Chapter 56 Hypertensive disorders of pregnancy
After reading this chapter, you will:
Introduction
The incidence of hypertensive disorders in the UK is difficult to gauge, owing to the variety of ways in which they present; however, approximately 12% of all pregnancies will be affected by pregnancy-induced hypertension and 3–5% by pre-eclampsia (Duley et al 2006, Walker 2000). Hypertensive disorders in pregnancy accounted for 18 maternal deaths in the UK between 2003 and 2005 (Lewis 2007); of these, six women developed eclamptic seizures (five in the antenatal period), and eight were diagnosed with HELLP syndrome.
Effects on maternal systems and effects on the fetus/neonate are as follows:
Terminology
Various terms have been applied to a condition arising in pregnancy characterized by hypertension, proteinuria, and a combination of other signs and symptoms including seizures (sometimes called convulsions, or fits) (Salas 1999). Box 56.1 shows the range of terms in use, together with the signs and symptoms relating to each. However, for midwives, more important than terminology, is the ability to recognize that a woman is unwell, and to refer swiftly and appropriately (Lewis 2007).
Box 56.1
Note: Not all women with severe pre-eclampsia present with all these symptoms and signs. Indeed any one symptom, with or without hypertension and proteinuria, is sufficient to indicate that the condition is worsening and that eclampsia may be imminent.
Features of hypertensive disorders in pregnancy
Pregnancy-induced hypertension (PIH)/Gestational hypertension (GH)
Pre-eclampsia
Definition
Most authorities agree that a blood pressure of 140/90 mmHg or more and/or an increase in diastolic pressure of 20 mmHg or more from the booking blood pressure, after the 20th week of pregnancy constitute grounds for further monitoring. A further recommendation is that women whose systolic blood pressure is above 160 on two separate occasions should be referred and treated for hypertension (Lewis 2007, PRECOG Development Group 2004, RCOG 2006).
Pre-eclampsia
Pre-eclampsia is the term used when hypertension is accompanied by one or more other signs and symptoms in the mother (see Box 56.1).
Pathophysiology
Despite much research, the cause of pre-eclampsia and HELLP syndrome is uncertain, and it remains a ‘disease of theories’ (Duley et al 2006, Redman & Walker 1996). There are some indications that low dietary calcium may be a factor and that antioxidant vitamins may help prevent the disease, but these areas remain under examination. Although it is associated with implantation of the placenta, there is a generalized response in the maternal endothelial system which leads to widespread platelet aggregation and vasoconstriction. It is thought that initial underperfusion of the placenta triggers a maternal circulatory response, leading to a generalized condition of hypovolaemia and vasospasm.
In normal pregnancy, physiological dilatation of the spiral arterioles in the placental bed occurs, by the stripping away of their muscle coating. This allows pooling of maternal blood in the intervillous spaces of the placental bed, creating a shunt, which lowers maternal blood pressure. This occurs around 16–18 weeks of pregnancy, and leads to the physiological fall in blood pressure commonly observed in the second trimester of normal pregnancy. In pre-eclampsia and HELLP syndrome, dilatation fails to occur and the blood pressure is raised as the blood is forced through constricted arterioles. There is generalized abnormal vascular tone and vasospasm, which leads to endothelial dysfunction and disturbances of maternal microcirculation. There is consequent hypoperfusion and vasoconstriction in the maternal brain, kidneys and liver, and in the placenta (Walker 2000). Similar circumstances arise in conditions where the placenta is large, such as diabetes and multiple pregnancy, and in hydatidiform mole, where the embryo dies, and only placental tissue remains (Roberts 2000).
Predisposing factors
Pre-eclampsia is more common in first pregnancies; even if the first pregnancy results in miscarriage, and more so after termination of pregnancy. There is a reduced incidence of pre-eclampsia in the second pregnancy, suggesting some immunological factors (Eras et al 2000). Li & Wi (2000) suggest that changing paternity also affects the woman’s likelihood of developing pre-eclampsia, while Smith et al (1997) suggest some partner-specific maternal immune response.
The Pre-eclampsia Community Guideline (PRECOG Development Group 2004) and National Institute for Health and Clinical Excellence guidelines (NICE 2008) suggest that women with any of the following are predisposed to the development of pre-eclampsia:
Diagnosis
Blood pressure
A reading of 140/90 mmHg is regarded as the upper limit of normal, but a rise in diastolic blood pressure of 15–20 mmHg or more above the level recorded prior to 20 weeks’ gestation is significant (Moran & Davison 1999). The diastolic pressure is no longer considered more significant than the systolic, and the Confidential Enquiry into Maternal and Child Health (CEMACH) suggests a systolic pressure of 160 mmHg on more than one occasion warrants investigation and treatment (Lewis 2007).
Accurate measurement of blood pressure is essential. The diastolic measurement is taken at stage V (five) of the Korotkov sounds (absence of sound) (PRECOG Development Group 2004, RCOG 2006). However, where there is no complete disappearance of sound (in about 15% of women), both ‘muffling’ and disappearance levels should be recorded (PRECOG Development Group 2004). The woman should be in a sitting or semi-reclining position, so that her arm and the sphygmomanometer cuff are at the same level as the left atrium. Large-size cuffs should be available for women weighing more than 85 kg (NICE 2008, PRECOG Development Group 2004, RCOG 2006). Although automated blood pressure devices are useful, they may underestimate blood pressure by as much as 30 mmHg (Natarajan et al 1999, RCOG 2006).
Urinalysis
Proteinuria of 1+ or more on dipstick, on two occasions more than 4 hours apart, in an uncontaminated specimen, and once infection has been excluded, is always serious in conjunction with hypertension (NICE 2008). It indicates damage to endothelial tissue, with leakage of albumin, the smallest plasma protein, from the blood into the urine. Protein loss changes osmotic pressure within the capillaries and may lead to pathological oedema. Hypertension and proteinuria occurring prior to 33 weeks’ gestation often has a poor prognosis (Mattar & Sibbai 1999).
Volume and concentration of urine affect random readings, and can result in false positives or negatives. Also, protein excretion is variable according to the time of day. A 24-hour urine collection remains the ‘gold standard’ for quantification of protein, and significant proteinuria is said to exist where protein exceeds 300 mg/24 hours (NICE 2008, RCOG 2006).
Oedema
Although 85% of women with pre-eclampsia develop oedema, it is no longer considered a cardinal sign, because it is a feature of normotensive as well as hypertensive pregnancies (see Ch. 31).
Pathological oedema associated with pre-eclampsia occurs in the pretibial area, hands, face and abdomen, and does not resolve with rest. Excessive weight gain may be due to occult oedema. During the latter half of pregnancy, normal weight gain should be approximately 0.5 kg per week. Weight gain significantly in excess of this, sometimes defined as exceeding 2 kg in a 1 week period, accompanied by hypertension and proteinuria is likely to be associated with pre-eclampsia (Duley et al 2006).
The midwife’s responsibility
It has been customary to describe pre-eclampsia as mild, moderate or severe, but it may also be helpful for midwives to consider ‘red flag’ signs and symptoms (Lewis 2007), as discussed below, which warrant immediate referral (see Ch. 55).
Early diagnosis is essential; so midwives must begin with an accurate recording of the woman’s history to identify risk factors and establish a baseline blood pressure using a standardized technique (Moran & Davison 1999). Thereafter, in addition to assessing general wellbeing, regular antenatal screening involves blood pressure readings, testing urine for protein at each visit, and assessing for significant non-dependent oedema.
Although hypertension and proteinuria, with or without oedema, occur in pre-eclampsia, the severity of these signs varies considerably. A small number of women present with symptoms such as headache, visual disturbances, epigastric pain or generally feeling unwell, which may be indicative of serious systemic complications, and may be followed by eclampsia (Lewis 2007). If a woman complains of these symptoms, the midwife must take her blood pressure, test the urine for protein, and then refer her to an obstetrician immediately, even if her blood pressure is not significantly raised. It is now known that convulsions may precede hypertension or proteinuria (Lewis 2007). Lewis (2007) suggests that same-day referral to an obstetrician is required for women who have hypertension >160 mmHg systolic and/or >90 mmHg diastolic or proteinuria >1+ on dipstick.
Following referral, women need follow-up care in a multidisciplinary team setting. With the changes in general practitioner (GP) involvement with antenatal care, and the arrangements for out-of-hours referral, midwives, in common with all health professionals, should make sure that GPs receive details of referrals and results of tests for their records (Lewis 2007).
Day assessment units (DAUs) or maternity day units (MDUs) offer facilities for ongoing assessment on an outpatient basis, with mothers actively involved in their own screening programmes (Moran & Davison 1999). All of the following tests can be carried out in this setting.
