The test can identify pregnant women who later will develop hypertensive complications in pregnancy. These include gestational high blood pressure and preeclampsia.
According to the National Institutes of Health, about 7 percent of first-time mothers and 1 to 2 percent of mothers having subsequent pregnancies develop preeclampsia. Usually, a pregnant woman with preeclampsia develops dangerously high blood pressure and begins excreting protein in the urine. In some cases, the condition may progress to eclampsia, a series of potentially fatal seizures. In cases where the condition does not progress to eclampsia, children born to mothers with preeclampsia may be born prematurely or have low birth weight.
Preeclampsia is still one of the leading causes of maternal mortality. Known risks for preeclampsia include: diabetes, pre-existing hypertension, kidney disease, and having previously had the condition during an earlier pregnancy.
"The only way to cure preeclampsia is to eliminate its cause, which is pregnancy itself," says lead author Ramon C. Hermida, Ph.D., director of the bioengineering and chronobiology laboratories and a professor at the University of Vigo in Vigo, Spain. "In most cases, that leads to delivering the baby early. Therefore, it is important to focus on prevention."
The test, called the tolerance-hyperbaric test (THT), can be used as early as the first trimester of pregnancy to identify 93 percent of women at high risk for blood pressure complications several months later. It's more accurate as time goes on, with the sensitivity – the ability to correctly identify those who will develop the condition – reaching 99 percent by the third trimester. The test has a 99 percent specificity, which is the ability to rule out those who are not at risk, starting in the first trimester.
"This is the first test that provides such high degrees of sensitivity and specificity at such a low gestational age," says Hermida. Blood pressure varies in predictable ways throughout the duration of pregnancy and also on a daily pattern known as a circadian rhythm. The THT compares the expected variability with a particular woman's blood pressure pattern over a 48-hour period to find those who are consistently outside the expected range. It involves wearing a portable blood pressure cuff and purse-sized monitor to record the readings at various times throughout the day and night, he says.
The researchers analyzed 2,430 ambulatory blood pressure monitoring sessions conducted every four weeks on 403 women – 235 with uncomplicated pregnancies and 168 who developed gestational high blood pressure or preeclampsia. "On average, the test provided early identification of gestational high blood pressure and preeclampsia 23 weeks before clinical confirmation of the disease," says Hermida. Hermida plans to make the software for the THT available to the scientific community this fall. The software application developed by Hermida and Ayala calculates blood pressure excess as an indication of disease. Many hospitals have the portable blood pressure monitoring equipment used with the THT, but it's relatively expensive. However, that limitation should ease as the test becomes more common, he says.
"The price of the equipment is insignificant compared to the cost of just one preterm delivery caused by preeclampsia," he says. "Ambulatory blood pressure monitoring should be performed at least once in the first half of pregnancy, and repeated in those women with a positive THT. Women at high risk for preeclampsia because of family or medical history, including those over 35 years of age, should have the test without question."