Background: The non-invasive photoplethysmographic (PPG) signal reflects blood flow and volume in a tissue. The PPG signal shows variation synchronous with heartbeat (PPGc), as used in pulse oximetry, and variations synchronous with breathing (PPGr). PPGr has been used for non-invasive monitoring of respiration with promising results. Our aim was to investigate PPG signals recorded from different skin sites in order to find suitable locations for parallel monitoring of variations synchronous with heartbeat and breathing.
Methods: PPG sensors were applied to the forearm, finger, forehead, wrist and shoulder on 48 awake healthy volunteers. From these sites, seven PPG signals were simultaneously recorded during normal spontaneous breathing over 10 min. Capnometry served as respiration and electrocardiogram (ECG) as pulse reference signals. PPG signals were compared with respect to power spectral content and squared coherence.
Results: Forearm PPG measurement showed significantly higher power within the respiratory region of the power spectrum [median (quartile range) 42 (26)%], but significantly lower power within the cardiac region [9 (10)%] compared with the other skin sites. PPG finger measurement showed the opposite; in transmission mode, the power within the respiratory region was significantly lower [4 (10)%] and within the cardiac region significantly higher [45 (25)%] than the other sites. PPGc coherence values were generally high [>0.96 (0.08)], and PPGr coherence values lower [0.83 (0.35)-0.94 (0.17)].
Conclusion: Combined PPG respiration and pulse monitoring is possible, but there are significant differences between the respiratory and cardiac components of the PPG signal at different sites.