Tolerance limits for ambulatorily monitored blood pressure in pregnancy

To recognize the highly statistically significant circadian variability of lood pressure (BP) in pregnancy [Hermida et al. Hypertension 2000; 36:149-158] is to admit that the diagnosis of hypertensive complications in pregnancy should be based not just on whether a casual BP value is too high or too low, but rather on more pertinent questions: How long is BP elevated above a given time-varying threshold? What is the excess BP? When most of the excess occurs? Answers to these questions may be obtained by establishing, first, an adequate reference BP threshold and, second, a proper measurement of BP elevation [Hermida et al. Hypertension 1998;31:83-89]. Accordingly, we derived time-specified reference standards for BP as a function of gestational age. We analyzed 1404 BP series systematically sampled from 234 women with uncomplicated pregnancies. BP was measured every 20 minutes during the day and every 30 minutes during the night for 48 hours with an ambulatory device once every 4 weeks from the first visit to the hospital (usually within the first trimester of gestation) until delivery. Data from each BP series were synchronized according to the rest-activity cycle of each individual in order to avoid differences among subjects in actual times of daily activity. Data were then used to compute 90% circadian tolerance intervals for each trimester of pregnancy, in keeping with the trends in BP along gestation previously documented [Ayala et al. Hypertension 1997;30:611-618]. The method, derived on the basis of bootstrap techniques, does not need to assume normality or symmetry in the data and, therefore, it is highly appropriate to describe the circadian pattern of BP variability [Hermida et al. Biomed Instrum Technol 1996;30:257-266]. Results not only reflect expected changes in the tolerance limits as a function of gestational age, but also upper limits markedly below the thresholds currently used for diagnosing hypertension in pregnancy. The use of these time-dependent tolerance limits for the computation of a hyperbaric index as a measure of BP excess [Hermida et al. Hypertension 1998;31:83-89] has already been show to provide high sensitivity and specificity in the early identification of gestational hypertension and preeclampsia.