Blood Pressure Measurement
Hypertension in pregnancy is defined as a systolic BP ≥ 140 mm Hg and a diastolic BP ≥ 90 mm Hg on two separate measurements at least 4–6 hours apart. However, the diagnosis of hypertension, in pregnancy or otherwise, requires first and foremost an accurate measurement of BP. Many automated BP cuffs have not been tested during pregnancy, and therefore obtaining a manual BP is the preferred technique. The 2000 NHBPEP Working Group Report on High BP in Pregnancy recommends that the Korotkoff phase V (disappearance) sound be used to determine the diastolic BP. In the outpatient setting, proper BP technique is essential and includes the subject being in a seated position, legs uncrossed, back supported, and no tobacco or caffeine for 30 minutes prior. In recumbent, hospitalized patients, the provider should measure the BP in the left lateral decubitus position to minimize the BP change caused by the compression of the inferior vena cava by the gravid uterus.
Blood pressure measurements should be interpreted in the context of the stage of pregnancy and the expected changes in blood pressure for each trimester. BP drops during the first and second trimesters, nadirs at around 20 weeks of gestation, and returns to preconception levels by the third trimester. Women who have not had regular medical care prior to pregnancy may be labeled as ‘gestational hypertension’ based on elevated BPs in the third trimester, when in reality, they were hypertensive prior to pregnancy, which was masked by the physiologic changes during mid-pregnancy. If a woman has gestational hypertension that does not resolve after delivery, she will subsequently be diagnosed as having chronic hypertension.
Ambulatory blood pressure monitoring (ABPM) and the hyperbaric index (HBI) have been suggested as alternative methods for diagnosing elevated blood pressure in pregnancy. The HBI is defined as the amount of BP excess during a given time period above a 90% tolerance limit, with units of mm Hg X hours. One promising study suggested that HBI calculated from a 48-hour ABPM performed in the first trimester had a 93% sensitivity and 100% specificity for predicting preeclampsia, although other researchers have not been able to replicate this high degree of accuracy and reliability. There is currently no official role of ABPM in the diagnosis of hypertensive pregnancy disorders. Home monitoring of blood pressure by automated cuffs in pregnancy has not been validated and some monitors have been shown to inaccurate in pregnancy and, therefore, in-office, manual BPs remain the gold-standard for the diagnosis and monitoring of hypertension in pregnancy. This may involve frequent outpatient visits, especially in those with severe hypertension.