Non-contact and through-clothing measurement of the heart rate using ultrasound vibrocardiography

We present a novel non-contact system for monitoring the heart rate on human subjects with clothes. Our approach is based on vibrocardiography, and measures locally skin displacements. Vibrocardiography with a laser Doppler vibrometer already allows monitoring of this vital sign, but can only be used on bare skin and requires an expensive piece of equipment. We propose here to use an airborne pulse-Doppler ultrasound system operating in the 20-60 kHz range, and comprised of an emitter focusing the ultrasound pulses on skin and a microphone recording the reflected waves. Our implementation was validated in vitro and on two healthy human subjects, using simultaneously laser vibrocardiography and electrocardiography as references. Accurate measurements of the heart rate on clothed skin suggest that our non-contact ultrasonic method could be implemented both inside and outside the clinical environment, and therefore benefit both medical and safety applications.