Numerical simulation of instantaneous backflow through central clearance of bileaflet mechanical heart valves at closure: shear stress and pressure fields within clearance.

Instantaneous backflow through central clearance of bileaflet heart valves at the instant of closure is investigated. An Edwards-Duromedics valve in the mitral position is employed to measure the transient pressures near the entrance and exit region in an in vitro flow chamber. A region surrounding the clearance is modelled, and two-dimensional quasi-steady-state numerical simulations are performed, with the measured transient pressure difference across the clearance as a driving force for the flow. The results show that pressure difference several times larger than the driving pressure used to close the valve is established across the clearance for about 0.5 ms at the moment of closure. The resulting average wall shear stress is an order of magnitude larger than the turbulent Reynolds stresses reported distal to the valve during opening. A local jump in the shear stress distribution and fall in the pressure distribution are observed at the entrance region. Rounding of the corners in the channel entrance attenuates these spikes. The results of the study indicate that backflow through clearance at closure may be one reason for the haemolysis and thrombosis associated with mechanical heart valves, despite the short duration of the flow field.