Spectral characteristics of mechanical heart valve closing sounds

In-vivo evaluation of cavitation is based on the registration of high-frequency pressure fluctuations that represent a mixture of both cavitation and valve closing sounds, and are difficult to separate. In order to extract the cavitation signal, a high-pass filter removing the closing sound is applied. Importantly, the cut-off frequency should be chosen based on the valve's resonance pattern. This could be determined in a cavitation-free, air-operated set-up. As air and water/blood have different physical properties that could influence resonance frequencies, it is necessary to correlate the frequency content of the closing sounds recorded in air to represent expected findings in fluid. The study aim was to characterize the impact of the surrounding media on resonance frequency of a sound source, and to develop a method capable of evaluating the spectral characteristics of mechanical heart valves.

Five different valves were investigated. An in-vitro set-up was developed where the valves were operated in an airflow-controlled setting without cavitation. The valve closing sounds were recorded and a spectral analysis was performed. The resonance frequency of a simple sound source was also recorded in water and air in order to evaluate the impact of the surrounding media.

Resonance frequencies from the sound source measured in air increased 14% compared with corresponding measurements in water. These data were used to correct findings from the five valves that showed different spectral characteristics in air. The frequency at which 97.5% of the signal energy was contained ranged from 40.9 to 65.8 kHz.

Using an airflow in-vitro model, it was possible to determine the frequency signature of different mechanical heart valves. This might provide the information needed to design the optimal high-pass filter when evaluating cavitation.