Key result
Cold exposure significantly increased the sympathetic low-frequency to high-frequency power ratio in the peripheral finger vasculature from 19.2 to 86.4, while the core ear canal ratio remained unchanged.
Why the study?
Differences in the degree of sympathetic nerve activity over cutaneous blood vessels between the periphery and core vasculature had not been thoroughly investigated quantitatively.
Does photoplethysmography quantitatively assess differences in sympathetic nerve activity between peripheral and core vasculature during cold stress in healthy volunteers?
Observational (n=24)
No
Does photoplethysmography quantitatively assess differences in sympathetic nerve activity between peripheral and core vasculature during cold stress in healthy volunteers?
Absolute Event Rate: 86.4% vs 19.2%
p-value: p=0.002
Photoplethysmography can quantitatively assess sympathetic nerve activity, demonstrating that central autoregulation is more effective than peripheral regulation during cold stress.
Suggests selective peripheral sympathetic activation by cold; hypothesis-generating for autonomic monitoring and requires validation before clinical use.
The differences in the degree of sympathetic nerve activity (SNA) over cutaneous blood vessels, although known to be more prominent in the periphery than the core vasculature, has not been thoroughly investigated quantitatively. Hence, two studies were carried out to investigate the 6 differences in SNA between the periphery and the core during the cold pressor test (CPT) (right-hand immersion in ice water) and cold exposure (whole body exposed to cold air) using photoplethysmography (PPG). Two methods utilizing PPG, namely differential multi-site PTT measurements and low-frequency spectral analysis were explored for quantitative determination of SNA. Each study involved 12 healthy volunteers, and PPG signals were acquired from the right index finger (RIF), left index finger (LIF) (periphery) and the ear canal (core). During CPT, Pulse Transit Time (PTT) was measured to the respective locations and the mean percentage change in PTT during ice immersion at each location was used as an indicator for the extent of SNA. During cold exposure, the low-frequency spectral analysis was performed on the acquired raw PPGs to extract the power of the sympathetic (low-frequency (LF): 0.04 - 0.15 Hz) and parasympathetic components (high-frequency (HF): 0.15 - 0.4 Hz). The ratio of LF/HF components was then used to quantify the differences in the influence of SNA on the peripheral and core circulation. PTT measured from the EC, and the LIF has dropped by 5% and 7% respectively during ice immersion. The RIF PTT, on the other hand, has dropped significantly (P<0.05) by 12%. During the cold exposure, the LF/HF power ratio at the finger has increased to 86.4 during the cold exposure from 19.2 at the baseline (statistically significant P= 0.002). While the ear canal LF/HF ratio has decreased to 1.38 during the cold exposure from 1.62 at baseline (P=0.781). From these observations, it is evident that differential PTT measurements or low-frequency analysis can be used to quantify SNA. The results also demonstrate the effectiveness of the central auto-regulation during both short and long-term stress stimulus as compared to the periphery.
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Budidha et al. (2019) conducted an observational in Healthy volunteers (Cold stress response) (n=24). Cold stress (whole body cold exposure and cold pressor test) vs. Baseline (room temperature) was evaluated on LF/HF power ratio at the finger during whole body cold exposure (p=0.002). Cold exposure significantly increased the sympathetic low-frequency to high-frequency power ratio in the peripheral finger vasculature from 19.2 to 86.4, while the core ear canal ratio remained unchanged.
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