Introduction. Since 1816 several ways of recording heart sounds have been in use. More recently, seismocardiography, electromagnetic wave Doppler, laser Doppler vibrometry and ultrasound echo Doppler have been shown to produce signals that in various ways reflect the movements of the heart. So far, low frequency ultrasound echo, either in a pulsed wave mode or as continuous wave mode, has been shown to give an accurate account of the thoracic skin vibrations due to the mechanical cardiac activity, although this was shown for low frequency (seismocardiographic) signals only. In the present work we investigate the use of ultrasound echo Doppler beyond the reproduction of seismocardiographic signals.
Methods. A 400EP250 Prowave ultrasound transceiver was used to transmit a 40 kHz acoustic sinusoidal wave directed to the chest wall. The reflected signal (echo) was recorded using a CM16/CMPA ultrasound microphone, a 116H US gate and USGH recording software from Avisoft. The signal was sampled at 1 MHz. The echo signal consists of the 40 kHz carrier, which is frequency modulated by the displacement of the chest wall. FM demodulation was done in MatLab using the quadrature FM demodulation method. Differentiation of the resulting signal gave the velocity and a second differentiation yielded the acceleration. The latter was compared with the seismocardiographic signal obtained using an accelerometer.
Results. Very low-frequency signals such as chest movement due to breathing as well as seismocardiographic signals were obtained with a very good correlation with actual seismocardiography. In addition, valve sounds could be isolated from the signal. Interestingly, signals obtained directly from the skin and signals obtained through a thin layer of clothes were similar.
Conclusion. Non-contact continuous-wave echo Doppler is a promising new modality for the recording of heart sounds, although its true potential has to be further investigated.