Thoratec HeartMate III, Instructions For Use Manual, Page: 24 / 458

1 Introduction
1-14 HeartMate III Left Ventricular Assist System Instructions for Use
Principles of Operation
The HeartMate III LVAD is a centrifugal pump that produces flow in the patient's circulatory system
by angularly accelerating and expelling blood that enters it. From a clinical viewpoint, this
mechanical pump works in concert with the native heart to which it is attached. It is a parallel
arrangement: Ventricular blood may flow either through the LVAD or the aortic valve to reach the
aorta, the proportion of which depends greatly on the degree of the patient's cardiac function and
the set-speed of the LVAD.
As with any continuous flow pump (axial, centrifugal, or mixed), the volume flow rate through the
Pump is directly related to the pressure across the Pump and inversely related to the resistance.
Clinically, the volume flow rate through the Pump is the difference between aortic and left
ventricular pressure, and systemic vascular resistance.
This relationship can be characterized at any Rotor speed, and the family of curves derived in
steady-state at different speeds is commonly termed
H-Q curves , or the pressure head
(H) - volume flow rate (Q) relationship. Figure 1.3 shows HeartMate III H-Q curves.
Figure 1.3 Pressure Head-Flow (H-Q) Relationship
Similarly, there is a characteristic relationship between pump power and volume flow rate.
Total power consumption includes hydraulic power (useful blood pressure and flow), viscous
losses, electrical resistance losses, and others. The relationship between hydraulic power and
volume flow rate is always direct, but the various losses have a multitude of dependencies that
make inflections in the relationship possible.
In general, if the speed is set optimally, LVAD flow will be unidirectional towards the aorta and
much greater than cardiac output. Cardiac output may be minimal or zero if the presence of the
LVAD keeps the aortic pressure above the ventricular pressure even during systole.