Chronological distribution of readings in ambulatory blood-pressure monitoring exams affects the nighttime average and the magnitude of blood-pressure dipping

Averaged nighttime blood pressure (BP) is superior to daytime BP for cardiovascular risk stratification, and the relative change between daytime/nighttime BP (dipping percentage) significantly predicts cardiovascular risk. Newer reports suggest that four measurements at night may be enough for cardiovascular risk stratification. Since BP oscillates across the night, the temporal distribution of measurements across the night may impact nighttime BP and dipping percentage. Therefore, we compared average nighttime BP and dipping percentage when using measurements in the first half (1st half), second half (2nd half), and a combination of both (combined). Forty-three (17 females and 26 males) midlife adults aged 50 ± 10 yr old wore an ambulatory BP monitor for 24 h at home, programmed to measure BP every 20 min when scheduled for daytime and every 30 min during a self-selected 8-h nighttime for time in bed. We compared the nighttime BP averages and dipping percentages when using either the first four measurements from the first half or second half of the nighttime and combined. Nighttime systolic BP was significantly different across first half, second half, and combined (111 ± 9 vs. 107 ± 11 vs. 109 ± 9 mmHg, P < 0.01), respectively, with significant pairwise differences across all categories (P < 0.01 for each). Systolic BP dipping percentage was significantly different across first half, second half, and combined (9.9 ± 5.5 vs. 13.5 ± 6.4 vs. 11.7 ± 5.0%, P < 0.01), respectively, with significant pairwise differences across all categories (P < 0.01 for each). Diastolic BP and diastolic dipping percentage were similar across the three different bins. In midlife adults, systolic nighttime BP and dipping percentage may depend upon when BP measurements are taken during the night. NEW & NOTEWORTHY The distribution of ambulatory blood-pressure monitoring (ABPM) readings affects the reproducibility of average blood pressure (BP) values as well as dipping percentage and dipping diagnoses. Our data suggest that the placement of measurements can affect nighttime average BP. Therefore, the temporal measurement distribution should be considered when interpreting ABPM.