Quantification of inspiratory‐induced vasoconstrictive episodes: a comparison of laser Doppler fluxmetry and photoplethysmography

Laser Doppler fluxmetry (LDF) or photoplethysmography (PPG) are frequently used as non‐invasive tools to assess skin blood flow. They are both thought to be suitable and interchangeable methods for the detection of sympathetically activated vasoconstrictive episodes that can be provoked by a voluntary deep inspiration (DI), the so‐called inspiratory gasp response (IGR). However, a rigorous comparison of the two methods has not been reported in the literature. The aim of this study was to compare the detection of IGR with LDF and PPG. We investigated 14 healthy volunteers (five female, nine male, 25·4 ± 2·7 years). A PPG probe was applied to the tip of the thumb of the dominant hand, a LDF sensor to the tip of the adjacent index finger. After adaptation and baseline measurements the subjects were asked to perform a DI with time intervals of 90, 60, 30 and 15 s. We found that both methods are useful to detect individual IGR. Overall correlation of IGR amplitude detected with LDF and PPG was significant (r = 0·433). Surprisingly, there was a continuous increase of the correlation coefficient from the first (r = 0·105) or second (r = −0·184) IGR to the fifth (r = 0·727) IGR. This implies that experimental data obtained with PPG and LDF are not equivalent and that one has to be cautious regarding the comparison and interpretation of results obtained with these two different methods.