Time-weighted vs. conventional quantification of 24-h average systolic and diastolic ambulatory blood pressures.

Background: Conventional calculation of mean 24-h ambulatory blood pressure (BP), SBP and DBP based on the average of all BP readings disregards the fact that a larger number of measurements is usually scheduled during the daytime than at night, an imbalance possibly leading to an overestimation of 24-h average BP. The aim of our study was to quantify this possible bias and to explore its determinants.

Methods: Four hundred and fifty untreated individuals were subdivided into three groups (150 individuals each) with three different ambulatory blood pressure measurement schedules for day/night: group I, four (day)/two (night) readings/h; group II, four (day)/three (night) readings/h; and group III, with BP readings every 30 min throughout 24 h. Hourly and 24-h averages were computed. The conventional 24-h averages of all SBP and DBP values were compared with the averages of hourly SBP and DBP mean values (time-weighted quantification). The difference between 24-h conventional and 24-h time-weighted BP was computed in each group and related to the degree of nocturnal BP dip and to the ratio between the number of readings of day and night.

Result: In the three groups, 24-h conventional and 24-h time-weighted BP values were highly correlated (r > 0.99), 24-h conventional SBP and DBP being significantly higher (P < 0.01) than the corresponding 24-h time-weighted values in groups I and II but not in group III (Bland-Altman analysis). The bias magnitude was related to the day/night ratio in number of readings and to nocturnal BP dip in groups I and II (P < 0.01) but not in group III.

Conclusion: The higher number of readings/h during daytime leads to an overestimation of conventional 24-h average BP, particularly in individuals with preserved nocturnal BP dipping. This can be avoided either by scheduling the same number of readings/h throughout 24 h or by performing a time-weighted quantification of 24-h BP. The clinical implications of these different approaches deserve further investigation.