Time–frequency analysis of the noise produced by the closing of artificial heart valves: an in vitro study

Clinical investigations showed that patients with an implanted mechanical heart valve are sometimes disturbed by the noise produced by the valve closing. The aim of this study was to investigate the noise generated by mechanical artificial heart valves with reference to the one produced by biological valves. This type of biological valve was used as a standard because it does not disturb the patient. We used an in vitro approach with a cardiovascular simulator. The noise signal was recorded simultaneously using a hydrophone and an accelerometer. During each cardiac cycle, because of the shortness of the signal (15-25 ms) and of the transient state, rapid frequency dynamics limited the usefulness of stationary analysis techniques for the first heart sound. Therefore, a time-frequency technique was much more convenient than a classic Fourier analysis. We used a discrete wavelet transform that pointed out the time fluctuation of each frequency component. Globally, mechanical valves produced a high sound level within the octave bandwidths centered on 64 Hz up to 512 Hz, while the amplitude of these components decreased rapidly for the biological valve.