Respiration from a photoplethysmogram (PPG) using fixed and adaptive filtering

Methods and systems for determining a respiration rate (RR) of a subject are disclosed. A photoplethysmogram (PPG) signal (101) is sampled at a first frequency. The signal is then filtering using first and second high-pass filters. In one embodiment, a zero-crossing filter is used 305 to interpolate positive-(rising) and negative-(falling) edge pulses of a portion of the PPG signal, thus to determine breath-time intervals caused by an influence of the respiration rate on the PPG signal. The average of the breath-time intervals for the positive-edge zero-crossings and the negative-edge zero-crossings is determined to give an estimate of RR 309. In other embodiments, a central frequency of components of the PPG signal is determined based on bandpass filters and a feedback mechanism to estimate Beta (ratio of RR to pulse rate) (Figure 6).