A Comparative Study of the Shear Stress Induced in the Leakage Backflow Produced by Four Types of Heart Valve Prostheses

Abstract
The clinical selection of a prosthetic heart valve requires the consideration of many factors. These include the pressure drop across the prosthesis, the amount of leakage backflow when the valve is closed as well as the associated shear stress within this flow for a particular valve design. Major emphasis has been placed upon the documentation of the pressure drop across prosthetic valves. Consideration of the shear stress induced in the leakage backflow associated with prosthetic valves, however, has received less attention. The purpose of this investigation, therefore, is to determine theoretically the extent of this particular shear stress for four types of currently used mechanical prosthetic cardiac valves. All valves have the same tissue annulus diameter of27 mm. The leakage backflow is relatively small compared to the main forward flow through the orifice. However, it is important from the haemolysis point of view. The effect of the shear stress on the suspected occurrence of haemolysis is analysed by two theoretical approaches: (a) the laminar flow approach, and (b) the entry flow approach, and then compared with the findings of previous investigators dealing with the critical shear stresses which may damage the red blood cells. It was found that both approaches give practically equal shear stresses under the same pressure differential for all valves investigated. Some haemolysis is expected to occur but it is generally compensated by bone marrow hyperfunction, since that type of shear stress is sustained for only a short duration of time.