Nocturnal Urination May Determine Dipping Status: A 24 Hour Ambulatory Blood Pressure Study.
Non-dipping, i.e. failure to reduce blood pressure (BP) by 10% or more, is considered an important prognostic variable of 24h ambulatory blood pressure monitoring (ABPM). However, some people wake up, rise and go to urinate at night. Usually, 24h ABPM derived means include these rises in the nighttime BP mean, by presenting total-day and total night (TDTN) levels. In 80 consecutive patients (33 females) age 63±10years, undergoing 24h ABPM who reported waking up to urinate at night we compared TDTN with actual day-awake and night-sleep (DANS) levels. Daytime awake BP, and nighttime awake BP were 141±15/83±11, and 140±18/82±12mmHg respectively p≥0.16. Nocturnal BP was 128±16/71±10mmHg by TDTN and 126±16/70±10mmHg by DANS p≤0.00001. The difference between TDTN and DANS calculations, increased from 1.2±0.9/1.0±0.8mmHg in those who woke once a night(n=51) to 1.8±1.4/1.7±1.3mmHg in those who woke more frequently(n=29) p≤0.02. Nocturnal decline of BP by TDTN was 13±11/11±7, and by DANS 15±12/13±7mmHg p≤0.0001. Although the absolute difference between TDTN and DANS was small, the effect on nocturnal dip was profound. Average systolic BP (SBP) dipping was 9.3±0.1% for TDTN, and 10.5±0.1% for DANS p≤0.0003, and that of diastolic BP (DBP), 13.5±0.1% and 15.6±0.1%, respectively p≤0.0001. The prevalence of SBP non-dipping decreased from 51.3% by TDTN, to 35% by DANS p≤0.038, and that of DBP from 36.3% to 21.3% respectively p≤0.036. The TDTN non-dipping status of waking once (n=51), twice (n=22), or thrice or more (n=7) at night was 45.1%, 54.5%, and 100% for SBP p≤0.024, and for DBP it was 27.5%, 45.5%, and 71.4%, respectively p≤0.044. By DANS the number of non-dippers decreased substantially, as compared with TDTN for each number of awakenings, 37.3%, 40.9%, and 85.6% for SBP p≥0.051, and 15.7%, 22.7%, and 57.1% for DBP, p≤0.042. Inclusion of awake BP measurements during the night obscures the normal dipping pattern of ABPM, in people who wake up to urinate. Non-dipping increases with number of awakenings, and leads to its substantial overestimation. Thus in 24h ABPM, as in anything else, taking into account people’s actual behavior increases accuracy of results.