Dipping status may be determined by nocturnal urination

Nondipping, ie, failure to reduce blood pressure by greater than or equal to 10% during the night, is considered an important prognostic variable of 24-hour ambulatory blood pressure monitoring. However, some people wake up at night to urinate. Usually 24-hour ambulatory blood pressure monitoring-derived blood pressure includes these rises in the nighttime blood pressure mean. We identified 97 subjects undergoing 24-hour ambulatory blood pressure monitoring who reported waking up at night to urinate. We assessed the 24-hour ambulatory blood pressure monitoring first using total daytime and total nighttime means and then using actual daytime awake and nighttime asleep (as reported by the patient) means. Nocturnal decline in blood pressure was 14.4+/-8.5/11.8+/-6.1 mm Hg with the first method and 17.1+/-8.3/13.8+/-5.9 mm Hg with the second one (P<0.00001). Although the absolute difference between the nocturnal blood pressure declines calculated by the 2 methods was small, the effect on nocturnal dip was profound. Average systolic blood pressure dipping was 10.1% by the total day-total night method and 12.0% by the actual day awake-night asleep method (P<less than or equal to>0.00001), and that of diastolic blood pressure was 14.2% and 16.7%, respectively (P less than or equal to0.00001). The prevalence of systolic blood pressure nondipping decreased from 42.2% by the first method to 31.9% by the second method (P less than or equal to0.0056), and that of diastolic blood pressure nondipping decreased from 22.6% to 11.3% (P less than or equal to0.00001). Inclusion of awake blood pressure measurements during the night obscured the normal dipping pattern in people who woke up to urinate. Thus, taking into account people's actual behavior increases the accuracy of the results.