Acute hot-water immersion augments the diastolic blood pressure nocturnal dip in healthy adults

Hot‐water immersion (HWI) has been shown to reduce 24 h ambulatory systolic blood pressure in hypertensive adults and might represent a preventative strategy for maintaining cardiovascular health in normotensive adults. The purpose of this study was to determine the time course of post‐HWI hypotension and test the hypothesis that a single HWI reduces subsequent 24 h ambulatory blood pressure (ABP) in healthy adults. In a randomized, crossover design, 23 participants [7 female and 16 male; 26 (4) years of age] underwent blood pressure assessments before, during, immediately after and for 24 h following 60 min of immersion in 40.4°C hot water (HWI) and a thermoneutral (24.7°C) air control (CON). Thermal and cardiovascular variables were assessed during and for 60 min after the intervention, after which the participants were instrumented with an ABP monitor for 24 h. At min 60 of the interventions, body core temperature was higher [CON, 36.93 (0.28)°C; HWI, 38.83 (0.19)°C; P < 0.001] and diastolic (P < 0.001) and mean (P < 0.001) arterial blood pressure lower in HWI than CON. HWI increased the diastolic blood pressure nocturnal dip compared with CON [CON, 15.4 (7.2)%; HWI, 19.1 (7.4)%; P = 0.022]. No differences in ABP were observed for 24 h, daytime or nighttime systolic, diastolic or mean arterial blood pressures (all P > 0.05). In conclusion, HWI transiently decreased mean arterial blood pressure and diastolic blood pressure for ≤20 min post‐heating and increased the subsequent nocturnal diastolic blood pressure dip in healthy adults.