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To compare the reproducibility of clinic and ambulatory blood pressure measurements in elderly hypertensive subjects.Twenty-two untreated elderly hypertensives, with a clinic systolic blood pressure (SBP) > 160mmHg and/or diastolic blood pressure (DBP) > 95mmHg, and a mean age of 76 years (range 66-86).Following three supine clinic blood pressure readings the subjects underwent 24-h non-invasive ambulatory blood pressure monitoring, measurements being taken at 20-min intervals from 0700 to 2200 h and at 30-min intervals from 2200 to 0700 h. Measurements were repeated during a further visit at a median interval of 10 weeks (range 1—10 months).Daytime ambulatory SBP levels were 20mmHg (95% confidence interval 14-27 mmHg, P<0.001) lower than clinic SBP, although DBP values were similar. The mean 24-h ambulatory SBP and DBP reproducibility [assessed by the standard deviation of differences (SDD) between visits] was significantly better than that for mean clinic blood pressure (SBP 6.3 versus 17.4 mmHg, P<0.001; DBP 4.8 versus 7.0 mmHg, respectively, P<0.05). With daytime defined as 1000-1959 h, the SDD between visits was 12.4 mmHg for SBP and 8.3 mmHg for DBP, but with daytime defined as 0700-2159 h, the SDD fell to 6.0 mmHg for SBP and 4.8 mmHg for DBP, values almost identical to those obtained with full 24-h blood pressure monitoring. There was no difference in night-time blood pressure reproducibility, whether night-time was defined as 2400-0559 h or 2200-0659 h.Both 24-h and daytime ambulatory blood pressure monitoring significantly improve the reproducibility of blood pressure measurements compared with clinic blood pressure readings in elderly hypertensive subjects. Increasing the number of daytime blood pressure readings by 50% (from 30 to 45) reduced the variability of blood pressure measurement by 50%. Twenty-four-hour ambulatory blood pressure monitoring is of value in obtaining reproducible blood pressure measurements in elderly hypertensive subjects. However, more than 30 readings are needed during a daytime recording to significantly reduce variability compared with repeated clinic measurements, although night-time variability is not significantly affected if the number of readings is reduced to 12 over a 6-h period.