Photoplethysmographic measurement of heart and respiratory rates using digital filters

A monitor for simultaneous recording of heart and respiratory rates has been developed using photoplethysmography (PPG) and digital filters. PPG signal includes both heart and respiratory synchronous components. Digital filtering technique was employed to differentiate between those. With increasing heart and respiratory rates, the cut-off frequency of filter was changed empirically. Obtained signals agreed with those obtained with ECG and thermistor. The digital filtering may provide on-line monitor for heart and respiratory rates.