1.
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What types of ventricular
assist devices are available for bridging to transplant?
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| Three devices are available and have been used in patients who are
transplant candidates but whose hemodynamics are inadequate, even with the use of
intravenous inotropic agents, to allow the patient to wait until a donor heart has been
identified.
The HeartMate ventricular assist pump is a pneumatically powered device that is
implanted in the left upper quadrant of the abdomen. The pneumatic air hose exits from the
lower half of the abdominal wall and is attached to a pneumatic power unit. This device is
FDA approved and is now being used in a number of centers in the United States and abroad.
An electric version is available on an investigational basis and allows the patient to be
considerably more mobile. With this system, a wire and vent tube pass through the lower
half of the abdominal wall but, in place of the pneumatic drive unit, a shoulder holster
with electric batteries is used to power the device.
The Thoratec assist pump, manufactured by Thoratec Laboratories, Inc., is a
pneumatically powered device that is placed on the anterior abdominal wall. Blood can be
taken from the left ventricular apex and pumped into the aorta. The cannulas pass through
the chest wall in a manner similar to that of a conventional chest tube. A pneumatic power
unit is used to provide the air pulses.
The Novacor ventricular assist pump is an electrically powered device built by the
Novacor division of Baxter Laboratories. The pump is implanted in the left upper quadrant
and the electric line and vent tube are passed through the lower anterior abdominal wall.
The device is available in this country on an investigational basis, but FDA approval for
open sale should be obtained in the near future. |
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2.
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What are the major
differences between the three types of ventricular assist devices that are being used?
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| The HeartMate assist pump is an implanted device that uses tissue
valves. The blood contacting component of the pump housing is lined with tiny titanium
beads while the flexible diaphragm is a textured surface. The patients are maintained on
aspirin. This allows a thin, fibrin layer to develop on the blood contacting surface. The
device has been generally free of thromboembolic complications. Blood can be taken only
from the left ventricular apex and is pumped into the ascending aorta. The device cannot
be used to access atrial blood, nor can it be used for right ventricular support.
The Thoratec pump uses tilting disk-type mechanical valves and a highly smooth blood
contacting surface. Patients in whom this device is employed are maintained on low dose
heparin or Coumadin. The unit is versatile in that blood can be taken from the left
atrium, left ventricle, and pumped into the aorta. Right heart support is readily provided
by installing the pump to fill from the right atrium and pump blood to the pulmonary
artery.
The Novacor system uses biological valves and a highly smooth blood contacting sac. An
interesting mechanical design employs pusher plates on the front and the back of the sac
insuring excellent washout. Patients with this device are maintained on long-term
anticoagulation. |
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3.
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What is the status of
permanent ventricular assist pumps?
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| Both the HeartMate electrical system and the Novacor unit were
ultimately designed to be used as permanent ventricular assist devices. The opportunity to
use these as bridge devices has provided the designers and the surgeons with an
opportunity to gain experience with these pumps but without the concerns regarding
long-term (greater than one year) reliability . The electrical devices are designed to be
implanted in the left upper quadrant, fill from the left ventricle, and eject into the
aorta. Both of these systems require vent tubes and electric wires that exit from the
lower abdominal skin. Ultimately, energy transfer by inductive coupling techniques can be
employed with these devices. Some type of implantable compliance chamber would also be
required to eliminate the need for any tube or wire to cross the skin.
Initial clinical use of these devices is just beginning to occur at the present time.
With these systems, the patients carry rechargeable battery packs that can be changed at
prespecified intervals.
The extreme shortage of donor hearts and the increasing population of patients with
ventricular failure suggests that these devices will play a major role in the treatment of
patients with end stage heart disease in the future. Since 1990, an average of 2217 heart
transplants are performed annually. Estimates suggest that as many as 10,000 patients may
be candidates annually for cardiac replacement therapy (heart transplantation, permanent
left ventricular assist pumps, and the artificial heart). |
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4.
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What is the status of the
artificial heart? Does the lack of news during the past decade mean that artificial heart
development has stopped?
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| In the mid 80's, William DeVries, originally at the University of
Utah and subsequently at Humana Hospital in Lewisville, implanted pneumatically powered
artificial hearts in a small series of patients who were not felt to be transplant
candidates. The disadvantage of having two sizable pneumatic tubes exit from the chest
wall and the need for the bulky pneumatic power unit led to the recognition that a
pneumatic artificial heart was unlikely to serve as an acceptable permanent cardiac
substitute.
About one decade ago, the Devices and Technology Branch of the National Heart, Lung,
and Blood Institute made funds available for the development of an electrically powered
artificial heart. As part of the requirements, the NIH requested that partnerships be
established between medical groups and engineering groups that were capable of
manufacturing such an artificial heart once the developmental work had been completed.
Accordingly, three such groups now exist in the United States and are making excellent
progress on artificial heart development. These groups are Abiomed and the Texas Heart
Institute, Nimbus, Inc. and the Cleveland Clinic Foundation, the 3M Company and The
Pennsylvania State University. The artificial hearts under development consist of separate
pumping chambers for the left and right ventricles. The devices are powered by small,
brushless DC motors that either power a hydraulic fluid or directly activate the
blood-containing sacs. Energy is to be transmitted by inductive coupling, thus there is no
break in the skin and no tubes or wires need to cross the skin. The devices are all placed
in the pericardial sac in a fashion similar to that of cardiac transplantation. The
anterior abdominal wall is the sit e for the electronic canister, implanted electric
battery to provide actuation of the device for 30-40 minutes, and an implanted electrical
coil which serves as the secondary coil of a transform repair. The external or primary
coil is energized either by conventional house current or by a portable rechargeable
battery.
Devices of this general nature have now functioned on the laboratory bench for many
months. The details of the control systems have been worked out and animal implantation in
calves has been carried out for as long as 13 months. Under the present NIH plan, the
device designs will be frozen within the next year. About a dozen of each of the devices
will be evaluated for reliability on a mock circulatory loop with continuo us operation
for at least one year and hopefully for two years. A series of animal implants will be
carried out, again in calves, with the final device.
With the current funding available and the time required for reliability testing and
animal implantation testing, it appears that the artificial heart should be ready for
initial human testing at the turn of the century. |
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5.
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Which patients will be
candidates for artificial hearts and which will be candidates for permanent ventricular
assist devices?
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| The simpler of the devices is the ventricular assist device. The blood
will be taken from the ventricular apex and the heart will remain in place as a fail safe
mechanism, should the ventricular assist pump fail. However, many chronically diseased he
arts have thrombus within the left ventricular chamber and will present an ever present
risk of thromboembolic complications. Moreover, a valvular disease may be present and
lethal arrhythmias may occur. In some patients, right ventricular failure is nearly as
prominent as left ventricular failure. In this latter group of patients, cardiectomy and
replacement of the entire heart will be the safer and preferred procedure. Experts
estimate that approximately 1/3 of the patients who require a permanent mechanical device
will be treated with the artificial heart whereas the remaining 2/3 will be able to be
adequately treated with a permanent ventricular assist pump. |
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