Please note: In 2003, the CTF updated its Grades of Recommendations to include an "I Recommendation" for situations where insufficient evidence exists to allow a recommendation to be made. (Formerly, these situations were captured under a "C Recommendation".) This change is not retroactive, and all "C Recommendations" made prior to 2003 have not been reevaluated in light of the new "I" recommendation grade. For a discussion of these recommendation grades, please link to the 2003 article in the Canadian Medical Association Journal here.
*Please note: Evidence from recently published studies has not yet been reviewed by the Task Force in terms of its potential effect on these recommendations.
Overview
Blood pressure (BP) measurement
should be part of the periodic health examinations of all pregnant women.
The technique for BP measurement should be consistent, with the patient
always in the same position (B Recommendation). There is no evidence to
recommend low dose aspirin prophylaxis as a primary preventive measure
in women at low risk of developing preeclampsia. Low dose aspirin prophylaxis
can be considered in women at high risk of preeclampsia and intra-uterine
growth retardation (IUGR) (C Recommendation).
Burden of
Suffering
Hypertension is the most common
medical complication of pregnancy, occurring in 6-8% of all pregnancies.<1,2>
It is seen in a group of disorders that include preeclampsia-eclampsia,
latent or chronic essential hypertension, a variety of renal diseases,
and transient gestational hypertension.
Preeclampsia, once called toxemia of pregnancy, is the most dangerous of these disorders, occurring in about 2.6% of pregnancies. Women with preeclampsia are at increased risk for abruptio placenta, acute renal failure, cerebral hemorrhage, disseminated intravascular coagulation, pulmonary edema, circulatory collapse, and eclampsia.
The fetus may become hypoxic, increasing risk of low birthweight, premature delivery, or perinatal death.<3> Women who suffered from preeclampsia are not at increased risk of developing chronic hypertension.
Risk factors for preeclampsia and eclampsia include black ancestry, nulliparity or first pregnancy with the actual partner, multiple gestations, chronic hypertension or diabetes, a family history of eclampsia or preeclampsia and possibly obesity.<4>
Although definitions differ, many define preeclampsia as acute hypertension presenting after the 20th week of gestation accompanied by abnormal edema and/or proteinuria (more than 0.3 g/24h), or both.<5> BP over 140/90, or a rise of 15 mmHg or 30 mmHg above the usual diastolic and systolic BP respectively, is considered abnormal. The appearance of edema and proteinuria alone is unreliable because edema is common in normal pregnancies.<6> BP levels should be normalized 6 weeks post-partum.
Transient gestational hypertension is defined as acute onset of hypertension in pregnancy or the early puerperium without proteinuria or abnormal edema and resolving within 10 days after delivery.<2> Chronic hypertension that had been latent prior to the pregnancy may become evident during gestation. Pregnant women with chronic hypertension are also at increased risk for stillbirth, neonatal death, and other fetal complications, but the risk is much lower than that of women with preeclampsia. Women with transient or latent chronic hypertension are more likely to develop chronic hypertension in later years.<4,5>
Maneuver
Two preventive maneuvers are
considered. Screening for early signs of preeclampsia followed by appropriate
clinical interventions has been for a long time the only strategy available.
Recently, low dose aspirin prophylaxis has been evaluated as a primary
prevention maneuver in low-risk and high-risk pregnancies.
Low Dose Aspirin Prophylaxis
Endothelial dysfunction caused
by the systemic effects of decreased placental blood flow is postulated
to be the pathophysiologic mechanism for preeclampsia. Compared to women
with normal pregnancies, women with preeclampsia have a relative excess
of thromboxane A2 compared to prostacyclin. It has been hypothesized that
the correction of the thromboxane: prostacyclin ratio by aspirin could
prevent preeclampsia and its complications. The aspirin dosage proposed
varies between 60 to 150
mg per day and has been initiated between 13
to 26 weeks of pregnancy, in different studies.<7-9>
Screening for Early Signs
of Preeclampsia
Much research has been done
to identify screening tests that would predict the risk of developing preeclampsia
before the classical triad of symptoms appears. The validity of proposed
tests is difficult to evaluate due to the absence of a gold standard to
confirm the diagnosis. Glomerular endotheliosis, the renal lesion characteristic
of preeclampsia, is present in only 54% of patients who meet the clinical
criteria for the disease,<10>
and is not specific for preeclampsia. For practical reasons, most studies
of potential screening tests for preeclampsia have relied on clinical criteria
to confirm the diagnosis.
Measurable proteinuria usually occurs late in the course of the illness and therefore is not useful for early detection.<2> However, screening for bacteriuria in pregnancy is recommended at 12 to 16 weeks (see Chapter 9). In a prospective study of women between 24 and 34 weeks of gestation, a urine albumin concentration ³11 mcg/L had a sensitivity of 50% in predicting subsequent preeclampsia.<11> The conventional dipstick test is unreliable in detecting the moderate and highly variable elevations in albumin that occur early in the course of preeclampsia but it can help ascertain the diagnosis when it is present.<12>
Other screening tests that have been suggested, include the angiotensin II infusion test and the supine pressor "rollover" examination, but these have also been found to be unsuitable, as the former is impractical and the latter lacks adequate sensitivity, specificity and positive predictive value.<1,12>
BP measurement remains the cornerstone of early diagnosis, although it has limitations. First, there are the usual sources of measurement errors associated with sphygmomanometry. In addition, maternal posture can affect BP in pregnant women significantly.<12> The results can be erroneous, for example, if BP is measured with the woman in the supine position. Most important, a single elevated BP reading is neither diagnostic of nor a reliable predictor of preeclampsia.
Absence of the normal decline in BP that occurs in the middle trimester or an increase in BP during the second trimester may be an early indicator of increased risk for preeclampsia.<3,13> Some experts recommend using the middle trimester mean arterial pressure (MAP), defined as ((2 x diastolic BP) + systolic BP)/3 as a screening test.<3>Studies indicate that a middle trimester MAP above 90 mmHg has a sensitivity of 61-71% and a specificity of 62-74% in predicting preeclampsia.<3,14>
Effectiveness of Prevention and Treatment
Low Dose Aspirin Prophylaxis
The effectiveness of prophylactic
acetylsalicylic acid (ASA) at doses ranging between 60 and 150
mg has been the object of some randomized controlled trials. Most were
conducted in women at high risk for preeclampsia. All showed that low-dose
aspirin can effectively reduce pregnancy-induced hypertension and preeclampsia
and IUGR associated with the conditions. The decrease in the incidence
of preeclampsia was not, however, associated with a decrease in neonatal
mortality but the cesarian section rate was significantly lower in the
aspirin group in two studies.<8,9>
A large randomized controlled trial was conducted recently in 3,135 low-risk nulliparous women.<7> In this trial 60 mg of ASA was effective in reducing preeclampsia incidence from 6.3% to 4.6% but was associated with a statistically significant increase in abruptio placenta which was not observed in a much larger study (CLASP).<15> The beneficial effect of ASA was observed only in women whose BP was >120 mmHg at recruitment.<7>
Screening for Preeclampsia
Followed by Early Intervention
Delivery is the only specific,
definitive treatment of preeclampsia. However, early detection of hypertension
during pregnancy permits clinical monitoring and prompt therapeutic intervention
should severe preeclampsia or eclampsia develop. Although some studies
support the use of bedrest, pharmacologic agents, and early delivery of
the fetus to prevent complications, there is little conclusive evidence
that these measures improve outcome.<12,16,17>
A randomized controlled trial found that antihypertensive therapy and hospitalization,
when compared with hospitalization alone, did not improve maternal or fetal
outcome.<18>
There have been no clinical trials to determine whether hypertensive women
treated early in pregnancy have a better prognosis than those who are not
detected early. Nonetheless, most obstetrical experts believe, based on
clinical experience, that early detection and preventive non-pharmacologic
treatment of preeclampsia may be beneficial to both mother and fetus.<1,6,18,19>
This view is based in part on inferences drawn from the apparent effectiveness
of regular prenatal care in reducing the risk of preeclampsia-eclampsia.
Studies conducted as early as the 1940s
suggested an inverse relationship between the extent of prenatal care and
the incidence of eclampsia, perhaps reflecting a beneficial effect due
to early detection.<20> These findings do not provide direct evidence,
however, that improved outcome is due solely to BP measurement itself,
rather than to other components of prenatal care or to the characteristics
of women who receive regular prenatal care.
Recommendations
of Others
The Canadian and American
Colleges of Obstetricians and Gynecologists recommend BP measurements at
the initial visit, every 4 weeks until 28 weeks’ gestation, every 2-3 weeks
until 36 weeks’ gestation, and weekly thereafter.<21,22>
In 1989, the U.S. Preventive Services Task Force recommended that all pregnant women should receive systolic and diastolic blood pressure measurements at the first prenatal visit and periodically until delivery or throughout the three trimesters.<23>
Conclusions
and Recommendations
There is no evidence to recommend
universal low-dose ASA prophylaxis in nulliparous women (C Recommendation).
Although there is grade I evidence of its capacity to reduce the incidence
of preeclampsia, it has not been shown to decrease neonatal mortality,
and has been associated with adverse outcome in some studies. The safety
of aspirin prophylaxis must be carefully evaluated. However, this treatment
may be considered for women at high risk for the condition.
To date, the only readily available screening strategy for preeclampsia is the early detection of an abnormal BP trend over time. A diagnosis of preeclampsia should not be made solely on the presence of elevated BP. There is no experimental evidence that these efforts will result in reduced maternal or perinatal morbidity and mortality. However, there is grade II-2 evidence that regular prenatal care is associated with a reduced incidence of preeclampsia.
Systolic and diastolic pressures should be measured on all obstetric patients at the first prenatal visit and periodically throughout the remainder of pregnancy (B Recommendation). The optimal frequency for measuring BP in pregnant women has not been determined and is therefore left to clinical discretion. The collection of reliable BP data requires consistent use of correct technique and a cuff of appropriate size encircling at least 2/3 of the upper arm length. The patient should consistently be in the same position, and the BP should be measured in the sitting position, after the patient’s arm has rested at heart level for 5 minutes.<5> For additional guidelines, see Chapter 53 on Screening for Hypertension.
Further diagnostic evaluation and clinical monitoring, including frequent BP monitoring and urinalysis, are indicated if BP does not decrease normally during the third trimester, if the diastolic pressure increases 15 mmHg above baseline or the systolic pressure increases 30 mmHg above baseline, or if the BP exceeds 140/90. Medical interventions upon the suspicion of a diagnosis of preeclampsia must include hospital admission to substantiate the diagnosis and its severity.
Unanswered
Questions (Research Agenda)
The following have been identified
as research priorities:
Acknowledgements
We would like to thank Jean-Marie
Moutquin, MD, FRCPC, Professor, Department of Obstetrics and Gynecology,
Université Laval, Québec City, Québec, for reviewing
this chapter.
Full Citation
Beaulieu M.D. Prevention of preeclampsia. In:
Canadian Task Force on the Periodic Health Examination. Canadian
Guide to Clinical Preventive Health Care. Ottawa: Health Canada,
1994; 136-43.