U.S. patent number 3,919,722 [Application Number 05/338,611] was granted by the patent office on 1975-11-18 for totally implantable artificial replacement heart.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the. Invention is credited to Lowell T. Harmison.
United States Patent |
3,919,722 |
Harmison |
November 18, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Totally implantable artificial replacement heart
Abstract
A totally implantable replacement heart system comprises a blood
pump, energy converter, heart rate control computer and an energy
storage unit. The blood pump is a four-chambered unit having two
reservoirs or atria with two large valves forming the inflow to the
pumping chambers or ventricles, and tricuspid pulmonary and aortic
valves. The artificial heart has been designed to adapt to
different types of internal power systems (electrical, nuclear, or
pneumatic). The heart rate control computer utilizes stroke volume
as one of the principal control parameters. This control system is
always seeking to converge on a rate that maintains stroke volume
at full value or at a predetermined high percent of full stroke
volume. A key part of the control logic is the automatic override
of heart rate from the right heart thereby providing sensitivity to
central venous conditions.
Inventors: |
Harmison; Lowell T. (Bethesda,
MD) |
Assignee: |
The United States of America as
represented by the Secretary of the (Washington, DC)
|
Family
ID: |
26656458 |
Appl.
No.: |
05/338,611 |
Filed: |
March 6, 1973 |
Current U.S.
Class: |
623/3.16 |
Current CPC
Class: |
A61M
60/40 (20210101); A61M 60/89 (20210101); A61M
60/538 (20210101); A61M 60/196 (20210101); A61M
60/876 (20210101); A61M 60/515 (20210101); A61M
60/871 (20210101); A61M 60/268 (20210101); A61M
60/50 (20210101); A61M 60/424 (20210101); A61M
60/894 (20210101); A61M 60/873 (20210101); A61M
60/859 (20210101); F02G 1/043 (20130101); A61M
60/443 (20210101); A61M 60/122 (20210101); A61M
60/562 (20210101); F02G 2280/005 (20130101); A61M
2205/8243 (20130101); A61M 2205/3334 (20130101); A61M
60/148 (20210101); A61M 60/857 (20210101) |
Current International
Class: |
A61M
1/10 (20060101); F02G 1/00 (20060101); F02G
1/043 (20060101); A61M 1/12 (20060101); A61F
001/24 () |
Field of
Search: |
;3/1,DIG.2,1.7
;128/1D,DIG.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Frinks; Ronald L.
Attorney, Agent or Firm: Browdy and Neimark
Claims
What is claimed is:
1. A totally implantable replacement heart comprising:
a blood pumping means for pumping blood to and receiving blood from
the lungs, and pumping blood to the venous system and receiving
blood from the arterial system;
pump drive means for periodically activating said pumping means at
a specific activation rate;
heart rate sensing means for detecting the physiological demand of
the body for a supply of blood thereto and for providing an output
signal in response to the physiological demand, said sensing means
including detection means associated with said pumping means for
sensing when a predetermined volume has been pumped by said pumping
means during any activation thereof; and
heart rate control means, connected to said means for periodically
activating said pumping means and said sensing means, for
controlling the activation rate of said pumping means in response
to said output of said sensing means, said control means
including:
a computer means for continuously monitoring the activation rate of
said pumping means and said output signal from said sensing means,
and after every series of a predetermined number of activations of
said pumping means generating an output signal to either increase,
decrease or maintain the activation rate.
2. A totally implantable replacement heart of claim 1, wherein said
pump drive means includes a power source to be totally implanted
within the body of the recipient of the replacement heart.
3. A totally implantable replacement heart of claim 2, wherein said
power source includes a heat engine containing a radioactive
isotope heat source.
4. A totally implantable replacement heart of claim 2, wherein said
power source includes a rechargeable battery, electrically
connected to a power transfer means to be implanted in the body of
the recipient of the replacement heart for transferring electrical
energy across intact layers of skin of the recipient.
5. The totally implantable replacement heart of claim 1, wherein
all blood contacting surfaces are coated with an endothelial or
pseudo-endothelial material to permit a living surface to
develop.
6. A totally implantable replacement heart comprising:
pumping means for pumping blood consisting of:
first and second pumping chambers corresponding respectively to the
left and right ventricles of a natural heart;
first and second reservoirs corresponding to the atria of a natural
heart;
driving means for periodically compressing said first and second
pumping chambers at a predetermined activation rate;
said first and second reservoirs each having a valve therein
leading to a respective one of said pumping chambers;
each of said pumping chambers having a valve therein;
a heart rate sensing means for detecting the physiological demand
of the body for a supply of blood thereto, and for providing an
output signal in response to the physiological demand, said sensing
means including first and second detection means, associated
respectively with said first and second pumping chambers, for
detecting the extent of filling of said first and second pumping
chambers and providing an output signal each time one of said first
and second pumping chambers is filled to a predetermined extent;
and,
heart rate control means, connected to said driving means and said
sensing means, for controlling the activation rate of said driving
means in response to the output signals of said first and second
detection means, said control means comprising a computer means,
electrically connected to said driving means and said first and
second detection means, for continuously monitoring the activation
rate of said driving means and the output signals of said first and
second detection means and after every series of a predetermined
number of activations generating a control signal dependent upon
the output signals of said first and second detection means to
either increase, decrease or maintain the activation rate.
7. The totally implantable replacement heart of claim 6, wherein
said computer means further includes:
first flow determining means for determining the number of times
said first pumping chamber is filled to a predetermined extent
during each said series of a predetermined number of
activations,
second flow determining means for determining the number of times
said second pumping chamber is filled to a predetermined extent
during each said series of a predetermined number of
activations;
means for generating a control signal to increase the activation
rate when either said first or said second pumping chambers is
filled to said predetermined extent for at least a first specified
percentage of activations during said series of activations;
means for generating a control signal to decrease the activation
rate when said first pumping chamber is filled to said
predetermined extent for less than a second specified percentage of
activations during said series of activations, and said second
pumping chamber is filled to said predetermined extent for less
than a third specified percentage of activations during said series
of activations;
means for generating a control signal to maintain the activation
rate without change when said first pumping chamber is filled to
said predetermined extent for more than a fourth specified
percentage but less than a fifth specified percentage of
activations during said series of activations, and said second
pumping chamber is filled to said predetermined extent for no more
than a sixth specified percentage of activations during said series
of activations; and
means for generating a signal to increase said activation rate when
said first pumping chamber is filled to said predetermined extent
for no more than a seventh specified percentage of activations
during said series of activations and said second pumping chamber
is filled to said predetermined extent during more than an eighth
specified percentage, greater than said seventh specified
percentage of activations during said series of activations.
8. A totally implantable replacement heart of claim 6, wherein said
driving means includes a power source to be totally implanted
within the body of the recipient of the replacement heart.
9. A totally implantable replacement heart of claim 8, wherein said
power source includes a heat engine containing a radioactive
isotope heat source.
10. A totally implantable replacement heart of claim 8, wherein
said power source includes a rechargeable battery, electrically
connected to a power transfer means to be implanted in the body of
the recipient of the replacement heart for transferring electrical
energy across intact layers of skin of the recipient of the
replacement heart.
11. The totally implantable replacement heart of claim 6, wherein
all blood contacting surfaces are coated with an endothelial or
pseudo-endothelial material to permit a living surface to
develop.
12. A totally implantable replacement heart comprising:
first and second A-V valve plates having first and second
sides;
right and left atria, each consisting of a flexible cup-shaped
bladder having an inlet valve at one end thereof and sealingly
engaging said first and second A-V valve plates at the other end
thereof, thereby forming, with said first side of respectively said
first and second A-V valve plates, right and left atria
chambers;
a right ventricle consisting of a first flexible bladder, said
first bladder being closed at one end thereof and the other end
thereof sealingly engaging said first A-V valve plate on said
second side thereof;
a left ventricle consisting of a second flexible bladder, said
second bladder being closed at one end thereof and the other end
thereof sealingly engaging said second A-V valve plate on said
second side thereof;
housing means for enclosing said right and left ventricles;
said right and left ventricles each having at least one outlet
valve;
check valve means in said first and second A-V valve plates for
allowing blood flow from respectively said right atria to said
right ventricle and said left atria to said left ventricle, while
preventing blood flow from respectively said right ventricle to
said right atria and said left ventricle to said left atria, said
check valve means comprising longitudinal openings in said first
and second A-V valve plates and flexible flaps hingedly attached to
said second sides of said A-V valve plates, overlaying said
longitudinal openings, and having a shape substantially the same as
said longitudinal openings, said flexible flaps being of slightly
larger dimensions than said longitudinal openings; and
means contained within said housing means for compressing said
first and second bladders.
13. A method of controlling pumping of an artificial heart having
left and right ventricles and left and right atria comprising the
steps of:
periodically compressing the left and right ventricles at a
specific activation rate;
detecting the degree of left and right ventricle filling;
providing a first output signal each time the left ventricle
receives a predetermined amount of blood;
providing a second output signal each time the right ventricle
receives a predetermined amount of blood; and
controlling the activation rate by analyzing said first and second
output signals during a period corresponding to a predetermined
number of activations in order to vary the activation rate at the
end of said period dependent upon the number of said first output
signals and the number of said second output signals.
Description
FIELD OF THE INVENTION
The present invention relates essentially to an artificial heart
and, more pertinently, to a totally inplantable artificial heart
designed and constructed for disposition with a human or animal
body as a complete replacement for the natural human or animal
heart.
BACKGROUND OF THE INVENTION
Of the vital organs, the heart has the most elementary function
since it acts simply as a pump. The heart is a four-chambered
device made up of two pumps, called ventricles, and two reservoirs,
called atria. The right heart pumps the venous blood to the lungs
at low pressure (20 mm Hg) where carbon dioxide is released and the
blood oxygenated. The left heart pumps the oxygenated blood to the
arterial system at high pressure (120 mm Hg). The mean blood flow
through the right and left heart is typically 6 liters/minute. The
left ventricle does about four times the work of the right
ventricle. The mechanical pumping power of the heart usually ranges
between 1.5 and 4 watts. (These power levels are over 10,000 times
higher than those required for cardiac pacing.)
The heart is an extremely complicated pump that must adapt to a
wide range of requirements to provide adequate blood flow to
satisfy physiologic needs. To meet these requirements, from sleep
to peak exercise, the heart must provide the flow necessary through
changes in rate and/or stroke volume.
The value and importance of the instant invention can be readily
understood by considering a patient afflicted with America's number
one killer, cardiovascular disease, who must be fully or partially
confined to a bed or wheelchair by virtue of his ailing heart. Such
a patient may undergo surgery in which the patient's heart would be
completely removed, and the instant invention mounted inside the
patient's body as a replacement or substitute for the removed
heart. Thereafter that patient, whose span of life prior to the
surgery was highly questionable and in any event of relatively
short duration, should assume a substantially normal as well as
active life, not only of an ambulatory character but even to the
extent of participating in physical sports and games, and maintain
that active life for an indefinite period. In other words, the
instant invention is designed to indefinitely prolong the lives of
patients, human or animal, which would otherwise be materially
foreshortened by virtue of heart trouble.
In the past artificial hearts designed to provide an actual
substitute for the entire human or animal heart have derived their
power from a source of energy carried by the user, externally of
the body. In order to connect the external power source to the
transplanted heart an orifice must be made in the body of the user.
The creation of the orifice and the insertion of the connecting
element results in discomfort and possible irritation or infection
to the user.
It is well known that the natural heart pumps less blood during
rest than during exercise and accomplishes this by changing its
frequency or number of beats, and the blood output per beat. The
speed of operation of prior artificial hearts has been controlled,
when a change in physical activity is contemplated, by the natural
adjustment of a rheostat or equivalent element which varies only
the pumping frequency while keeping the blood output per beat
substantially constant. Thus the range of operation of prior
artificial hearts is not quite as great or as flexible as that of
the natural heart from complete rest to very violent exercise.
SUMMARY OF THE INVENTION
The shortcomings of prior art artificial replacement hearts are
satisfactorily overcome by the present invention. An object of the
present invention is thus to overcome the defects of the prior art
such as indicated above.
With the foregoing in mind, it is an important object of the
instant invention to provide a totally implantable artificial heart
capable of being mounted in the chest cavity of a human or
animal.
Also, a principal object of this invention is a totally implantable
artificial heart capable of functioning as a complete replacement
for a human or animal heart, and which may automatically vary the
frequency of beats and the stroke volume or output per beat.
Another object of the present invention is an implantable energy
system that would accept power from an internal storage unit and
convert the energy into useful driving energy for squeezing the
chambers of the total heart in a controlled fashion.
Another object is to provide a control system that would regulate
the flow of energy from the implanted power system to the
artificial heart and provide the controlled output energy into the
blood with the proper physiologic waveforms.
Another object of the instant invention is a total artificial heart
that would accept all returning venous and pulmonary blood with any
fixed demands.
Another object of this invention is a completely implanted power
unit that would provide the necessary energy at the current,
voltage and frequency necessary for operating the total heart
without creating excessive heat.
A further object of the present invention is to provide a control
system responsive to changing physiological needs and to
instantaneous energy demands.
Yet another object is the dissipation of heat within the system and
its transfer to the surrounding biological tissue and fluid.
A still further object is the provision of adequate flow to the
pulmonary and systemic tissues without damage to the blood.
Another object of the instant invention is to provide an artificial
heart capable of developing psuedoendothelial surfaces on all its
blood contacting surfaces.
Yet another object is to provide a total heart which does not
demand blood from either the lungs or the venous tree and would be
fully responsive to the returning flow.
Another object is to provide a system which functions normally
without creating undesirable noise, vibrations, or other side
effects.
A still further object is to provide a complete heart designed to
adapt to three types of internal power control systems--electrical,
nuclear, or pneumatic.
In furtherance of these and other objects, a principal feature of
the present invention is a totally implantable replacement heart
comprising four pumping chambers, two atria and two ventricles with
an energy unit nested therebetween.
Another feature is a totally implantable replacement heart
comprising of a left and right side.
Another feature is a totally implantable replacement heart
comprising an energy converter.
A further feature of the present invention is a totally implantable
replacement heart comprising a heart rate control computer.
Yet another feature is a totally implantable artificial heart
comprising an energy storage pack.
A still further feature of the instant invention is a totally
implantable replacement heart comprising an energy recovery
unit.
The artificial heart of the present invention is similar to the
natural heart. It is a four-chambered pump having two atria with
two large valves forming the inflow to the ventricles, and
tricuspid pulmonary and aortic valves. Blood flows into the atria
through surgical quick connects that are designed to conform to the
size and shape of the natural vessels and the artificial heart. The
atria are designed to accommodate wide changes in returning venous
flow and to be responsive to intrathoracic pressure. Large
atrioventricular valves were developed to provide large flow areas,
to minimize pressure drop, to achieve rapid filling, and to
minimize low flow regions within the atria and ventricles. The
ventricles are designed to provide a uniform pumping action without
causing damage to the blood or stagnant flow regions. Also, they
are so arranged as not to permit the moving internal surfaces to
contact each other during the pumping cycle. This is done in order
to eliminate potential wear points and to permit living cellular
surfaces to be put on the internal surface of the pump. All
blood-contacting surfaces have been designed to permit a living
surface to develop, i.e., endothelial or pseudo-endothelial
surfaces. All components may well be fabricated from materials that
do not require a living surface. The compression of the ventricles
accomplished through a pressure plate bonded to the external
surface of the ventricle. The ventricular ejection volumes are
approximately equal to the natural heart so as to achieve an
operating heart beat range similar to the natural heart. The
components of the total implanted heart are individually
replaceable.
The energy converter is designed to nest within the blood pump
assembly or in the abdominal assembly and provide the controlled
power for actuating both the left and right hearts. The system
converts rotary motion into linear motion within the mechanical
energy converter to actuate each ventricle. After the actuator has
moved completely to the ejected position, it is free to return to
its original position. It will not return unless there is
sufficient blood available in the atria to flow into the
ventricles. Hence, the degree of filling or return of the actuator
is controlled entirely by the available supply of blood in the
atria on a beat-by-beat basis. This automatically varies stroke
volume from 0 to a completely full stroke. The power changes due to
filling variation are reflected back to the energy converter and to
the motor control electronics. The motor control electronics
provide variable electrical input power to satisfy the
instantaneous load conditions and maintain a constant speed which
in turn maintains the prescribed heart rate commanded by the heart
control computer. A single implanted package contains the heart
control computer, motor control circuits, batteries, battery charge
control units and sensors that monitor the degree of left or right
ventricular filling.
The artificial heart of the instant invention has been designed to
adapt to electrical, nuclear, or pneumatic internal power
systems.
The electrically powered, totally implantable, artificial heart
comprises: (a) a blood pump and energy system which are placed in
the thoracic cage; (b) a control system, power conditioning
circuitry and energy storage unit which rest in the abdominal
cavity under the diaphragm; and (c) an energy receiving coil,
placed under the skin, that receives the energy from an external
transmission unit.
The electrical power to operate this system is transmitted across
intact skin to the implanted transformer. The energy is used
directly to power the converter and/or charge the implanted
batteries. If the external power source is interrupted, the system
will automatically switch to internal battery power (up to 4 hour
discharge capacity). The complete heart system has the capacity to
satisfy all cardiac output requirements from rest to peak exercise
(15 L/min). It may be possible to consider the use of biological
fuel cells that would make the electrical system completely
implantable and self-contained.
The nuclear powered, totally implantable, artificial heart
substitutes a thermal engine with a nuclear heat source for the
energy storage unit of the electrically powered heart. Hydraulic
pressure generated by the engine is conveyed by a tube to the blood
pump and its control unit located within the chest. The heat-energy
source is about 100 grams of plutonium-238 (Pu-238) in a 3-layer
metal capsule designed and proven to withstand corrosion, high
impact and crush pressures, and cremation to insure against leakage
of the radio-isotope in an accident.
One of two nuclear thermal engines may be employed. One is a vapor
cycle steam engine, the other a modified Stirling cycle engine.
In the vapor cycle engine system, the water alternates many times
each second between the vapor and liquid phases. The fundamental
problem of valve and sliding seal leakage associated with
conventional Rankine engines is obviated by the vapor cycle engine
which does not use either valves or sliding seals. Linear motion is
obtained by means of bellows.
The vapor cycle engine can be controlled electronically via any
electromechanical actuator such as a solenoid torque motor or
piezo-electric bimorph. Pressurization of the engine is achieved by
vaporizing a single drop of water or transferring it from the
condenser to the boiler.
The modified Stirling engine is incorporated in a power source
which produces hydraulic energy which powers an automatically
controlled actuator. Heat is supplied by a radio-isotope and stored
in a molten-salt reservoir. Liquid is pumped at a pressure
difference of about 200 psig.
A displacer oscillates in a closed engine cylinder heated at one
end and cooled at the other. The annular space between the cylinder
and the displacer functions as the gas heater regenerator, and gas
cooler, with a minimum of dead volume. In place of the more
conventional power piston, the cold end of the engine is fitted
with inlet and outlet check valves which allow pressure surges
produced by the oscillating regenerator to generate a pumped gas or
liquid output. The displacer is supported at the hot end by a
flexure and driven from the cold end by a controllable displacer
drive and support system which is powered by engine pressure
pulses.
Operation as a direct pumping system eliminates the large bearing
loads. The clearance regenerator, rather than the usual displacer
with a fixed regenerator, eliminates the displacer gas seal problem
and considerably simplifies this miniature engine. In the displacer
drive a metal bellows seal hermetically isolates all moving bearing
and seal surfaces from the engine working gas, allowing latitude in
choice of bearing lubrication. The displacer support at the hot end
is a flexure which operates at a stress level sufficiently low to
ensure long life.
Pneumatic power may be supplied by a thermocompressor engine which
is a variant of the Stirling engine. The modified Stirling cycle
system of the thermocompressor engine converts heat directly to
pressurized gas by heating and cooling the working media in a
closed cycle. The gas is expanded when hot and compressed when
cold, and work is extracted since the expansion work exceeds the
compression work. The implanted thermocompressor engine converts
the heat from radiosotope source to usable energy through a working
medium.
The pressure difference in the system accomplished through a single
displacer piston chamber (the hot end temperature above
1200.degree.F and the cold end below 250.degree.F). The implanted
engine utilizes a regeneration external to the driving piston. The
displacer piston moves the gas from end to end of the cylinder
through a regenerator and heater. When the displacer piston moves
gas into the hot end of the cylinder, the gas is heated flowing
through the regenerator and heated by the hot end of the engine,
thus increasing the gas pressure.
The system employs the pneumatic power to modulate and actuate the
operation of the artificial heart. A pneumatic pump actuator
controller may be used in conjunction with the other artificial
heart components.
The thermocompressor engine utilizes a thermodynamic cycle in which
helium is alternately heated and cooled to provide pressure
fluctuations which can be utilized to drive an artificial heart.
The thermocompressor engine is coupled to the blood pump by means
of a pneumatic logic unit.
In place of these internal power control systems which are
implanted in the abdomen of the patient, the artificial replacement
heart of the instant invention may be provided with a
self-contained nuclear drive system.
The control theory of the totally implantable artificial heart
rests on the fundamental requirement that the total artificial
heart must functionally satisfy the physiologic demands for the
perfusion of the body with blood. To accomplish this, the control
system must ensure that at any given time the total artificial
heart will pump all of the blood returned to it. It must be capable
of pumping any given venous return, without demanding more than is
available, at normal venous pressure, into the pulmonary artery and
aorta at normal pressures. If systemic and pul monary venous
returns are transiently unequal, pulmonary and systemic outputs
must also be appropriately unequal. In addition, the control system
must require no intravascular transducers or sensors, must maximize
total system performance at any given power output level (for
example, during sleep, rest or exercise), and must be insensitive
to changes in system spatial orientation and attitude.
An analysis of the physiologic control mechanisms of the natural
heart was made. The physiologic cardiac control systems are various
and complex; the end result is that the natural heart can rapidly
change its output to meet a wide range of physiologic needs. In
basic terms, there are intrinsic and extrinsic mechanisms of
cardiac control. Intrinsic control results from the nature of the
heart's pressure-volume or cardiac-muscle-fiber's (tension-length)
relationship. Therein lies the basis of the Frank-Starling law,
which says that (in the absence of extrinsic influences) the heart
will increase its output in response to increased input. Extrinsic
cardiac control is the result of a complex neurohumoral system
which has the capacity to regulate heart rate, peripheral vascular
tone (arteriolar and venous), and mechanical state of the heart
muscle (e.g. position in the pressure-volume curve) in the
interests of meeting physiologic needs. In addition, there are
extrinsic and intrinsic mechanisms which "autoregulate" the
coronary circulation so that metabolic supply (fuel) satifies the
heart's metabolic need and the products of metabolism (waste) are
removed.
In a manner of speaking, the control theory for the total
artificial heart also involves both intrinsic and extrinsic
mechanisms. In this case the intrinsic control is the built in
relationship among venous return (ventricular filling), stroke
volume, and heart rate. As a result of this intrinsic control, the
extrinsic physiologic control mechanisms can still influence
recipient. Direct input to the total artificial heart from the
central nervous system is of course absent. Cardiac output will,
however, increase in response to the peripheral neurohumoral
effects of the psychic stimulus: general adrenegic levels increase
as the sympathetic nervous system and adrenal medulla are
activated; arteriolar tone increases to increase systemic pressure;
venous capacitance volume decreases, resulting in increased venous
return. Cardiac output is increased as the total artificial heart
responds by pumping all of the increased return (through automatic
increase in stroke volume and heart rate) into a higher systemic
pressure; more pressure-volume work is done, the additional power
for which is instantly available.
In short, by being intrinsically controlled by venous return, and
by having large amounts of power instantaneously available to it,
the total artificial heart in effect is completely controlled by
physiologic demands. This control theory has been demonstrated to
be hemodynamically stable and completely responsive to
physiological needs in multiday experiments and under a variety of
induced cardiovascular conditions. Further experimental study is
needed to establish, under chronic conditions, the physiological
effectiveness of this theory.
Two Heart Control Computers have been developed for controlling the
complete heart. One is an analog system and the other a digital
system. The control philosophy utilizes stroke volume as one of the
principal control parameters. When blood flow requirements, either
pulmonary or systemic, increase and full stroke volumes are being
used, the beat rate of the heart will automatically be increased
until the stroke volume decreases to less than full stroke. When
this condition is attained, the heart rate is decreased to
establish a balance between the number of full and partially full
stroke volumes. System convergence is achieved because the totally
implanted heart is always seeking to restore and maintain full or
near full stroke conditions within either the left or right
ventricle. Any combination of conditions involving varied stroke
volume or heart rate in either the left or right ventricle which
could potentially produce divergent instability (i.e. extreme
tachycardia or bradycardia) has been eliminated.
Experimental studies confirmed the computer control approach in
which the heart automatically (a) increases its rate if all the
blood returning to either the left or right heart cannot be pumped
through increase of stroke volume to the limit; (b) decreases its
rate if the blood returning to either the left or right heart is
low and requires only a very limited number of full strokes to pump
all of the returning blood; and (c) continues at a given rate if
the number of full strokes is approximately the same as the number
of non-full strokes. This control logic operates on a convergency
principle. The system is always seeking to converge on a rate that
maintains stroke volume at full value or at a predetermined high
percent of full stroke volume.
The principal input into the heart control computer is full stroke
signals from both left and right ventricles. The computer
continuously monitors beat rate and stroke volume for each beat and
performs an analysis based on the immediately preceeding series of
heart beats, e.g. every five preceeding heartbeats. One of the
following conditions will prevail and produce the resulting action
based on a 5 heart beat analysis:
a. If either the left or right ventricle has taken a sufficient
number of strokes, e.g. four or five full strokes, the computer
will increase heart rate, e.g. by five beats per minute and
automatically sample the next series of beats;
b. If the left ventricle has taken two or three full strokes and
the right ventricle has had no more than three full strokes, the
computer will signal to hold the heart rate constant, e.g. for the
next five or ten seconds;
c. If the left ventricle has taken none or one full stroke and the
right ventricle had no more than three full strokes, the computer
signals the heart to decrease its heart rate, e.g. by five beats
per minute and hold for five or ten seconds;
d. If the left ventricle has taken three or less full strokes but
the right ventricle has had four or five full strokes, the computer
increases the rate, e.g. by five beats per minute. (A key part of
the control logic is this automatic override of heat rate from the
right heart providing sensitivity to central venous
conditions.)
The Heart Control Computer (HCC) automatically regulates heart rate
over a wide range, e.g. from approximately 50 to 200 beats per
minute in response to physiologic needs. Sensors located in the
heart but not inside the ventricle provide an output signal each
time a ventricle (left or right) receives sufficient blood to
achieve a full stroke. No sensors in the blood stream or tissue are
required. The computer stores information on recent blood pump
filling history and processes it every beat or multiple beats, e.g.
every 2 to 5 seconds (depending upon heart rate) to provide the
control previously described. All numerical constants can be
altered to either increase or decrease sensitivity of the Heart
Control Computer. The above constants have proven to be very
effective in achieving the appropriate degree of sensitivity and
responsiveness to all normal and abnormal (drug induced)
physiological conditions. The complete HCC can be reduced to the
size of a dime.
A number of possible control methods and approaches were considered
in arriving at the control system. They are:
a. Continuous trend method--as the stroke volume increased or
decreased, the heart rate would increse or decrease on a percentage
basis for each incremental change in stroke volume on a
beat-by-beat basis with both right and left ventricles being
analyzed for change;
b. Rate of filling method--as changes occur in filling rate and in
the percentage of the stroke volume, the heart rate would be
increased or decreased;
c. Complete stroke volume method--when either ventricle becomes
filled, the heart automatically goes into systole ejecting all of
the blood in each ventricle, or the ventricle that is filled ejects
and the other ventricle does not eject until it is completely
filled; however, the ventricle ejecting first cannot eject a second
time until the second ventricle has completed its ejection. Between
these two limits, there are a variety of synchronous and
asynchronous conditions that can be established.
None of the above approaches require any instrumentation or sensors
to be placed in the blood stream, they are not sensitive to
movement or to any spatial arrangement. These concepts permit
control of rate and stroke volume with electrical, hydraulic or
pneumatic logic circuitry. This permits the nuclear heart systems
to function without the generation of electrical energy for
control. The totally implanted heart system under animal evaluation
utilizes the more simplified concept of control that monitors the
condition in each ventricle based on percentage of full stroke as
previously discussed. When the system has the capacity to
automatically vary stroke volume and rate, it permits determination
of the physiological trends and requirements very accurately and
with the appropriate sensitivity so that the total heart
automatically increases, decreases, or holds its rate constant
based on physiological demand to provide a stable control
system.
Satisfactory performance physiologically is dependent upon having
surfaces that do not shed emboli or thromboses, or cause damage to
the blood during pumping. Toward this goal, all blood contacting
surfaces have been designed to permit the development of living
endothelial or pseudoendothelial type surfaces. All surfaces of the
pump develop a smooth glistening living interface surface from the
blood stream. Early in the development program, some difficulty
existed with regard to flow and thrombosis formation; however, in
later experiments involving changes in fabrication, these problems
have been minimized. No significant adverse physiologic effects
have occurred, i.e., peripheral emboli and body damage. The
leaflets of the valve, both atrial and tricuspid, remain very
flexible and responsive to dynamic flow conditions without
increasing changes in pressure drop. Based upon the accumulated
experience, prolonged total cardiac replacement may be
achieved.
The report by the instant inventor, Lowell T. Harmison, entitle
"Totally Implantable Nuclear Heart Assist And Artificial Heart",
National Heart And Lung Institute (1972), is hereby incorporated by
reference into the present specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a perspective view of an electrically powered, totally
implantable artificial heart.
FIG. 1 is a perspective view of the artificial heart of FIG. 1a
implanted in a human body.
FIG. 2 is a perspective view of a nuclear powered, totally
implantable artificial heart and its location within the human
body.
FIG. 2a is a partial cutaway view of the nuclear powered artificial
heart of the present invention.
FIG. 3 is a perspective view of the artificial heart of FIG. 3a
implanted in a human body.
FIG. 3a is a broken away view of a totally implantable artificial
heart with a self-contained nuclear drive system.
FIG. 4 is a top view of the totally implantable artificial
heart.
FIG. 5 is an elevational view of the left side of the artificial
heart shown in FIG. 4.
FIG. 6 is an elevational view of the right side of the artificial
heart shown in FIGS. 4 and 5.
FIG. 7 is a cross-sectional view of the artificial heart taken
along line B--B of FIG. 5 and along line A--A of FIG. 6.
FIG. 8 is an exploded view of the total heart assembly.
FIG. 9 is a block schematic diaphragm of the electrically powered
total heart system shown in FIG. 1.
FIG. 10 is a block schematic diagram of the heart rate control
computer for the totally implantable artificial heart.
FIG. 12 is a series of graphs showing the relationship of venous
return flow; heart rate, and residual atrial volume to time.
For a better understanding of the invention a possible embodiment
thereof will now be described with reference to the attached
drawing, it being understood that this embodiment is to be intended
as merely exemplary and in no way limitative.
DETAILED DESCRIPTION
Referring to FIG. 1, the totally implantable artificial heart 101
is shown in place in the thoracic cage of a human as a replacement
for an ailing natural heart. The electrical storage unit 105 is
located in the abdominal cavity under the diaphragm and is
connected to the artificial heart by means of an electrical cable
102. Electrical energy from the outside of the body is transmitted
by an external transmission unit 103 across intact skin to an
energy receiving coil 104 placed under the skin. The external
transmission unit 103 may be supplied with power from batteries
carried in a vest or attached case (See also FIG. 9). The need for
external recharging may be eliminated through the use of bio-fuel
cells. FIG. 1a shows the artificial heart 101, the electrical
storage unit 105 and the transmission unit 104 in more detail.
FIG. 2 shows a nuclear powered artificial heart system. The total
artificial heart 201 is connected to a nuclear engine 203 located
in the abdominal by a hydraulic cable 202. As shown in FIG. 2a the
nuclear engine 203 comprises a control housing 204 and a thermal
energy storage unit 205. Contained within the thermal energy
storage unit 205 is a plutonium fuel capsule 206. Vacuum foil
insulation 207 is disposed between the control housing 204 and the
thermal energy storage unit 205.
FIG. 3a shows a totally implantable artificial heart 301 with a
self-contained nuclear drive system 302. As can be seen, the
approach eliminates the necessity of an energy source located in
the abdominal cavity. FIG. 3 shows the artificial heart of FIG. 3a
in place in the body of the recipient.
Referring now to FIGS. 4-8, the total artificial heart 1 basically
comprises a left atrium 60, a left atroventricular (A-V) valve
plate 7, a left ventricle 61, an energy converter 54, a right
ventricle 51, a right A-V valve plate 55, and a right atrium 50
(See FIG. 7).
The left atrium 60 comprises a flexible cup-shaped bladder and an
open-ended branch-off 62 for receiving arterial return from the
lungs. The open end of the bladder faces towards the center of the
artificial heart 1 and forms a complete enclosure about the
exterior surface of the left A-V valve plate 7.
The left A-V valve plate 7 is a rigid circular disk comprising a
plurality of longitudinal openings 63 (See FIG. 8). Each
longitudinal opening 63 is overlaid with flexible flaps of A-V
valves 64. Metal stays are embeded in the A-V valves 64 which are
hingedly attached to the interior surface of the left A-V valve
plate 7. The A-V valves 64 are substantially the same shape as the
longitudinal openings 63 and when in the closed position completely
cover the longitudinal openings 63.
The left ventricle 61 also comprises a flexible cup-shaped bladder
and an opened-ended branch-off 65 for supplying arterial flow to
the body. The open end of the bladder faces outward from the center
of the artificial heart 1 and forms a complete enclosure about the
interior surface of the left A-V valve plate 7. Disposed within the
open end of the branch-off 65 is a tricuspid valve 66. The
tricuspid valve 66 is provided with three slits or channels in its
upper surface to allow for the passage of blood therethrough (See
FIG. 8). The left ventricle housing 3 is sleeve-shaped so as to
receive the left ventricle 61 and includes an externally threaded
outer end 67. The externally threaded outer end 67 mates with an
internally threaded, O-shaped, left atrial clamp ring 5.
The left atrial clamp ring 5 fits over the left atrium 60 and when
screwed onto the left ventricle housing 3 secures the left A-V
valve plate 7 between the left atrium 60 and the left ventricle 61.
The union between the ventricle housing and the atrial clamp ring
is seated by an atrium washer 4.
The left ventricle housing 3, like the right ventricle housing 2,
is provided with an aperture in its lower surface. A tubular
CO.sub.2 puncture housing 43 is inserted into the aperture in the
ventricle housing and a self-sealing washer 53 is inserted therein.
This arrangement permits a hyperdermic needle or similar device to
communicate with the interior of the ventricles and thereby
provides a means for the ingress or egress of fluids thereto. A
washer 45 and top cover 44 are employed to cap the CO.sub.2
puncture housing 43 and the self-sealing washer 53. (See FIG.
7).
Bonded to appropriately the lower 3/4 of the outer surface of the
interior side of the left ventricle 61 and right ventricle 51 are
compression plate assemblies 70 and 10, respectively.
An energy converter 54 is disposed in the center of the artificial
heart. The energy converter 54 which converts electrical energy to
mechanical energy is employed to actuate any suitable driving means
which in turn moves the compression plates 70 and 10 outward. The
outward movement of the compression plates need not be simultaneous
as shown in FIG. 7 where the right ventricle 2 is shown being
compressed by the outward movement of the right compression plate
10 while the left ventricle 61 and the left compression plate 70
are shown in a non-compressing state. Once the compression plate
assemblies have been driven outward and the ventricles compressed,
a reset spring 52 or similar device is used to return the
compression plate assemblies to their original positions.
The energy converter 54 is enclosed within the left ventricle
housing 3 and the right ventricle housing 2. The housings are
secured together by means of a "V" clamp 11. The V-clamp comprises
two jaws which are pivotally joined to a third connecting section
by hinges 12,12 and 13,13 and dowel pins 16. The open ends of the
jaws may be joined and tightened by means of angle brackets 15 and
nut 14 (See FIG. 5).
Electrical energy is supplied to the energy converter from either
an electrical energy storage unit or a nuclear engine located in
the abdominal cavity by a multi-stranded wire 23. The
multi-stranded wire 23 which is enclosed within an insulated sleeve
17 communicates with the energy converter 54 through corresponding
apertures in the V-clamp 11 and the bottom of the joined left
ventricle housing 3 and the right ventricle housing 2 (See FIG. 7).
A sliding bushing 22 is disposed in the aperture in the V-clamp 11
and acts as a sleeve for the multi-stranded wire 23 passing
therethrough. Just prior to its entrance into the sliding bushing
22 the multi-stranded wire 23 passes through a dome-shaped clamp
bushing 21 which caps the aperture in the V-clamp 11.
Of course, the use of a bio-fuel cell or an artificial heart with a
self-contained nuclear power source would eliminate the necessity
of the separate electrical energy storage unit of nuclear engine
and the interconnecting cable.
The right side of the artificial heart 1 is similar to the left
side except that, instead of having a single branch-off 62 like the
left atrium 60, the right atrium has a dual branch-off. The dual
branch-off comprises one branch 68 which receives venous return
from the trunk and another branch 69 which receives venous return
from the head. The right atrium may also have single inflow similar
to that shown for the left atrium 60.
The right ventricle 51, like the left ventricle 61, has a simple
branch-off 71. The branch-off 71 which supplies venous flow to the
lungs also comprises a three channeled tricuspid valve.
The tricuspid valves are housed in metallic cylindrical tricuspid
valve housing 25,25 which are disposed at the ends of the
branch-offs 65 and 71. The tricuspid valve housings 25,25 are
secured to the branch-offs by a spacer ring 26 and clamp ring 27
(See FIG. 5). All of the valves in the artificial heart are
interchangeable and may be of different types, e.g. tricuspid,
disc, ball, and flap.
Metallic cylindrical coupling rings 33 are attached to the open
ends of the branch-offs 62, 68 and 69. Disposed over the outer ends
of the coupling rings 35 and the tricuspid valve housings 25,25 are
cylindrical Dacron flocked metallic quick connects 30. The quick
connects 30 comprise multi-fingered retainer means 28 which secure
the quick connects to the tricuspid valve housings 25,25 or the
coupling rings 33. Tissue grabbing barbs 34 are disposed on the
outer surface of the quick connects 30.
Tubular elbow adapts 31 may be disposed within the surgical quick
connects 30, if necessary to enhance the tissue connection. A
protecting ring 32 encircles the outer diameter of the end of the
elbow adapt which is not disposed within the surgical quick
connects 30 (See FIG. 5). The artificial heart may also be
connected to the appropriate flow channels of the body via
connectors sutured to the tissue.
The atria, ventricles, and A-V valves, may be fabricated from
Silastic or polyurethane or other similar material. The ventricle
housings, atrial clamp rings, V-clamp assembly, surgical quick
connects and tricuspid valve housings may be made of stainless
steel or other similar material. The A-V valve plates may be
manufactured out of any suitable metal. All blood contacting
surfaces are flocked with Dacron or a like material to promote the
development of a living surface, i.e. endohelilial or
pseudo-endothelical surfaces.
The basic assembly procedure for the total heart comprises the
following steps:
1. the tricuspid valve housings are clamped and bonded to the left
and right ventricles;
2. surgical quick-connect adapter rings are bonded to the inlets of
the left and right atria;
3. the right and left ventricles are positioned in their respective
metal housings;
4. the right and left A-V valves are located onto their respective
ventricles; and
5. the right and left atria are positioned and the atrial clamp
rings are tightened down. The following is one example of a heart
rate control computer operative in the present invention.
The Heart Rate Control Computer illustrated diagramatically in FIG.
10 is contained within the implantable electronics package and is
constructed on circuit board.
The Heart Rate Control Computer regulates heart rate over a range
from 50 to 200 beats per minute in response to signals indicating
ventricular filling. A sensor is arranged to provide an output
signal each time a ventricle receives sufficient blood to fill it
to 95 percent or more of its capacity. The computer stores this
information on recent blood pump filling history and processes it
to derive one of the three following commands.
1. The computer commands a five beat per minute increase in rate if
left or right side gets five full indications out of five.
2. The computer commands a five beat per minute decrease in rate if
the rate has not changed for 10 seconds and left side has at most
one full indication out of five.
3. The computer commands that there be no change in heart rate if
neither increase or decrease criteria are satisfied.
Referring to the block diagram of FIG. 10 after the start of the
computer cycle when a Fill Detector output exceeds the threshold of
the corresponding Fill Pulse Generator, a pulse is transmitted to
the Fill Counter. This will occur whenever the corresponding
ventricle is at least 95 percent full.
The Count Detectors determine if either of the Fill Counters has
reached a count of five, or if the left count is less than two.
When the Reset Pulse occurs, the Rate Change Calculator logic
circuits determine if a rate change is required. If either 5 Count
Detector has an output, the Rate Change Calculator emits a rate
increase pulse, which increases the value in the Computer Heart
Rate Storage by an amount equivalent to about 5 beats per minute.
If the 0 or 1 Count Detector has an output and the 10 Second Delay
Pulse Generator has no output, the Rate Change Calculator emits a
rate decrease pulse, which decreases the stored value of computer
heart rate by five beats per minute. The 10 Second Delay Pulse
Generator will have an output if either a rate increase or decrease
pulse has been emitted within the last 10 seconds. Therefore, after
a heart rate change, the heart rate will not decrease for at least
ten seconds.
The value in the Computer Heart Rate Storage determines the speed
at which the Motor Drive Control Circuits will try to drive the
motor in the Energy Converter. The actual heart rate is
proportional to motor speed. The Rate Control Error Generator
therefore compares the Motor Speed Calculator output to the
Computer Heart Rate Storage to determine the error signal input to
the Motor Drive Control Circuits.
The Motor Speed signal is also used to drive the Heart Rate
Counter. When the Heart Rate Counter reaches a count of five beats,
a Reset Pulse is emitted by the Reset Pulse Generator. This resets
all the counters and triggers the Rate Change Calculator outputs.
The computer cycle then begins again.
Since extremely low power operational amplifiers are now available,
it was decided to use analog, rather than digital circuitry in the
Rate Computer.
In the Fill Pulse Generators, linear amplifiers amplify the signals
from the strain gage Fill Detectors. Their outputs are applied to
threshold amplifiers. The gains of linear amplifiers must first be
adjusted to compensate for differences in strain gage sensitivity.
Then a right and left thresholds, as determined by the voltage
divider, must be adjusted to produce Fill pulses at the point where
the blood pump is 95 percent full. The two 150 ms one-shot
multivibrators provide Fill Pulse Generator outputs of constant
width to drive the Fill Counters.
The Fill Counters are simply analog integrators. The integrators
drive threshold circuits, which perform the Count Detector
functions. The logical operations of the Rate Change Calculator are
performed by diodes which drive two amplifiers.
The 10-Second Delay Generator is a one-shot multivibrator circuit
which is reset each time it receives an input, whether or not it
has completed a count. The 10 ms delay multivibrator associated
with this circuit eliminates a race condition in the logic
circuit.
The Computer Heart Rate Storage is performed by an analog
integrator, which also includes limiting diodes to control the
extremes of the heart rate. Provision has been made for manual
adjustment of heart rate as determined by this circuit. The manual
rate is obtained either by operation of a toggle switch in the Test
Point Box, or by operation of an internal magnetic switch by means
of a large magnet outside of the electronics package.
The Rate Control Error Generation is performed by a differential
amplifier, which compares the output of the Heart Rate Storage to
the Motor Speed Signal, and drives the Drive Pulse Generator.
The Heart Rate Counter is an analog integrator operating on the
Motor Speed signal. When the integrator indicates that the time of
five heart beats has elapsed, a threshold amplifier fires and
triggers the 15 ms delay multivibrator. This multivibrator triggers
the logic circuitry output and then fires the 60 ms one-shot
multivibrator circuit which drives the reset transistors of the
three integrating counters. The 15 ms multivibrator is designed to
free run if its input stays high, so that a lockup condition cannot
occur in the Heart Rate Counter and Reset circuitry.
In the basic motor drive control loop the phase of the moving rotor
is indicated by Hall effect device signals. These signals are
converted to drive pulses of the appropriate phase by the Drive
Pulse Generator, the drive pulse widths being controlled by the
error signal from the Rate Computer. These pulses are then
amplified in the Motor Drive Circuits and used to drive the two
phase motor winding. A signal proportioned to motor speed is
derived by circuits in the Drive Pulse Generator. This signal is
compared to the desired motor speed signal in the Rate Computer to
obtain the Rate Control Error signal.
In the circuitry associated with the generation of the motor drive
pulses. Amplifiers produce amplified Hall signals of the desired
amplitude and dc level. Common mode variations in the Hall device
outputs, due to electrical noise, bias variations, and thermal
variations, are cancelled out by the balanced input circuitry of
these amplifiers.
Two further amplifiers are used as unity gain inverters. Since the
Hall devices are located on the motor stator so as to produce rotor
position signals which are 90.degree. out of phase with each other,
the addition of the inverters produces signals at all four
quadrature phases.
The phases of the four Hall signals produced do not in general
coincide with the optimum phases for the motor drive pulses.
However, since these signals are nearly sinusoidal, they can simply
be combined in weighted resistive adders to produce four signals of
the desired phases. The signals are again nearly sinusoidal and
have the phases needed for the motor drive pulses.
The actual drive pulses are generated in threshold amplifiers. At
the negative input of one of the threshold amplifiers, for
instance, three signals are added:
1. The Hall signal which has a negative maximum at the time when
the phase A pulse should occur,
2. A positive going full-wave rectified signal formed from the
phase B and B Hall signals, and
3. The dc Rate Control Error signal from the computer.
The resulting waveform has a negative triangular peak at the
desired phase for the A drive pulse. A positive output pulse is
produced during the time that this signal is below the amplifier
threshold. The width of the pulse increases as the Rate Control
Error signal is made more negative. Other amplifiers operate in
similar fashion to produce the A, B, and B drive pulses.
This technique provides a smooth, reasonably linear, variation of
drive pulse width with control error signal for drive pulse widths
even up to 130.degree., so that the feedback gain of the control
loop is nearly constant over the range of drive pulse widths
normally required.
The motor drive circuits consist of two parts:
1. The power amplifiers which drive the two phase motor winding;
and
2. the analog circuits which calculate motor current and motor
speed from the motor input signals.
The Driver amplifiers are conventional, except that the amount of
drive current available is varied in proportion to the peak motor
current required, as determined by the motor current calculation
circuit. This drive level signal determines the output amplitude of
the device level amplifiers. The power output stages which follow
these amplifiers are simply switched on or off by the amplifier
output pulses, thereby applying nearly the full battery voltage
(+14V nominal) across the drive motor winding for a period of time
equal to the input pulse width from the Drive Pulse Generator. The
advantage of the current level drive control is that less current
is drawn by transistors when low output currents are required, and
therefore these amplifiers maintain a reasonable efficiency at low
drive levels.
The current calculating circuit simply amplifies the voltage drop
across the 0.0300 resistors in series with each motor winding in
balanced differential amplifiers. These amplifier outputs are then
inverted in further amplifiers, so that both positive and negative
going replicas of each current pulse are available. The outputs of
these four amplifiers are applied to a four-phase full-wave peak
detector, the output of which is smoothed in a filter-amplifier to
give the dc current signal which controls the Driver amplifier
output capability.
The voltage applied to the motor winding is also obtained through
balanced differential amplifiers. These signals are combined in RC
networks with the current waveforms and the result amplified to
obtain a replica of the motor back-EMF voltage. Since motor
back-EMF voltage is proportional to motor speed, a signal with dc
level properties to motor speed is obtained from the back-EMF
signal in a four-phase detector, necessary waveform inversions
being performed. The resulting signal is sent to the Rate Computer
for comparison with the desired motor speed signal, and the
resultant Rate Control Error signal is used to control drive pulse
width.
The implantable battery power supply shown schematically in FIG. 11
receives input power from the implanted secondary coil of an
intact-skin transformer. A controllable rectifier circuit converts
this ac power to dc at a voltage suitable for both charging the
battery pack and running motor control circuits. The battery pack
as shown is wired in parallel with the load so that it takes both
the ripple current from the charger and the ac fluctuations in load
current.
The battery control circuits derive information from each cell of
the battery pack to control the charger output.
If at any time the battery pack charge content declines below
optimum, the control circuit provides maximum available power
throughout to charge the pack in the shortest possible interval.
When any one or more cells of the battery pack reaches a fully
charged condition, the control circuit terminates the rapid charge
and reverts to a normal trickle charge mode of operation.
When the power output is unavailable, for instance, when the skin
transformer is decoupled, power is supplied from the implanted
battery pack to operate the heart. Sufficient capacity is provided
to operate a total heart device for about 3-4 hours. If ac power is
not restored before the battery becomes discharged, the battery
control circuit terminates battery discharge when any one cell of
the battery pack is discharged to a cell voltage of approximately
0.7 volts. This protects the battery pack from over discharge and
possible cell damage.
The battery output current supplies power directly to the Motor
Drive Circuit output stages, and also provides power to the Low
Power Voltage Supply which provides regulated voltages for the low
power circuitry in the Motor Drive Control and Rate Computer.
In operation, electrical energy is supplied to an external
transmission unit 103 by batteries carried in an attache case or
vest (See: FIG. 1). The electrical energy is then transmitted from
the external transmission unit 103 across intact skin to an energy
receiving coil 104 implanted just under the skin. The electrical
energy then travels to an electrical energy storage unit 105 where
it is finally transmitted to the energy converter of the artificial
heart.
The energy converter converts the electrical energy to mechanical
energy through an electric motor which actuates any suitable
mechanical driving means. In the preferred form, the electric motor
through a series of reduction gears drives cam members. As the cam
members are rotated to their high point, they force the compression
plate assembly 70, 10 outward, thereby compressing the ventricle.
It is this compression of the ventricle which produces the actual
pumping of the blood. When the cam is rotated past its high point,
the cam surface loses contact with the compression plate assembly
70, 10 which is now free to return to its original position by
means of reset springs 52 or like devices. However, the compression
plate 70, 10 will not return unless there is blood available in the
atria to flow into the ventricles. Hence, the degree of filling or
return of the compression plate is controlled entirely by the
available supply of blood in the atria on a beat-by-beat basis.
If a nuclear power source, as shown in FIG. 2, is utilized, energy
in the form of heat is converted to electrical energy in the
thermal energy storage unit 205. The electrical energy is then
transmitted to the artificial heart's energy converter which
converts the electrical energy to mechanical energy in the manner
described above. The same basic process takes place if an
artificial heart with a self-contained nuclear drive system is
employed.
In a typical cycle, blood carrying CO.sub.2 is supplied to the
right atrium. From the right atrium the blood passes through the
right A-V valves into the right ventricle. As the right ventricle
is compressed by the outward movement of the right compression
plate assembly, the blood contained therein is forced out or pumped
to the lungs where the CO.sub.2 is released and the blood
oxygenated.
The oxygenated blood is then transported to the left atrium. From
the left atrium, the oxygenated blood passes through the left A-V
valves into the left ventricle. As the left ventricle is compressed
by the outward movement of the left compression plate assembly the
oxygenated blood contained therein is forced out or pumped to the
arterial system. The cycle then begins again.
A-V valve-blowthrough and the subsequent backflow of blood from the
ventricles to the atria during pumping is prevented by the metallic
stays which are mebedded within the flexible flaps or A-V
valves.
Also, as the ventricular blood volume changes during pumping, the
internal air volume between the ventricles must change. This is
accomplished by a flexible bag, fabricated from Silastic or a
similar material, which is clamped to the outside of the metallic
ventricle housings. This bag also forms a body fluid seal for the
electric motor.
As noted earlier, the pumping of the left and right ventricles need
not be simultaneous and is, in fact, regulated by the heart rate
control computer. In operation, computer response to a decrease in
inlet blood flow is fully accommodated within a single beat
interval by decreased ventricle filling, but increases in inlet
flow must be accommodated by one or more of the following three
mechanisms:
1. A 10 percent increase in ventricle filling can occur within a
single beat (assuming the fill sensor trigger is set at 90 percent
of ventricular volume).
2. The computer can increase heart rate but within the limiting
rate of change of 4.25 bpm/beat.
3. When the capability of the above two mechanisms has been
exceeded, or during a speed change transient, the excess of
inflow-outflow must be accumulated in the atria.
If an increase occurs which finally exceeds atrial storage
capacity, then the inevitable result will be an increase in atrial
pressure which implies an increase in vena caval or pulmonary vein
pressure --either of which should be avoided. The most critical
situation therefore arises from a step increase in blood inflow
that exceeds the 10 percent excess ventricle capacity. The atria
should be designed to accommodate reasonable changes in inflow that
would frequently occur. The following analysis results in a method
of predicting the required atrial capacity necessary to accommodate
a given step change in venous return without causing excessive
atrial pressure.
If blood return to the heart increases at a rate faster than the
computer can accommodate by increasing heart rate (such as a step
increase in venous return), then blood must accumulate in the atria
until such time that the rate increases to pump the new inlet flow.
This situation is illustrated in FIG. 12.
The maximum rate of change of the computer-controlled heart rate,
ignoring fine-grain variations within a beat, is given by the
expression: ##EQU1## where R is the heart rate and .delta. is the
maximum change in rate per beat. (.delta. .congruent. 4.25 beats
per min/beat.)
The general expression for the rate of change of heart rate is:
##EQU2## where K is the fraction of heart beats in which a fill
pulse occurs. If a fill pulse occurs on every beat, K = 1, and the
maximum positive change of heart rate occurs. If there are no fill
pulses, K = 0, and the maximum negative change occurs. If fill
pulses occur 50 percent of the time, K = 1/2, and the heart rate
remains constant.
Equation (2) is a differential equation with the well-known
solution:
where R.sub.o is the initial value of R.
The output flow rate pumped by the heart is ideally:
where V is the ventricle volume. The accumulated volume of blood in
the atrium when the input flow rate, Q.sub.1, exceeds the output
is: ##EQU3## assuming that the initial value of v is zero.
Consider a large step increase in Q.sub.1 at t=0. For t>0:
where Q.sub.0 = R.sub.0 V is the maximum initial output flow-rate
possible, without any accumulation in the atrium and X is the
fraction by which the step increase exceeds Q.sub.0. When this step
increase occurs, fill-pulses will occur every beat, K=1, until some
point after the time, t.sub.m, when the heart rate reaches the
value necessary to make Q.sub.2 = Q.sub.1. During this time excess
blood will accumulate in the atrium, so that the residual atrial
volume, diastolic atrial volume, V, will reach a maximum when
Q.sub.2 = Q.sub.1. After time, t.sub.m, fill-pulses will continue
to occur on each beat until the accumulation in the atria is pumped
out and the ventricle filling drops back to the fill threshold
level, 90 percent. Therefore, R will continue to increase until the
fill-pulses begin to disappear, after which time R will decrease
until it reaches the required steady-state value. The heart rate
when ##EQU4## so that: ##EQU5## The maximum value of V, which
occurs at t = t.sub.m is the volume which the atrium must be able
to accommodate if venous pressure buildup is to be avoided. The
maximum value is: ##EQU6## For 0 <t<t.sub.m
since K = 1.
at t = t.sub.m, R = R.sub.1, so that
or ##EQU7## since Q.sub.0 = VR.sub.o. From Eqs. (8), (9), and (10):
##EQU8## Since ##EQU9## we have: ##EQU10## or ##EQU11## where
##EQU12##
EXAMPLES TABULATE
f(0.1) = 0.00440, f(0.15) = 0.00933, f(0.2) = 0.01565, f(0.3) =
0.0316, f(1) = 0.1931.
__________________________________________________________________________
Symbols t -- Time variable R -- Instantaneous heart rate R.sub.o --
Initial value of R R.sub.1 -- Heart rate for Q.sub.2 = Q.sub.1
.delta. -- Maximum change in R per beat (.congruent. 4.25
beats/min/beat) K -- Fraction of time that fill pulses are
occurring Q.sub.1 -- Blood flow rate into the heart atrium Q.sub.2
-- Blood flow rate out of the heart ventricle Q.sub.O -- Value of
maximum ventricle output when R = R.sub.o V -- Effective ventricle
volume v -- Residual volume of blood accumulated in the atrium with
time Q.sub.1 x = - 1 Q.sub.0 x f(x) = ln(1+ x) - 1+x
__________________________________________________________________________
As one example, the artificial heart operating at a steady state
under computer control is presented with a 10 percent increase in
venous return. This will be fully accommodated by an immediate
increase in ventricle filling followed by a 10 percent increase in
rate. No atrial expansion is required.
As a second example, assume that the artificial heart is operating
at steady state at a nominal 100 bpm and is presented with an
abrupt 20 percent increase in venous return. This will require
transient blood storage in the atria and therefore increase the
atrial residue volume. The final heart rate that results in this
example will be a 20 percent increase or 120 bpm. The rate at which
output flow equals input flow is R.sub.1 = 110 bpm. During the
transient, the maximum atrial residual volume ratio will be
##EQU13## and will occur at a response time ##EQU14## Evaluating
these quantities we see that ##EQU15##
Further, ##EQU16## and f(x) = 0.00440 therefore ##EQU17## so the
maximum required atrial residual volume is 11.4 percent of
ventricle volume.
Mock loop data tell us that the artificial heart under computer
comtrol at 90 percent filling expells about 80 ml per stroke. Thus
we may assume that the ventricle capacity with complete filling is
about 89 ml. Using V = 89 ml we get v.sub.m = 0.114(89) = 10.1 ml
of residual atrial volume.
Given the cycle timing used in the present face cam systems, the
atrial capacity variation during each heart beat is 0.36 V = 32 cc.
From tests of A-V valve regurgitation during systole, we determined
that the atria must also accommodate about 17 ml from this source
bringing the atrial capacity variation during the heart cycle to 32
= 17 = 49 ml.
To accommodate the 20 percent step increase in venous return would
require an additional 10 ml bringing the required total atrial
capacity to 59 ml. Measurements of actual atrial capacity indicate
that 60 ml is about all that is really available without excessive
excursions in atrial pressure, so we may conclude that the present
artificial heart with reinforced atrial bladder and the new fast
computer triggered at 90 percent filling is just capable of
handling the 20 percent step increase in venous return.
The above example assumes the new fast computer response and a 90
percent fill sensor trigger point. In all in vivo tests to date,
the computer has been slower (.delta..apprxeq.1.0 bpm/beat) and the
fill sensor was set at 95 percent. If we assume the same 20 percent
step increase in venous return, one would predict the following
response.
The speed at t = tm is R.sub.1 = 115 bpm, but the response time =
tm = 8.39 seconds since the fill sensor trigger is at 95 percent,
only 5 percent of the step increase can be accommodated by
increased filling; this leaves 15 percent or x = 0.15 and f(x) =
0.00933 so: ##EQU18## i.e., the residual atrial volume = 1.073 (89)
= 95.5 ml. Total atrial capacity therefore should be 96 + 32 + 17 =
145 ml. This is obviously not available and if such a step change
were imposed, it would result in increased atrial inlet pressures
that would impede venous return flow for most of the eight-second
interval.
In arriving at the control theory described above, a number of
possible control methods were also considered. They are:
1. Continuous trend method -- as the stroke volume increased or
decreased, the heart rate would increase or decrease on a
percentage basis for each incremental change in stroke volume on a
beat-by-beat basis with both right and left ventricles being
analyzed for change;
2. Rate of filling method -- as changes occur in filling rate and
in the percentage of the stroke volume, the heart rate would be
increased or decreased; and
3. Complete stroke volume method -- when either ventricle becomes
filled, the heart automatically goes into the systole ejecting all
of the blood in each ventricle, or the ventricle that is filled
ejects and the other ventricle does not eject until it is
completely filled; however, the ventricle ejecting first cannot
eject a second time until the second ventricle has completed its
ejection. Between these two limits, there are a variety of
synchronous and asynchronous conditions that can be
established.
None of the above approaches require any instrumentation or sensors
to be placed in the blood stream, they are not sensitive to
movement or to any spatial arrangement. These concepts permit
control of rate and stroke volume with electrical, hydraulic or
pneumatic logic circuitry. This permits the nuclear heart system to
function without the generation of electrical energy for
control.
The foregoing description of the specific embodiment will so fully
reveal the general nature of the invention that others can, by
applying current knowledge, readily modify such specific embodiment
and/or adapt it for various applications without departing from the
generic concept, and therefore, such adaptations and modifications
should and are intended to be comprehended within the meaning and
range of equivalents of the disclosed embodiment.
It is to be understood that the phraseology or terminology employed
herein is for the purposes of description and not of
limitation.
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