HEMODYNAMIC EVALUTION OF PROSTHETIC HEART VALUES WITH PARTICLE IMAGE VELOCIMETRY TECHNIQUES

Abstract

Flow characteristics, such as velocity profiles and Reynolds stresses downstream of heart valve prostheses are vital parameters in the study of hemolysis and thrombus formation associated with heart valve prostheses. They have previously been evaluated using single point measurement techniques such as laser Doppler anemometry and hot film anemometry. Limitations of these single point measurement techniques have prompted us to develop particle image velocimetry (PIV) techniques to study the dynamic fluid flow characteristics associated with prosthetic aortic heart valves in vitro under both steady and pulsatile flow conditions. PIV is a multipoint measurement technique that allows full field measurement of instantaneous velocity vectors in a flow field, thus allowing us to map the entire velocity or Reynolds stress field over the aortic root (where single point measurements are difficult). Other quantitative data which were obtained include: pressure drop across valves (resistance), pressure loss coefficients and pulsatile pressure flow versus time tracings. The performances of four prosthetic heart valves; a porcine bioprostheses, a caged ball valve, and two single leaflet tilting disc valves with different opening angles, were evaluated under steady flow conditions. Comparisons were made with an empty aortic root and an empty aortic root with valve mounting ring (to study the effects of orifice reduction). Under pulsatile flow conditions, three prosthetic heart valves were evaluated, a porcine bioprostheses, a caged ball valve, and a single leaflet tilting disc valve. For the porcine valve under pulsatile flow conditions, a Lagrangian concept of stress-induced blood damage was introduced. PIV and flow visualization images obtained from both steady and pulsatile flow conditions showed that all the heart valve prostheses gave rise to areas of turbulence and eddy formation immediately distal of the valve and in the aortic sinus cavity. All valves showed a strong recirculating region in the sinus cavity. Regions of stasis were observed for the tilting disk and caged ball valves, located immediately downstream of the valve occluder. Flow expansion, turbulent mixing and strong momentum exchange were observed further downstream of the valves.

Under steady flow conditions, all of the prosthetic heart valves had pressure drops which increased in a non-linear (almost parabolic) trend with increasing flowrates. The St. Vincent Meditech tilting disc valve recorded the lowest pressure drop (5.1 mm Hg) while the St. Vincent Porcine Tissue valve recorded the highest pressure drop (38 mm Hg) at 35 liters/min. The high pressure drop recorded for the St. Vincent porcine tissue valve can be attributed to the reduction in the effective orifice area. The St. Vincent Meditech tilting disc valve recorded the lowest Reynolds stresses (shear stress of 924 dynes/cm2) among the four valves tested, with the St. Vincent Porcine Tissue valve recording the highest stresses (shear stress of 2090 dynes/cm 2). It is also interesting to note that the porcine valve exhibited Reynolds shear stresses which are twice that of those generated by the three mechanical valves. In general, zones of elevated stresses were found to coincide with zones of turbulent intermixing in the flow, a consequence of the interactions of wake regions, major flow areas and the recirculating region in the sinus cavity. All of the prosthetic heart valves had average pulsatile pressure drops ranging from 10 mm Hg for the Bjork-Shiley valve, 18 mm Hg for the StarrEdwards valve, and 22 mm Hg for the St. Vincent Porcine valve. Peak Reynolds shear stresses of 3469, 3188, 2159 dynes/cm2 were measured at peak systole for the porcine tissue, tilting disk and caged ball valves respectively. From the Lagrangian approach of determining shear stress related blood damage (whereby the shear stresses experienced by a fluid particle as it moves across the elevated shear stress zones are tabulated over time), the St Vincent Porcine Tissue valve exhibited the highest total percentage of possible blood damage (both red blood cells and platelets). The total percentages were higher than that reported in literature. Both the Bjork-Shiley and the Starr-Edwards valve exhibited low total percentages of possible blood damage. Zones of elevated stresses were found to coincide with those found from the steady flow studies. The overall view of the velocity and stress mappings help to identify regions of flow disturbances which otherwise may be lost with single point measuring systems. Although the PIV measurements may lack the accuracy of single point measuring systems, the overall view of the flow in the aortic root region compensates for the shortcoming.