Instantaneous back flow through peripheral clearance of Medtronic Hall tilting disc valve at the moment of closure.

Abstract

An investigation of the flow dynamics through the peripheral clearance (the gap formed between the occluder tip and the metal housing in the closed position) of a tilting disc heart valve at the moment of valve closure is presented. A Medtronic Hall valve in the mitral position of an in vitro experimental set up is employed to measure the transient pressure pulses near the entrance (ventricular side) and exit (atrial side) of the peripheral clearance at valve closure. Flow within the peripheral clearance is analyzed employing a two-dimensional quasisteady computational fluid dynamics model with the measured peak pressures specified as the boundary conditions inducing the flow. The valve is visualized from its inflow (atrial) side using a stroboscopic lighting technique to investigate the presence of cavitation bubbles within the clearance. The pressure measurements showed that a relatively large pressure drop exists between the entrance and the exit to the clearance for about 0.5 msec at the moment of valve closure. The numerical simulation resulted in relatively large magnitudes of wall shear stress and pressure reduction within the clearance due to the flow established by the large pressure drop between the entrance and the exit. Cavitation bubbles visualized within the peripheral clearance at higher loading rates for valve closure correlated with the presence of large pressure reduction within the clearance. Analysis of the results of this study indicates that the back flow through the clearance at the instant of valve closure may contribute toward injury to formed elements in blood in spite of the short duration of the flow.