Biomedical Engineering Reference
In-Depth Information
Recent results have been quite impressive, with some patients showing sufficient
improvement while using an LVAD that can be removed. Even so, the overall mortality
rate due to end-stage heart failure remains high, mainly due to the limited number of donor
heart transplant organs available. Hence, researchers have carefully studied a possible use
of the LVAD as a long-term therapy for patients who are not candidates for heart transplant.
Though most LVADS are mounted within the body (intracorporeal), some are mounted
outside the body (extracorporeal) with tubes that pass through the skin to convey blood to
and from the device.
The following sections of this chapter document some of the VAD devices that have
been developed by research institutes and medical companies over the past 2 decades.
8.5.2 Extracorporeal Ventricular Assist Devices
Extracorporeal ventricular devices, as shown in Figure 8-21, consist of an external con-
troller and air pump connected to one or two pneumatically powered heart pumps with
cannulae that pass through the abdominal wall and the diaphragm where they connect to
the left ventricle and the aorta and to the right ventricle and the pulmonary artery. These
devices include the Berlin Heart Excor, BioMedicus BP-80, Medos, Thoratec, and the
Abiomed BVS500 devices. They are primarily BTT devices, though occasionally just de-
creasing the stress to the diseased heart has allowed it to recover sufficiently for the system
to be removed, relieving the patient of needing a transplant. This is known as a BTR event.
Most extracorporeal VADS can be tailored to an individual's heart characteristics.
Blood volumes ranging from 10 ml required by a child to 80 ml per stroke required by a
large adult are available, while the silicon cannulae are available in a number of different
diameters to accommodate the different flow rates. Different types of drive units are also
available for every conceivable blood pressure and volume, depending on whether the
patient will be mainly stationary or mobile. Most of the devices also include features such
as a heparin coating, titanium connectors, and multiple membranes.
In the Berlin Heart, the inner surfaces of the blood chambers are extremely smooth
and are designed to ensure optimum flow characteristics. Additionally, the heparin coating
prevents thrombogenesis. The pump has three membranes separated by a thin graphite
FIGURE 8-21
Thoratec
extracorporeal
biventricular assist
device
(a) Photograph of
installation.
(b) Graphic showing
internal connections
to the heart.
(Courtesy of
Thoratec
Corporation,
reproduced with
permission.)
