U.S. patent number 3,597,766 [Application Number 04/744,204] was granted by the patent office on 1971-08-10 for artificial heart pumping system powered by a modified stirling cycle engine-compressor having a freely reciprocable displacer piston.
Invention is credited to Keith E. Buck.
United States Patent |
3,597,766 |
Buck |
August 10, 1971 |
ARTIFICIAL HEART PUMPING SYSTEM POWERED BY A MODIFIED STIRLING
CYCLE ENGINE-COMPRESSOR HAVING A FREELY RECIPROCABLE DISPLACER
PISTON
Abstract
A modified Stirling cycle engine operable as a compressor and
having its displacer piston directly connected to a reversing
piston to which high and low pressures are alternately applied at
opposite ends for reciprocably driving the pistons. In one
embodiment, the reversing piston is mounted in a double acting
compression cylinder which acts as an oscillating spring to drive
the pistons whereby energy to sustain oscillation of the reversing
piston within the compression cylinder is supplied directly from
the displacer piston. In another embodiment, the pistons are driven
by high and low pressure gases alternately applied from respective
reservoirs to opposite sides of the reversing piston through valves
operated by the reversing piston. The modified Stirling cycle
engine is capable of unattended operation over a period of years in
an inaccessible location and therefore, is especially well suited
for supplying motive power for an artificial heart-pumping
system.
Inventors: |
Buck; Keith E. (Alamo, CA) |
Assignee: |
|
Family
ID: |
24991865 |
Appl.
No.: |
04/744,204 |
Filed: |
July 11, 1968 |
Current U.S.
Class: |
623/3.22;
600/16 |
Current CPC
Class: |
A61M
60/435 (20210101); A61M 60/871 (20210101); F02G
1/043 (20130101); A61M 60/50 (20210101); A61M
60/148 (20210101); A61M 60/562 (20210101); A61M
60/268 (20210101) |
Current International
Class: |
A61M
1/10 (20060101); A61M 1/12 (20060101); F02G
1/00 (20060101); F02G 1/043 (20060101); A61f
001/24 () |
Field of
Search: |
;3/1,1AH ;128/1,214
;60/34,203 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
1 "The Artificial Heart-Exemplar of Medical-Engineering Enterprise"
by Nilo Lindgren, IEEE Spectrum, Vol. 2, No. 9, Sept. 1965, pages
67--83. Copy available in group 335, 3-1AH.
|
Primary Examiner: Gaudet; Richard A.
Assistant Examiner: Frinks; Ronald L.
Claims
I claim:
1. In an artificial heart system including at least one fluid
actuated artificial ventricle and a fluid-pumping means connected
to said ventricle, wherein said pumping means is provided with a
pressure input and a pumping fluid output connected to said
ventricle, a fluid pressure-operated control system for
reciprocating fluid through said pumping means to actuate the
ventricle in simulating systole and diastole, and a source of
high-pressure working gas for operating the control system; the
improvement wherein the source of high-pressure working gas
comprises a Stirling cycle engine, said Stirling cycle engine
comprising:
a cylinder having opposite end portions respectively disposed in
hot and cold zones,
a source of thermal energy operably associated with the end portion
of said cylinder disposed in the hot zone for heating the working
gas in the hot zone,
a regenerator having opposite ends respectively extending into the
opposite end portions of said cylinder within the hot and cold
zones,
a piston mounted for reciprocation in said cylinder to transfer the
working gas between said zones through said regenerator,
respective high and low pressure reservoirs of working gas
maintained at said high and low pressures,
means separately connecting each of said high and low pressure
reservoirs to said cylinder at the end portion thereof disposed in
the cold zone,
a first valve means in said means connecting said high-pressure
reservoir to said cylinder,
a second valve means in said means connecting said low-pressure
reservoir to said cylinder,
said piston being positioned in the end portion of said cylinder
disposed in the hot zone at the beginning of its stroke and said
first and second valve means being closed to prevent communication
between the end portion of said cylinder disposed in the cold zone
and said high and low pressure reservoirs respectively,
said first valve means opening in response to the movement of said
piston toward the end portion of said cylinder disposed in the cold
zone increasing the pressure of the working gas in said cylinder to
a magnitude at least equal to the pressure of the working gas in
the high-pressure reservoir for admitting working gas into the
high-pressure reservoir from said cylinder until the end of the
piston stroke,
said piston in its return stroke moving toward the end portion of
said cylinder disposed in the hot zone so as to decrease the
pressure of the working gas in said cylinder closing said first
valve means, and
said second valve means opening in response to the continued
movement of said piston in its return stroke decreasing the
pressure of the working gas in said cylinder to a magnitude no
greater than the pressure of the working gas in the low-pressure
reservoir for drawing working gas from said low-pressure reservoir
into said cylinder,
whereby the working gas is pumped from said low-pressure reservoir
to said high-pressure reservoir which serves as the source of
high-pressure working gas;
means communicatively connecting the high-pressure reservoir to
said control system for admitting high-pressure working gas
thereinto to operate said control system, and
means communicatively connecting said control system to the
low-pressure reservoir for discharging working gas from said
control system to the low-pressure reservoir for subsequent
recycling through said Stirling cycle engine.
2. In an artificial heart system as set forth in claim 1, wherein
said piston of said Stirling cycle engine comprises a displacer
piston, and said Stirling cycle engine further includes
a reversing piston,
a connecting rod extending between said displacer piston and said
reversing piston and having its opposite ends respectively secured
thereto, and
means for alternately applying forces at the opposite ends of said
reversing piston to effect reciprocation of said reversing piston
and said displacer piston.
Description
BACKGROUND OF THE INVENTION
The present invention relates to Stirling cycle engines, and more
particularly, the invention pertains to a modified Stirling cycle
engine having a freely reciprocable displacer piston.
Characteristic of conventional engines and compressors is the
requirement that there be a number of rotating, load-bearing,
lubricated parts. Commonly, such machines are accessible for
lubrication, repair and servicing, and it is not required that they
be operated unattended over a period of years. However, it is
required in some systems, for example, in an implantable heart
system or in an outer space life support system, that a device for
supplying motive power be operated continuously over a period of
years in an inaccessible location without benefit of servicing or
repair. Conventional engines and compressors are found to be
unsatisfactory for use in these systems; the relatively large
number of parts of conventional engines and compressors increases
the possibility of a malfunction, while their seals and bearings
bear relatively high loads, are subject to wear and require
lubrication.
SUMMARY OF THE INVENTION
In brief, the present invention pertains to a Stirling cycle engine
operable as a compressor. The engine is provided with a single main
moving part that is guided in dry running bearings which are loaded
only by the weight of the moving part. More particularly, the
invention is a modification of a type of Stirling engine which
utilizes a displacer piston for reciprocating a working gas between
a hot zone and a cold zone. The hot and cold zones are separated by
a regenerator through which the working gas is moved with the
displacer piston. When the gas is in the hot zone it is heated and
expanded, thereby raising the pressure in both zones to the point
that high-pressure gas may be extracted from the engine through a
valve. During transfer of the gas from the hot zone to the cold
zone, heat is transferred to the regenerator to thereby cool and
contract the gas and lower the pressure in both hot and cold zones
to the point that low-pressure gas may be introduced into the
system through another valve. When the engine is used in a closed
system, the working gas may be recirculated, whereby the
high-pressure gas is extracted from the engine, applied to a load
where it is expanded for operation of the load, and then exhausted
from the load at a low pressure suitable for reintroduction into
the engine.
The main feature of the invention is the provision of a reversing
piston that is integrally connected to the displacer piston with a
connecting rod. Kinetic energy is extracted from the engine and
differentially and alternately applied to opposite ends of the
reversing piston for reciprocating the reversing piston and
displacer piston without resort to rotating mechanism or high load
seals and bearings. In one embodiment, the energy may be
differentially applied to the reversing piston with opposing
oscillative springs, such as coil springs, magnetic springs or
compressible gas mass springs, acting on opposite ends of the
reversing piston. Energy to sustain spring oscillations is supplied
to the springs through the connecting rod and the reversing piston
by the expanding engine-working gas acting on an area of the
displacer piston equal to the cross-sectional area of the
connecting rod where it passes through the engine wall.
Alternatively, the pistons may be driven with successive masses of
compressed gas whereby high and low pressure working gas is
alternately supplied through valves to opposite ends of the
reversing piston.
An object of the invention is to provide a Stirling cycle engine in
which the number of parts, the wear, and the need for lubrication
are minimized.
Another object is to directly drive the displacer piston of a
Stirling cycle engine with energy derived from the engine.
Another object is to drive the displacer piston of a Stirling cycle
engine with a differentially operated reversing piston that is
integral with the displacer piston.
Other objects and advantageous features of the invention will be
apparent in a description of a specific embodiment thereof, given
by way of example only, to enable one skilled in the art to readily
practice the invention, and described hereinafter with reference to
the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagram partially in cross section of a modified
Stirling cycle engine according to the invention.
FIG. 2 is a graph showing cylinder pressure as a function of
position of the displacer piston of the Stirling cycle engine shown
in FIG. 1.
FIG. 3 is a diagram partially in cross section of a second
embodiment of the invention showing a reversing piston in a first
position.
FIG. 4 is a diagram partially in cross section of the embodiment of
FIG. 3 showing the reversing piston in a second position.
FIG. 5 is a schematic diagram of a portion of an artificial heart
pumping system that is driven with a modified Stirling cycle engine
such as shown in FIGS. 1 or 3.
FIG. 6 is a schematic diagram of a control system for the
artificial heart pumping system of FIG. 5.
FIG. 7 is a cross-sectional diagram of a bellows used in the
artificial heart-pumping system shown in FIGS. 5 and 6.
FIG. 8 is a top view of the bellows of FIG. 7 with portions broken
away.
DESCRIPTION OF AN EMBODIMENT
Referring to the drawing, there is shown in FIG. 1 a modified
Stirling cycle engine 20 having a displacer piston 21 mounted for
reciprocation within a displacer cylinder 23. A connecting rod 24
has one end integrally connected to the piston 21, while the other
end extends through a bearing 22 for cooperation with a spring
system 25 which is used to oscillate the piston 21 within the
cylinder 23. Interaction between the spring system 25 and the
piston 21 is by means of a reversing piston 26 mounted on the lower
end of the connecting rod 24. The spring system 25 is comprised of
a reversing cylinder 27 within which the reversing piston 26 is
reciprocated. The cylinder 27 defines a reversing chamber 28, the
upper portion of which connects with an upper compression chamber
30, while the lower portion connects with a lower compression
chamber 31. Heat is continuously supplied to the engine 20 by means
of a heat source 33 which may conveniently be a radioisotope such
as plutonium-238. The entire engine 20 including the heat source 33
may be encapsulated in a tungsten alloy radioactivity shield which
is provided with an outer polyethylene shield to reduce the surface
neutron dose. Heat is transferred from the source 33 to a heating
chamber 34 for heating a working gas that fills the chamber 34 and
the entire interior space of the engine 20. Helium has been found
to be a suitable working gas.
For purposes of explanation, the interior of the engine 20 may be
referred to as being divided into a hot zone and a cold zone. The
hot zone includes the heating chamber 34 and the space and walls
adjacent thereto, in particular the upper portion of the cylinder
23. The cold zone includes a space 36 in the lower part of the
cylinder 23, all connecting passages thereto and adjacent engine
walls. The hot and cold zones are separated with a regenerator 37
which acts as a heat sink and permits free flow of the working gas
between the hot and cold zones. It should present a large surface
area to the gas for efficient transfer of heat between the
regenerator and the gas. The regenerator could be comprised of a
stack of steel wool, or alternatively, it could be comprised of a
bundle of thin metal straws that provide many small vertical gas
passages. The displacer piston 21 also separates the hot and cold
zones and should be fabricated of light gauge material having a low
thermal conductivity to provide a small mass and to limit heat
transfer between the two zones. Thermal radiation from one end of
the piston to the other may be limited by means of a radiation
baffle such as 35. The walls of the displacer cylinder 23, as well
as other portions of the engine, should also be made of material
having a low thermal conductivity to limit heat loss by conduction
from the engine.
The engine 20 is suitable for supplying a load 41 with successive
compressed masses of the working gas. High-pressure working gas may
be extracted from the engine 20 through a check valve 39 and then
accumulated in a high-pressure reservoir 40 from which a continuous
supply of high-pressure working gas may be applied to the load.
Energy transfer from the gas to the load causes a reduction in gas
pressure. The gas is then exhausted to a low-pressure reservoir 43
from which a continuous supply of low-pressure gas is available for
reintroduction into the engine 20 through a check valve 44.
The gas within the chamber 28 and the connecting chambers 30 and 31
is made to be equal to the mean pressure of the working gas in the
interior space of the engine 20 when the piston 26 is centralized
in the chamber 28. Upon application of heat to the chamber 34 by
means of the source 33, the mass of working gas in the hot zone is
raised in temperature, causing a corresponding rise in pressure.
Since the hot and cold zones are separated through open passages in
the regenerator 37, the pressures in both hot and cold zones are
always virtually equal. Consequently, as the pressure is raised in
the hot zone, the pressure of all of the working gas within the
engine 20 is raised. Continued heating of the working gas will
cause the pressure to rise above the mean pressure maintained
within the chamber 31 and the lower part of the chamber 28. The
rise in pressure of the working gas acts on an area of the
displacer piston 21 that is equal to the small cross-sectional area
of the connecting rod 24 where it extends through the bearing 22.
As the pressure of the working gas rises above the mean pressure, a
downward force is exerted on the displacer piston 21, driving it
and the reversing piston 26 downward, compressing the gas within
the chamber 31. The pressure of the working gas continues to rise
until it is equal to the pressure within the high-pressure
reservoir 40, at which point the check valve 39 opens for transfer
of working gas to the high-pressure reservoir 40. There will,
therefore, be no further rise in engine pressure. The compressed
gas in the chambers 28 and 31 results in a force acting upward on
the reversing piston 26 that becomes greater than the force acting
downward. The displacer piston 21 is driven upward thereby, causing
the large mass of working gas in the hot zone to be transferred
through the regenerator 37 to the cold zone. As the hot gas is
moved through the regenerator, heat is transferred from the gas to
the regenerator. Thus, the gas moved into the space 36 has been
cooled by its passage through the regenerator. Since the greatest
mass of working gas is now in the cold zone and is at a reduced
temperature, the pressure of all of the working gas is
correspondingly reduced according to the ratio P.sub.2 /P.sub.1
=T.sub.2 /T.sub.1, typical of a constant volume process. A
reduction in the pressure of the working gas to a point slightly
below that in the low-pressure reservoir 43 causes the check valve
44 to open for reintroduction of working gas from the reservoir 43
to the cold zone of the engine 20. Movement of the displacer piston
21 to top dead center is aided by the kinetic energy imparted to
the pistons 21 and 26 by the previously compressed gas in the
chambers 31 and 28. The upward movement of the reversing piston 26
causes the gas in the chamber 30 to compress so that upon the
displacer piston reaching top dead center a force is applied to the
reversing piston 26 from the compressed gas in the chamber 30,
driving the reversing piston, and therefore the displacer piston,
downward. This downward movement again transfers working gas from
the cold zone through the regenerator 37 to the hot zone. As the
gas moves into the hot zone it picks up heat from the regenerator
and is further heated by the radioisotope source. A large mass of
gas is heated thereby, causing a corresponding rise in pressure of
all the working gas. The rise in pressure of the working gas acts
on the small area of the displacer piston 21 equal to the
cross-sectional area of the portion of the connecting rod that
passes through the bearing 22. The force acting through the
displacer piston plus the force due to the compressed gas in the
chamber 30 drives the reversing piston 26 and displacer piston 21
downward. The forces acting downward, including the kinetic energy
imparted to the displacer piston 21, the connecting rod 24 and the
reversing piston 26, carry the piston 26 to the bottom dead center,
compressing the gas in the chamber 31. Upon a rise in pressure of
the working gas to that in the high-pressure reservoir, the working
gas is extracted through the check valve 39 and the cycle is
continued in the manner described hereinbefore.
It will be observed that the alternate compression and expansion of
gas in the chambers 30 and 31 constitutes an oscillating spring for
reciprocating the displacer piston 21. These oscillations are
sustained with energy supplied by the displacer piston on its
downward stroke to make up for frictional fluid and mechanical
losses. The energy for driving the displacer piston downward is
derived from the expansion of engine working gas into the increased
engine space resulting from movement of the connecting rod from the
interior of the engine. During this period the working gas has a
high average pressure. During the upward stroke of the displacer
piston, the working gas has a low average pressure. Thus, the
driving energy during the downward stroke is greater than the
energy returned to the displacer piston during the upward stroke,
the difference being available to drive the piston.
A more precise description of the operation of the engine 20 may be
had by reference to FIG. 2 which is a graph of pressure as a
function of displacer piston position. The displacer piston 21 is
shown in FIG. 1 in its top dead center position which corresponds
to point a in FIG. 2. At this point, the valves 39 and 44 are
closed. From point a the displacer piston is driven toward bottom
dead center by the force of the compressed gas in the chamber 30
and by the increasing pressure in the interior of the engine
resulting from increased gas temperature. Upon the displacer piston
21 reaching its centralized position corresponding to point b in
FIG. 2, a sufficient mass of gas will have been transferred to the
hot zone to cause a rise in pressure that opens the valve 39. From
point b to the end of the downward stroke, gas is transferred from
the engine to the high-pressure reservoir 40. The velocity of the
piston is reduced by the increasing pressure in chamber 31 until
the piston is stopped at bottom dead center, point c. From this
point, the piston is moved upward, initially by the force of the
compressed gas in the chamber 31 and also by the decreasing
pressure in the interior of the engine resulting from decreased gas
temperature. Upon the displacer piston 21 reaching its centralized
position corresponding to point d in FIG. 2, a sufficient mass of
gas will have been transferred to the cold zone to cause a pressure
decrease that opens valve 44. For the remainder of the upward
stroke gas is transferred from the low-pressure reservoir to the
engine.
It will be appreciated from the description of the engine 20 that
the displacer piston 21, the connecting rod 24, and the reversing
piston 26 constitute a single-integral part, and that this part is
guided by bearing surfaces which can experience a load no greater
than the weight of the part. The bearings, therefore, may be dry
running and they will experience little or no wear over extended
periods. High performance seals are not required since the chambers
30 and 31 are operated around the mean engine pressure and any
leakage will be balanced. It will be further appreciated that the
single moving part is self-reversing without the aid of other
moving mechanism such as cranks or wheels which would require
lubrication and are subject to higher loads and a higher degree of
wear.
A second embodiment of the invention is shown in FIG. 3 in which
the spring system 25 of FIG. 1 is replaced with a displacer piston
reversing arrangement 45 that is directly actuated with
high-pressure working gas alternately applied to opposite sides of
a reversing piston 46. The reversing piston 46 is integrally
connected to the connecting rod 24 and is mounted for reciprocation
within a reversing cylinder 47. The interior of the cylinder 47 is
separated by the piston 46 into an upper chamber 48 and a lower
chamber 50. The piston 46 is provided with valve passages 51 and 52
for alternate connection with a passage 53 from the high pressure
reservoir 40. Thus, during reciprocation of the piston 46, the
passages 51 and 52 are alternately and briefly connected to the
chambers 50 and 48 respectively to thereby force the reversing
piston first in one direction and then the other. To relieve the
pressure in the chamber opposite the one to which the high-pressure
working gas is applied, a pair of valve passages 54 and 55 are
provided in the piston 46 for alternate and brief connection
through a valve passage 56 to the low-pressure reservoir 43.
In operation, with the reversing piston 46 in the position shown in
FIG. 3, high-pressure working gas is briefly applied through the
valve passage 52 to the chamber 48, while the gas within the
chamber 50 is exhausted through the passage 55 to the low-pressure
reservoir. The differential pressures applied to opposite ends of
the reversing piston 46 cause the piston 46 and the piston 21 to be
driven downward. Upon movement of the piston 46 to the position
shown in FIG. 4, the gas in the chamber 48 has been expanded to a
pressure slightly above that in the low-pressure reservoir 43, and
the gas in the chamber 50 has been compressed to a pressure
slightly below that in the high-pressure reservoir 40. The pressure
difference between the two chambers is further increased by brief
gas flow from high-pressure reservoir 40 to chamber 50 and from
chamber 48 to low pressure reservoir 43. Differential pressures are
thereby applied to opposite ends of the piston 46 in a direction
opposite to those previously applied, thereby exerting a net upward
force on the piston 46.
Thus, as with the first embodiment, the displacer piston 21 may be
reciprocated by means of the reversing arrangement 45 using a
single moving part that may be guided with dry running bearings
that are not subject to a load greater than the weight of the part.
Similarly, there will be little wear, no high performance seals or
bearings are required, and no additional moving mechanism is
required.
For the reasons discussed, the engines shown in FIG. 1 and FIG. 3
are susceptible of unattended operation over extended periods. A
particular application of this type of engine is to transduce heat
energy to a compressed working gas for operation of a heart-pumping
system. A heart-pumping system for one artificial ventricle 59 is
shown in FIG. 5. The ventricle 59 is comprised of a flexible sac 60
mounted within a semirigid case 61. The space between the sac 60
and case 61 and the connecting passage thereto is filled with a
pumping fluid, preferably a liquid having the heat transfer
characteristics and viscosity of water but which is a nonpolar
fluid in order to minimize any reaction between the components of
the blood and the pumping fluid which may result from permeation of
the blood components through the thin flexible sac 60. Liquid
fluorocarbons or silicones have the characteristics desired for the
pumping fluid. The pumping fluid between the sac and the case is
alternately pumped into the space and removed therefrom to cause
rhythmic contraction and expansion of the sac 60. During expansion
of the sac 60 there is a diastolic blood flow through a check valve
64 into the sac, while during contraction of the sac, blood is
forced through the check valve 63 for systolic blood flow.
Reciprocation of the pumping fluid to cause rhythmic contraction
and expansion of the sac 60 is by means of a pumping chamber 66
that is connected to the case 61 through a line 67. The chamber 66
is comprised of a movable plate 68 to which three sets of bellows
69 and 69', 70 and 70', and 71 and 71' are attached. The bellows 70
receives the pumping fluid through line 67, while the bellows 70'
is directly opposite the bellows 70 and is filled with a fluid
which is maintained at ambient atmospheric pressure at all times.
This pressure is maintained in the bellows 70' by means of a
flexible sac 72 which may be surgically located in the pleural
cavity so as to be exposed to ambient atmospheric pressure at all
times. The atmospheric pressure in the bellows 70' is used in a
manner more fully described hereinafter, as a reference for the
pumping fluid in the bellows 70 so that diastolic blood flow is at
a pressure substantially equal to atmospheric pressure. Since the
veins are easily collapsed, any venous pressure less than
atmospheric could cause collapse of the veins with consequent
blocking of diastolic blood flow.
The pumping chamber 66 is actuated with high-pressure working gas
from the engine 20 under control of a control system 75. The
bellows 69 and 71 are commonly connected over a line 73 to the
control system, while the bellows 69' and 71' are commonly
connected over the line 74 to the system 75. High-pressure working
gas from the engine 20 is applied to the system 75 over a
high-pressure line 77, which line also constitutes a high-pressure
reservoir, while working gas is returned to the engine 20 from the
system 75 over a low-pressure line 78, which constitutes a
low-pressure reservoir. The control system 75 is operable for
alternately connecting the lines 73 and 74 to the high and low
pressure lines 77 and 78 at a rate equal to the natural pulse rate.
Differential pressures are thereby applied to the plate 68 through
the bellows 69 and 69', and the bellows 71 and 71' to cause the
plate to reciprocate leftwards and rightwards. This causes the
pumping fluid in the bellows 70 to be reciprocated through the line
67 to actuate the artificial ventricle 59.
Excessive heat may be removed from the working gas by transferring
it to the blood stream. However, care must be taken not to overheat
the blood and thereby raise the temperature of the recipient body
above normal levels. This may be avoided by first transferring heat
from the working gas to the pumping fluid in the line 67 by means
of a heat exchanger 80 serially connected in the low-pressure line
78. The pumping fluid preferably is a liquid having a heat capacity
and mass flow rate that is much higher than that of the working
gas. Thus, the temperature rise of the pumping fluid due to the
excessive heat in the working gas is only a fraction of a degree as
opposed to the relatively high temperature in the line 78. The heat
added to the pumping fluid is distributed to the blood-circulating
system in two ways. One is by means of the heat exchanger 81
serially connected with the line 67 with its input connected to the
line 67 and its output serially connected with the circulatory
system, for example, with an aortic bypass. The heat in the pumping
fluid is also transferred to the blood directly through the walls
of the sac 60.
It is believed to be highly desirable that an artificial heart pump
should have a stroke profile and pulse rate similar to those of the
natural heart in order to accommodate the dynamic changes in
vascular impedance. This requires that an artificial heart be
capable of pulsatile flow and be operable for varying the duration
of systole and diastole, as well as heart rate and stroke volume,
as required by the changing needs of the vascular system. A control
system to accomplish these objectives is shown in FIG. 6 which is a
diagram of portions of FIG. 5 and further including detailed
controls and pumping means for actuating both left and right
ventricles of an artificial heart.
The pumping chamber 66, described with reference to FIG. 5
previously, is shown in FIG. 6 for supplying the pumping fluid to
an artificial right ventricle, while a second pumping chamber 83 is
shown for supplying the pumping fluid to an artificial left
ventricle. The pumping chamber 83 is comprised of three sets of
bellows 84 and 84', 85 and 85', and 86 and 86' with the bellows of
each set symmetrically arranged on opposite sides of a movable
plate 87. The chamber 83 operates in the same manner as the pumping
chamber 66 described hereinbefore. A prime objective of the control
system is to simultaneously actuate the pumping chambers 66 and 83
to effect synchronous systolic and diastolic cycles simultaneously
in the artificial right and left ventricles. This is accomplished
by means of a conventional bistable fluidic amplifier used as a
fluid switch and indicated in FIG. 6 as switch 88 with its input
connected to the high-pressure working gas line 77. The
high-pressure working gas is transferred through the switch to one
or the other of a pair of output lines 89 and 90. Flow through
either one of the outputs is stable until output flow is blocked at
which time output flow from the switch is transferred to the
opposite output line.
During a systolic cycle, output of the switch 88 will be into the
line 89 which is directly connected through a check valve 91 to the
bellows 69' and 71' of the pumping chamber 66. The line 89 is also
connected to an input of a spool valve 93 so as to drive a movable
valve element 94 leftward to the position shown in FIG. 6. With the
element 94 in the position shown, a valve passage 92 in the element
connects the bellows 69 and 71 through the line 73 to the
low-pressure working gas line 78. Thus, high-pressure working gas
is applied to the bellows 69' and 71', while the bellows 69 and 71
are connected to the low-pressure line 78. The differential
pressure forces thereby applied to the plate 68 cause it to be
moved leftward to carry out a systolic cycle. Similar but larger,
differential pressure forces are applied to the plate 87 to effect
systolic actuation of the artificial left ventricle simultaneous
with actuation of the right ventricle by means of a spool valve 95
having a movable valve element 96. The output line 89 is connected
to an input of the valve 95 such that upon application of the
output of the switch 88 to the line 89, the movable element 96 is
driven leftward to the position shown in the FIG. 6. With the
element 96 in this position, the low-pressure line 78 is connected
through a valve passage 117 directly to the bellows 84 and 86,
while the high-pressure line 77 is connected directly to the
bellows 84' and 86' through a valve passage 118. The differential
pressure forces thereby applied to the movable plate 87 drive it
leftward to carry out a systolic cycle simultaneous with actuation
of the plate 68. Upon movement leftward of the plate 68 to the
limit of its travel, the gas in the line 89 can no longer expand in
the bellows 69' and 71', thereby causing a high pressure to be
built up in the line 89. This causes the output of the switch 88 to
become unstable and to switch to the line 90. The line 90 is
connected to an input of each of the valves 93 and 95 such as to
drive the movable elements 94 and 96 fully rightward. The high
pressure in the line 90 is also connected through a check valve 98
to the bellows 69 and 71, while the bellows 69' and 71' are
connected through a valve passage 99 and a throttle valve 100 to
the low-pressure line 78. The differential pressure forces thereby
applied to the plate 68 drive it rightward to actuate the
artificial right ventricle to carry out a diastolic cycle. A
diastolic cycle is simultaneously carried out with respect to the
artificial left ventricle by means of the pumping chamber 83
whereby the movable plate 87 is driven rightward by differential
pressure forces applied thereto. The high-pressure line 77 is
connected directly to the bellows 84 and 86 through a valve passage
101 in the element 96, while the bellows 84' and 86' are connected
to the low pressure line 78 through a valve passage 102 and a
throttle valve 104. The diastolic cycle is continued until the
movable plate 68 reaches its end-of-travel at which time a higher
pressure is built up in the line 90 causing the output of the
switch 88 to switch to the line 89 to initiate a systolic cycle.
Systole and diastole are thereby rhythmically and synchronously
accomplished with the described system.
In order to increase or decrease blood pumping rates in response to
physiological needs, the pumping chambers 66 and 83 are made
responsive during diastole to respective atrial pressures. This is
accomplished with the throttle valves 100 and 104 being placed
respectively under control of right and left atrial pressures by
means of pressure sensors (not shown) surgically attached to the
right and left atriums. The rate of diastolic exhaust from the
pumping chambers may thereby be made to conform to the current
physiological need as exhibited by atrial pressures.
Because of the relaxed condition of the pumping chambers 66 and 83
during diastole, especially during very relaxed bodily activities,
it is desirable to ensure initiation of systole after a
predetermined period even though the pumping chamber 66 has not
completed its diastolic movement. This may be accomplished by
applying high-pressure working gas from the line 77, through the
spool valve passage 97, to a fluidic capacitor 105 during systole.
The capacitor 105 is thereby normally maintained at a high
pressure; its output is applied to the main input of a monostable
fluidic switch 107 through a restrictor 108; and the output is also
applied through a second restrictor 109 to a switching input of the
switch. The switch 107 has an unstable output connected to the
low-pressure line 78 and a stable output connected over a line 111
to a switching input of the switch 88. Under normal conditions, the
high-pressure gas applied through restrictor 109 to the control
input of the switch 107 causes the main input to the switch through
the restrictor 108 to be directed to the low-pressure line 78.
Normally the capacitor 105 has a sufficient capacity to maintain
the output of the switch 107 directed to the line 78 throughout
each diastolic cycle. However, should the diastolic cycle not be
terminated by the end of a predetermined period, the pressure in
the capacitor 105 becomes insufficient to maintain the output of
the switch 107 directed to the line 78. This causes the output of
the switch 107 to be directed over the line 111 to the switching
input of the switch 88 to ensure that its output is into the line
89 to initiate systole and thereby maintain the minimum desired
pulse rate.
The provision of right and left pumping chambers 66 and 83, in the
system described with reference to FIG. 6, is particularly
advantageous since such an arrangement permits synchronous pumping
of respective right and left artificial ventricles and yet it still
permits independent control of the ventricles with respect to blood
flow rates. The bellows of the chambers also provide a convenient
interface between the gaseous portion of the system and the liquid
portion, wherein the gaseous system is preferable for energy
transfer from the heat source and for operation of the control
portion of the system, while the pumping liquid is preferable for
its optimum heat transfer characteristics and as a means for
positively actuating the artificial ventricles. The particular
bellows arrangement further provides convenient means for
maintaining venous pressures substantially at ambient atmospheric
pressure during diastole to prevent collapse of the veins as
mentioned hereinbefore. The chambers 66 and 83 may be made
substantially identical. The chamber 66 is shown in greater detail
in cross section in FIG. 7. The chamber 66 includes a housing 113
in which the bellows 69, 70 and 71 are mounted on one side of the
plate 68 while the bellows 69', 70' and 71' are mounted on the
opposite side. A top view of the pumping chamber 66 is shown in
FIG. 8 to better illustrate the relative sizes and positions of the
bellows. The housing 113 is filled with a gas having a pressure
that is always higher than the internal pressure of any of the
bellows in order to prevent bellows squirm. A chamber 115 is
centrally located within the bellows 70 and a vacuum atmosphere is
provided within the chamber during its manufacture. The chamber 115
is required to provide a reduced area on the plate 68 on which the
pressure within the bellows 70 acts as compared with the area on
the side of the plate 68 on which the atmospheric reference
pressure acts. This arrangement provides a compensating force that
counteracts the mean arterial hydrostatic pressure which is
transmitted through the pumping fluid to bellows 70. The
compensating force causes centering of the plate 68 when the
bellows 70 is subjected to mean arterial pressure. This permits the
application of equal gas pressure differentials on each side of the
plate during systole and diastole. The throttle valve control
system 100 ensures that no significant vacuum (suction) is
transmitted by the bellows 70 to the corresponding atrium during
diastole, and further ensures that the rate of movement of plate 68
is determined by venous pressure. Thus, during diastole, the rate
of movement of plate 68 is determined by venous pressure with
respect to atmospheric. This corresponds to the natural heart
function whereby the ventricles relax during diastole to be filled
under venous pressure.
While an embodiment of the invention has been shown and described,
further embodiments or combinations of those described herein will
be apparent to those skilled in the art without departing from the
spirit of the invention or from the scope of the appended
claims.
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