Near valve flows and potential blood damage during closure of a bileaflet mechanical heart valve

Abstract

Blood damage and thrombosis are major complications that are commonly seen in patients with implanted mechanical heart valves. For this in vitro study, we isolated the closing phase of a bileaflet mechanical heart valve to study near valve fluid velocities and stresses. By manipulating the valve housing, we gained optical access to a previously inaccessible region of the flow. Laser Doppler velocimetry and particle image velocimetry were used to characterize the flow regime and help to identify the key design characteristics responsible for high shear and rotational flow. Impact of the closing mechanical leaflet with its rigid housing produced the highest fluid stresses observed during the cardiac cycle. Mean velocities as high as 2.4 m/s were observed at the initial valve impact. The velocities measured at the leaflet tip resulted in sustained shear rates in the range of 1500-3500 s(-1), with peak values on the order of 11,000-23,000 s(-1). Using velocity maps, we identified regurgitation zones near the valve tip and through the central orifice of the valve. Entrained flow from the transvalvular jets and flow shed off the leaflet tip during closure combined to generate a dominant vortex posterior to both leaflets after each valve closing cycle. The strength of the peripheral vortex peaked within 2 ms of the initial impact of the leaflet with the housing and rapidly dissipated thereafter, whereas the vortex near the central orifice continued to grow during the rebound phase of the valve. Rebound of the leaflets played a secondary role in sustaining closure-induced vortices.

Fig. 1

A portion of the valve housing was removed to expose the leaflet tip

Fig. 2

High speed images of the near valve closing flow and vortices were taken. (a) The closing volume was observed as streaklines. (b) Vortex initiation and (c) growth were identified. (d) The window in the housing enabled optical access to near valve flows.

Fig. 3

A schematic of the LDV experiments

Fig. 4

The three velocity components, (a) U, (b) V, and (c) W, were plotted on individual axes to study velocity fluctuations and beat-to-beat variations in a point-wise fashion

Fig. 5

Three-dimensional reconstructions of the flow taken 2.65 mm upstream of the leaflet tip (a) at impact, (b) 2 ms after impact, (c) 4 ms after impact, and (d) 9 ms after impact

Fig. 6

Velocity maps of the centerline plane (a) 0.5 ms before impact, (b) 4 ms after impact, during rebound, (c) 10 ms after impact, and (d) as the flow began to dissipate 16 ms after impact

Fig. 7

Centerline velocity profiles generated using PIV (a) at impact, (b) 2 ms after impact, and (c) 4 ms after impact

Fig. 8

Mean and peak shear rates were computed across the edge of the leaflet