Ear and Finger PPG Wearables for Night and Day Beat-to-Beat Interval Detection

In this work, we study the accuracy of ear and finger photoplethysmography (PPG) based inter-beat interval (IBI) detection and estimation compared to the R-to-R interval (RRI) values derived from the electrocardiography (ECG). Seven male subjects with a mean age of 34.29±5.28 years are asked to wear simultaneously the Senbiosys earbud SBE2200 and the Senbiosys ring SBF2200 together with the Shimmer3 ECG development kit. The study includes 43 recordings with a total duration of 72.21 hours divided into 37.10 and 35.11 hours of sleep and wake recordings, respectively. The obtained results show that the earbud PPG enables a higher beat detection rate and a more accurate IBI estimation than the ring. They also show that the performance of the beat detection and estimation is significantly better for the sleep recordings compared to the wake recordings with an increase of ∼ 1.5% in the detection rate and a decrease of ∼ 1 ms and ∼ 4 ms in the mean absolute error (MAE) and the root mean square error (RMSE), respectively. Moreover, we propose a novel fusion scheme that smartly combines the IBI values from both devices and achieves a superior performance with a beat detection rate of 99.22% and an IBI estimation with MAE and RMSE values of 7.42 ms and 13.45 ms, respectively.