Vortex formation in the aortic root during LVAD support

Introduction:: The vortex formation in the sinuses of Valsalva is believed to play an important role in allowing
for rapid opening and closure of the aortic valve. The swirling flow of blood in the sinuses
facilitates closure of the aortic valve leaflets at the end of systole, preventing backflow of blood
into the left ventricle and washing the aortic root in diastole. Cardiovascular support with a Left
Ventricular Assist Device (LVAD) reduces forward flow through the native aortic valve, resulting
in complete hemodynamic closure at high LVAD speeds. The disruption of the sinus vortices
may contribute to the development of aortic root thrombus, a dangerous condition that poses
a high risk for subsequent stroke. The goal of this study was to measure the flow field in a
model of the aortic root during LVAD support and characterize the sinus vortex formation as a
function of LVAD speed.

Materials and Methods:: Experiments were performed with a benchtop model of the cardiovascular system that allows
mimicking multiple operation conditions for the LV-LVAD system. A transparent silicone left
ventricle (LV) with a HeartMate II LVAD was enclosed in a fluid-filled tank and attached to a
pump (Vivitro Superpump) that pressurized the tank in a cyclic manner, causing the LV to beat.
A chamber mounted to the aortic outflow enabled flow visualization of the aortic valve and
root as shown in Figure 1. A baseline level of function corresponding to a heart failure patient
was established with a mean aortic pressure of 62±3 mmHg, cardiac output of 2.5 L/min and
heart rate of 60 bpm, corresponding to a 27% ejection fraction. Pressure and flow were
measured in the aorta, LV and LVAD, and the midplane flow field of the aortic root was
measured with particle image velocimetry (PIV) as LVAD speed was increased from 7.2 krpm to
8.2 krpm by 400rpm increments. 2-D velocity field data were analyzed to identify vortex
formation using the Q criterion, and the circulation, kinetic energy and size computed.

Results, Conclusions, and Discussions:: Pressure increased to 80 mmHg at 7.2krpm LVAD speed, which corresponded to a total flow
rate of 3.0 L/min. At this speed, 85% of flow passed through the LVAD and 15% through the
native aortic valve. Further increases in speed increased pressure and flow as well as the
fraction of flow through the LVAD which reached 100% when the aortic valve could no longer
open. Velocity images of the aortic root showed that vortex formation begins in early systole
and grows in strength and energy then decaying when the leaflets close. Two vortices are
visible, clockwise and counter-clockwise, which travel distally as they attenuate (see Figure 2).
During low LVAD support, flow through the aortic valve continues but at a reduced rate, and
the vortices continue to form but are smaller and weaker as shown in Figure 3. Velocity profiles
through the vortex centers, illustrated in Figure 4, show both forward and rotational flow are
affected as is the distance between the vortex centers. As LVAD speed is increased, the aortic
valve stops opening at 8.2 krpm and no vortices are observed in the aortic root. The time
course of velocity in the aortic root during the cardiac cycle (Figure 5) shows two phases
corresponding to vortex formation during systole and vortex washing during diastole. As systole

becomes shorter with increased LVAD support, the LVAD opens for less time and allows less
flow, which impacts not only vortex formation but also diastolic washing.
Conclusion: Vortex formation in the aortic root plays an important role in washing the leaflets
and is dramatically reduced during LVAD support. Without these vortices, the risk of thrombus
formation in the aortic root is increased. These studies support the clinical practice of setting
LVAD support levels to provide intermittent aortic valve opening.

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