Background

Passive heat exposure may be a therapeutic strategy to reduce blood pressure and improve cardiovascular health. Evidence within hypertensive populations indicates acute passive hot water immersion (HWI) causes hypotension that persists over 24 hours. It is currently unknown whether a 24-hour post-heat-exposure hypotensive effect exists in young, healthy individuals. This study aimed to determine the influence of HWI on blood pressure responses within the first hour and the following 24 hours after a single session of HWI.

Methods

To date, nine healthy males (26 ± 4 years; 85 ± 11 kg; 180 ± 5 cm) completed two experimental trials in a randomised order, separated by at least 48 hours. In the HWI trial, participants were immersed to the neck in 40.5°C water for 30 min, followed by an additional 30 min at waist level. In the time-of-day matched control (CON) trial participants rested seated in 27°C air. Thermoregulatory and cardiovascular measures were assessed before, during, and 1 hour following HWI and CON interventions. At the end of the 1-hour recovery period, an ambulatory blood pressure monitor was fitted to the participant for 24 hours. All statistical analysis was conducted in GraphPad Prism, with two-way analysis of variance conducted to assess differences in dependent variables during HWI and CON trials. Where significance was identified, using the critical P value of 0.05, post-hoc analysis was conducting using Bonferroni adjustment. Two-tailed, paired samples t-tests were conducted to assess 24-hour blood pressure data.

Results

Preliminary analyses demonstrate at 60 min of the interventions, core temperature and heart rate were higher during HWI than CON (CON, 36.9 ± 0.4°C; HWI, 38.8 ± 0.2°C, P <0.001; CON, 67 ± 6 bpm; HWI, 102 ± 11 bpm, P < 0.001) and mean arterial blood pressure (MAP) and diastolic blood pressure (DBP) was lower (MAP; CON, 84 ± 6 mmHg; HWI, 73 ± 6 mmHg, P =0.002; DBP; CON, 70 ± 7 mmHg; HWI, 55 ± 7 mmHg, P =<0.001). Systolic blood pressure (SBP) was lower during HWI compared to CON at 30 min of the intervention only (CON, 112 ± 5 mmHg; HWI, 105 ± 7 mmHg, P =0.049). Throughout the 1-hour recovery period, core temperature was higher after HWI than CON (trial effect; P <0.05). DBP was lower immediately post-HWI compared to CON (CON, 68 ± 9 mmHg; HWI, 57 ± 7 mmHg, P =0.013) returning to pre-immersion values within 10 minutes post-HWI. No differences were detected between HWI and CON for SBP and MAP during the 1-h recovery period (all P >0.058). 24-hour MAP, SBP and DBP responses were not different for daytime, nighttime or overall mean values between CON and HWI interventions when assessed using ambulatory blood pressure monitoring (all P >0.169). 

Conclusion

Preliminary data from this study indicate that HWI reduces arterial blood pressure in young healthy adults. However, the hypotension did not persist beyond 10 minutes after ceasing HWI despite core temperature remaining elevated during the 1-hour recovery period.