The effect of drysuit diving in warm water on body temperature and post immersion orthostatic hypotension

Introduction: Warm-water diving can limit heat dissipation, particularly when performed in fully encapsulating protective gear, leading to substantial thermal and cardiovascular strain that may impair diver safety. Following immersion, removal of hydrostatic support combined with heat-induced vasodilation may reduce central blood volume and increase susceptibility to orthostatic intolerance during egress and recovery. The extent to which this thermal strain impairs post-immersion orthostatic tolerance remains unknown. Methods: Four randomised, crossover immersion trials were conducted at 28°C, 33°C, 38°C without precooling (38°C), and 38°C with precooling (38°C + Cool), with subjects wearing fully encapsulating dive gear. Subjects walked for up to 60 minutes at approximately 50% of O2max heart rate (HR) or until core temperature (Tc) reached 38.5°C, or they voluntarily stopped. Tc, HR, and perceptual measures were recorded every 10 minutes. Orthostatic tolerance was assessed after immersion via a 70° head-up tilt test. Results: Eight healthy adults completed all aspects of the study. Tc and HR were higher during both 38°C conditions compared with 28°C and 33°C (all P < 0.01) with no differences between 38°C and 38°C + Cool. Sweat loss exceeded 1.2 (SD 0.67) L⋅h-1 in both 38°C conditions compared with ≤ 0.3 (0.32) L⋅h-1 at 28°C and 33°C (P < 0.01). Survival analysis showed orthostatic tolerance decreased with increasing thermal stress (log-rank P = 0.027; trend P = 0.003). Precooling did not reduce peak Tc or HR, nor did it improve tolerance time in 38°C water. Conclusions: Encapsulated warm-water diving causes heat stress and cardiovascular strain that persists after immersion, impairing orthostatic tolerance. Precooling does not significantly reduce these outcomes.