Comparison among low-cost portable systems for thoracic impedance plethysmography

Impedance plethysmography is a technique which allows the monitoring of respiratory activity through the measurement of variations in the trans-thoracic impedance. Provided the measurement system has enough sensitivity, the same technique can also detect heartbeats, thus adding cardiac activity monitoring capabilities. In this paper, a comparison is performed between 2-electrode and 4-electrode systems, considering both volt-amperometric and AC-bridge solutions. Attention is focused on low-cost, easily portable, and reconfigurable measurement set-ups, whose performances are analyzed in terms of sensitivity and accuracy. To this end, LabVIEW virtual instruments based on a data acquisition card are employed. An experimental metrological assessment of the systems shows that their basic accuracy is definitely adequate for patient monitoring. However, based on a theoretical study on measurement resolution, only the volt-amperometric solution in 4-electrode configuration achieves enough sensitivity to correctly detect the cardiac activity. The above conclusion is verified by acquisitions made on several volunteers.