Myths and methodologies: Reliability of forearm cutaneous vasodilatation measured using laser-Doppler flowmetry during whole-body passive heating.

Laser-Doppler flowmetry (LDF) is commonly used to assess cutaneous vasodilatation responses, but its reliability (i.e. consistency) during whole-body passive heating is unknown. We therefore assessed the reliability of LDF-derived indices of cutaneous vasodilatation during incremental whole-body heating. Fourteen young men (age: 24 (SD 5) years) completed three identical trials, each separated by 1 week. During each trial, a water-perfused suit was used to raise and clamp oesophageal temperature at 0.6°C (low-heat strain; LHS) and 1.2°C (moderate-heat strain; MHS) above baseline. LDF-derived skin blood flow (SkBF) was measured at three dorsal mid-forearm sites, with local skin temperature clamped at 34°C. Data were expressed as absolute cutaneous vascular conductance (CVCabs ; SkBF/mean arterial pressure) and normalised to maximal conductance (%CVCmax ) achieved via simultaneous local skin heating to 44°C and increasing oesophageal temperature to 1.8°C above baseline. Between-day reliability was characterised as measurement consistency across trials, while within-day reliability was characterised as measurement consistency across adjacent skin sites during each trial. Between- and within-day absolute reliability (coefficient of variation) generally improved with increasing heat strain, changing from poor (>25%) at baseline, poor-to-moderate (15-34%) at LHS, and moderate (10-25%) at MHS. Generally, these estimates were more consistent when expressed as %CVCmax . Conversely, relative reliability was mostly acceptable (intraclass correlation coefficient ≥0.70) during LHS and when data were expressed as CVCabs . These findings indicate that the consistency of LDF-derived CVC estimates during heat stress depends on the level of heat strain and method of data expression, which should be considered when designing and interpreting experiments.