Increasing the dynamic range of a pulse oximeter using heart rate characteristics

Aims: This theoretical investigation aimed to increase the dynamic range of a pulse oximeter by reducing electronic noise in the photoplethysmogram (PPG) using characteristics of the heart rate (HR) signal. The PPG is used to measure blood oxygen saturation (SpO₂). Methods: We developed a novel algorithm for dynamically tuning a band-pass filter in real-time to pass the SpO₂ information while maximally rejecting the electronic noise. The algorithm tunes the filter based on characteristics of the HR signal recordedfrom a separate source (e.g. an electrocardiogram, or ECG). We derived a theoretical model of the signal and noise levels in a physiological monitor, and calculated the expected increase in dynamic range resultingfrom the new filter. Results: The dynamically tuned band-pass filter achieved a narrow bandwidth of just 0.1 - 0.2 Hz, increasing the PPG SNR by 14 dB (5x). If the filter is applied to the PPG signal from the physiological monitor´s SpO₂ AFE, the device could theoretically measure signal levels 5x lower than without the filter. Conclusions: A band-pass filter, tuned based on characteristics of the HR signal, can be used to reduce the electronic noise in the PPG signal, increasing SNR and thus increasing the dynamic range of a pulse oximeter.