Hemodynamic Measurements and Dynamics
of
Assisted Circulation
237
typically in the subendocardium of the left ventricle in the normal,
border, and ischemic zones. Pumping was initiated at various times after
the onset
of
coronary artery occlusion. Contraction, assessed by
segmental muscle shortening was significantly improved in the border
zone, but unchanged either in the central ischemic zone, or in the normal
zone. This was attributed to the border zone regional increase in
myocardial blood flow and in the availability of oxygen due
to
augmented diastolic perfusion pressure.
There are disputes as to which appropriate hemodynamic variables
are suitable as controls to optimize the efficacy of cardiac assist devices
(CADS) either in-series or in-parallel. Two such variables utilized are the
aortic pressure and the cardiac output. Their use
as
input control signals
has been found to be inadequate from both experimental and model
studies.
Apart from IABP, left ventricular assist devices (LVADs) exist in
several types. They may be either pulsatile or non-pulsatile, synchronous
or non-synchronous to the pumping of the natural heart. These may be
the sac type, the diaphragm type, tbe pusher-plate type, the roller pump,
or the centrifugal pump type.
The
LVADs
are used in patients with either
reversible or
nonreversible cardiac failure and can be used for temporary or permanent
assistance. These types of cardiac assist devices are sometimes referred
to as parallel assist devices or bypass devices because they direct blood
from either the left atrium or the left ventricle to the aorta, hence acting
in parallel with the natural heart pump. As with all cardiac assist devices,
the primary aim of the LVAD is to provide adequate perfusion to vital
organ vascular beds.
Of
secondary concern is to unload the heart, or to
reduce afterload and hence myocardial oxygen consumption.
Blood pressure
is
often higher when the LVAD is on rather than off.
This tends to increase the load on the left ventricle, which is already in
failure with reduced external work capability. The counterpulsation
mode allows
the
reduction of afterload
however,
if
properly
synchronized.
Synchronous actuation,
with
left
ventricular
apical
cannulation, provides the greatest amount of ventricular unloading.
Nevertheless, synchronization does not appear to be a priority of many of
