Commentary: Photoplethysmography for Quantitative Assessment of Sympathetic Nerve Activity (SNA) During Cold Stress

In relation to the paper “Photoplethysmography for Quantitative Assessment of Sympathetic Nerve Activity (SNA) During Cold Stress,” we have recommendations that may be of importance when considering future research aiming to assess SNA with photoplethysmography (PPG). The aim of this study was to investigate the differences in SNA between the periphery and the core during the cold pressor test (right-hand immersion in ice water) and cold exposure (whole body exposed to cold air) using PPG. Based on significant changes in the Low Frequency (LF) to High Frequency (HF) (LF/HF) ratio, the authors concluded that PPG is an effective method to measure SNA. From this conclusion we have highlighted three main issues that need to be addressed. Firstly, the LF/HF ratio is used as evidence for SNA change, based on the assumption that LF reflects SNA, a concept that has long been rejected in psychophysiology. Secondly, given the organization of the sympathetic nervous system, it is not possible to talk about SNA as a single unique concept. Finally, frequency components are investigated without considering breathing frequency, which is very likely to change during the cold pressor and cold exposure test. These critical points are now discussed in detail below.