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

Methods and systems for determining a respiration rate (RR) of a subject are disclosed. In one embodiment, a method includes sampling a PPG signal at a first frequency, filtering the PPG signal with a first high-pass filter, receiving an output from the first high-pass filter and filtering the output at a second frequency in a second high-pass filter, counting positive- and negative-edge pulses of a portion of the PPG signal to determine breath-time intervals caused by an influence of the respiration rate on the PPG signal, and determining an average of the breath-time intervals for the positive-edge zero-crossings and the negative-edge zero-crossings to derive an estimate of the RR. In other embodiments, a central frequency of components of the PPG signal is determined based on bandpass filters and a feedback mechanism to estimate β and select an appropriate adaptive filter to determine the RR. Other methods and systems are disclosed.