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Estimation of spontaneous respiratory rate from photoplethysmography by cross time-frequency analysis

In this paper, a methodology to indirectly estimate the respiratory rate from the photoplethysmography (PPG) signal is presented. The possibility to reliably estimate respiratory rate from the PPG signal is particularly appealing since PPG is simple, comfortable and cheap. The underlying hypothesis of this methodology is that respiration provokes simultaneous changes in the pulse interval, amplitude and width of the PPG signal. These respiratory related changes are combined together by cross time-frequency analysis, performed by smoothed pseudo Wigner-Ville distribution, in order to obtain indirect estimates of respiratory rate. The algorithm is designed to yield estimates only when the estimation is robust. In 17 spontaneous breathing subjects, among which 7 were characterized by a respiratory rate lower than 0.15 Hz, this methodology provided accurate estimates, being the median error 0.00 ±1.95 mHz (0.00 ± 0.75%) and the interquartile range error 7.81 ± 6.10 mHz (3.00 ± 4.03%).