U.S. patent number 3,911,898 [Application Number 05/520,046] was granted by the patent office on 1975-10-14 for heart assist method and device.
Invention is credited to Frank A. Leachman, Jr..
United States Patent |
3,911,898 |
Leachman, Jr. |
October 14, 1975 |
Heart assist method and device
Abstract
A heart assist device is controlled in a normal mode of
operation to counterpulsate 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 ventricle discharge and a hydraulically powered,
closed-loop servomechanism controls the displacement of a piston in
an expansible chamber receiving the blood from the ventricle, in
such a way that programmed pressure is maintained in the chamber.
Means are provided for recording the piston displacement as a
function of time during ventricular systole. During diastole, the
piston motion is reversed, and servo-controlled to duplicate the
recorded displacement waveform while the piston contracts the
chamber volume and expels blood into the vascular system. In this
way the output blood from wave-form produced by the pump during
diastole is the same as the output flow waveform produced by the
ventricle during the previous systole. 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: |
27039036 |
Appl.
No.: |
05/520,046 |
Filed: |
November 1, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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458535 |
Apr 5, 1974 |
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Current U.S.
Class: |
600/17;
623/3.1 |
Current CPC
Class: |
A61M
60/148 (20210101); A61M 60/562 (20210101); A61M
60/50 (20210101); A61M 60/258 (20210101); A61M
60/40 (20210101); A61M 60/857 (20210101); A61M
60/894 (20210101); A61M 60/268 (20210101) |
Current International
Class: |
A61M
1/10 (20060101); A61M 1/12 (20060101); A61F
001/24 () |
Field of
Search: |
;3/1.7 ;23/258.5
;128/1D,2.5E,2.5R,419P,419PG,DIG.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Hiller et al., "American Journal of Medical Electronics" July-Sept.
1963, pp. 212-221..
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: McCormick, Paulding & Huber
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of copending application
Ser. No. 458,535, filed Apr. 5, 1974, entitled HEART ASSIST DEVICE.
Claims
I claim:
1. A counterpulsation heart assist device comprising:
blood pumping means connectible between a ventricle of the natural
heart and the associated vascular system of the body, and including
an expansible and contractible pumping chamber for ingesting blood
from the ventricle and expelling blood into the associated vascular
system; and
pumping control means connected with the heart and the pumping
means for regulating the pumping means in counterpulsation with the
heart and including means for detecting and storing the blood flow
waveform produced by the heart during systolic ventricle
contractions and means for regulating the expulsion of blood from
the pumping chamber to duplicate the stored flow waveform.
2. The heart assist device of claim 1 wherein:
the blood pumping means has a displaceable piston associated with
the pumping chamber for expanding and contracting the chamber
volume and correspondingly causing blood to be ingested and
expelled; and
the pumping control means includes driving means for regulating the
piston displacement.
3. A heart assist device as in claim 2 wherein:
the means for detecting and storing is connected to the
displaceable piston.
4. A heart assist device as defined in claim 3 wherein:
the means for detecting and storing includes a piston position
transducer and waveform recorder connected with the transducer for
recording piston displacement.
5. A heart assist device as in claim 4 wherein:
the waveform recorder has a memory programmed on a
first-in-first-out basis.
6. A heart assist device as in claim 5 wherein:
the waveform recorder includes a clock-controlled, sequentially
addressed memory; and
the pumping control also includes an adjustable clock connected to
the memory for adjusting the memory access rate.
7. A heart assist device as defined in claim 1 wherein the pumping
control means includes:
pressure responsive means for controlling the expansion of the
pumping chamber at a rate establishing a programmed pressure at the
ventricle discharge during systole; and
the means for detecting and storing includes memory means for
recording the chamber expansion as a function of time.
8. A heart assist device as in claim 7 wherein:
the blood pumping means includes a displaceable piston connected
with the pump chamber for causing chamber expansion and
contraction;
the pressure responsive means comprises a closed loop
servomechanism connected with the piston to control the piston
displacement; and
the means for detecting and storing includes a pressure transducer
sensing ventricle discharge pressure and providing a pressure
signal as an input to the servomechanism.
9. A heart assist device as in claim 7 wherein:
the means for regulating the expulsion of blood from the pumping
chamber also includes the closed loop servomechanism connected with
the piston in the pumping means.
10. A heart assist device as defined in claim 9 wherein:
the control means includes switching means operated in synchronism
with the heart cycle for connecting the input of the servomechanism
alternately to receive as inputs the pressure transducer signal and
the stored blood flow waveform.
11. A heart assist 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 puming 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.
12. A heart assist device as defined in claim 1 wherein:
the pump control means includes programmed means defining the
programmed systolic pressure as a constant pressure level
throughout the systolic period.
13. A heart assist device as defined in claim 1 wherein:
the pump control means includes programmed means defining the
programmed systolic pressure as a variable pressure waveform during
the systolic period.
14. A heart assist device as defined in claim 13 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.
15. A method of assisting the heart to produce pulsatile blood flow
in the vascular system of the body with the blood flow waveform
duplicating that of the heart comprising:
maintaining a programmed pressure at the ventricle discharge of the
heart during systole;
detecting the blood flow waveform from the heart while the
programmed pressure is maintained;
recording the blood flow waveform from the heart as the waveform is
detected; and
pumping blood into the vascular system in a controlled manner to
duplicate the recorded blood flow waveform.
16. The method of assisting the heart as in claim 15 wherein:
the step of pumping includes the steps of retrieving the recorded
flow waveform and driving a blood pump in accordance with the
retrieved waveform.
17. The method of assisting the heart as in claim 15 wherein:
the detecting step includes receiving a volume of blood equal to
that discharged by the ventricle during systole in a chamber and
wherein,
the step of pumping comprises expelling the received volume of
blood from the chamber during diastole.
18. The method of assisting the heart as defined in claim 15
wherein:
the step of pumping is performed during the diastole immediately
following the systole in which the blood flow waveform was
detected.
19. The method of assisting the heart as defined in claim 15
including the step of:
synchronizing the step of pumping with the systolic and diastolic
periods of the heart by sensing the blood pressure emanating from
the ventricle.
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 counterpulsation 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. A common approach involves the concept of
counterpulsation in which a blood pump in series with the heart is
operated in synchronism with ventricular contractions to receive
blood from the ventricle at low pressure during the systolic phase
of heart action, and to expel this blood at arterial pressure
during the diastolic phase of heart action. Counterpulsation by
itself, however, can be achieved in various manners and does not
necessarily result in duplication of the heart 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 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 preforms 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 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.
A heart assist device which operates in copulsation with the
natural heart to produce a blood flow waveform substantially the
same as that of the natural heart is disclosed in my copending
application Ser. No. 458,535 identified above. In the copulsation
device, blood is expelled into the vascular system by the blood
pump during systolic contractions of the heart. My present
invention relates to a counterpulsation heart assist device which
is capable of expelling blood into the vascular system with a blood
flow waveform substantially the same as that produced by the
heart.
It is, therefore, a general object of this invention to disclose a
counterpulsation heart assist device which provides a blood flow
waveform that duplicates closely the natural heart output.
SUMMARY OF THE INVENTION
The present invention resides in a heart assist method and device
which operates in counterpulsation with the heart and duplicates
the blood flow waveform of the natural heart, that is the blood
flow versus time function during systolic contraction, as long as
the heart beat 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, to produce a pulsatile blood flow. Pumping
control means is connected with the heart and the pumping means for
regulating the pumping means in counterpulsation with the heart.
The pumping means includes an expansible and contractible pumping
chamber connected with the ventricle, and a displaceable piston
which controls chamber expansion and contraction is operated by the
pumping control means to maintain a programmed systolic pressure or
back pressure at the ventricle during systole. Means are provided
in the pumping control means for detecting and recording or storing
the expansion of the chamber as a function of time during
ventricular systole. The stored expansion defines the blood flow
waveform produced by the heart, and means for regulating the
pumping means reproduces the flow waveform during ventricular
diastole by controlling chamber contraction and blood expulsion
into the vascular system in a proportional manner. Accordingly,
during diastole, blood is expelled from the chamber into the
vascular system with a flow waveform which duplicates the flow
waveform produced by the contracting ventricle during the previous
systole.
By duplicating the blood flow waveform produced by the heart, the
vascular system experiences the same stress that would ordinarily
be produced by the healthy heart. At the same time, the heart is
permitted to operate against a reduced load at the ventricle
discharge. Thus, the counterpulsation heart assist device
duplicates the flow characteristics of the heart while reducing the
heart load.
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 to the heart.
FIG. 2 is a schematic diagram showing the controls which regulate
the blood pump operation for both counterpulsation 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 expansible and contractible
pumping chamber 12 which is connected by means of a blood
compatible inlet 14 to the aorta between the aortic valve at the
left ventricle discharge and a ligation in the aorta a short
distance from the discharge. Chamber 12 is fitted with a check
valve 42 at its discharge, and the downstream side of this valve is
connected to the aorta at the opposite side of the ligation from
the tube 14 by a blood compatible outlet 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 during
systole, and to expel blood into the aorta during diastole with a
flow waveform corresponding to that produced by the heart during
the previous 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 the ventricle supported
by the assist device.
Within the pump 10 a displaceable piston 20 is provided with piston
head 22 inside chamber 12. The piston head 22 is sealed at the
cylindrical wall of the 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. 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.
During ventricular contraction, piston head 22 moves from top to
bottom in FIG. 1, ingesting blood into chamber 12. During diastole,
the aortic valve of the ventricle closes, preventing back-flow of
blood, and the piston head 22 moves from bottom to top in FIG. 1,
expelling blood through the check valve 42 into the vascular
system.
Hydraulic fluid used to operate the actuator including the piston
20 may be derived from a self-contained and battery-operated power
source installed within the body of the heart assist recipient or
carried externally of the body.
FIG. 2 illustrates the pump control which regulates the blood pump
in FIG. 1 to achieve either counterpulsation of the pump and left
ventricle or autonomous operation of the pump.
A principal operation of the heart assist device is
counterpulsation 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 waveform from the ventricle during systolic
contractions. The counterpulsation 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. The first portion
of the cycle is substantially coextensive with a systole and during
this portion the displacement of the piston 20 is regulated by a
closed-loop servomechanism in the pump control to maintain a
programmed pressure at the ventricle discharge. Piston displacement
is recorded or memorized as a function of time during systole and
the recorded displacement accordingly represents the blood flow
waveform produced by the ventricle. The other portion of the
counterpulsation cycle is coextensive with the ventricular diastole
and during this portion the recorded piston displacement is
supplied as an input to the pump control to regulate piston
displacement and the expulsion of blood from the chamber 12 with a
flow waveform that is a duplicate of that produced by the heart
during the previous systole. The recorded piston displacement,
therefore, represents a reference waveform that the pump duplicates
during diastole.
During the first portion of a counterpulsation operation cycle the
piston 20 is controlled to maintain a programmed pressure at the
ventricle discharge. A pressure sensor or transducer 50 supplies
electrical signals as feedback in the 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 chamber 12 of the pump 10 as illustrated in FIG.
1.
As the systolic contraction begins, pressure at the discharge of
the ventricle, and correspondingly in the chamber 12, begins to
rise, and an electrical signal from the transducer 50 passes to an
input amplifier 52 in FIG. 2 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 an 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 the
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 control pulse 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.
Control of the piston 20 during systolic contraction displaces the
piston head 22 downward in FIG. 1, and increases the volume of
chamber 12 at the same instantaneous rate that the volume of the
ventricle is decreasing. A position transducer 80, mounted in the
pump 10 and cooperating with the piston head 22, produces an
electrical piston position signal that represents the ventricle
output volume as a function of time. This signal is fed into a
waveform recorder 84, which is turned on at the start of systole by
the leading edge of the control pulse from pulse stretcher 61. The
waveform recorder consists of an analog-to-digital converter, a
random access memory (preferably a state-of-the-art semiconductor
memory), timing and control circuits for storing and retreiving
information from the memory, and a digital-to-analog converter. As
the piston head 22 moves during systole, the output from the
position transducer 80 is sampled and digitized at closely spaced
intervals of time by the analog-to-digital converter within the
waveform recorder 84. The digitally encoded data is sequentially
addressed and stored in the semiconductor memory by the internal
timing and control circuits. Since this data is read in at a
constant clock rate, as described below, the information stored in
the memory represents the displacement waveform of piston head 22,
and hence the volume output waveform of the heart ventricle.
During systole 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 top to bottom 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, the chamber 12
receives a quantity of blood equal to the quantity expelled from
the left ventricle, and as long as the servomechanism including the
piston head 22 holds the systolic pressure near its reference value
with small error signals, the displacement signals produced by the
position transducer 80 and stored in the waveform recorder 84 will
represent the blood flow waveform produced by the ventricle during
the systolic period.
The foregoing description of the pump control which causes blood to
be ingested into chamber 12 entails a pressure mode of operation
which mode is coextensive with the systolic period of the
ventricle. During the diastolic period of the ventricle, blood then
filling chamber 12 is expelled through the check valve 42 into the
aorta. To accomplish this return, the pump control switches to a
position mode of operation.
Since it is an object of this invention to supply the vascular
system with a blood flow waveform which duplicates the flow
waveform produced by the heart ventricle, the piston head 22 in the
pump 10 is controlled during its return by the displacement
waveform stored in the waveform recorder 84. In pumping, the piston
head 22 must displace a volume of blood in each successive time
increment of its return stroke equal to that ingested during a
corresponding time increment of the previous ingesting stroke when
the piston head followed the volume output waveform of the
ventricle. This process and the apparatus regulating the pump 10
generate a blood flow waveform in the aorta that duplicates the
flow waveform produced by the ventricle during the previous
systolic period. The durations of the systole and diastole of the
natural heart, of course, vary with physiological demand, but
within the range of heart beats that is expected of a recipient of
the heart assist device, the systole is shorter than the succeeding
diastole so that there is always adequate time to duplicate the
blood flow waveform of the previous systole during a diastole. 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 pressure waveform is generated as the vascular
system response to a flow waveform produced by the heart. At the
same time, the load or pressure against which the left ventricle
operates is reduced to a 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.
At the end of a systolic contraction and the beginning of the
diastole, the pressure in chamber 12 falls below its reference
value and causes the comparator 58 and pulse stretcher 61 outputs
to turn off. This pulse transition causes the timing and control
circuits in the waveform recorder 84 to retrieve the information
stored in its memory, on a first-in-first-out basis. The
sequentially addressed digital data is processed by the
digital-to-analog converter within waveform recorder 84 and
generates a series of output voltage levels which closely
approximate the waveform recorded.
Because the semiconductor memory in the waveform recorder 84 can
store a limited number of digital words, and the size and cost of
the memory increase with increasing word storage capacity, the
minimum word capacity required for adequate waveform amplitude
resolution should be used. Although the memory must have a capacity
capable of storing the number of words necessary to digitize the
entire stroke of piston 20 to the required amplitude resolution
(even though the full stroke may not be utilized during each beat),
excess memory capacity would be required if a constant clock
frequency were used. This is because the heart beat rate and the
period of systolic contraction vary widely to meet various
physiological demands. If a constant clock rate were used, a
digital word would be recorded with each clock pulse after a
constant delay between clock pulses had elapsed. The clock rate
would be that required to achieve the necessary amplitude
resolution for the shortest expected systolic period. If this same
clock rate were used while digitizing the waveform with the longest
expected period, however, the clock would produce several digital
words for each amplitude resolution level selected for the faster
waveform. These redundant digital words would waste memory
space.
To avoid this, an adjustable clock rate based on the previous
systolic period is utilized. A fast clock rate is used for the
shortest waveform duration, and this rate is proportionally lowered
as the systolic period increases. This assures a more uniform
amplitude increment for each clock pulse, so that memory capacity
is dictated mostly by amplitude resolution considerations.
The clock rate is derived by sampling and storing a voltage
proportional to the previous systolic period, and using this stored
voltage as the input to a voltage-to-frequency converter
circuit.
To this end, a reset time delay 130 is triggered at the beginning
of a systolic period by the leading edge of the control pulse from
pulse stretcher 61. After a delay of only several milliseconds,
delay 130 triggers a ramp function generator 120 to reset to its
initial value, and to begin generating a ramp voltage. This ramp
voltage decreases lineraly with time and reaches a minimum value
only at the end of the longest systole. At the end of a systole,
the trailing edge of the control pulse from the pulse stretcher 61
causes the generator 120 to stop and hold its last ramp voltage. At
the beginning of the following systole, the leading edge of the
control pulse from stretcher 61 triggers a single-pulse generator
124, generating an output pulse that is shorter than the delay
generated by circuit 130. A sample-and-hold circuit 122 responds to
the pulse from the pulse generator 124, forming a sample time
aperture coextensive with the duration of this pulse, and during
which the circuit 122 stores the voltage of the ramp generator 120
representing the length of the previous systolic period. It should
be noted that the stored voltage representing the systolic period
is smaller if the period is longer.
A voltage-to-frequency converter 126 is connected to the output of
the sample-and-hold circuit 122, and produces an output pulse train
having a pulse rate proportional to its input voltage. This pulse
train determines the clock rate for reading and recording
information in the waveform recorder 84. Note that although the
clock rate is adjusted from heart beat to heart beat on the basis
of the previous systolic period, it remains constant during each
heart cycle of one systole and diastole, assuring that the waveform
is read into and out of the memory at the same rate. The ramp
function generator 120, the sample-and-hold circuit 122, the
converter 126 and associated control components therefore form an
adjustable clock for controlling the read and record rate for the
memory in the waveform recorder 84.
During its return stroke, piston 20 is regulated in a closed loop
by the pump control to reproduce the position waveform stored in
recorder 84. To do this, the output of the position transducer 80
is compared with the waveform output of recorder 84 and hydraulic
control is exercised to minimize the error between these signals.
When the position signal is fed into recorder 84, however, the
transducer 80 output begins at a zero level when the piston 20 is
in its extreme top position, and increases as the piston travels
downward in FIG. 1. When the waveform is read out of the memory,
however the piston must travel in the opposite direction, that is
from bottom to top in FIG. 1. This means that the position
transducer 80 output signal cannot be directly compared with the
waveform recorder 84 output during the piston return stroke. To be
compared with the transducer 80 output, the waveform recorder 84
signal must be substracted from a constant representing the
transducer 80 voltage output when it begins its return stroke.
To achieve this, the trailing edge of the control pulse from the
pulse stretcher 61 causes a sample-and-hold circuit 82 to hold the
last transducer 80 voltage occurring at the end of systole. This
voltage is fed into a differential amplifier 92. Another input of
this amplifier receives the output signal from the waveform
recorder 84 during diastole. Differential amplifier 92 generates a
signal representing the difference between these signals. This
difference signal is then compared directly with the position
transducer 80 output signal during the piston 20 return stroke,
allowing reproduction of the volume displacement waveform, and
hence the blood flow waveform produced by the ventricle during
systolic contractions. In particular, the output signal from the
differential amplifier 92 is applied 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 control pulse from
pulse stretcher 61 deenergizes 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 68 and torque motor 32 which drives the
piston 20 upwardly to the starting position in accordance with the
stored waveform in recorder 84.
Accordingly, the servomechanism assumes a position mode of
operation in response to the trailing edge of the control 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 systolic pressure pulse of the ventricle which
switches the servomechanism, or more specifically the electronic
switches 60 and 63, between the pressure and the position modes of
operation, the pressure pulse generated by the piston 20 during its
return stroke will also be detected by the transducer 50, and could
inadvertently switch the servomechanism into the pressure mode of
operation. To prevent inadvertent mode switching in this fashion,
the electronic switch 56 serving as a control gate for the pressure
signal remains open during diastole and is closed only if the
piston is in its starting (extreme top in FIG. 1) position.
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 top position or if the piston is moving from top to bottom
during a systolic contraction of the left ventricle.
To this end, a logic circuit 100 controls the electronic switch 56
in 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 produces an output
to the logic circuit only when the piston 20 is in the starting
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 counterpulsation 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
control switches to an autonomous operation in which the pulsation
rates are no longer controlled by the heart. The control for the
pump as disclosed includes 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
output of the sample-and-hold circuit 82 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 sample-and-hold circuit 82 is therefore placed in
the sample mode of operation with zero output. A separate input
from the pulse width discriminator 104 resets and holds the
waveform recorder 84 at 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, the
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
discriminator 104. Since the discriminator signal and control
pulses have contradictory effects, the isolation provided by the
rectifier 70 is needed.
To warn the 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 counterpulsation 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 utililze the defibrillator, he can again
determine whether it has been successful by resetting the pulse
width discriminator 104 and listening for the alarm signal.
Although the programmed systolic pressure or backpressure
established by the pressure reference 62 has been described above
as being constant throughout systole, it is contemplated that the
pressure reference may also generate a variable, programmed analog
pressure signal during each ventricular contraction. FIG. 3
discloses one form of the pressure reference 62 and associated
components described above that are capable of producing a
variable, programmed pressure waveform with an adjustable time
base. The time base is derived from the previous systolic
period.
The output of the voltage-to-frequency converter 126 is a pulse
train having a pulse rate proportional to the input or stored
voltage in circuit 122, which in turn is proportional to the
previous systolic period. These pulses are applied as an input to
the waveform generator 128 in FIG. 3, and serve as clockpulses for
reading a pressure program memory. The waveform generator 128 is
basically a read-only memory having analogue pressure voltages or
signals stored as digital words at sequentially addressed memory
sites. The clock pulses cause sequential reading of these digital
words. An internal digital-to-analog converter processes these
digital words into a sequence of analog voltages, producing a
desired systolic pressure or 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
120 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 waveform generator 128 is turned on by the leading edge of the
control pulse from pulse stretcher 61, and is reset to the initial
value by its trailing edge.
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 waveform recorder 84 may
include random or controlled access memory devices which store the
ingested blood flow waveform in either analog or digital form. The
ramp function generator 120, the sample-and-hold circuits 82 and
122, the flow waveform generator 106 and other components may be
digital or analog devices. The specific blood pump shown and
described is not the only design available for counterpulsation,
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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