pressure circulatory assist apparatus.

FIG. 5 is a perspective view of a platen assembly used to transmit pressure to a leg-surrounding bladder.

FIG. 6 is a perspective view of the rigid casing into which the patients leg and bladders are placed and the means for quick connection of the casing.

With reference to FIGS. 1 and 2, it is seen that legs 20 are placed within blanket-like bladder 22 which is filled with a non-compressible liquid, e.g., water 23. Bladder 22 completely encloses legs 20 and is adapted for a snug fit within a rigid casing 24 formed of an upper shell 26 and a lower shell 28. Just beneath bladder 22 and also within casing 24 is a platen 30 mounted for reciprocal vertical motion imparted by a mechanical linkage 32. This linkage is driven by a hydraulic cylinder as better seen in FIG. 5. A pressure-sensitive transducer 34 is placed in hydraulic communication with the bladder for use in monitoring the pressure therein; the particular function of this transducer will be described below.

FIG. 2 illustrates how an envelope 36 encloses the casing 24. The envelope 26 and casing 24 are broken away to illustrate the relative position of platen 30. It will be noted that the envelope extends beyond casing 24 and is adapted to form a snug fit with waist 38 of a patient 40. A suction pump 39 will normally be used to maintain the normal environment within envelope 36 at a sub-atmospheric pressure.

There are certain cycle-phasing problems solved by the use of the more advantageous embodiment of the invention. If a patient is to be treated by pressure applied to his legs by pressure on bladder 22, it will typically take about milliseconds (ms) for such pressure to reach the heart and be synchronized with the hearts beating as monitored by an ECG signal device placed on the patients'chest. It will take another ms for the pressure effect initiated at the legs to reach the radial artery as monitored by a sensing device placed thereon. FIG. 3 graphically (but still somewhat schematically) shows the time phenomena related to operation of the apparatus.

In one trace, there is shown an ECG curve 402 related in real time to a typical cardiac period of 840 ms. In this trace, the beginning of diastole is typically signitied by the start of the drop of the curve at 403. This drop at 403 should follow by about 80 ms, the rise 405 in leg pressure curve 404. This is to allow the pressurewave induced by the apparatus of the invention to arrive at the heart at the beginning of diastole after completion of a trip taking 80 seconds.

The curve 407 is of radial arterial pressure as measured at the wrist. The beginning of diastole in pressure curve 407 is signified by the so-called dicrotic notch 409 which takes place about 90 ms after the ECG reports the beginning of diastole. For convenience, the apparatus is normally triggered by sensing the prominent QRS peak 415 of the EKG.

Now referring to FIGS. 4a and 4b, an advantageous method of utilizing the above-described phenomena in phasing and control of the apparatus is set forth schematically and can be described as follows:

A transducer-type pressure sensor 34 including a sensing bladder 35 is utilized to sense hydraulic pressure in bladder. If the pressure so sensed is less than millimeters of mercury (mm of Hg), then a pressurelevel amplifier 46 will provide an output to the andtype gate 48. if during this low-pressure signal situation, the operator closes a water fill switch 50, he will cause the and gate 48 to transmit a signal which will activate a fill pump 52 through pump control circuit 54 and simultaneously cause a multi-position, solenoid valve 56 to shift to its fill position, i.e., the position allowing water to flow from conduit 58 to conduit 60. Thereupon water will flow into the bladder 22 until a pressure of 20 mm Hg is sensed by pressure sensor 34. The signal from amplifier 46 will then drop consequently closing and-gate 48. Thereupon, fill pump 52 is deactivated and solenoid valve 56 is shifted to its off position.

Water is emptied from bladder 22 by using an analagous system whereby pressure is sensed by sensor 62. In a typical mode of operation, if the temperature sensed is above a minus 15 mm Hg value, a signal is sent to and-gate 64. If empty switch" 66 is closed, the gate 64 will allow a signal command to actuate pump control 68 and empty pump 69 and cause solenoid valve 56 to shift to its empty position, i.e., the position allowing water to flow from conduit 60 to conduit 70 and thence back to water reservoir 72 through 74. The pump 69 will empty until a pressure of less than the illustrative minus 15 mm Hg is sensed. At that time the and-gate 64 will no longer sense the required signal from sensor 62, and the pump 69 will be turned off and valve 56 will return to its off position.

It is noted that the above-described filling and emptying operations are generally used at the beginning and ending of use and not as continuous control means. Nevertheless it is emphasized that the ability to fill the bladder to a particular pre-determined pressure by automatic means rather than judgement means is highly advantageous.

During operation of the apparatus, the pressure within bladder 34 is maintained at its desired level by adding or removing water from the bladder system as follows:

A displacement sensing type transducer 76 is mounted on hydraulically-actuated rod 78. Transducer 76 is adapted to provide an electrical signal output proportional to the linear advance of rod 78 at any given moment.

A normalized value of the position of cylinder 78 is provided by feeding the signal from transducer 76 into a displacement level sensor 80. Thence the normalized signal is provided to a level comparator 82.

A second imput to comparator 82 is derived as follows: An ECG signal from a patient being treated is fed into a trigger generator 84. The trigger generator is so selected that it recognizes and responds to the so-called QRS complex rise of the ECG signal. (This rise is identified by numeral 415 in FIG. 4). Thus when the QRS rise periodically occurs with each heart beat, an output is triggered which is fed to a manually controlled delay 86. The purpose of delay 86 is to convert the original signal from first trigger generator 84 to a second timedelayed signal corresponding in real time with the beginning of cardiac diastole. This second signal enters a duration signal generator 88. Generator 88 is so selected that the signal therefrom continues for the length of time that leg pressurization (as caused by pressure in bladder 34) desired. This signal from generator 88 is fed to level comparator 82 through a level stroke signal generator 89.

The signal from generator 88 is also fed to a waveform generator 90 which produces a wave whose amplitude and frequency is analogous to that desired for the leg pressurization sequence. A typical such wave will be trapezoidal-like in shape with a rise of 60 ms, a plateau of 250 ms, and a fall of 60 ms. This wave from generator 90 is fed to a wave form comparator 92 wherein it is continually compared to a signal derived from the actual bladder pressure wave as experienced by sensor 34. The output of comparator 92 is a socalled error signal, i.e., a signal which is indicative of the quality and strength of the difference between the desired wave signal from generator 90 and the actual wave signal from sensor 34.

The error signal is used to control servo-valve 94 through a servo-amplifier 93 and thereby controls the supply of fluid to, and consequently the movement of, hydraulic cylinder 78.

As has been described above, level comparator 82 receives two signals: one, a periodic signal from a duration signal generator 88, via strobe signal generator 89 and the other from displacement level sensor 80. Displacement level sensor compares these two signals, one indicative of the actual position of hydraulic cylinder 78 at a given time. It is the periodic, or strobe signal from duration signal generator 88, as modified by a level strobe signal generator 89, which determine when this comparison is to be made. The comparison is most usefully made at the time platen 30 is in the upper-most portion of its stroke and pressing firmly against bladder 22.

If level comparator 82 senses that the platen 30 is too high, output signal is provided to a one shot (monostable multivibrator) 96. This one shot 96 will then turn on till pump 52 and also position solenoid valve 56 for a 0.5-second fill period. The 0.5-second fill periods will continue during each cardiac period (i.e., each heartbeat) until the water volume in bladder 22 is sufficient so that the platen 30 is within an acceptable displacement range at the top of its stroke.

if, on the other hand, the level comparator 82 receives a signal that the platen 30 is too low in its stroke, the empty pump is turned on for one half second in each cardiac cycle by activation of the one shot device 98 and the empty pump 69. Water is then pumped out of the bladder until the platen achieves a desired rise during its stroke.

The general operation of the system is described above. The manner in which the operation is phased into correct synchronization with the heartbeat is described below:

A signal from an arterial pressure-sensing device 99 is fed into a dual trace oscilloscope 100 along with a signal from ECG device 102. The duration of an intensification of these signals is the duration of a pressure command signal as received from duration signal generator 88 and appears as intensifications of ECG trace 104 and arterial trace 106. Delay

devices

108 and 110, of milliseconds and 80 milliseconds respectively, cause the pressure command signal to appear on both waveforms in approximately correct time relationship referenced to the actual ECG and arterial pressure. Delay 86 is used to bring the intensification signal into register with diastolic period as viewed on either the ECG or arterial traces, thereby simultaneously phasing the pressurization wave to the cardiac cycle.

The operator of the apparatus can also have variable control means for changing the characteristics of the duration signal generator 88 to provide the intensified signal duration which is required for a particular patient. On the oscilloscope 101, this duration will show up as intensifications 103 of the oscilloscope traces.

his to be noted in reference to waveform generator 90 that a typical generated wave will normally have a total positive pressure period of from about 250 to about 300 milliseconds. The rise and fall in pressure will take about 60 ms each and rise will start about 80 ms before diastole. In general, it is desirable to select period-determining device which. can maintain posifive-pressure periods of from about 150 ms to about 500 Referring generally to FIG. 5, a mechanical assembly 112 comprises a base 122 on which a hydraulic cylinder 123 is mounted. Attached to cylinder 123 is a reciprocating, operating rod 124 connected, through a mechanical actuating means 114, to platen 116. Platen 116; has a rigid truncated-triangle like planar surface 117 for engagement with the bladder. The cylinder moves through a short stroke sufficient to import about a 2-inch vertical travel to platen 16.

FIGS Sand 6show the hydraulically-actuated platen assembly 112.

It is iinportant to note in connection with this platen assembly that one of the problems encountered in design of hospital installations is to assure they can be used with a reasonably available power source. This problemis particularly critical in design of the apparatus of the invention with its suction requirement, its hydraulic-pressurizing feature and its control circuitry. Thus platen assembly 112 is constructed to minimize the power requirement by utilizing a lifting means 114 whereby the product of the vertical travel of platen 116 and the resistance it meets during travel is maintained at a relatively constant value. This is to say that in the lower portion of the stroke where the reaction pressure of the bladder against the platen is relatively low, the mechanical linkage between the hydraulic cylinder and theplaten is so designed to provide a greater displacement in trade-off for pressure. At the upper portion of the stroke, where bladder reaction pressure increases displacement of the platen is reduced to obtain a pressure advantage. In this manner, the stroke requirement of the hydraulic cylinder and, consequently the power losses from pressure drops in the servo-valve, are held to a minimum value. Since power used in the hydraulic system is proportional to stroke this feature and minimize input power requirements.

This effectis achieved by use of pivotally mounted support levers 118. and pivotally mounted lift levers 120. Because of the relative length and angles of

levers

118, and 120, as operating rod 124moves to the right, platen ll6 drops at an accelerating velocity to its lower position. As rod 124 moves back to the left, the platen is raised until the angle between

lever

118 and 120 is about 56. As the platen rises, it decelerates but also meets more resistance from the bladder.

As has been illustrated schematically in FIG. 2, a rigidhousing or casing 24 is used to enclose the bladder and its mechanical pressurizing means. This casing should be as light as is possible without compromising the rigidity thereof.

As is seen in FIGS. 6 through 11, leg casing 24 comprises a unitary upper shell 131 and a unitary lower shell 132. The shells are advantageously formed of plastic material and are internally reinforced with a rigid, low-density, organic-resin foam such as a polyurethane foam 134. l

A particularly advantageous aspect of the casing 24 is the quick connect and disconnect means. Bolt connectors 136, mounted generally between the leg rest are formed of bolts which are normally captive within upper shell 131. The bolts are provided with large heads which facilitate their quick connection by screwing into lower shells 132.

Latch connectors mounted in inserts along the outer periphery of lower and outer shells comprise a pin which is so positioned on connector member 142 that when connector member 144 is moved downwardly, it will not contact pin 140 but, once lowered a lateral movement of the upper shell wil cause the pin to lock into aperture 146 of member 144. Once this latching is achieved, bolts 136 may be tightened and the operation may proceed.

Another important aspect of the invention is tha whereby applicants have been able to combine a rela tively simple construction, having only a single fluid bladder and an upper and lower casing member, and yet achieve a device suitable for excellent pressure control. This result was accomplished by providing a unitary upper leg casing member 131 and a unitary lower leg casing member 132, each of which comprise two truncated semi-conical leg-receiving compartments 135. These compartments are connected by a thinner zone (see FIG. 1), which allows a single blanket-shaped bladder to be wrapped about the patients legs. To avoid any substantial deflection by the bladder, say deflection of over about onesixteenth of an inch at the pressure mid-point, it was found necessary to place triangular brace means between the leg-enclosing compartments and easing member and connect them to gether. This arrangement allows even light plastic materials of construction, such as fiberglass reinforced polyester to be used in constructing the device. The leg-enclosing zones themselves tend to resist substantial deflection or deformation because of the resistance of hoop stresses which is a result of their conical shape.

In FIG. 6, connectors 136 connect an upper traingular brace means 150 to a lower brace means 152. The upper brace means is formed out of a resin-type honeycomb material 154.

Necessary connections, both electrical and hydraulic are made through the envelope 36 and casing 34 as are required.

In the illustrated embodiment of the invention, the platen has a surface area of about 100 square inches and a stroke of about 2 inches. This apparatus provides a displacement capacity of about 200 cubic inches and this has been found to be entirely adequate not only to meet the displacement requirements resulting from the compressability of a patients legs, but also when combined with pressure-wave monitoring and control system as disclosed above, to enable the use of materials of construction which have some limited distensibility.

In general, the platen and its actuating device should be selected to have a displacement capacity of at least about 100 cubic inches. The vertical stroke of the platen is advantageously maintained at less than about 3.5 inches to avoid design and control problems associated with excessive velocity and the mechanical and hydraulic inertice associated therewith.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which might be said to fall therebetween.

What is claimed is:

1. ln apparatus for applying external pressure circulatory assist action to the legs of a patient in cyclic synchronization with the heartbeat of said patient by pressurizing said legs, the improvement wherein said apparatus comprises:

A. a unitary bladder adapted for wrapping about a patients legs and holding a quantity of liquid,

B. mechanical displacement means in contact with outside wall of said bladder and forming means to press against said bladder, said means forming the primary means for obtaining the aforesaid pressure assist action, and

C. a unitary upper leg casing member and a unitary lower leg casing member having means, said casing members adapted for attachment to form a compartment to enclose said bladder, said casing members having sufficient brace means between leg sections thereof to avoid any substantial outward deflection of said leg casing members during said pressurizing of said fluid.

2. Apparatus as defined in claim 1 wherein said brace means comprise:

A. triangular brace means between each of legenclosing zones of each of the upper and lower casing members, and

B. a plurality of fastening means connecting said brace means together.

3. Apparatus as defined in claim 2 wherein said legenclosing zones are conical in shape and adapted to dissipate stresses exerted thereon as hoop stresses, thereby resisting any substantial deflection or deformation of said zones.

4. Apparatus as defined in claim 3 wherein said pressurizing means has a displacement capacity of at least 100 cubic inches and comprises means for actuating said platen through a stroke length of up to about 3.5 inches.

5. Apparatus as defined in claim 2 wherein said casing members, bladder and displacement means are all placed within an envelope forming means to maintain sub-atmospheric pressure therearound.

6. Apparatus as defined in claim 1 wherein said mechanical displacement means is placed within said compartment formed by said casing and also comprises a platen and a power linkage forming means to impart a power stroke thereto characterized by a substantial loss of velocity of said platen as the platen rises against said bladder.

7. Apparatus as defined in claim 6 wherein said pressurizing means has a displacement capacity of at least 100 cubic inches and comprises means for actuating said platen through a stroke length of up to about 3.5 inches.

8. Apparatus as defined in claim 1 wherein said casing members, bladder and displacement means are all placed within an envelope forming means to maintain sub-atomspheric pressure therearound.

9. Apparatus as defined in claim 8 wherein said pressurizing means has a displacement capacity of at least cubic inches and comprises means for actuating said platen through a stroke length of up to about 3.5 inches.

10. In apparatus for applying external pressure circulatory assist action to the limbs of a patient in synchronization with the heartbeat of said patient, and wherein said assist is achieved by pressurizing a fluid-filled bladder surrounding said limbs, the improvement wherein said apparatus comprises a mechanical pressurizing member positioned generally between said limbs and in contact with an outside wall of said bladder and forming means to press against said bladder, said bladder adapted for reciprocal movement in contact with said bladder, said mechanical means comprising a powertransmitting linkage forming means to impart a power stroke characterized by a substantial loss of velocity of said pressurizing member as the pressure of said contact is increased in the bladder during said stroke.

11. Apparatus as defined in claim 10 wherein saidbladder and mechanical pressurizing means are enclosed in a rigid casing, wherein the casing is enclosed in an envelope, and wherein a suction pump is operably connected to the interior of said envelope forming means to maintain a sub-atmospheric pressure within said envelope of between atmospheric pressure and about minus 100 mm of mercury below atmospheric pressure.

12. Apparatus as defined in claim 11 wherein said pressurizing means has a displacement capacity of at least 100 cubic inches and comprises means for actuating said platen through a stroke of up to about 3.5 inches.

13. Apparatus as defined in claim 10 wherein said pressurizing means has a displacement capacity of at least 100 cubic inches and comprises means for actuating said platent through a stroke of up to about 3.5 inches.

14. In apparatus for applying external pressure circulatory assist action to the limbs of a patient in cyclic synchronization with a heartbeat of said patient, and wherein said assist is achieved by means for pressurizing a fluid-filled bladder surrounding said limbs, the improvement wherein pump means is provided for adding to or subtracting fluid from said bladder, said pump means being responsive to the pressure within said bladder, through a bladder-pressure sensing means and a control circuit, and wherein said pump is responsive to said sensing means to receive an independent command signal therefrom on each heartbeat cycle of said patient.

15. Apparatus as defined in claim 14 wherein said pump means has an operating period of about 0.5 seconds.

16. Apparatus as defined in claim 14 comprising a reciprocating mechanical member forming displacement means to engage said bladder and pressurize the fluid therein.

17. Apparatus as defined in claim 16 wherein said pressure-sensing means is an indirect sensing means comprising a displacement-sensing monitor adapted to sense the position of said mechanical pressuretransmitting device, to transmit a signal indicative of said position, and consequently indicative of said pressure in said bladder.

18. Apparatusfor applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist isachievedwith means for transmitting pressure to said limbs through a fluid-filled bladder surrounding said limbs, the improvement wherein the apparatus includ'es a visual cycle-phasing monitor comprising:

A. a multi-channel display device adapted to display 1. an EKG trace, and 2. a trace derived from an arterial pressure sensor,

B.- means for visually displaying of the relative time duration of a pressure command signal on said traces, and

C. manual means to adjust the relative timing and duration of said pressure command signal, while simultaneously achieving a visual adjustment of said signal to bring it into the desired relationship to said EKG trace, or arterial trace.

19. Apparatus-for applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist is achieved with means for transmitting pressure througha hydraulic cylinder to said limbs through a fluid filled bladder surrounding said limbs, the improvement wherein the apparatus includes A. an ideal pressure waveform generator means,

B. actual leg pressure waveform sensing means,

G. means for continually comparing said ideal and actual waveforms, and

D. variable valve means, responsive to a signal from said comparing means, to modify fluid flow to said hydraulic cylinder and thereby more nearly achieve said ideal-pressure waveform.

20. Apparatus as defined in claim 19 comprising:

A. a multi-channel oscilloscope adapted to display 1. an EKG trace, and 2. a trace derived from an arterial pressure sensor,

B. means for visually displaying of the relative time duration of a pressure command signal on said traces, and i C. manual means to adjust the relative timing and duration of said pressure command signal, while simultaneously achieving a visual adjustment of said signal to bring it into the desired relationship to said EKG trace.

21. Apparatus as defined in claim 20 wherein pump means is provided for adding to or subtracting fluid frornsaid bladder, said pump means being responsive to the pressure within said bladder, through a bladderpre'ssu're sensing means and a control circuit, and whereinlsaid pump is responsive to said sensing means to receive an independent command signal therefrom oneach heartbeat cycle of said patient.

22. Apparatus as defined inclaim 1 wherein said pressurizing means has a displacement capacity of at I least 100 cubic inches and comprises means for actuatin'gsaidplaten th'roughastroke length of up to about 3.5 inches.

-23. In apparatus as'defined in claim 1 for applying external pressure circulatory assist action to the limbs of a patient in cyclic synchronization with a heartbeat of said patient, and wherein said assist is achieved by meansfor. pressurizing a fluid-filled bladder surrounding said lirn bs, the improvement wherein pump means is provided for adding to or subtracting the fluid from 14 said bladder, said pump means being responsive to the pressure within said bladder, through a bladderpressure sensing means and a control circuit, and wherein said pump is responsive to said sensing means to receive an independent command signal therefrom on each heartbeat cycle of said patient.

24. Apparatus as defined in claim 23 wherein said pump means has an operating period of about 0.5 seconds.

25. Apparatus as defined in claim 24 wherein said leg-enclosing zones are conical in shape and adapted to dissipate stresses exerted thereon as hoop stresses, thereby resisting any substantial deflection or deformation of said zones.

26. In apparatus as defined in claim 23 for applying external pressure circulatory assist action to the limbs of a patient in cyclic synchronization with a heartbeat of said patient, and wherein said assist is achieved by means for pressurizing a fluid-filled bladder surrounding said limbs, the improvement wherein pump means is provided for adding to or subtracting fluid from said bladder, said pump means being responsive to the pressure within said bladder, through a bladder-pressure sensing means and a control circuit, and wherein said pump is responsive to said sensing means to receive an independent command signal therefrom on each heartbeat cycle of said patient.

27. Apparatus as defined in claim 26 wherein said pump means has an operating period of about 0.5 seconds.

28. Apparatus as defined in claim 27 wherein said leg-enclosing zones are conical in shape and adapted to dissipate stresses exerted thereon as hoop stresses, thereby resisting any substantial deflection or deformation of said zones.

29. Apparatus as defined in claim 26 comprising a reciprocating mechanical member forming displacement means to engage said bladder and pressurize the fluid therein.

30. Apparatus as defined in claim 29 wherein said pressure-sensing means is an indirect sensing means comprising a displacement-sensing monitor adapted to sense the position of said mechanical pressuretransmitting device, to transmit a signal indicative of said position, and consequently indicative of said pressure in said bladder.

31. Apparatus as defined in claim 1 for applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist is achieved with means for transmitting pressure to said limbs through a fluid-filled bladder surrounding said limbs, the improvement wherein the apparatus includes a visual cycle-phasing monitor comprising:

A. a multi-channel display device adapted to display 1. an EKG trace, and

2. a trace derived from air arterial pressure sensor, B. means for visually displaying of the relative time duration of a pressure command signal on said traces, and

32. Apparatus as defined in claim 31 wherein said leg-enclosing zones are conical in shape and adapted to dissipate stresses exerted thereon as hoop stresses, thereby resisting any substantial deflection or deformation of said zones.

33. Apparatus as defined in claim 31 wherein said pressurizing means has a displacement capacity of at least 100 cubic inches and comprises means for actuating said platen through a stroke of up to about 3.5 inches.

34. Apparatus as defined in claim 16 for applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist is achieved with means for transmitting pressure to said limbs through a fluid-filled bladder surrounding said limbs, the improvement wherein the apparatus includes a visual cycle-phasing monitor comprising:

B. means for visually displaying of the relative time duration, and

35. Apparatus as defined in claim 33 wherein said leg-enclosing zones are conical in shape and adapted to dissipate stresses exerted thereon as hoop stresses, thereby resisting any substantial deflection or deformation of said zones.

36. Apparatus as defined in claim 34 wherein said pressure-sensing means is an indirect sensing means comprising a displacement-sensing monitor adapted to sense the position of said mechanical pressuretransmitting device, to transmit a signal indicative of said position, and consequently indicative of said pressure in said bladder.

37. Apparatus as defined in claim 23 wherein said leg-enclosing zones are conical in shape and adapted to dissipate stresses exerted thereon as hoop stresses, thereby resisting any substantial deflection or deformation of said zones.

38. Apparatus as defined in claim 1 for applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist is achieved with means for transmitting pressure through an hydraulic cylinder to said limbs through a fluid-filled bladder surrounding said limbs, the improvement wherein the apparatus includes A. an ideal pressure waveform generator means, B. actual leg pressure waveform sensing means, C. means for continually comparing said ideal and actual waveforms, and D. variable valve means, responsive to a signal from said comparing means, to modify fluid flow to said sydraulic cylinder and thereby more nearly achieve said ideal-pressure waveform. 39. Apparatus as defined in claim 38 comprising: A. a multi-channel oscilloscope adapted to display 1. an EKG trace, and 2. a trace derived from an arterial pressure sensor, B. means for visually displaying of the relative time duration of a pressure command signal on said traces, and C. manual means to adjust the relative timing and duration of said pressure command signal, while simultaneously achieving a visual adjustment of said signal to bring it into the desired relationship to said EKG trace. 40. Apparatus as defined in claim 39 wherein pump means is provided for adding to or subtracting fluid from said bladder, said pump means being responsive to the pressure within said bladder, through a bladderpressure sensing means and a control circuit, and

wherein said pump is responsive to said sensing means to receive an independent command signal therefrom on each heartbeat cycle of said patient.

41. Apparatus as defined in claim 40 wherein said pump means has an operating period of about 0.5 seconds.

42. Apparatus as defined in claim 38 wherein said pump means has an operating period of about 0.5 seconds.

43. Apparatus as defined in claim 39 wherein said pump means has an operating period of about 0.5 seconds.

Claims

1. In apparatus for applying external pressure circulatory assist action to the legs of a patient in cyclic synchronization with the heartbeat of said patient by pressurizing said legs, the improvement wherein said apparatus comprises: A. a unitary bladder adapted for wrapping about a patient''s legs and holding a quantity of liquid, B. mechanical displacement means in contact with outside wall of said bladder and forming means to press against said bladder, said means forming the primary means for obtaining the aforesaid pressure assist action, and C. a unitary upper leg casing member and a unitary lower leg casing member having means, said casing members adapted for attachment to form a compartment to enclose said bladder, said casing members having sufficient brace means between leg sections thereof to avoid any substantial outward deflection of said leg casing members during said pressurizing of said fluid.

2. Apparatus as defined in claim 1 wherein said brace means comprise: A. triangular brace means between each of leg-enclosing zones of each of the upper and lower casing members, and B. a plurality of fastening means connecting said brace means together.

2. a trace derived from an arterial pressure sensor, B. means for visually displaying of the relative time duration, and C. manual means to adjust the relative timing and duration of said pressure command signal, while simultaneously achieving a visual adjustment of said signal to bring it into the desired relationship to said EKG trace, or arterial trace.

2. a trace derived from an arterial pressure sensor, B. means for visually displaying of the relative time duration of a pressure command signal on Said traces, and C. manual means to adjust the relative timing and duration of said pressure command signal, while simultaneously achieving a visual adjustment of said signal to bring it into the desired relationship to said EKG trace.

2. a trace derived from an arterial pressure sensor, B. means for visually displaying of the relative time duration of a pressure command signal on said traces, and C. manual means to adjust the relative timing and duration of said pressure command signal, while simultaneously achieving a visual adjustment of said signal to bring it into the desired relationship to said EKG trace, or arterial trace.

3. Apparatus as defined in claim 2 wherein said leg-enclosing zones are conical in shape and adapted to dissipate stresses exerted thereon as hoop stresses, thereby resisting any substantial deflection or deformation of said zones.

9. Apparatus as defined in claim 8 wherein said pressurizing means has a displacement capacity of at least 100 cubic inches and comprises means for actuating said platen through a stroke length of up to about 3.5 inches.

10. In apparatus for applying external pressure circulatory assist action to the limbs of a patient in synchronization with the heartbeat of said patient, and wherein said assist is achieved by pressurizing a fluid-filled bladder surrounding said limbs, the improvement wherein said apparatus comprises a mechanical pressurizing member positioned generally between said limbs and in contact with an outside wall of said bladder and forming means to presS against said bladder, said bladder adapted for reciprocal movement in contact with said bladder, said mechanical means comprising a power-transmitting linkage forming means to impart a power stroke characterized by a substantial loss of velocity of said pressurizing member as the pressure of said contact is increased in the bladder during said stroke.

11. Apparatus as defined in claim 10 wherein said bladder and mechanical pressurizing means are enclosed in a rigid casing, wherein the casing is enclosed in an envelope, and wherein a suction pump is operably connected to the interior of said envelope forming means to maintain a sub-atmospheric pressure within said envelope of between atmospheric pressure and about minus 100 mm of mercury below atmospheric pressure.

17. Apparatus as defined in claim 16 wherein said pressure-sensing means is an indirect sensing means comprising a displacement-sensing monitor adapted to sense the position of said mechanical pressure-transmitting device, to transmit a signal indicative of said position, and consequently indicative of said pressure in said bladder.

18. Apparatus for applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist is achieved with means for transmitting pressure to said limbs through a fluid-filled bladder surrounding said limbs, the improvement wherein the apparatus includes a visual cycle-phasing monitor comprising: A. a multi-channel display device adapted to display

19. Apparatus for applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist is achieved with means for transmitting pressure through a hydraulic cylinder to said limbs through a fluid-filled bladder surrounding said limbs, the improvement wherein the apparatus includes A. an ideal pressure waveform generator means, B. actual leg pressure waveform sensing means, C. means for continually comparing said ideal and actual waveforms, and D. variable valve means, responsive to a signaL from said comparing means, to modify fluid flow to said hydraulic cylinder and thereby more nearly achieve said ideal-pressure waveform.

20. Apparatus as defined in claim 19 comprising: A. a multi-channel oscilloscope adapted to display

21. Apparatus as defined in claim 20 wherein pump means is provided for adding to or subtracting fluid from said bladder, said pump means being responsive to the pressure within said bladder, through a bladder-pressure sensing means and a control circuit, and wherein said pump is responsive to said sensing means to receive an independent command signal therefrom on each heartbeat cycle of said patient.

22. Apparatus as defined in claim 1 wherein said pressurizing means has a displacement capacity of at least 100 cubic inches and comprises means for actuating said platen through a stroke length of up to about 3.5 inches.

23. In apparatus as defined in claim 1 for applying external pressure circulatory assist action to the limbs of a patient in cyclic synchronization with a heartbeat of said patient, and wherein said assist is achieved by means for pressurizing a fluid-filled bladder surrounding said limbs, the improvement wherein pump means is provided for adding to or subtracting the fluid from said bladder, said pump means being responsive to the pressure within said bladder, through a bladder-pressure sensing means and a control circuit, and wherein said pump is responsive to said sensing means to receive an independent command signal therefrom on each heartbeat cycle of said patient.

30. Apparatus as defined in claim 29 wherein said pressure-sensing means is an indirect sensing means comprising a displacement-sensing monitor adapted to sense the position of said mechanical pressure-transmitting device, to transmit a signal indicative of said position, and consequently indicative of said pressure in said bladder.

31. Apparatus as defined in claim 1 for applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist is achieved with means for transmitting pressure to said limbs through a fluid-filled bladder surrounding said limbs, the improvement wherein the apparatus includes a visual cycle-phasing monitor comprising: A. a multi-channel display device adapted to display

34. Apparatus as defined in claim 16 for applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist is achieved with means for transmitting pressure to said limbs through a fluid-filled bladder surrounding said limbs, the improvement wherein the apparatus includes a visual cycle-phasing monitor comprising: A. a multi-channel display device adapted to display

36. Apparatus as defined in claim 34 wherein said pressure-sensing means is an indirect sensing means comprising a displacement-sensing monitor adapted to sense the position of said mechanical pressure-transmitting device, to transmit a signal indicative of said position, and consequently indicative of said pressure in said bladder.

38. Apparatus as defined in claim 1 for applying external pressure assist action to the limbs of a patient in cyclic synchronization with the heartbeat of said patient, wherein said assist is achieved with means for transmitting pressure through an hydraulic cylinder to said limbs through a fluid-filled bladder surrounding said limbs, the improvement wherein the apparatus includes A. an ideal pressure waveform generator means, B. actual leg pressure waveform sensing means, C. means for continually comparing said ideal and actual waveforms, and D. variable valve means, responsive to a signal from said comparing means, to modify fluid flow to said sydraulic cylinder and thereby more nearly achieve said ideal-pressure waveform.

39. Apparatus as defined in claim 38 comprising: A. a multi-channel oscilloscope adapted to display

40. Apparatus as defined in claim 39 wherein pump means is provided for adding to or subtracting fluid from said bladder, said pump means being responsive to the pressure within said bladder, through a bladder-pressure sensing means and a control circuit, and wherein said pump is responsive to said sensing means to receive an independent command signal therefrom on each heartbeat cycle of said patient.