Optimal fiducial points for pulse rate variability analysis from forehead and finger PPG signals.

OBJECTIVE: The aim of this work is to evaluate and compare five fiducial points for the temporal location of each pulse wave from forehead and finger photoplethysmographic pulse waves signals (PPG) to perform pulse rate variability (PRV) analysis as a surrogate of heart rate variability (HRV) analysis. 
 
 Approach: Forehead and finger PPG signals were recorded during tilt-table test simultaneously to the ECG. Artifacts were detected and removed and, five fiducial points were computed: apex, middle-amplitude and foot points of the PPG signal, apex point of the first derivative signal and, the intersection point of the tangent to the PPG waveform at the apex of the derivative PPG signal and the tangent to the foot of the PPG pulse defined as intersecting tangents method. Pulse period (PP) time intervals series were obtained from both PPG signals and compared to the RR intervals obtained from the ECG. Heart and pulse rate variability signals (HRV and PRV) were estimated and, classical time and frequency domain indices were computed.
 
 Main Results: The middle-amplitude point of the PPG signal (n<sub>M</sub>), the apex point of the first derivative (n<sub>A</sub><sup>*</sup>), and the tangents intersection point (n<sub>T</sub>) are the most suitable fiducial points for PRV analysis, which result in the lowest relative errors estimated between PRV and HRV indices, higher correlation coefficients and reliability indexes. Statistically significant differences according to the Wilcoxon test between PRV and HRV signals were found for the apex and foot fiducial points of the PPG, as well as the lowest agreement between RR and PP series according to Bland-Altman analysis. Hence, they have been considered less accurate for variability analysis. In addition, the relative errors are significantly lower for n<sub>M</sub> and n<sub>A</sub><sup>*</sup> features by using Friedman statistics with Bonferroni multiple-comparison test and, we propose n<sub>M</sub> as the most accurate fiducial point. Based on our results, forehead PPG seems to provide more reliable information for a PRV assessment than finger PPG. 
 
 Significance: The accuracy of the pulse wave detections depends on the morphology of the PPG. There is therefore a need to widely define the most accurate fiducial point to perform a PRV analysis under non-stationary conditions based on different PPG sensor locations and signal acquisition techniques.&#13.