K-Band Doppler Radar is feasible and accurate to record and assess overnight respiratory rate

Sleep-disordered breathing (SDB) occurs in 10% to 17% of the male and 3% to 9% of the female population, and is identified as one of the leading causes of hypertension and cardiovascular morbidity. Polysomnography as the current state-of-the-art monitoring and diagnosis technique is highly resource intensive and significantly obtrusive for long term overnight usage. In this study, we aimed to develop a contact-less respiratory measurement device to facilitate long-term home based SDB diagnosis and monitoring. We used a dual channel Doppler transceiver to transmit electromagnetic waves (K-band, 24 GHz) and a non-linear arc-tangent demodulation scheme to measure the resulting phase variations in the received waves, reflected from the thorax of the subject. A band-pass filter isolated the periodic respiratory chest wall movement from the received signal. A peak detection algorithm counted the inspiratory maxima during a given period of measurement (respiratory rate). We compared the respiratory rates from our prototype (RR-PTP) with the gold standard respiratory inductance plethysmography (RR-RIP) in overnight studies with two volunteers. Linear regression yielded: RR-PTP= 1.06*RR-RIP – 1.22, r2 = 0.96, n=405. Bland-Altman analysis showed a mean bias of -0.14, and upper and lower limit of agreement of 0.86, -1.1 respectively. The current prototype for contact-less estimation of overnight respiratory rate correlates and agrees very well with the gold standard respiratory inductance plethysmography.