Vascular, inflammatory, and perceptual responses to hot water immersion; impacts of water depth and temperature in young healthy adults

Repeated hot water immersion can improve cardiovascular health; however, the respective effects of distinct immersion protocols remain unclear. Twenty-two healthy adults completed three 30-min hot water immersion bouts of different water temperatures and immersion depths (40°C shoulder-deep immersion, 40-Shoulder; 42°C waist-deep immersion, 42-Waist; and 40°C waist-deep immersion, 40-Waist) in a randomised crossover design. Vascular, inflammatory and perceptual responses were collected via brachial and superficial femoral artery ultrasound, venous blood sampling and perceptual scales. Rectal temperature increased less in the 40-Waist (Δ0.5 ± 0.1°C) condition than the other conditions (40-Shoulder: Δ0.9 ± 0.3°C, 42-Waist: Δ0.9 ± 0.3°C, P < 0.001). Arm skin temperature increased more in the 40-Shoulder (Δ5.2 ± 1.9°C) condition than the other conditions (40-Waist: Δ2.6 ± 1.0°C, 42-Waist: Δ3.6 ± 1.1°C, P < 0.001), whilst thigh temperature had a greater increase in the 42-Waist (8.6 ± 1.3°C) condition than either the 40-Waist (7.8 ± 0.2°C) or 40-Shoulder (Δ7.8 ± 1.0°C) conditions (P < 0.001). Brachial artery shear rate was greatest post-immersion following the 40-Shoulder condition (40-Shoulder: Δ121 ± 94/s, 42-Waist: Δ47 ± 73/s, 40-Waist: Δ-21 ± 41/s, P < 0.001) whereas superficial femoral artery shear rate was largest following the 42-Waist condition (40-Shoulder: Δ143 ± 61/s, 42-Waist: 196 ± 85/s, 40-Waist: 131 ± 93/s, P < 0.001). IL-6 (P = 0.16) and cortisol (P = 0.83) responses did not differ between conditions. Perceptual responses were more favourable in the 40-Waist condition. Taken together, these data demonstrate that the distinct region-specific arterial responses align with increases in local skin temperature to alterations in hot water immersion protocols, whilst showing that beneficial physiological responses may be accompanied with less favourable perceptual responses.