Time-qualified reference values for ambulatory blood pressure monitoring in pregnancy

To recognize the highly statistically significant circadian variability of blood pressure in pregnancy is to admit that the diagnosis of gestational h ypertension or preeclampsia should be based not just on whether a casual bl ood pressure value is too high or too low, but rather on more pertinent que stions: How long is blood pressure elevated above a given time-varying thre shold? What is the excess blood pressure? When does most of the excess occu r? Answers to these questions may be obtained by establishing (1) an adequa te reference threshold for blood pressure and (2) a proper measurement of b lood pressure elevation. Accordingly, we derived time-specified reference s tandards for blood pressure as a function of gestational age. We analyzed 1 408 blood pressure series systematically sampled by ambulatory monitoring f or 48 consecutive hours every 4 weeks from the first obstetric visit (usual ly within the first trimester of pregnancy) until delivery in 235 women wit h uncomplicated pregnancies. Data from each blood pressure series were sync hronized according to the rest-activity cycle of each individual to avoid d ifferences among women in actual times of daily activity. Data were then us ed to compute 90% circadian tolerance intervals for each trimester of pregn ancy, in keeping with the trends in blood pressure along gestation previous ly documented. The method, derived on the basis of bootstrap techniques, do es not need to assume normality or symmetry in the data, and therefore, it is highly appropriate to describe the circadian pattern of blood pressure v ariability. Results not only reflect expected changes in the tolerance limi ts as a function of gestational age, but also upper limits markedly below t he thresholds currently used for diagnosing hypertension in pregnancy. The use of these time-qualified tolerance limits for the computation of a hyper baric index as a measure of BP excess has already been show to provide high sensitivity and specificity in the early identification of gestational hyp ertension and preeclampsia.