STUDY OF AN ACOUSTIC TECHNIQUE TO DETECT CAVITATION PRODUCED BY A TILTING DISC VALVE

Background and aim of the study:Transient cavitation has been directly observed near operating mechanical heart valve's in vitro and inferre d in vivo via the observation of pitting on explanted clinically used valves. Visual detection of cavitation bubbles, however, cannot be acc omplished in vivo or when any opaque fluid, e.g. blood, is used. Metho ds: This study examines a passive acoustic technique for detecting cav itation caused by a 27 mm tilting disc valve. We captured, valve closi ng sounds in vitro and attempted to detect a shift of energy into high er frequencies due to emission of broad-band noise caused by collapsin g bubbles. The valve tester consists of a piston pump which directly d rives the valve, a 16 Cm diameter cylindrical lucite atrium and an air chamber in parallel to provide compliance. Water was used as a blood analog fluid. The cycle rate was altered to vary valve loading and pro duce cavitation. Acoustic signals were detected by a miniature hydroph one and a large area transducer and the waveforms were spectrum analyz ed to 200 kHz and 500 kHz respectively. Cavitation onset was determine d rising a highspeed video camera. Results: It was found that even und er non-cavitating conditions, significant energy was produced at frequ encies greater than 100 kHz, and this energy increased with increased load. The proportion of energy in high frequency bands, however, remai ned fairly constant when cavitation was not present and began to rise only after the cavitation threshold was reached. To isolate cavitation as an independent variable, data were taken with all system parameter s constant, but using water under two different conditions. Degassed 1 7 degrees C water produced no visualizable bubbles, while aerated tap water at 43 degrees C showed a high degree of cavitation. Conclusions: The results indicate that cavitation, while causing a shift of energy to higher frequencies, is not the only mechanism responsible for the shift of energy Into higher frequencies.