Artificial Heart

CARDIOVASCULAR DISEASE IS THE MAIN CAUSE
of death in the United States, Canada, UK, Ireland, and
Europe. It accounts for more than 40% of total deaths
in these countries. Many of these deaths are due to heart
failure. Heart failure affects approximately five million
Americans with more than half a million new cases diagnosed
annually. The aggregate five-year survival rate of
patients with heart failure is approximately 50%, but the
one-year mortality rate is close to 50% for patients with
severe heart failure (New York Heart Association class IV
heart failure). Patients who remain in class IV heart failure
for several months despite optimal medical therapy may
require heart transplantation.

There is no doubt that cardiac transplantation has a
role and provides beneficial relief of suffering in selected
individuals. The one-year survival rate is more than 80%
and the 10-year survival rate is close to 50% for
transplantation. This far exceeds the dismal survival rate
obtained with the left ventricular assist devices that at two
years is less than 20%. Following left ventricular assist
device a transplant is necessary within one to six months.
This calls for two operations within six months in very
sick individuals. This is a major disadvantage of assist
devices. Most important, there are fewer than 4000 donor
organs available worldwide per year. The cost of two
operations within six months is prohibitive as well as
physically and mentally traumatic. A functional total artificial
heart would be a dream come true.

I. ELECTRIC TOTAL ARTIFICIAL HEART
The first artificial heart implant was carried out by
Dr. Denton Cooley in Houston, Texas, in a 47-year-old
man with intractable heart failure using the Liotta artificial
heart developed by Domingo Liotta. This artificial heart
was based upon the laboratory work of Dr. Michael
DeBakey.

The electric total artificial heart was designed for
permanent use and was completely implantable. Pneumatic
total artificial hearts have not proved successful in
small group studies. The Jarvik-7-100 artificial heart,
a pneumatic system developed by the physician-engineer,
Robert Jarvik, was tried in four patients as a permanent
implant. All four patients died because of infectious,
hematologic, and thromboembolic complications. One
patient lived for 20 months. This device was first use
by Dr. William DeVries who implanted the device in
Barney Clarke in 1982. The presence of percutaneous
drive lines causes infectious complications and the bulky
external drive unit makes pneumatic artificial hearts unsuitable
for permanent implantation.

From 1970 to the present much investigational work
has been done in the development of left ventricular assist
devices as opposed to the development of a total artificial
heart. This setback has delayed the development of the
true total artificial heart. During the past decade, through
a contract program established by the National Heart and
Lung Institute, two research teams have been developing
an electric total artificial heart that is undergoing
clinical testing in selected areas in the United States.

A. AbioCor
A completely novel approach to the artificial heart was
conceived by engineers from the Abiomed company.
The AbioCor is a totally implantable artificial heart
that is electric as opposed to the failed Jarvik-7, which is
a pneumatic system.
AbioCor is the Abiomed (Danvers, MA) total artificial
heart. It has been tried in a few patients (see Fig. 1). The
prosthesis replaces the right and left ventricles of the
recipient and is an electrohydraulically triggered device
that can provide a cardiac output of 4–8 L/minute with
a maximum of 10 L/minute. The design allows for
physiologic flow in a pulsatile manner, and has been shown
to provide satisfactory end organ perfusion. This device
has the potential to provide a satisfactory outcome for
patients suffering from end-stage heart failure who are not
suitable for transplantation or when donor hearts are not
available.

1. Implantable Components
A chest unit containing two blood-pumping sacs fill
and empty alternately, supplying pulsatile blood flow
through the aorta and pulmonary arteries, respectively.
An artificial septum is situated between the two blood
sacs and contains a miniaturized centrifugal pump that
rotates at 5000–90,000 rpm. The pump then transports
the hydraulic fluid alternatively to the right and left
sides. Blood flows through artificial plastic trileaflet valves
which act as atrioventricular and ventriculoarterial valves.
Rechargeable lithium ion cells provide energy to the
pump for up to 30 minutes. The internal rechargeable
batteries allow patients freedom for 15–30 minutes as
the batteries are constantly charged from an external
source. A microprocessor in the chest unit provides physiologic
parameters: beat rate, motor speeds, and balance
between the right and left pressures that control the
pumping function of the heart. A disk-shaped internal
transcutaneous energy transfer (TET) coil is placed in the
subpectoral region.

2. External Components
The internal TET receives magnetic waves from an external
TET coil, which transfers energy across the skin to the
implanted device by a process of inductive coupling that
converts the external energy to electrical energy. Thus, the
device is totally internal with no cables through the skin.
A console the size of a laptop computer houses a battery
that powers the device for approximately 45 minutes.
The external TET coil attaches to the console and provides
a constant power supply. The console is plugged into a wall
outlet and is disconnected for activities and travel.

B. Lionheart
1. Chest Size
Size constraints remain the most important hurdle.
Patients must have an adequate chest cavity size or anteroposterior
thoracic dimension.
2. Power Source
The Lionheart (Arrow, Reading, PA) electrical artificial
heart uses a single, implantable energy converter to drive
both ventricles that are implanted within the pericardium.
It avoids the problem of using a separate external drive
unit for each ventricle as is done with the pneumatic
system. The electrical unit uses a single implantable energy
converter to drive both ventricles and requires a minimum
energy of 14 W. Unfortunately, implanted batteries
are not capable of supplying the power that is required.
The development of high-energy density batteries hopefully
should eliminate the need for the present use of
a primary external power source as this is a major hurdle
still left to overcome.

II. LEFT VENTRICULAR ASSIST DEVICE
A. Systems
Left ventricular assist devices are implanted only in
patients who are eligible for cardiac transplantation.
These devices are used as bridges to transplantation.
There are three left ventricular assist devices presently
available:
Thoratec, the HeartMate, and Novacor.
The Thoratec system is paracorporeal: the pneumatic
pump resides externally on the surface of the abdomen and
is connected by cannulas to the heart and the ascending
aorta. Figure 2 is taken from the Rose et al. study which
reviewed long-term use of left ventricular assist devices.
In the second and third systems the device is placed
entirely within the chest cavity and abdomen. The energy
conduit and vent, a drive line, is brought through the
skin to the external energy source. Major problems with
these systems include: a high incidence of infection of the
device that usually occurs between 3 and 6 months, a high
rate of bleeding by 6 months, and a probability of device
failure greater than 33% at 2 years.

NEW FRONTIERS
A ray of hope has been generated by the work of
Dr. M. H. Yacoub, who describes a novel strategy: a combination
of surgery and physiologic hypertrophy. The
surgical process involves implantation of a left ventricular
assist device and medical therapy with a drug, clenbuterol,
a beta-2-agonist which induces reverse remodeling of the
left ventricular myocardium and subsequent physiologic
myocardial hypertrophy. This strategy improves left ventricular
contractility and ejection fraction sufficiently to
allow explantation of the assist device (the Harefield
protocol).
The small study of 19 patients resulted in 4 deaths.
Among the 15 patients, 11 had sufficient recovery from
heart failure and had the left ventricular assist device
explanted. Ten patients are alive at two and a half years
follow up and showed excellent exercise capacity with
remarkable improvement in ejection fraction and lead
a relatively normal life.
Clenbuterol changes phenotype, genotype, and gene
expression in myocytes; in animal studies the agent has
been shown to improve pressure volume relationships,
increase myocyte size, and enhance organization of
myofibrils. Dr. Yacoub indicates that not all hypertrophy
is maladaptive. The strategy is to rest the heart and
make it as small as possible, then activate the genes
associated with the fetal heart and make it mature
again. Once the heart is atrophied it is appropriate to
enhance physiologic hypertrophy with clenbuterol or
similar agents, which leads to improved left ventricle
function.

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