Key result
Arterial pressure and MSNA showed high coherence with low-frequency fluctuations in ear PPG, with midfrequency powers positively correlated between MSNA and ear PPG (r=0.77, P=0.002).
Observational (n=13)
Effect estimate: r = 0.77
p-value: p=0.002
Spontaneous PPG variability in the ear includes a major contribution from arterial pressure and muscle sympathetic nerve activity, highlighting differential mechanisms governing PPG waveform fluctuations across body sites.
Ear PPG may better track sympathetic and pressure signals in research; leaves open clinical translation beyond healthy volunteers.
Assessment of spontaneous slow waves in the peripheral blood volume using the photoplethysmogram (PPG) has shown potential clinical value, but the physiological correlates of these fluctuations have not been fully elucidated. This study addressed the contribution of arterial pressure and muscle sympathetic nerve activity (MSNA) in beat-to-beat PPG variability in resting humans under spontaneous breathing conditions. Peripheral PPG waveforms were measured from the fingertip, earlobe, and toe in young and healthy individuals (n = 13), together with the arterial pressure waveform, electrocardiogram, respiration, and direct measurement of MSNA by microneurography. Cross-spectral coherence analysis revealed that among the PPG waveforms, low-frequency fluctuations (0.04-0.15 Hz) in the ear PPG had the highest coherence with arterial pressure (0.71 ± 0.15) and MSNA (0.44 ± 0.18, with a peak of 0.71 ± 0.16 at 0.10 ± 0.03 Hz). The normalized midfrequency powers (0.08-0.15 Hz), with an emphasis on the 0.1-Hz region, were positively correlated between MSNA and the ear PPG (r = 0.77, P = 0.002). Finger and toe PPGs had lower coherence with arterial pressure (0.35 ± 0.10 and 0.30 ± 0.11, respectively) and MSNA (0.33 ± 0.10 and 0.26 ± 0.10, respectively) in the LF band but displayed higher coherence between themselves (0.54 ± 0.09) compared with the ear (P < 0.001), which may suggest the dominance of regional vasomotor activities and a common sympathetic influence in the glabrous skin. These findings highlight the differential mechanisms governing PPG waveform fluctuations across different body sites. Spontaneous PPG variability in the ear includes a major contribution from arterial pressure and MSNA, which may provide a rationale for its clinical utility.
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Chan et al. (2011) conducted an observational in Healthy (n=13). Arterial pressure and muscle sympathetic nerve activity (MSNA) was evaluated on Cross-spectral coherence between PPG waveforms and arterial pressure/MSNA (r = 0.77, p=0.002). Arterial pressure and MSNA showed high coherence with low-frequency fluctuations in ear PPG, with midfrequency powers positively correlated between MSNA and ear PPG (r=0.77, P=0.002).
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