Both hot- and thermoneutral-water immersion reduce 24-h blood pressure in people with hypertension: A randomized crossover study.

 The objective of this study was to characterize the 24-h blood pressure response following an acute hot-water immersion exposure, specifically examining the effect of immersion duration and water temperature, in people with hypertension. Sixteen participants (11 females; 62 ± 7 y; 31.7 ± 7.5 kg.m^-2) with hypertension (taking at least one anti-hypertensive medication) completed four randomized sessions: 1) 20-min and 2) 40-min hot-water immersion (40°C), 3) 40-min thermoneutral immersion (36.5°C), and 4) control (seated, no immersion). Blood pressure, heart rate, perceptual and affective responses were recorded throughout exposure. Immediately after exposure, participants were fitted with an ambulatory blood pressure monitor for the following 24 h. Twenty-four-hour SBP was 7 mm Hg lower (95% CI: -11, -2; p = 0.001) after the 40-min hot-water immersion and 6 mm Hg lower after both the 20-min hot-water immersion (-10, -1; p = 0.006) and 40-min thermoneutral immersion (-9, -3; p = 0.002) compared to control; these effects were similar across nighttime (i.e. 10 pm-6 am) and daytime periods. Twenty-four-hour DBP was not statistically different across any exposure (p = 0.093). The percentage of 24 h in target range for SBP (110-130 mm Hg) more than doubled (39% vs. 18%) following the 40-min hot-water immersion, compared to control. Clinically meaningful decreases in systolic blood pressure were evident with hot-water immersion in people with hypertension; these effects were present irrespective of immersion duration (i.e. 20 min vs. 40 min) or water temperature (36.5°C vs 40°C).