Chronic Hypertension

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Historically, there have been many classifications of hypertension in pregnancy with no apparent consensus. In this chapter we use the now-accepted classification as developed by the National Institutes of Health (Report of the National High Blood Pressure Education Program Working Group on High Blood Pressure in Pregnancy). Hypertension during

Table 21-8 Normal Arterial Blood Gas Values in Pregnancy

pH

Pco2 (mm Hg)

Po2 (mm Hg)

Normal

7.40

35-40

75-100

Pregnancy

7.45

27-32

90-108

Box 21-5 Classification of Hypertensive Disorders in Pregnancy

Chronic hypertension Preeclampsia/eclampsia

Preeclampsia superimposed on chronic hypertension Gestational hypertension

1. Transient hypertension of pregnancy if preeclampsia is not present at delivery and blood pressure returns to normal by 12 weeks postpartum.

2. Chronic hypertension if the elevation persists.

pregnancy is categorized as (1) preeclampsia/eclampsia, (2) gestational hypertension, (3) chronic hypertension, or (4) preeclampsia superimposed on chronic hypertension (Box 21-5). These categories identify different disorders that at times overlap but with different epidemiologic characteristics, pathophysiology, and risks for mother and baby. Pregnancy-induced hypertensive disorder is an older, less specific term no longer in general use.

Preexisting hypertension often complicates pregnancy. A diagnosis of chronic hypertension is made when hypertension precedes pregnancy or is diagnosed before 20 weeks of gestation. In normal pregnancy, arterial blood pressure (BP) and peripheral vascular resistance (PVR) fall shortly after implantation of the conception. Normal first-trimester and early second-trimester BP ranges from 92 to 114 mm Hg systolic, 46 to 66 diastolic sitting, and 103 to 123 systolic, 47 to 67 diastolic supine. At 28 to 30 weeks' gestation, BP in normal pregnant women increases to near prepregnancy range. It is generally accepted that BP greater than 130/80 mm Hg at any time during pregnancy is abnormal.

Fetal effects of chronic or essential hypertension depend on severity of hypertension and range from none to IUGR, fetal distress, abruptio placentae, prematurity, and fetal death (Haddad and Sibai, 1999). Women with preexisting hypertension have approximately a 20% chance of developing superimposed preeclampsia. The maternal and fetal morbidity and mortality are higher in chronic hypertension with superimposed preeclampsia than in each disorder alone.

Most women with chronic hypertension will present at their first prenatal visit with the diagnosis. As many as one third of pregnant women with essential hypertension will become normotensive in the first half of pregnancy. Often, their antihypertensive medications can be discontinued and only restarted when elevations recur, typically after 28 weeks. Controversy exists as to whether medical treatment of essential hypertension in pregnancy improves fetal outcome or decreases the risk of superimposed preeclampsia. However, maternal complications can occur when diastolic BP is persistently elevated. Thus, it is generally accepted that therapy to decrease maternal BP be instituted or modified to keep maternal diastolic BP lower than 100 to 105 mm Hg. A profound drop in maternal BP over a short period should be avoided to prevent decreased cardiac output to the placenta and fetus.

Alpha-methyldopa (Aldomet), a central-acting, alpha-adrenergic, false neurotransmitter, is the drug of choice for treatment of chronic hypertension in pregnancy. The initial starting dose is 250 mg every 8 hours. This dose can be increased to 2 g/day if needed. If adequate control cannot be obtained, a second drug, most often nifedipine or hydralazine, can be added. Also, with its safety confirmed in pregnancy, labetalol is gaining popularity for use as first-line single-agent therapy. ACE inhibitors and diuretics for BP control are con-traindicated in pregnancy (Witlin and Sibai, 1998).

Obstetric management includes baseline laboratory studies early in pregnancy that will later help in diagnosis of superimposed preeclampsia. Specifically, in addition to routine prenatal testing, this includes renal function studies, liver enzymes, platelets, uric acid, and 24-hour urine test for protein and cre-atinine clearance. If chronic hypertension is a new diagnosis, pheochromocytoma should be ruled out by catecholamine levels in serum or 24-hour urine (Keely, 1998). Early ultrasonography to confirm dating and intermittently to evaluate fetal growth will aid in the diagnosis of IUGR. Women with moderate to severe hypertension benefit from more frequent prenatal visits as well as home BP monitoring. Antenatal testing should be performed at least for women with superimposed preeclampsia or IUGR. Preterm delivery may occur.

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