Wrist02: Reliable Peripheral Oxygen Saturation Readings from Wrist-Worn Pulse Oximeters
arXiv preprint, Technical Report arXiv:1906.07545, June 2019
Peripheral blood oxygen saturation (SpO2) is a vital measure in healthcare. Modern off-the-shelf wrist-worn devices, such as the Apple Watch, FitBit, and Samsung Gear, have an onboard sensor called a pulse oximeter. While pulse oximeters are capable of measuring both SpO2 and heart rate, current wrist-worn devices use them only to determine heart rate, as SpO2 measurements collected from the wrist are believed to be inaccurate. Enabling oxygen saturation monitoring on wearable devices would make these devices tremendously more useful for health monitoring and open up new avenues of research. To the best of our knowledge, we present the first study of the reliability of SpO2 sensing from the wrist. Using a custom-built wrist-worn pulse oximeter, we find that existing algorithms designed for fingertip sensing are a poor match for this setting, and can lead to over 90% of readings being inaccurate and unusable. We further show that sensor placement and skin tone have a substantial effect on the measurement error, and must be considered when designing wrist-worn SpO2 sensors and measurement algorithms. Based on our findings, we propose WristO2, an alternative approach for reliable SpO2 sensing. By selectively pruning data, WristO2 achieves an order of magnitude reduction in error compared to existing algorithms, while still providing sufficiently frequent readings for continuous health monitoring.