SUBJECTS AND METHODS
Study population
Children from 4 to 18 years of age, selected from one study that is part of a larger project in which reference values and determinants of ambulatory blood pressure have been assessed, were included in the present study. All had been selected from the Pediatric Outpatient Clinic of the General Hospital of the University of Valencia, Spain, which the children attended for the purpose of routine health maintenance. Systemic and renal disease were discounted through physical examination, serum biochemistry and urinalysis. In the present study, children included were born at term (equal or higher than 37 weeks), after a normotensive pregnancy. The gestational age and the birth weight were obtained from routine obstetrical records. The study was approved by the Committee for the Protection of Humans Subjects of the General Hospital, University of Valencia, Spain. All parents, and those children older than 12, gave their consent to participate in the study.
The subjects were divided according to birth weight: from 2.000 to 2.499 kg, from 2.500 to 2.999 kg, from 3.000 to 3.299 kg, from 3.300 to 3.599 kg, and >3.599 kg. The number of subject per weight group was representative of that for the Comunidad Valenciana over the last five years, which averaged 30,982 births per year. The subset of those children whose birth weight was from 2.000 kg to 2.499 kg, 5.5% of the total population, was also representative of the population born with intrauterine growth retardation during the same period.
Blood Pressure Measurements
Office and ambulatory measurements were performed following a previously published protocol (10). On the day of the monitoring, nurses measured the BP of the subject three times consecutively using a mercury sphygmomanometer, in the sitting position and after a rest of at least 5 minutes. Korotkoff phase I was used to measure systolic blood pressure (SBP), and phase IV for diastolic blood pressure (DBP) in children up to 13 years of age and phase V in those over 13 (11). The mean of the three measurements was taken as the office BP.
Ambulatory BP monitoring was performed using a Spacelabs model 90207 monitor (Spacelebs, Inc., Redmond, Washington, USA) weighing 340g (including batteries). This device employs an oscillometric method with a deflation rate of 8 mmHg/s. The proper cuff, placed on the non-dominant arm, was selected from among the three sizes supplied by the manufacturer (10x13, 13x24 and 24x32 cm) according to the subject´s upper arm length; the cuff extended completely around the arm and the bladder width covered at least two thirds of the upper arm. The accuracy of the monitor was tested in each subject under resting conditions at the beginning of the monitoring period.
Monitoring was performed on a regular school day with normal recreational activities, although the children were instructed not to engage in vigorous physicial exercise or contact sports on monitoring days. Recording began between 08:30 and 09:00 h. The reading frequency was programmed for every 20 minutes from 06:00 to midnight and for every 30 minutes from midnight to 06:00 h. During the daytime period (08:00-22:00 h) an acoustic signal before the measurement was automatically programmed to remind the child to relax the arm. Readings were automatically rejected when: 1) SBP >220 mmHg or <70 mmHg and 2) DBP >140 mmHg or <40 mmHg. Whenever a reading could not be succesfully completed, the measurement was automatically repeated after two minutes while retaining the pre-established sequence. Non-edited recordings in which greater than 30% of measurements were erroneous were excluded from the analysis. The average of the total number of readings obtained at each monitoring session was 62±5.
Three different time periods were defined: 24-hour, daytime and night-time. The 24-hour period included all valid readings performed during the monitoring, daytime was defined as the interval from 08:00 to 22:00 h and nighttime from midnight to 06:00 h, because these periods corresponded closely to activity and sleep in the majority of the subjects. The following parameters were calculated for each subject: a) total number of readings; b) average of SBP, DBP, and heart rate (HR) over the 24-h, daytime and nighttime periods; c) circadian variability, estimated as the day/night ratio of the BP averages; d) variability, estimated as the standard deviation of the SBP, DBP and HR valid measurements during 24 hour, daytime and nighttime.
Statistical analysis.
The difference in BP values and variability estimates within birth weight groups were examined through analysis of variance. Covariance analysis by sex, current age, weight, and height was performed to control for differences in each of the birth weight groups. Associations between two parameters were assessed by Pearson's correlation coefficient, controlling for the potential confounders, age, sex, current weight and height. Multiple linear regression analyses were calculated using ambulatory blood pressure values or variability as dependent variables and birth weight, age, sex, current weight and height as independent ones.