title: Time-weighted vs. conventional quantification of 24-h average systolic and diastolic ambulatory blood pressures.
abstract: 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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Experimental Cardiology, Tropical Medicine Institute, Central University, Caracas, Venezuela.
authors.forename: José A
authors.initials: JA
authors.lastname: Octavio
authors.forename: Jesús
authors.initials: J
authors.lastname: Contreras
authors.forename: Pablo
authors.initials: P
authors.lastname: Amair
authors.forename: Bernardo
authors.initials: B
authors.lastname: Octavio
authors.forename: Domenico
authors.initials: D
authors.lastname: Fabiano
authors.forename: Federico
authors.initials: F
authors.lastname: Moleiro
authors.forename: Stefano
authors.initials: S
authors.lastname: Omboni
authors.forename: Antonella
authors.initials: A
authors.lastname: Groppelli
authors.forename: Grzegorz
authors.initials: G
authors.lastname: Bilo
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Mancia
authors.forename: Gianfranco
authors.initials: G
authors.lastname: Parati
mesh_terms: D001794:Blood Pressure; D018660:Blood Pressure Monitoring, Ambulatory / Q000379:methods*; D002940:Circadian Rhythm; D003971:Diastole; D006801:Humans; D013599:Systole