Thermal physiology of open water wetsuited swimming: A cohort study.

Swimming in cold water is intrinsically unsafe. One of the threats is a fall in deep-body temperature, which adversely affects all body systems and increases the risk of death. Wetsuits mitigate, but do not negate this threat. Environmental conditions may confound findings from laboratory studies or mathematical models. The study aimed to elucidate associations and patterns of deep-body temperature change in open water wetsuited swimmers, thus facilitating risk assessment and improving swimmer safety. In this exploratory cohort study, following collection of anthropomorphic data including a Dual-energy X-ray Absorptiometry scan, deep-body temperature was continuously measured in 76 open water swims by 31 wetsuited swimmers in water temperatures ranging from 8.4°C to 24.5°C. The temperature trajectories were analyzed by piece-wise and linear mixed model regression. Registered with clinicaltrials.gov NCT06685627. Following immersion, there was a tri-phasic deep-body temperature trajectory, although not all phases were present in all swims: an initial rise, then a plateau phase and then thermal decompensation. Water temperature was correlated with the duration of these phases, as was body habitus for the initial rise. Minimum deep-body temperature was correlated with pre-swim temperature. Despite environmental and individual variations, this paper identified a tri-phasic deep-body temperature trajectory for wetsuited swimmers and proposed physiological mechanisms that may account for them. The tri-phasic pattern of an initial rise, a plateau phase and then thermal decompensation, may be useful in assessing the risks of open water swimming and may facilitate development and validation of deep-body temperature models for wetsuited swimmers.