What is the Role of Out-of-Office BP Measurement in Hypertensive Pregnancy?

Keywords:

• ABPM
• Home BP
• Obstetrical day unit
• Prediction
• Pregnancy

INTRODUCTION

‘Out-of-office’ or ambulatory blood pressure (BP) measurement refers to measurement of BP: by 24-hour ambulatory BP monitoring (ABPM), by serial measurement in an obstetrical day unit, or by home BP monitoring (with or without an associated antepartum home care program). These approaches are not necessarily equivalent.

Out-of-office BP measurement is widely used ([1]), but its role has not been defined in international guidelines regarding the diagnosis and evaluation of women with the hypertensive disorders of pregnancy (HDP). The Australasian pregnancy-specific guidelines (year 2000) recommend that automated devices and ABPM should not yet be used in routine clinical practice until more detailed information becomes available about their accuracy and effectiveness ([2];[3]). The American guidelines (2000) do not make recommendations about ambulatory monitoring ([4]). The Canadian pregnancy-specific hypertension guidelines (1997) do not make specific recommendations about the use of ambulatory or home BP monitoring, although they acknowledge that home BP measurement is practiced in some Canadian centers ([5]).

Out-of-office BP measurement has two potential roles. The first is preeclampsia prediction among women at increased risk (e.g., previous preeclampsia); 24-hour ABPM has been studied for this purpose, but it is not sufficiently predictive on its own to be clinically useful ([6]). There is current enthusiasm for inclusion of 24-hour ABPM with other clinical and laboratory risk markers in multivariable models ([7];[8]). The second potential role of out-of-office BP measurement is prognosis (i.e., prediction of adverse maternal and perinatal outcomes) of women with office hypertension. This is the focus of this editorial for if useful, out-of-office BP measurement for this purpose is within the realm of clinical care.

WHICH DEVICE(S) SHOULD BE USED FOR OUT-OF-OFFICE BP MEASUREMENT?

Automated BP measurement devices eliminate the observer bias inherent in manual BP measurement, although there is still potential for error due to other factors, such as positioning and cuff size. These devices work by use of either an auscultatory device or an oscillometer that detects arterial oscillations. Devices that detect oscillations depend on a number of factors, all of which are affected by normal pregnancy and further affected by preeclampsia: arterial compliance, systemic vascular resistance, and intravascular volume. Also, the edema of preeclampsia can dampen the transmission of pressure waves to the cuff, thereby leading to an underestimate of the true BP ([9];[10]). It follows, therefore, that automated BP devices may not be accurate in pregnancy and/or preeclampsia. For example, the OMRON HEM 705 CP ([11]), Welch Allyn ‘Vital Signs’ ([12]), and Spacelabs 90207 [British Hypertension Society (BHS) grade C/C, AAMI pass] devices ([10]) were all found to be acceptable for use in pregnancy but not in women with preeclampsia, according to standardized criteria ([13]). Automated devices may underestimate BP in preeclampsia by an average of 5mmHg in systolic and diastolic, but there is wide variation between patients ([14]). To date, two oscillometric devices have received acceptable grading for BP measurement in preeclampsia: Microlife 3BTO-A (BHS grade A for sBP and B for dBP) ([15]) and the OMRON MIT (BHS grade B for sBP and A for dBP) (now withdrawn from the market) ([16]).

HOW PREVALENT IS ISOLATED OFFICE HYPERTENSION AND WHAT DOES THIS MEAN?

The prevalence of isolated office hypertension varies widely. Approximately 30% of women will have isolated office hypertension by 24-hour ABPM. The prevalence has been as high as 70% when measurements have been made late in the third trimester, in an obstetrical day unit ([17]) or by home BP assessment ([18]).

What constitutes an elevated out-of-office BP measurement should be based on a level that is associated with an excess of adverse outcomes. It should be noted that all relevant studies used the Spacelabs 90207 ([19]) which is acceptable for use in pregnancy but not in preeclampsia ([10]).

24-Hour Ambulatory BP Monitoring (ABPM)

Twenty-four-hour ABPM has limited availability. Patients are frequently billed for each study, and are not normally reimbursed by insurance providers. Although 24-hour ABPM is well tolerated with few complications, 49% of pregnant women have reported some interference with normal activity and 76% some disruption of sleep ([20]).

One prospective study followed women with isolated office hypertension in early pregnancy, performed serial 24-hour ABPM, and reported on evolution into persistent hypertension. Among 78 such women, 40% developed gestational hypertension (without proteinuria), and 8% developed preeclampsia ([21]).

Five prospective observational studies have enrolled women with gestational ([17];[22–24]) or preexisting hypertension ([21]), and examined the relationship between 24-hour ABPM and adverse pregnancy outcomes. 24-hour ABPM (compared with standard office BP measurement) is better at identifying hypertensive pregnant women at increased risk of adverse pregnancy outcomes such as severe hypertension, preterm delivery, Cesarean section, small-for-gestational age infants, and admission to a neonatal intensive care unit ([17];[21–24]).

In these prospective validation studies, the BP cut-offs have varied widely, as follows: average 24-hour ambulatory BP of ≥ 135/85 mm Hg ([24]); average daytime ambulatory BP of ≥130/85 mm Hg ([17]); average daytime ambulatory BP >130/80 mm Hg at ≤ 26 weeks or > 135/85 mm Hg after 26 weeks ([21]); one or more of either average 24-hour BP of ≥ 125/74, average daytime BP of >128/78, or average nighttime BP of ≥ 121/70 ([22]); or 48-hour ambulatory BP ‘excess’ above a gestational age specified reference limit ([23]). [Two other prospective studies have found significant inverse relationships between 24-hour ambulatory (but not clinic) BP measurements, and both higher 24-hour proteinuria levels in preeclampsia ([25]), and lower birthweight ([26]).]

Data from these prospective validation studies (i.e., sensitivities and specificities) and the prevalence of adverse outcomes among hypertensive pregnant women can be used to calculate the positive (PPV) and negative predictive values (NPV) for dBP and sBP. Table 1 shows that based on NPV, 24-hour ABPM may be somewhat useful for prediction of adverse outcomes, using the most common criterion of average daytime dBP cut-off of 85mm Hg.

Table 1: Diagnostic accuracy of out-of-office BP measurement in pregnancy

Outcomes	Prevalence	sBP		dBP
		PPV	NPV	PPV	NPV
Severe hypertension	33%	55%	73–78%	50%	74–81%
Preterm delivery	20%	28–35%	82–98%	29–32%	84–94%
Admission to NICU	10%	12–20%	91–99%	12–16%	91–97%

ABPM (ambulatory blood pressure monitoring), NICU (neonatal intensive care unit), NPV (negative predictive value), PPV (positive predictive value)

* Defined as 130–140 mm Hg by 24-hour ABPM or as 140 mm Hg in the obstetrical day unit.

^† Defined as ≥85 mm Hg for 24-hour ABPM or ≥90 mm Hg in the obstetric day unit.

Twenty-four-hour ABPM also has the ability to detect women with masked hypertension in pregnancy (i.e., elevated BP out-of-office with normal BP values by office measurement). The prevalence was 15%, similar to that outside of pregnancy, among primiparous women in one prospective study ([23]). Of interest was the fact that the frequency of adverse pregnancy outcomes among women with masked hypertension was similar to women with persistent gestational hypertension. The role of ABPM in detecting masked hypertension in pregnancy is uncertain.

Serial BP Measurement in Obstetrical Day Units

Many obstetric units now have a ‘day unit’, where women can be followed serially over daytime working hours by an experienced obstetric nurse. Three of the prospective cohort studies that examined the predictive value of 24-hour ABPM also performed serial BP assessment in an obstetrical day unit ([17]; [21–24]) (Table 1). ABPM was more predictive than were the BP measurements in the obstetrical day unit, using a dBP cut-off of 90 mm Hg (date not shown).

Home (Self) BP Monitoring

Home BP monitoring is widely available, economical, comfortable, and easy to repeat when disease evolution is suspected. How knowledgeable women are about BP measurement technique and whether or not women are instructed in the use of these units is uncertain outside of the context of associated antepartum home care program. With instruction, women can accurately record BP readings ([27]).

Pregnant women prefer home BP monitoring to 24-hour ABPM. In a prospective study that included healthy pregnant women and those with preeclampsia, only 28% of women rated the 24-hour device as very good, compared with 64% who rated the home BP monitor as very good ([20]).

Home BP values have not been validated against adverse pregnancy outcomes in hypertensive pregnancy.

In summary, 24-hour ABPM is the out-of-office BP measurement technique that best identifies hypertensive pregnant women at increased risk of adverse pregnancy outcomes. However, the negative predictive values for adverse outcomes are not sufficiently high that women with isolated office hypertension can be followed substantially differently from those with persistent hypertension (i.e., in the office and out of the office). Twenty-four-hour ABPM may perform better if evaluated using devices approved for women with preeclampsia. At present, there is insufficient information to define the role of home BP monitoring in hypertensive pregnancy.

To date, no RCT has been performed to assess the impact of any out-of-office BP measurement technique (vs. conventional office values) on maternal or perinatal outcomes ([28]).

WHAT ARE THE IMPLICATIONS FOR ANTIHYPERTENSIVE THERAPY?

Unlike the situation outside pregnancy, identifying subjects with persistent (‘true’) hypertension will not necessarily identify those who would most likely benefit from antihypertensive therapy, or more intensive antihypertensive therapy, because it is unclear how best to manage nonsevere hypertension in pregnancy, in order to do more good than harm. Theoretical concern about harm is based on the fact that the placenta does not autoregulate blood flow. Therefore, lowering BP may decrease placental perfusion, and because of poorer fetal growth, babies may do more poorly.

The relative benefits and risks of antihypertensive therapy in pregnancy have not been established, based on the results of systematic reviews of randomized controlled trials (46 trials, 4,282 women) ([29]). Allowing BP to be somewhat elevated (to non-severe levels below 160/110 mm Hg) may decrease the risk of SGA infants, but may also increase the risk of severe maternal hypertension. The trials do not provide sufficient evidence on which to base clinical decisions.

CONCLUSIONS

If out-of-office BP monitoring is used, a pragmatic approach may be to ensure accuracy of a given device for BP measurement in a given woman, according to established criteria. When out-of-office BP is <135/85 mm Hg, women are less likely to be at risk for pregnancy complications than are persistently hypertensive pregnant women, but the degree of clinical overlap mandates that all such women be followed closely. Confirmation that the office hypertension is isolated may minimize use of antihypertensive medication in pregnancy. Not only may this be of benefit to the fetus ([1]), but most women would prefer to avoid drug therapy in pregnancy, if possible.