Biomedical Engineering Reference
In-Depth Information
FIGURE 8-56
Circuit model for the
left ventricle and its
load [Adapted from
(Paden, Ghosh et al.,
2000).]
8.6.3 Estimation and Control of Blood Flow
Most LVAD systems receive blood through an inflow cannula that is inserted into the apex
of the left ventricle through a “cored” hole. The device then pumps the blood back into
circulation through an outflow cannula grafted into the ascending aorta. Thus, the LVAD
operates in parallel with the left ventricle. Since the pressure in the left ventricle varies
as the heart beats, the LVAD is subjected to cyclical variations in loading that must be
accommodated.
System models for simulation often replace the mechanical components with their
electrical equivalents to form a circuit in which current corresponds to blood flow and
voltage corresponds to blood pressure. An example of such a model for the left ventricle
is shown in Figure 8-56.
The electrical model of the LVAD is placed in parallel with the two series resistor-diode
sections.
In general, pulsatile systems concentrate on regulating flow or arterial pressure syn-
chronously with the natural beating of the heart by using the passive filling of the LVAD
pumping chamber to regulate stroke volume and stroke rate. In centrifugal and axial-flow
pumps typical of the newer generation of LVADs, only the rotation speed can be controlled
to maintain the required pressure and flow. In these cases flow is mostly governed by sup-
ply from the venous system, and innate feedback is provided by resistance to blood flow.
This open-loop control is adequate over a small operating range, but as soon as the patient
becomes rehabilitated and starts to resume a normal lifestyle LVADs must be capable of
responding to demand.
Axial-flow pumps do not use valves, so it is possible for blood to flow backward
through the pump if it is run too slowly. Conversely, if it runs too quickly it may attempt
to pump more blood from the ventricle than is available, causing ventricular collapse
or kinking in the inlet cannula. This is illustrated in vivo using a Thoratec Heartmate
implanted in a calf in Figure 8-57. At 9000 rpm the minimum pump flow is close to zero
and the ventricular pressure is always positive, reaching about 70 mmHg at its peak. At
10,000 rpm the flow is much less pulsatile with a minimum value significantly greater than
zero. The pressures are also less pulsatile with the peak ventricular pressure reaching only
about 30 mmHg. Finally, if the speed is increased to 11,000 rpm, the pulsatile pattern is
lost and the pump flow oscillates erratically. This last condition corresponds to ventricular
suction and can result in damage to the heart muscle if it is not identified and corrected
quickly (Antaki, Boston et al., 2003).
The normal heart is controlled via the nervous system to maintain a mean arterial
blood pressure (MABP) of 90 mmHg. As discussed earlier in this chapter, in the natural
heart this set point is maintained by feedback from baroreceptors in the atria.
