The importance of blood pressure measurement during pregnancy has been recognized for more than a century (Seligman, 1987) and is a fundamental part of antenatal care. For many women their pregnancy will be the first point of medical contact and they may have been unaware of any pre-existing hypertension up to this point.
Early diagnosis of hypertension has important implications for the management and prognosis of both the mother and the fetus. It is dependent on the accurate measurement of blood pressure, as hypertension is often the only early sign of impending pre-eclampsia. It is not, however, necessarily indicative of pre-eclampsia or indeed eclampsia.
A survey of eclamptic women in the United Kingdom in 1992 demonstrated that a significant proportion of women did not have a blood pressure measurement that adequately distinguished them from the general antenatal population and fits occurred in hospital without severe hypertension (Douglas and Redman, 1994). There is also evidence of an increased perinatal mortality rate associated with blood pressures recorded below 140/90 mmHg (Browne and Dodds, 1942; Chesley, 1976; Rippmann, 1968).
If eclampsia occurs at normal pressures, it may be that blood pressure measurement will become much less significant in identifying women prone to eclampsia. Currently clinicians rely mainly on blood pressure to identify, diagnose and manage these women and an increasing proportion of women will be ''missed'' according to current blood pressure criteria. It is uncertain to what degree the current errors associated with mercury sphygmomanometer as well as automated devices could contribute to the failure of current screening methods.
Women with chronic hypertension are usually essentially hypertensive and sometimes have underlying renal disease. They are at an increased risk (Ferrazzani et al., 1990; Mabie et al., 1986; Rey and Couturier, 1994; Sibai and Anderson, 1986; Sibai et al., 1983), regardless of whether they present with superimposed pre-eclampsia or not. Compared to normotensive pregnancies, women with chronic hypertension have an increased risk of fetal loss (McCowan et al., 1996). However, they have lower associated fetal and maternal morbidity compared to pre-eclamptic women (Ferrazzani et al., 1990; Mabie et al., 1986; Rey and Couturier, 1994; Sibai and Anderson, 1986; Sibai et al., 1983). Maternal mortality in women who have pre-eclampsia is mainly contributed to intracerebral hemorrhage (CEMD, 2004; HMSO, 1996). This direct arterial injury commonly occurs as a result of severe hypertension, when the limits of the auto regulation of the brain circulation have been exceeded (Strandgaard et al., 1973). An association between abruptio placentae and hypertension in pregnancy has been shown (Brosens and Renaer, 1972) and if severe hypertension is noted before 20 weeks gestation, it could result in adverse effects in the fetus.
Indirect determinants of blood pressure, whether it be Korotkoff sounds or oscillations, are dependent on the hemodynamics of the circulation. Blood pressure is a function of peripheral vascular resistance and cardiac output, both of which change during pregnancy and even more dramatically in pre-eclampsia. These changes will result in a change in blood pressure and have the potential to influence the characteristics of arterial oscillations that are generated during a blood pressure measurement cycle. As the vast majority of automated blood pressure devices rely on oscillometry to determine blood pressure, their accuracy in the chronic hypertensive or normotensive state cannot be assumed to be equivalent in pre-eclampsia.
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