U.S. patent number 3,911,897 [Application Number 05/458,535] was granted by the patent office on 1975-10-14 for heart assist device.
Invention is credited to Frank A. Leachman, Jr..
United States Patent |
3,911,897 |
Leachman, Jr. |
October 14, 1975 |
Heart assist device
Abstract
A heart assist device is controlled in a normal mode of
operation to copulsate with the heart and produce a blood flow
waveform corresponding to the flow waveform of the heart being
assisted. A blood pump in the device is connected serially between
the discharge of a heart ventricle and the vascular system, and
during the normal mode of operation, the pump is operated to
maintain a programmed pressure at the ventricle discharge during
systolic cardiac pulsation. A pressure transducer detects the
pressure at the discharge and controls the pump through a
hydraulically powered, closed-loop servomechanism. In the event
that the heart beat stops or becomes severely arrhythmic, the
device switches to an autonomous mode of operation and a waveform
generator in the pump controls provides an ideal blood flow
waveform independent of cardiac pulsations.
Inventors: |
Leachman, Jr.; Frank A.
(Bristol, CT) |
Family
ID: |
23821163 |
Appl.
No.: |
05/458,535 |
Filed: |
April 5, 1974 |
Current U.S.
Class: |
600/17;
623/3.1 |
Current CPC
Class: |
A61M
60/148 (20210101); A61M 60/50 (20210101); A61M
60/40 (20210101); A61M 60/894 (20210101); A61M
60/857 (20210101); A61M 60/562 (20210101); A61M
60/268 (20210101) |
Current International
Class: |
A61M
1/10 (20060101); A61M 1/12 (20060101); A61F
001/24 () |
Field of
Search: |
;128/1D,2.5E,2.5R,419P,419PG ;3/1,DIG.2 ;417/12,38 ;23/258.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Sato et al., "Transactions of the American Society for Artificial
Internal Organs," Vol. XV, 1969, pp. 449-453..
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: McCormick, Paulding & Huber
Claims
I claim:
1. A heart assist device comprising:
blood pumping means connectible between a ventricle of the natural
heart and the associated vascular system of the body; and
pumping control means connected with heart and the pumping means
for regulating the pumping means in synchronism with heart
pulsations including:
a pressure sensor detecting blood pressure at the ventricle during
systole; and
servo control means connected in driving relationship with the
pumping means and having programmed means defining a programmed
systolic pressure and a servo control loop responsive to signals
from the pressure sensor and the programmed means for driving the
pumping means to maintain the programmed systolic pressure at the
ventricle discharge during systole and to produce from the pumping
means during intervals synchronized with heart pulsations a blood
flow waveform which duplicates the ventricular blood flow waveform
generated while maintaining the programmed systolic pressure.
2. A heart assist device as defined in claim 1 wherein:
the servo control means is a hydraulically powered
servomechanism.
3. A heart assist device as defined in claim 1 wherein:
the blood pumping means comprises a displacement pump having inlet
and discharge chambers which vary in volume by corresponding
amounts in opposite senses.
4. A heart assist device as in claim 3 wherein the displacement
pump also includes a flow-checking passageway ducting blood from
the inlet chamber to the discharge chamber.
5. A heart assist device as defined in claim 3 wherein: the inlet
chamber of the displacement pump is connected with the heart
ventricle; and the pumping control means has the pressure sensor
mounted in the inlet chamber of the displacement pump to detect
systolic pressure.
6. A heart assist device as defined in claim 1 wherein the blood
pumping means comprises a piston pump having coaxially aligned
cylindrical inlet and discharge pumping chambers of the same
diameter and a reciprocating piston in one chamber fixedly
connected with a reciprocating piston of the other chamber.
7. A heart assist device as in claim 6 wherein the piston pump
further includes a passageway leading between the inlet and
discharge chambers and a check valve in the passageway preventing
flow from the discharge chamber into the inlet chamber.
8. A heart assist device as defined in claim 1 wherein:
the blood pumping means includes an inlet chamber connectible with
the heart ventricle, a discharge chamber connectible with the
vascular system and a cooperating reciprocating member which moves
from a starting position in one direction to ingest blood from the
ventricle into the inlet chamber and expel blood from the discharge
chamber and moves in the opposite direction to transfer ingested
blood from the inlet chamber to the discharge chamber; and
the servo control means of the pumping control means has means for
reversing the direction of movement of the reciprocating member in
synchronism with the respective systolic and diastolic cardiac
periods.
9. A heart assist device as defined in claim 8 wherein:
the pumping control means comprises copulsation pumping control
means providing a pressure mode during the systolic cardiac period
causing the servo control means to respond to the systolic pressure
signals detected by the sensor to stroke the reciprocating member
in said one direction and maintain said programmed systolic
pressure, and providing a positioning mode of operation during the
diastolic cardiac period causing the servo control means to stroke
the reciprocating member in said opposite direction to the starting
position.
10. A heart assist device as defined in claim 9 wherein the
copulsation pumping control means includes:
a switch connected with and controlled by the pressure sensor,
timing and sampling means controlled by the switch and sensor for
measuring the diastolic period and
means connected to the servomechanism for returning the
reciprocating member to the starting position after the systolic
period and within a time period less than the preceding diastolic
period measured.
11. A heart assist device as defined in claim 10 wherein the
copulsation pumping means further includes means for returning the
reciprocating member to the starting position within said time
period at a minimal velocity.
12. A heart assist device as defined in claim 1 further including
in the pumping control means:
an autonomous pump control having a waveform generator providing a
predetermined flow waveform for regulating the pumping means
independently of the heart pulsations; and
discriminator means responsive to systolic pressure for energizing
the autonomous pump control in the presence of severely arrhythmic
heart pulsations.
13. A heart assist device as defined in claim 1 wherein:
the programmed means defines the programmed systolic pressure as a
constant pressure level throughout the systolic period.
14. A heart assist device as defined in claim 1 wherein:
the programmed means defines the programmed systolic pressure as a
variable pressure waveform during the systolic period.
15. A heart assist device as defined in claim 14 wherein:
the programmed means includes an adjustable time-base generator
responsive to a heart function for producing the variable pressure
waveform in periods of time dependent upon the heart function.
16. A heart assist device for operation in synchronism with
ventricular heart contractions comprising:
blood pumping means serially connectible between the heart and the
vascular system of the body for producing a pulsatile blood flow;
and
control means connected to the pumping means including a pressure
sensor connected with a heart ventricle and detecting blood
pressure pulsations produced by the contractions of the ventricle,
and means responsive to the pressure sensor for driving the pumping
means to maintain a programmed reference pressure at the heart
ventricle during systolic contractions and to reproduce the blood
flow waveform from the ventricular contractions obtained when the
programmed pressure reference is maintained.
17. A heart assist device as defined in claim 16 wherein:
the blood pumping means is a piston pumping having an inlet
connectible with a heart ventricle; and
the pressure sensor is mounted in the pump adjacent the inlet to
sense ventricular discharge pressure.
18. A heart assist device comprising in combination:
a pacemaker connected to the natural heart for electrically
stimulating regular heart contractions;
a cyclically driven blood pump connectible to the vascular system
of the body and producing pulsatile flow when driven; and
blood flow waveform generating means connected with both the
pacemaker and the blood pump for driving the pump and pacemaker in
synchronized relationship to produce pulsatile flow from the pump
in synchronism with the heart stimulated by the pacemaker.
19. A heart assist device as in claim 18 wherein the blood flow
waveform generating means is connected to control the cyclically
driven blood pump and to trigger the pacemaker.
20. A heart assist device as in claim 18 wherein:
the blood pump is connectible with the discharge of a heart
ventricle and the vascular system; and
pump control means responsive to the waveform generating are
provided for cyclically driving the pump in copulsation with the
contractions of the ventricle connected with the pump.
Description
BACKGROUND OF THE INVENTION
The present invention relates to prosthetic devices and, more
particularly, is concerned with a heart assist device which can
operate in copulsation with the heart or establish autonomous
control of blood flow from the heart.
Several approaches to the problem of providing artificial
assistance to a weak or diseased heart have been proposed or
developed. One of the most promising approaches involves the
concept of copulsation in which a blood pump in series with the
heart is operated in synchronism with ventricular contractions to
augment blood flow through the vascular system and reduce the work
load on the heart at the same time. Copulsation by itself, however,
can be achieved in various manners and does not necessarily result
in duplication of the blood pressures and flows that would normally
be experienced from a healthy heart. It is desirable that the heart
assist device produce a pulsatile flow which the vascular system is
accustomed to and in this respect, the blood flow or velocity
waveform during each pulsation should be such that there is no
breakdown or damage to the blood or the vascular system.
The need for an optimum blood flow waveform is apparent from the
physical characteristics of the heart and the vascular system into
which it empties. The vascular system properly performs its
function only when it transports a needed amount of blood in a
given time to the appropriate cells of the body. Examination of
blood vessels at different locations in the vascular system reveals
that the wall of each vessel has the minimum cellular structure
required to withstand the most severe stresses imposed upon it by
the circulatory system. Significantly, this minimum structure of
the vessels is designed for the particular blood flow waveform
produced by the heart. It can be demonstrated that other flow
waveforms delivering the same average blood flow during a given
ventricular contraction generate greater arterial wall stresses
than the natural heart waveform. Greater stresses applied
cyclically over long periods of time produce deleterious effects
which the body may not be able to withstand.
Furthermore, development of a heart assist device to implement the
concept of copulsation has previously been unsuccessful. One
state-of-the-art device in this area utilizes a pressure-ratio
system in which the pressures of the ventricle discharge and of the
blood pump discharge connected to the vascular system are held at a
preselected and constant ratio. A closed-loop servosystem controls
the pump and relies upon pressure sensors detecting the ventricular
discharge pressure and the pump discharge pressure to maintain the
fixed ratio. A description of the pressure-ratio copulsation system
is given in Volume XV of the Transactions of the American Society
for Artifical Internal Organs, 1969beginning at page 449.
Because the vascular system represents a complex elastic impedance
network to blood flow, however, rapid changes in blood pump output
can give rise to arterial pressure oscillations unrelated to hear
pulsation. These arterial oscillations are sensed by the pressure
sensor at the pump discharge in the pressure-ratio system and
introduce spurious errors into the feedback loop which cause the
pump to depart from rather than amplify the ventricular pressure
waveform. Instabilities of this type render the pressure-ratio
system ineffectual and unusable.
The more useful interpretation of heart action recognizes the blood
flow or velocity waveform rather than the pressure waveform. As
indicated above, the pressure waveform is more significantly the
result of vascular impedance to heart action than an indication of
the heart action itself. It is known also that the heart produces a
positive flow output even when no significant pressure pulse is
generated. Duplication of the flow waveform rather than the
pressure waveform is, accordingly, considered to be a more
significant goal to be achieved by a heart assist device.
It is, therefore, a general object of this invention to disclose a
heart assist device which provides a blood flow waveform that
duplicates closely the natural heart output regardless of pressure
fluctuations which may be produced in the vascular system.
SUMMARY OF THE INVENTION
The present invention resides in a heart assist device which
operates in copulsation with the heart and duplicates the blood
flow waveform of the natural heart, that is the blood velocity-time
function during systolic contraction, as long as the heart is not
severely arrhythmic.
The heart assist device includes blood pumping means connectible
between a ventricle of the natural heart and the associated
vascular system of the body, which may include the coronary
arteries themselves. A closed-loop pumping control means is
connected with the heart and the pumping means for regulating the
pumping means in copulsation with the heart. Blood is pumped away
from the ventricular discharge and into the vascular system with a
flow waveform established by maintaining programmed systolic
pressure of backpressure at the ventricle discharge.
The pumping control means includes a pressure sensor which is
responsive to the discharge pressure of the ventricle to which the
pumping means is attached. A closed-loop servomechanism is
connected in driving relationship with the pumping means to operate
the pumping means in synchronous response to the sensed pressure
and maintain the programmed systolic pressure. By controlling the
pumping means in this manner, blood passes from the ventricle
through the pump and into the vascular system with the same flow
waveform as that produced by the ventricle. The vascular system,
therefore, experiences the stress that the blood flow waveform from
the natural heart produces and the heart is permitted to operate
against a reduced pressure load at the same time.
The pumping control means is also capable of autonomous pumping
operation to assume complete control of heart pulsations and blood
circulation. Switching of the pumping control means between
autonomous operation and copulsation operation is performed by a
discriminator which initiates autonomous operation in the presence
of severely arrhythmic heart pulsations. A pacemaker may also be
driven synchronously with the blood pumping means by the control
means to stimulate heart contractions simultaneously with the
reduction in the heart load by the pumping means.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates in cross-section the blood pump
and the manner in which the pump is connected into the vascular
system adjacent the heart.
FIG. 2 is a schematic diagram showing the controls which regulate
the blood pump operation for both copulsation and autonomous
operation.
FIG. 3 is a schematic diagram of a variable time-base, programmed
pressure reference.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows the blood pump, generally designated 10, of a heart
assist device in one embodiment of the present invention. The pump
10 is surgically connected to the aorta immediately adjacent the
aortic valve in the left ventricle and preferably is installed
within the body.
The pump 10 includes a cylindrical inlet chamber 12 which is
connected by means of a blood compatible inlet tube 14 to the aorta
between the left ventricle discharge and a ligation in the aorta a
short distance from the discharge. A cylindrical discharge chamber
16 of the pump is coaxially aligned with the inlet chamber 12 and
is connected to the aorta at the opposite side of the ligation from
the tube 14 by another blood compatible tube 18. The coronary
arteries (not shown) are preferably connected to the aorta at the
same side of the ligation as the tube 18 discharging blood from the
pump 10. With the surgical installation indicated, all blood
leaving the left ventricle must pass serially from the heart
through the pump 10 and then into the aorta. The pump is operated
as explained in greater detail below to maintain programmed
pressure or back pressure at the ventricle discharge which causes
the blood flow or velocity waveform entering the aorta from the
discharge chamber 16 to correspond with a blood flow waveform
produced by the systolic contraction of the ventricle.
Since the major circulatory load of the heart is borne by the left
ventricle, the heart assist device of the present invention is most
commonly employed with that ventricle. However, the utility of the
device is not so limited and it is also possible to connect the
pump to the right ventricle to transmit the venous blood to the
lungs through the pulmonary arteries. Within the scope of the
present invention, the term "vascular system" is used generally to
refer to the arterial system connected to either the right or left
ventricles.
Within the pump 10 a reciprocating piston 20 is provided with
piston heads 22 and 24 inside the inlet chamber 12 and discharge
chamber 16 respectively. The piston head 22 is sealed at the
cylindrical wall of the inlet chamber 12 by means of a flexible
diaphragm 26 which rolls back and forth along the walls of the
chamber as the piston 20 is reciprocated. A corresponding diaphragm
28 seals the piston head 24 at the walls of the discharge chamber
16. The piston 20 is mounted within a cylinder 29 and together they
form a hydraulic actuator in a servomechanism including a spool
valve 30 and an electrical torque motor 32. In conventional
fashion, the torque motor 32 receives electrical signals from a
pump control and positions the spool valve to regulate the flow of
hydraulic fluid between the hydraulic actuator and an inlet port 34
and discharge port 36.
The diameters of the cylindrical chambers 12 and 16 are the same
and since the piston heads 22 and 24 are fixedly connected to the
piston 20 for simultaneous movement with the piston, the volumes of
the inlet and discharge chambers vary by the same amounts but in
opposite senses when the piston is displaced. In other words, when
a given volume of blood is ingested into the inlet chamber 12 from
the left ventricle, a matching volume of blood is expelled from the
discharge chamber 16 into the aorta.
Furthermore, the pump 10 includes a flow-checking passageway 40
interconnecting the inlet chamber 12 and the discharge chamber 16,
and a check valve 42 in the passageway allows blood flow only from
the inlet chamber to the discharge chamber. Therefore, when the
piston head 22 moves from left to right as viewed in FIG. 1 to
ingest blood into the inlet chamber 12, the piston head 24 must
expel blood from the discharge chamber 16 into the tube 18 because
the passageway 40 is temporarily closed by the check valve 42. On
the other hand, when the piston 20 moves from right to left in FIG.
1, the check valve 42 opens and permits blood then filling the
inlet chamber 12 to flow through the passageway 40 into the
discharge chamber 16. As described in greater detail below, the
reciprocation of the piston 20 is synchronized with the cardiac
pulsations or periods so that blood is ingested into the inlet
chamber 12 and expelled from the discharge chamber 16 during the
systolic period of the left ventricle, and blood is transferred
from the inlet chamber to the discharge chamber during the
diastolic period.
Hydraulic fluid used to operate the actuator including the piston
20 may be derived from a self-contained and batteryoperated power
source installed within the body or carried externally of the
body.
FIG. 2 illustrates the controls which regulate the blood pump in
FIG. 1 to achieve either copulsation of the pump and left ventricle
or autonomous operation of the pump.
A principal operation of the heart assist device is copulsation of
the heart ventricle and pump 10 in a manner which causes the blood
flow waveform emanating from the pump 10 to duplicate the flow
waveflow from the ventricle during systolic contractions. In
contrast to the pressure-ratio systems of the prior art having two
pressure sensors, a single pressure sensor or transducer 50
supplies electrical signals as feedback in a closed-loop
servomechanism including the torque motor 32 located in the pump 10
but illustrated separately in FIG. 2. The transducer 50 must be
located in a position to detect the blood pressure at the discharge
of the left ventricle and in one form of the invention, the
transducer is located in the inlet chamber 12 of the pump 10 as
illustrated in FIG. 1.
The copulsation operation of the heart assist device is a cyclic
operation synchronized with the heart and it will be assumed that
the beginning of each cycle corresponds with the beginning of each
heart beat when the ventricle is filled with blood and about to
begin a systolic contraction.
As the systolic contraction begins, pressure at the discharge of
the ventricle, and correspondingly in the inlet chamber 12, begins
to rise, and an electrical signal from the transducer passes to an
input amplifier 52 where it is amplified to a maximum level of
several volts. The electrical signal produced by the transducer is
proportional to the pressure in the chamber and hence, represents
an analogue pressure signal. The amplified signal passes through a
shaping network or signal conditioner 54, which filters out
unwanted noise, and then through an electronic switch 56 into and
input signal comparator 58 and a double pole/double throw
electronic switch 60. The electronic switch 56 serves as a gate
transmitting the analogue pressure signal during the systolic
period if the heart is beating properly as explained in greater
detail below.
In the signal comparator 58, the analogue pressure signal is
compared with an electrical, programmed pressure level signal
determined by the setting of the pressure reference 62 which, for
example, may be an adjustable potentiometer. The programmed
pressure reference signal represents a desired pressure to be
maintained at the discharge of the left ventricle and can be
adjusted to increase or decrease the load against which the
ventricle operates during the systolic period.
If the input pressure detected by the transducer 50 is greater than
the programmed reference pressure, the output of the comparator 58
is turned on, and when the input pressure again drops below the
reference pressure the output is turned off. Hence, the comparator
58 normally produces a control pulse in synchronism with and
coextensive with the systolic period of the left ventricle. This
control pulse is used as a mode signal for controlling the
servomechanism.
To insure that perturbations of the pressure signal detected by the
transducer 50 do not prematurely cut off the mode signal, the
control pulse is transmitted through a pulse stretcher 61 which
effectively delays the trailing edge of the pulse a few
milliseconds to insure that the systolic contraction has in fact
ended. The output of the pulse stretcher 61 is transmitted to the
electronic switch 60 and a single pole/double throw electronic
switch 63 to pull in and set these switches in the positions
opposite the deactivated positions illustrated.
The analogue pressure signal from the electronic switch 56 and the
pressure reference signal from the reference 62 are then
transmitted through parallel circuits in the switch 60 to the
differential error detector 64 which produces an error signal
representative of the difference between the two signals. That
error signal is transmitted through the electronic switch 63, a
rectifier 66 and a servo-amplifier 68 to drive the torque motor 32
and piston 20.
The rectifier 66 prevents the servo-amplifier 68 and torque motor
32 from responding to negative error signals which are generated
when the pressure sensed by the transducer 50 drops below the
reference pressure. Such negative error signals occur, for example,
at the end of each systole during the several milliseconds that the
control pulse from comparator 58 is extended by the pulse stretcher
61. The error signal during this period could rapidly reverse the
movement of the pump piston 20 and interfere with the programmed
return of the piston described below.
In the pressure mode of operation, therefore, the torque motor 32
is in a closed servo loop which attempts to hold the ventricle
discharge pressure at the level of the reference pressure and
causes the piston 20 in the pump 10 to move from left to right in
FIG. 1 as long as the pressure sensed by the transducer 50, that is
the systolic pressure of the left ventricle, does not drop below
the programmed pressure established by the reference 62. At the
same time, a quantity of blood equal to the quantity expelled from
the left ventricle is expelled from the discharge chamber 16 into
the aorta, and as long as the servomechanism including the piston
22 follows the systolic pressure closely with small error signals,
the flow waveform produced by the piston head 24 will be the same
as the flow waveform produced by the ventricle during the systolic
period. The vascular system, therefore, receives a quantity of
blood matching the ventricle output during each systole and
experiences a pressure waveform which should be easily tolerated
without deleterious effects since the waveform is effectively
generated by the heart. At the same time, the load or pressure
against which the left ventricle operates remains substantially
uniform at a reduced level determined by the pressure reference 62.
It is contemplated that the pressure reference may be adjusted
upwardly during a recuperative period to permit a damaged or
diseased heart to eventually reach a functional level approximating
that of a healthy heart.
The foregoing description of the servomechanism entails the
pressure mode of operation which is coextensive with the systolic
period of the ventricle. During this mode, the piston 20 moves from
left to right in FIG. 1 to ingest and expel blood as described.
During the diastolic period of the ventricle, the piston 20 is
returned to its extreme left hand position and to accomplish this
return, the servomechanism switches to a position mode of
operation.
The return of the piston 20 to its extreme left hand or starting
position must be completed during the diastolic period of the
ventricle and during this period, blood is transferred from the
inlet chamber 12 of the pump 10 to the discharge chamber 16. Since
rapid motion of the piston 20 can create turbulance which is
injurious to the blood cells, it is desirable to return the piston
to its starting position within the longest possible period and at
the lowest possible velocity. To do this, the controls generate a
linear piston displacement program or signal which has the smallest
slope permitting the piston to be returned to the starting position
within some portion of the previous diastolic period, for example,
80 percent or 90 percent of that period. Utilizing only a portion
of the previous diastole reserves some time to accommodate
accelerations between heart beats.
The durations of the systolic period and the diastolic period vary
significantly because the beat rate of the heart is not constant
and the lengths of the periods relative to one another also vary
with beat rate. In order to return the piston 20 to its starting
position within the variable length diastolic period, the controls
driving the servomechanism measure the duration of each diastolic
period and temporarily store the duration of the preceding
diastolic period.
To measure the duration of a diastolic period, the control pulse
from the pulse stretcher 61 is transmitted through the blocking
diode of an isolation rectifier 70 and its trailing edge triggers a
ramp function generator 72 into operation. The ramp voltage from
the generator decreases linearly with time and reaches a minimum
value only at the end of the longest diastole. At the end of a
diastole, the leading edge of the next control pulse from the pulse
stretcher 61 triggers a single pulse generator 74 and a reset time
delay 76 connected with the ramp function generator 72. The
generator 72 also responds to the leading edge of the pulse by
stopping and holding the ramp voltage. A sample-and-hold circuit 78
responds to the pulse from the pulse generator 74 and stores the
voltage of the ramp generator representing the length of the
diastolic period. Shortly thereafter, the time delay circuit 76
resets the ramp function generator 72 for the next diastolic
period. It should be noted that the stored voltage representing the
diastolic period is smaller if the period is longer.
Since the quantity of blood pumped by the ventricle during each
systolic period is not constant, the piston 20 will normally not
utilize its full stroke which must be large enough to accommodate
the maximum quantity of blood ever expelled by the ventricle in a
single systole. Therefore, to return the piston to its starting
position at the lowest possible velocity in the available time,
consideration must be given to the distance to be traveled by the
piston and, accordingly, the right-hand or stopping position of the
piston at the end of the systolic period must be detected.
To determine the position at which the piston 20 stops and the
distance to be traveled at the end of a systolic period, a position
transducer 80 mounted in the pump 10 as shown in FIG. 1 and
cooperating with the piston head 24 produces a position signal
which is a voltage having the smallest value at the right-hand end
of the stroke and largest value at the left-hand end or starting
position. The position signal generated by the transducer 80 is
held in a position sample-and-hold circuit 82 when that circuit is
triggered by the trailing edge of the pulse emanating from the
isolation rectifier 70. The sampled position signal in the circuit
82 is transmitted as an input voltage to a differential input
multiplier 84 and a start position reference 86 transmits a voltage
representative of the starting position to another input of the
differential input amplifier 84. The two position input voltages
are internally subtracted and the resulting voltage difference
represents the return distance. The return distance is also divided
in the multiplier 84 by the maximum stroke distance to obtain a
fraction representing a normalized return distance or that portion
of the maximum stroke which will be traversed to reach the starting
position.
The normalized return distance is then multiplied in the
differential input multiplier by the voltage stored in the
sample-and-hold circuit 78 representing the time of the previous
diastolic period. The product of this multiplication represents the
desired piston velocity since the product takes into consideration
the distance to be traveled and the period of time available. It
should be noted that the voltage in the sample-and-hold circuit 78
is smaller if the diastolic periods are longer, therefore, the
product or velocity is as it should be, smaller if the periods are
longer and larger if the periods are shorter. Also, of course, the
product or velocity is greater if the return distance is
greater.
The product generated in the multiplier 84 is transmitted to an
integrator 90 where it is integrated with respect to time between
the trailing edge of the pluse received from the pulse stretcher 61
at the end of the systolic period and the leading edge of the pulse
at the beginning of the next systolic period. In other words, the
desired velocity signal is integrated during the diastolic period
by the integrator 90 to produce a ramp voltage rising linearly with
time. This voltage represents a displacement program for returning
the piston to the starting position. The slope of the ram voltage
is the velocity established by the multiplier 84 and is corrected
during each cardiac period to insure return of the piston to its
starting position within a time period no greater than the previous
diastolic period and at the minimum velocity needed to reach the
starting position safely within the period.
The integrator 90 is reset by the leading edge of the control pulse
of the next systole so that the ramp voltage always starts from a
zero voltage level. The position transducer 80, however, produces
the zero output at the extreme right-hand position of the piston
20. Since the piston normally begins its return stroke from a
position other than the extreme right-hand position, a large
negative error signal would be generated in the servomechanism
operating in a closed loop with the position transducer 80 if the
ramp voltage were applied directly as an input signal at the
beginning of the return stroke. To compensate for the initial
piston position, the voltage stored in the sample-and-hold circuit
82 representing the piston stopping position is added to the ramp
voltage of the integrator 90 at a summing junction 92. The output
voltage of the summing junction, therefore, is a voltage rising
from some initial value representing the piston position at the
beginning of the return stroke.
This output voltage is applied through a voltage limiter 94 to an
input of the electronic switch 60. Also, the output of the position
transducer 80 representing actual piston position is fed back to
another input of the electronic switch 60. The two position signals
are transmitted through the switch when the trailing edge of the
pulse from pulse stretcher 61 de-energizes the switch and the
switch assumes the position illustrated schematically. The error
detector 64 compares the position signals and produces a position
error signal. At the same time, the electronic switch 63 is
de-energized so that the position error signal is applied through
the electronic switch 63 to the servoamplifier and torque motor 32
which drives the piston to the starting position.
The voltage limiter 94 is provided to stop the piston motion at the
same starting position during each cycle of the pump 10 in
copulsation with the heart. The slope of the ramp function
generator 72 is selected to insure return of the piston within 80
percent or 90 percent of the previous diastolic period, that is,
before the integrator 90 stops integrating; therefore, without the
voltage limiter, the piston would be driven past the starting
position. The limiter, then, holds the output of summing junction
92 at the limited value until subsequent commands are received.
Accordingly, the servomechanism assumes a position mode operation
in response to the trailing edge of the pulse from the pulse
stretcher 61 and the piston 20 is returned to its starting position
by the servomechanism under closed loop control.
As long as the heart continues to beat in a regular manner, the
pump control illustrated in FIG. 2 continues to switch between the
pressure and position modes of operation in synchronism with the
systolic and diastolic periods of the left ventricle. This
synchronism is maintained by utilizing the systolic pressure pulse
detected by the transducer 50 as the synchronizing signal.
Since it is the pressure pulse which switches the servomechanism,
or more specifically the electronic switches 60 and 62, between the
pressure and the position modes of operation, it is possible that
during a return stroke of the piston 20 a pressure pulse could be
generated and detected by the transducer 50 and inadvertently
switch the servomechanism into the pressure mode of operation
before the piston actually reaches the starting position. To
prevent inadvertent mode switching in this fashion, the electronic
switch 56 serving as a control gate for the pressure signal remains
open during a diastole and is closed only if the piston 20 is in
its starting position in FIG. 1. Furthermore the switch remains
closed thereafter only if a control pulse from the stretcher 61 is
present. In other words, the electronic switch 56 is held closed
only if the piston is at its extreme left hand position or if the
piston is moving from left to right during a systolic contraction
of the left ventricle.
To this end, a logic circuit 100 controls the electronic switch 56
is response to signals received from a position voltage comparator
102 and the pulse stretcher 61. The voltage comparator 102 receives
information from the position transducer 80 and the start position
reference 86 and produces an output to the logic circuit only when
the piston 20 is in the start position. If the logic circuit
receives a signal from either the voltage comparator 102 or the
pulse stretcher 61, and as long as an additional signal from a
pulse discriminator 104 described in greater detail below is
absent, the logic circuit 100 closes the electronic switch 56 and
analogue pressure signals will pass to the signal comparator 58 and
the electronic switch 60.
At the end of a systolic pressure pulse, the logic circuit 100
opens and disables the electronic switch 56 and further pressure
pulses sensed by transducer 50 during the return stroke of the
piston do not cause the switches 60 or 63 to be positioned in the
pressure mode condition.
The pump 10 continues cyclic operation in copulsation with the left
ventricle as long as a regular heart beat persists. If, however,
the heart beat should cease or become severly arrhythmic, that is
either too long in the case of severe bradycardia or too short
indicating ventricular tachycardia or fibrillation, the pump
controls switch to an autonomous operation in which the pulsation
rates are no longer controlled by the heart. The controls for the
pump as disclosed include a secondary control system that
automatically provides an artificially generated blood flow
waveform from the pump to establish blood circulation completely
independent of the natural heart.
The secondary control system includes a pulse width discriminator
104 which monitors the delay between the control pulses received
from the pulse stretcher 61. The discriminator produces two delay
periods in response to the trailing edge of each pulse, the delay
periods representing the maximum and minimum delay periods
considered acceptable for the heart. If the leading edge of the
subsequent control pulse occurs either before the minimum delay
period has elapsed or after the maximum delay period has elapsed,
the discriminator produces a latched output signal. This
discriminator signal is applied to the logic circuit 100 to disable
the electronic switch 56 and prevent further signals from passing
through it. These signals could trigger the signal comparator 58
and pull the electronic switches 60 and 63 into the pressure mode
positions. The discriminator signal is also transmitted to a flow
waveform generator 106 which produces a time-voltage waveform
representing the piston position signals that would be generated by
the transducer 80 during a normal ventricular contraction and
dilation. This waveform signal is applied as the input to the
electronic switch 60 along with the feedback signal from the
position transducer 80 and is repeated continuously as long as the
input from the pulse discriminator 104 is present.
During autonomous operation of the pump 10, the servomechanism
drives the piston 20 in a closed-loop positioning mode of operation
in accordance with the waveform generator 106. To prevent the
integrator 90 and the associated ramp function generator 74 and
differential input multiplier 84 from interfering with the position
commands from the waveform generator 106, the signal from the
discriminator 104 is applied through a blocking diode of the
rectifier 70 in the same manner as the control pulse from the
stretcher 61. The integrator 90 is, therefore, reset to zero and
the sample-and-hold circuit 78 is placed in the sample mode of
operation with zero output. The only signals then received at the
operative inputs of the electronic switch 60 are the voltage
waveform produced by the generator 106 and the position feedback
signal from the position transducer 80. Accordingly, closed loop
positioning control of the piston 20 continues as long as the
discriminator output persists.
The blocking diodes of isolation rectifier 70 prevent the signal
from the pulse width discriminator from reaching the electronic
switches 60 and 63 during autonomous operation of the pump control
and prevent the control pulses from the pulse stretcher 61 from
reaching the logic circuit and other components connected with the
discriminator 104. Since the discriminator signal and control
pulses have contradictory effect, the isolation provided by the
rectifier 70 is needed.
To warn an individual wearing the heart assist device that the pump
control is operating autonomously, the signal from the
discriminator 104 is also applied to an alarm 108.
The flow waveform generator 106 can also produce a timing pulse to
trigger a pacemaker 110 and cause the heart to receive a mild
electrical shock in synchronism with the leading edge of the flow
waveform. In this manner, normal contraction may be reinduced in
the heart in synchronism with the autonomous operation of the pump
10.
The pulse width discriminator 104 may be manually reset by the
user. If the heart has recovered and assumed normal contractions,
the alarm 108 remains off and the entire pump control system
reverts to copulsatile rather than autonomous operation. If the
alarm turns back on immediately after manual resetting of the
discriminator 104, the individual will know that normal heart
action has not been resumed. At this point he has the option of
remaining on autonomous operation or utilizing the manual
defibrillator 112 which raises a single pacemaker pulse to the
level of defibrillator action and administers a severe electrical
shock to a fibrillating heart in order to return to normal
operation. If he elects to utilize the defibrillator, he can again
determine whether it has been successful by resetting the pluse
width discriminator 104 and listening for the alarm signal.
Although the programmed systolic pressure or back pressure
established by the pressure reference 62 has been described above
as being constant throughout the systole, it is contemplated that
the pressure reference may also generate a variable, programmed
pressure during each ventricular contraction. FIG. 3 discloses in
greater detail one form of the pressure reference 62 that is
capable of producing a variable, programmed pressure waveform with
an adjustable time base. The time base is derived by sampling and
storing the previous systolic period in a manner similar to the
sampling and storage of the previous diastolic period during the
positioning mode of operation which returns the piston 20 to its
starting position.
A ramp function generator 120 is triggered at the beginning of a
systolic period by the leading edge of the control pulse from pulse
stretcher 61 and produces a steadily decreasing ramp voltage. At
the end of the systolic period the ramp generator is stopped by the
trailing edge of the control pulse and a sample-and-hold circuit
122 responds to a pulse from the pulse generator 124 to store the
ramp voltage held by the generator 120. The single-pulse generator
124 also responds to the trailing edge of the control pulse to
trigger the sample-and-hold circuit 122. The voltage stored in the
circuit 122 is, therefore, representative of the duration of a
systol and it will be noted that the longer the systole, the
smaller the stored voltage.
During the next systole a voltage-to-frequency converter 126,
commonly referred to as a voltage controlled oscillator (VCO), is
triggered into operation by the leading edge of the control pulse.
The output of the converter 126 is a pulse train having a pulse
rate proportional to the input or stored voltage in circuit 122.
The pulses serve as clock-pulses for reading a pressure program
memory or waveform generator 128. The waveform generator is
basically a read-only memory having analogue pressure voltages or
signals stored at sequentially addressed memory sites. The clock
pulses cause sequential reading of the sites and produce a desired
systolic pressure of backpressure waveform that becomes the
variable pressure program to be maintained by the blood pump during
each systole.
Since the frequency of the clock pulses from the converter 126
determines the rate at which the pressure waveform is produced by
the generator and since the frequency depends upon the voltage
stored in the sample-and-hold circuit 122, the pressure waveform is
produced in a period of time related to the last preceding systolic
period. By setting the slope of the ramp produced by the generator
122 in conjunction with the converter 126, the programmed pressure
waveform from the generator 128 can be produced in a period of time
equal to the last preceding systolic period.
The reset time delay 130 responds to the trailing edge of the
control pulse from pulse stretcher 61 to reset the ramp generator
120 during the diastolic period. The waveform generator 128 would
be reset at the same time and the converter 126 is also shut off.
To provide an input to the signal comparator 58 before the end of a
diastolic period, the output of the waveform generator 128 may be
latched or automatically stepped back to its initial value during
the diastolic period.
Accordingly, the components producing the pulse train input for the
waveform generator 128 form an adjustable time-base generator which
derives its time base from the previous systolic period. Either the
variable pressure program or the constant pressure program produced
by a potentiometer may be selected to most advantageously serve the
recipient of the heart assist device.
While the present invention has been described in several
embodiments, it will be understood that numerous modifications and
substitutions can be had without departing from the spirit of the
invention. Many of the components described and illustrated in the
pump control of FIG. 2 may be replaced by other components
performing equivalent functions. The ramp function generator 72,
the sample-and-hold circuits 78 and 82, the integrator 90, the flow
waveform generator 106 and other components may be digital devices
rather than the analogue devices described. The specific blood pump
shown and described is not the only design available for
copulsation and suitable modifications of the servomechanism
driving the disclosed pump or other pumps can be made as long as
the blood flow waveform produced by the ventricle is matched at the
pump output. Accordingly, the present invention has been described
in a preferred embodiment by way of illustration rather than
limitation.
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