A Study on the Mechanism for Cavitation in the Mechanical Heart Valves with an Electrohydraulic Total Artificial Heart

Abstract

It has been conceived that the mechanical heart valves mounted in an artificial heart close much faster than in vivo use, resulting in cavitation bubbles formation. In this study, the mechanisms for cavitation in mechanical heart valves (MHVs) is investigated with monoleaflet and bileaflet valves in the mitral position with an electrohydraulic total artificial heart (EHTAH). The valve-closing velocity and pressure-drop through the valve were done, and a high-speed video camera was employed to investigate the mechanism for MHVs cavitation. The valve-closing velocity and pressure-drop of the bileaflet valves were less than that of the monoleaflet valves. Most of the cavitation bubbles in the monoleaflet valves were observed next to the edge of the valve stop and the inner side of the leaflet. With the bileaflet valves, cavitation bubbles were concentrated along the leaflet tip. Also, the number density of cavitation bubbles in the bileaflet valves was less than that of the monoleaflet valves. The number density of cavitation bubbles increased with an increase in the valve-closing velocity and the valve stop area. It is established that squeeze flow holds the key to cavitation in the mechanical heart valve. In a viewpoint of squeeze flow, the bileaflet valve with slow valveclosing velocity and small valve stop area, is safer to prevent of blood cell damage than the monoleaflet valves.Key Words: Mechanical Heart Valve, Cavitation Bubble, Closing Velocity, Squeeze Flow InroductionThe cavitation phenomenon in mechanical heart valves is well documented and has been visualized by stroboscopic photography(1) -(3) . Cavitation in a liquid flow field occurs at locations where the local pressure falls below the vapor pressure of the liquid, creating vaporous bubbles in the field (4) . The collapse of the cavitation bubbles generates a high-speed micro-jet and shock waves that may damage the valve surface and blood components.In general, the major causes of cavitation occurrence * Received 21st May, 2004 (No. 04-4120) * * Department of Artificial Organs, Research Institute, National Cardiovascular Center, 5-7-1 Fujishiro-dai, Suita, Osaka 565-8565, Japan. E-mail: hslee@ri.ncvc.go.jp * * * Japan Association for the Advancement of Medical Equipment, 3-42-6 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan in a mechanical heart valve were as follows: venturi effect due to flow after valve closure at the narrow gap between leaflet and valve housing, water hammer effect due to the sudden stop of the MHV leaflets, and squeeze flow that can take place in the narrow gap between the closing leaflet and valve stop (5) . Bluestein et al. (6) reported that the caivtation bubbles cause to squeeze flow and its maximum velocity reach as high as 30 m/s. Lee et al.( 7) and Shu et al. (8) examined the cavitation threshold of dp/dt for different mechanical heart valves using stroboscopic photography. He et al. (3) investigated the mechanism of the formation of cavitation bubbles in cases involving Medtronic Hall valves; in that study, the average v...