Assessment of In-ear Photoplethysmography as a Surrogate for Electrocardiography in Heart Rate Variability Analysis

Heart rate variability (HRV) analysis is a valuable tool in the investigation of cardiovascular regulation by the autonomic nervous system. Generally, beat-to-beat interval time series, which are necessary for calculating quantitative HRV parameters, are extracted from electrocardiographic (ECG) recordings. However, in situations like home monitoring, acute medical care or the perioperative setting, the recording of ECG signals is inconvenient. Here, in-ear photoplethysmography (PPG) is a promising alternative technology for the acquisition of beat-to-beat intervals. In this work, the accuracy of HRV parameters derived from in-ear PPG recordings is compared to ECG-derived parameters in order to the accuracy of the in-ear PPG as a surrogate for ECG in HRV analysis. For this purpose, recordings of 28 volunteers were collected. Common HRV features from both the time and frequency domain have been calculated from the in-ear PPG signal and a reference ECG signal. For comparison, HRV parameters were also derived from the common fingertip PPG. It could be shown that the in-ear PPG is a viable alternative measurement modality for continuous HRV monitoring when ECG recording is not applicable. Nevertheless, care has to be taken in the selection of HRV parameters that are calculated from the in-ear PPG.