Small-form wearable device for long-term monitoring of cardiac sounds on the body surface

Sound monitoring from sources inside the human body can have important diagnostic relevance in medicine. Cardiac sounds originated from the pumping activity of the heart structure is such an example, with valuable cardiovascular parameters being extracted from the signal, including heart rate (HR) and the systolic intervals. Novel non-invasive methods for early detection of potential life-threatening risks convoyed by unbalanced cardiovascular parameters are essential to reduce the mortality rates associated with cardiac diseases nowadays. In this paper, we propose a small-form wearable device for long-term monitoring of the cardiac sounds through a miniaturized microphone in contact with the body surface at specific locations, which extend from the chest region to the upper and lower body parts. Powered by battery, the device can measure signals for a consecutive period of 28 h in continuous recording mode that is extensive up to 7 days in discontinuous mode, achieving signal amplitude resolution of 0.81 V and optimal bandwidth between 5 to 20 Hz (infrasound range). The proposed device was able to detect cardiac sound patterns in locations as distant as the forehead, wrist, or ankle, thus paving the way to the use of acoustic signals for wearable heartbeat estimators still relying on optical or bio-potential methods, while replacing the obtrusive and expensive cardiography equipment dedicated to the estimation of the systolic intervals directly from the chest.