Monitoring breathing rate by fusing the physiological impact of respiration on video-photoplethysmogram with head movements

Objective: The simple observation of breathing rate (BR) remains the first and often the most sensitive marker of acute respiratory dysfunction. In fact, there is evidence that drastic changes in BR are a predictive indicator of adverse events (i.e. cardiac arrest). The aim of this study is to develop a camera-based technology that may provide near-continuous estimation of BR considering the effect of respiration on video-PPG (vPPG). Approach: The technology has been tested in two different experimental settings, including controlled BR and more challenging scenarios with spontaneous breathing patterns. Video data were processed offline to derive the vPPG signal. The method derives respiration from beat-to-beat PPG rate and morphology changes in amplitude and width driven by the physiological relationships between vPPG and respiration. Moreover, respiratory-induced head movements were used as an additional source of information for the vPPG system. A combination of these methods has been exploited to estimate the respiratory rate every 10 seconds. Main results: According to the results, respiratory frequencies in the central range (0.2-0.4 Hz) may be estimated using the vPPG system with a low relative error, epsilon(R) < 2% and interquartile range of the order IQR < 5%. However, the vPPG system showed a drop in performance at respiratory range boundaries, around 0.1 Hz and 0.5 Hz. Significance: This camera-based technology can be used as an ubiquitous BR monitoring system. However, vPPG-based systems should consider the effect of the BR in the estimation, mainly in applications where the respiratory rate is out of the 0.2-0.4 Hz range.