Low-cost and portable impedance plethysmography system for the simultaneous detection of respiratory and heart activities

Impedance plethysmography is a technique that allows monitoring changes in the volume of specific tissues or organs inside the human body, through the measurement of variations in the bio-impedance between electrodes placed on the body surface. One of its best-known applications is the monitoring of respiratory activity based on trans-thoracic impedance measurements. Provided the measurement system has enough sensitivity, trans-thoracic impedance can also allow the detection of heartbeats, thus adding cardiac activity monitoring capabilities. In this paper, a low-cost and easily portable bio-impedance measurement system based on a volt-amperometric scheme, suitable for continuous wearable cardio-respiratory monitoring, is presented. The system measurement performances in 2-electrode and 4-electrode configurations are compared and the resulting sensitivity is investigated, in order to assess the feasibility of heartbeat detection. The theoretical study on measurement resolution shows that only the 4-electrode configuration achieves enough sensitivity to detect the cardiac activity, as confirmed by experimental acquisitions made on several volunteers. The performances of the impedance plethysmography system in detecting cardio-respiratory activities are evaluated through a comparison with a respiratory belt transducer and an ECG device, showing the suitability of the proposed solution. In particular, the average difference between heart rates evaluated with the impedance plethysmograph and with the ECG instrument is below 3%. Altogether, the evaluated performances are comparable with alternative proposals available in the literature, in spite of the extreme simplicity of the device.