Influence of nighttime bathing on evening home blood pressure measurements: how long should the interval be after bathing?

The Japanese Society of Hypertension has recommended that evening home blood pressure measurement be taken just before bedtime. In this study, to elucidate the influence of nighttime bathing on evening home blood pressure and heart rate, measurements were performed for 7 days using volunteers who were employees of a single company and who had no alcohol intake during the study period. We used data obtained from 158 subjects (78 males and 80 females; mean age, 41.6 years) whose evening data consisted of a combination of pre-bathing and post-bathing measurements. We divided the subjects into four groups according to the time interval from bathing: blood pressure was measured at 30 min after bathing in group I (n=40), at 31-60 min after bathing in group II (n=89), at 61-120 min after bathing in group III (n=74) and at more than 121 min after bathing in group IV (n=53). We evaluated the changes after bathing in each group. For all subjects combined, the evening home blood pressure measured after bathing (114.0+/-17.1/69.4+/-10.9 mmHg) was significantly lower than the value before bathing (116.3+/-17.1/70.7+/-11.2 mmHg). However, there was no difference in heart rate. Both systolic and diastolic blood pressure after bathing in group I (109.1+/-15.2/66.3+/-10.8 mmHg) and II (112.0+/-15.2/66.5+/-10.1 mmHg) were significantly lower than those before bathing (group I: 113.2+/-15.8/70.2+/-10.6 mmHg; group II: 115.2+/-15.8/69.3+/-10.3 mmHg), but these differences disappeared in group III and IV. On the other hand, there was no difference in heart rate after bathing in group I, II, or III, but group IV showed a slight but significant decrease after bathing (67.7+/-10.0 --> 65.8+/-10.7 beats/min). In conclusion, if evening home blood pressure is to be measured after bathing, subjects should be instructed to wait more than 60 min after bathing before performing the measurement in order to eliminate the depressor effect of bathing.