Abstract
Hypertension affects more than 30% of adults, increasing the risk of developing heart, brain, and kidney diseases. Hot-water bathing has been used for centuries as a form of healing, with recent research revealing it improves resting blood pressure and cardiovascular health. It is unknown whether heat per se and/or the hydrostatic effect of immersion are responsible for acute blood pressure responses, nor what duration is most effective. Therefore, the objective of this study was to characterise 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 (10 females) with hypertension (taking at least one anti-hypertensive medication) completed four randomised sessions: 1) 20 min and 2) 40 min hot-water immersion (HWI, 40 °C), 3) 40 min thermoneutral immersion (36.5 °C), and 4) control (seated, no immersion). Immediately after exposure, participants were fitted with an ambulatory blood pressure monitor for the following 24 h.
Mean 24 h systolic blood pressure was 7 mm Hg lower (95%CI: 2, 11; p=0.001) after the 40 min HWI and 6 mm Hg lower after both the 20 min HWI (1, 10; p=0.006) and 40 min thermoneutral immersion (2, 10; p=0.002) compared to control; these effects were similar across night-time (i.e., 10 pm-6 am) and daytime periods. Mean 24 h diastolic blood pressure was not different across any exposure (p=0.093). The percentage of 24 h in target range for systolic blood pressure (110-130 mm Hg) more than doubled (39% vs. 18%) following the 40 min HWI, 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). Future work should explore if similar effects occur in more ecologically valid settings (e.g., home bath or lagoon bathing).