(H-306) The Impact of LVAD Outflow Graft Diameter on Flow in the Aortic Arch

Introduction:: Left Ventricular Assist Devices (LVAD) are implantable rotary pumps used to treat heart failure patients by redirecting blood flow from the Left ventricle to the aorta, bypassing the aortic valve. New LVAD models such as the HeartMate3 (Abbott Laboratories, Chicago IL) are equipped with an artificial pulse to restore flow pulsatility. However, the non-planar curvature and complex geometry of the aortic arch give rise to unique hemodynamic and flow features that may not be preserved during LVAD support. The LVAD flow outlet is attached to the ascending aorta by an outflow graft, with the aortic root perfused in a retrograde manner. Previous studies have found that LVAD outflow graft angle and position affect flow patterns in the aortic arch and can increase the risk of stroke. Less research has been performed to investigate the effect of outflow graft diameter, despite evidence that links small graft size to increased neurological event risk. Our goal in this study was to measure the effect of outflow graft diameter on 3-D flow and pulsatility in the aortic arch during support with a HeartMate3 LVAD.


Materials and Methods:: A mock circulatory loop consisting of a silicone left ventricle, HeartMate3 LVAD, Tygon tubing, outflow graft and a glass aorta model with three branches representing the brachiocephalic (B), carotid (C), and subclavian (S) arteries. The aortic valve was closed with a silicone plug to ensure full bypass flow conditions, when the LVAD provides all flow into the aorta. The HeartMate3 artificial pulse consists of a speed change every two seconds that is ± 400rpm from the set speed. Flow and pressure sensors were placed on the vessels and aorta and measured at 200Hz using LabChart. Three-dimensional velocity in the aortic arch and branch vessels was measured with particle image velocimetry (PIV). 63-micron fluorescent particles were injected into the circulatory loop and imaged with the LaVision Mini-shaker camera and an LED light for 4 sec periods at four LVAD speeds (3900, 4000, 5000 and 5800 rpm) with no cardiac function (asystolic heart). Experiments were performed for two clinically relevant OGDs, 10mm and 14mm, at matched hemodynamic conditions. LaVision DaVis Shake-the-box and Tomo-PIV algorithms were used to track the particles as they entered the aortic arch. Average velocity in rectangular regions of interest (ROIs) were computed for the proximal and distal aorta, and the brachiocephalic, carotid, and subclavian blood vessels.


Results, Conclusions, and Discussions:: Pressure and flow increased with LVAD speed as shown in Table 1, with the highest speed (5800 rpm) corresponding to clinical patient values. As the aortic valve was closed off in these studies, all flow entered the aorta via the LVAD outflow graft and 20-25% entered the arch branches with the remainder proceeding downstream. The 14mm outflow graft developed higher pressure and flow for the same LVAD speed, reflecting the lower resistance compared to the 10mm outflow graft.

For all four LVAD speeds, average velocity was highest at the proximal aorta and decreased as flow traveled downstream to the distal aorta. Overall, average velocity increased with increasing speed but was consistently higher for the 10mm outflow graft condition in the proximal, distal and branch vessels. Velocity measured in the individual branch vessels was also higher for the 10mm outflow graft but the ratio of flow entering the three branches increased slightly at higher speeds, especially for the 14mm outflow graft.
The results of these studies indicate that overall flow and pulsatility is higher with the 14mm outflow graft, but that the 10mm graft imparts higher velocity to the flow which increases flow velocity in the aorta and branch vessels.



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