Mechanical Heart Valve Cavitation: Valve Specific Parameters

Abstract

Several aspects of mechanical heart valve cavitation, in particular of "severe" vapor cavitation, have been investigated in order to describe the phenomenon of cavitation itself and to classify various mechanical heart valves with respect to their tendency to cavitation. Furthermore, following the results of the measurements, a model for determination of time-dependent physical properties and dynamics of cavitation bubbles, such as size, pressure and temperature was developed. In order to classify the cavitation tendency of mechanical valves, a pulsatile hydraulic-driven circularly mock loop was used. Besides measurements of the relevant hemodynamic parameters, the leaflet velocities of the valves were also determined. In addition, numerous high-resolution pressure measurements, in particular the pressure drops necessary for the initiation of cavitation (local atrial pressure drop), were performed. For the investigation of bubble dynamics, a second pulsatile electro-magnetically-driven tester was used. The influence of density, viscosity and temperature of the fluid on the onset of cavitation was investigated. Cavitation events were recorded with a digital high-speed video camera (up to 40,500 frames/sec) for all investigated heart valves and under different conditions. A critical local upstream pressure drop (located within the model atrium after valve closure) of 450 mmHg was found for all valves as well as a valve specific correlation between left ventricular pressure gradient and local upstream pressure drop. Also, a valve dependent correlation between left ventricular pressure gradient and the local upstream pressure drop was provided. Finally, valve specific parameters were found to predict the cavitation tendency for a specific heart valve. The implementation of a suitable theoretical model allowed conclusions on bubble physics. High pressures (up to 800 bar) and temperatures (up to 1,300 degrees C) at bubble collapse have been determined. The influence of fluid parameters such as density, viscosity and temperature on the onset of cavitation is negligible within physiological range. Critical regions for cavitation for all mechanical heart valves were detected. All mechanical heart valves investigated show cavitation under different conditions (dp/dt) associated with high pressures and temperatures at bubble collapse. Cavitation bubble occurrence depends on valve design and location.