Myths and methodologies: Reliability of non‐invasive estimates of cardiac autonomic modulation during whole‐body passive heating

Abstract

Observed individual variability in cardiac baroreflex sensitivity (cBRS) and heart rate variability (HRV) is extensive, especially during exposure to stressors such as heat. A large part of the observed variation may be related to the reliability (consistency) of the measurement. We therefore examined the test–retest reliability of cBRS and HRV measurements on three separate occasions in 14 young men (age: 24 (SD 5) years), at rest and during whole‐body heating (water‐perfused suit) to raise and clamp oesophageal temperature 0.6°C, 1.2°C and 1.8°C above baseline. Beat‐to‐beat measurements of RR interval and systolic blood pressure (BP) were obtained for deriving HRV (from RR), and cBRS calculated via (i) the spontaneous method, α coefficients and transfer function analysis at each level of heat strain, and (ii) during forced oscillations via squat–stand manoeuvres (0.1 Hz) before and after heating. Absolute values and changes in all cBRS estimates were variable but generally consistent with reductions in parasympathetic activity. cBRS estimates demonstrated poor absolute reliability (coefficient of variation ≥25%), but relative reliability (intraclass correlation coefficient; ICC) of some frequency estimates was acceptable (ICC ≥0.70) during low‐heat strain (ICC: 0.56–0.74). After heating, forced oscillations in BP demonstrated more favourable responses than spontaneous oscillations (better reliability, lower minimum detectable change). Absolute reliability of HRV estimates were poor, but relative reliability estimates were often acceptable (≥0.70). Our findings illustrate how measurement consistency of cardiac autonomic modulation estimates are altered during heat stress, and we demonstrate the possible implications on research design and data interpretation.

Revisiting regional variation in the age‐related reduction in sweat rate during passive heat stress

“…Further, all participants were habitually active (i.e., performed ≥30 min of structured physical activity at least twice per week) as determined by a standardized questionnaire (Baecke et al, 1982). Data from the young men have been reported as part of a larger project assessing the reliability of microvascular, sudomotor, and cardiovascular autonomic function (Akerman et al, 2021; Gemae et al, 2021; Rutherford et al, 2021). Given the exploratory nature of the study, no a priori power analysis was performed.…”

“…Participants completed one screening session and one experimental session in a temperature‐controlled laboratory (~25°C). The experimental sesssion for the young group was the first of three sessions reported in our previous work (Akerman et al, 2021; Gemae et al, 2021; Rutherford et al, 2021). The firstexperimental session for the young group was chosen as it provided the most naïve assessment between both age groups.…”

Cardiovascular autonomic modulation during passive heating protocols: a systematic review

“…On the other hand, the αLF index represents the oscillations between systolic blood pressure and R-R intervals at a low-frequency band (0.1 Hz), where these signals commonly have a high coherence. Also, spontaneous baroreflex sensitivity methods have low reliability during heat stress, while forced low-frequency oscillations in BP could be an alternative to increase the coherence and reliability of BP and R-R oscillations (Akerman et al 2020). Hence, further work is needed to investigate the possible effects of a heated environment (dry or humid), employing different approaches of baroreflex control analysis.…”