Biomedical Engineering Reference
In-Depth Information
FIGURE 8-24
Thoratec Heartmate
VAD. (Courtesy of
Thoratec
Corporation,
reproduced with
permission.)
8.5.4.2 Thoratec/Heartmate
The Thoratec Heartmate VAD System includes three major components: blood pump,
cannulae, and the portable driver. This system can provide partial or total circulatory
assistance when the natural heart is unable to maintain adequate circulation to perfuse
vital organs. As with all VADs, the Heartmate receives blood from the ventricle of the
natural heart at low pressure and then pumps it back into the arterial system at high pressure.
Manufactured by Thermo Cardiosystems Inc. (Woburn, MA), the device is fully im-
plantable with wearable external components. As shown in Figure 8-24, the pump consists
of a flattened titanium cylinder about 50 mm thick and 100 mm in diameter, weighing
approximately 1150 g. It is made up of two chambers: the blood chamber and the air
chamber. The former contains an electric motor-driven cam mechanism that actuates a
pusher plate. This in turn displaces a flexible polyurethane diaphragm that separates the
two chambers. A textured interior surface helps prevent blood clots and reduces the need
for the patient to take anticoagulants. Each conduit contains a 25 mm porcine valve within
a woven Dacron fabric graft. The pump has a maximum stroke volume of 83 ml. It can be
operated at up to 120 BPM, resulting in flow rates of up to 10 L/min.
The device is implanted in the upper part of the abdominal wall or in the peritoneal
lining, as illustrated in Figure 8-25. At the start of each cycle, blood drains from the left
ventricle into to the blood chamber within the LVAD, at which point an external control
system triggers pumping. An electric motor-driven pusher plate forces the polyurethane
diaphragm upward to pressurize the blood chamber. This motion propels the blood through
an outflow conduit that connects to the aorta. The valves in each of the conduits ensure that
blood flows only in one direction. A second tube extends outside the body to a battery pack
that is carried in a shoulder holster. This external tube also maintains near atmospheric
pressure in the air chamber and can also provide pneumatic drive for the LVAD in case of
motor failure. The pump is designed to respond to changing flow demands of the body,
with variable flow rates up to a maximum of 10 L/min.
As of July 2006, over 4100 patients had been supported by the Heartmate LVAD. It was
approved by the FDA as a destination therapy with overall costs of $160,000 being similar
to those for a heart transplant. However, unlike a natural heart, problems associated with
the device include infection of the surrounding area, bleeding, and device malfunction.
Statistics show that there is a 52% chance of survival after 1 year but also that the device
failed 35% of the time after 2 years.
