Contactless Measurement of Heart Sounds and Heart Rate with Laser Doppler Vibrometry

Authors: Kristian Kroschel, Armin Luik, Jürgen Metzler

Abstract:

Currently heart sounds are picked up by a stethoscope from the thorax. In case of preterm infants and burnt clients a contactless method is preferred which might be based on laser Doppler vibrometer signals. The raw vibrometer signal is sampled with the sampling frequency fs = 480 Hz so that the usable upper frequency is fu = 240 Hz. The mean of the vibrometer signal is suppressed. The heart sounds are extracted with a linear-phase band-pass filter with the passband 15 Hz ≤ f ≤ 200 Hz. In the filtered signal the two heart sounds S1 and S2 of an adult client without any disease are visible and after transformation also audible. Parallel to the vibrometer signal the electrocardiogram or ECG is monitored. After synchronisation of the filtered vibrometer signal with the ECG the coincedence of the S1 sound with the R-spike of the ECG is visible. The same applies for the S2 sound at the end of the T-wave. The synchronism between the ECG and the heart sounds can be exploited to calculate the heart rate. First, the vibrometer signal is down sampled and filtered to extract the sound signal. The sound signal is rectified, filtered with a linear-phase band-pass filter with the passband adapted to the frequency range of the heart rate. Finally the heart rate extracted from the ECG and the vibrometer signal are compared to each other.