Finger and forehead PPG signal comparison for respiratory rate estimation

Objective: an evaluation of the location of the photoplethysmogram (PPG) sensor for respiratory rate estimation is performed. Approach: finger-PPG, forehead-PPG, and respiratory signal were simultaneously recorded from 35 subjects while breathing spontaneously, and during controlled respiration experiments at a constant rate from 0.1 Hz to 0.6 Hz, in 0.1 Hz steps. Four PPG derived respiratory (PDR) signals were extracted from each one of the recorded PPG signals: pulse rate variability (PRV), pulse width variability (PWV), pulse amplitude variability (PAV) and the respiratory- induced intensity variability (RIIV). Respiratory rate was estimated from each one of the 4 PDR signals for both PPG sensor locations. In addition, different combinations of PDR signals, power distribution of the respiratory frequency range and differences of the morphological parameters extracted from both PPG signals have been analysed. Main results: results show a better performance in terms of successful estimation and relative error when: i) PPG signal is recorded in the finger; ii) the respiratory rate is less than 0.4 Hz; iii) RIIV signal is not considered. Furthermore, lower spectral power around the respiratory rate in the PDR signals recorded from the forehead was observed. Significance: these results suggest that respiratory rate estimation is better at lower rates (0.4 Hz and below) and that finger is better than forehead to estimate respiratory rate