Physiology

Important physiological changes in the cardiovascular system need to occur during pregnancy in order to facilitate the increased demand for oxygen and nutrients by the uteroplacental unit, and that of the growing fetus. Blood flow to the uterus increases from 50 mL/min at 10 weeks’ gestation to 850 mL/min at term. This is accomplished by a 50% rise in cardiac output (CO) and blood volume. There is an increase in heart rate from 6 weeks. There is a reduction in systemic and pulmonary vascular resistance due to vasoactive prostaglandins and nitric oxide production, thus preventing a rise in pulmonary artery pressure from the increased circulating volume. The systolic and diastolic pressures decrease, reaching their lowest values during the second trimester, before increasing as term approaches, although never reaching prepregnancy values. Therefore, there is a risk of cardiac decompensation as pregnancy progresses. If the cardiac output does not increase adequately, placental insufficiency will occur, which will lead to fetal growth retardation. Changes in vascular resistance can lead to changes in flows through shunts. In patients with Marfan’s, repaired coarctation or bicuspid aortic valve with ascending aortopathy, the increase in cardiac output can lead to aortic dilatation leading to dissection and rupture.

There is also a 15% reduction in colloid osmotic pressure during pregnancy, which can increase the risk of pulmonary oedema in patients with cardiac disease.

During labour CO increases by a further 25–50% with up to 500 mL autotransfusion of blood from an increased return from the intervillous spaces during contractions. There is a further increase in blood volume after delivery due to uterine contraction in the third stage. This can result in a clinical deterioration in patients with cardiac disease due to the associated rise in ventricular filling pressure and end-diastolic volume.

General approach to cardiac disease in pregnancy

Ideally, preparation should occur before pregnancy occurs. The responsible cardiologist and obstetrician should undertake preconceptual counselling. Risks involved to the patient should be discussed openly so informed decisions can be made. Cardiac function can be quantified with cardiopulmonary exercise testing. Cardiac function should be optimized, which may include surgical interventions such as valve replacement. Known teratogenic medications should ideally be discontinued. Women should be seen early in established pregnancy in joint cardiology and obstetric clinics. A multidisciplinary care plan can be made, including consideration of the services required at the intended delivery unit such as cardiology, cardiothoracic surgery and cardiac or general intensive care. An anaesthetist should review the woman at an early stage to develop a plan for delivery and postpartum care.

In general, predictors of a primary cardiac event during the pregnancy include:

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  Prior cardiac event or stroke

  
  
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  Prior arrhythmia

  
  
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  New York Heart Association (NYHA) functional class >2

  
  
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  Left-sided heart obstruction (mitral valve area <2 cm², aortic valve area <1.5 cm² or outflow tract gradient >30 mmHg

  
  
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  Left ventricular ejection fraction <40%.

Congenital heart disease

At almost 1% (7/1000), congenital heart disease is the most common inborn defect, with 60% diagnosed before 1 year of age, 30% as children and 10% after 16 years of age. Of those surviving infancy, 96% will live for at least 15 years, with the majority of deaths occurring after 20 years age. There are approximately 150,000 patients with CHD in the UK and this number is increasing as surgical and medical management improves. At least 17% are recognized as having a genetic basis. The common defects are illustrated in Table 16.1.

In most cases of congenital heart disease the diagnosis, functional status and any therapeutic strategies will be well established prepregnancy. Depending on the condition, pregnancy could be well tolerated or be classified as risk class 4 in the modified WHO classification of maternal cardiovascular risk (Table 16.2). Risk Class IV is where pregnancy is relatively contraindicated and termination should be discussed, if the patient presents as pregnant. If pregnancy is to continue, treat the same as Risk Class III, with expert input for the duration of the peripartum period. High-risk groups are illustrated in Tables 16.3 and 16.4. Pregnancy is associated with a thrombotic tendency, which is exacerbated by the polycythaemia that occurs in patients with cyanotic heart disease. Congenital heart disease patients also have a higher incidence of pre-eclampsia, which is associated with high mortality in patients with Eisenmenger’s. In addition, they also have a tendency to develop new arrhythmias.

In 50% of patients there will be intrauterine growth retardation (IUGR) with fetal indication of early delivery. The babies have an increased risk of heart defect, which is often similar to that of the parents (3–5%) (Table 16.5).

Assessment

The most import part of the assessment is to identify patients that have complete repair, palliative surgery or no repair. Those with complete correction should behave in a similar manner to normal patients. Palliative correction or no correction is likely to be associated with primary and secondary cardiac, pulmonary or systemic dysfunction.

These patients should be seen in clinic at fortnightly intervals in the first 24 weeks and then on a weekly basis, looking for specific signs of deterioration (Table 16.6).

History of shortness of breath and palpitations should be specifically sought and examination should focus on signs of failure. Periodic transthoracic ECHO may also be helpful in identifying deterioration and the need for early hospitalization and delivery. Pulse oximetry particularly before and after exertion can be helpful in identifying patients that are deteriorating.

Fetal scans at 20 weeks will pick up 80% of major cardiac defects and should be repeated at 24 weeks. Nuchal scans help indicate fetal cardiac disease (>4 mm = Down’s and cardiac disease likely; <4 mm = 1:1000 risk of CHD).

In addition, many patients with CHD have other syndromes, which will have specific anaesthetic implications. Noonan’s, Turner’s and trisomy can be associated with problems with airway management as well as communication and consent. Di George syndrome, which is also called CATCH 22 (Cardiac defect, Abnormal facies, Thymic hypoplasia, Cleft palate, Hypocalcaemia due to chromosome 22), is associated with potential airway problems, electrolyte problems and immune deficiency requiring the use of irradiated blood products for transfusion.

Ventriculoseptal defects (VSD)

These are graded according to the flow through the defect and its site. Therefore, unrestricted defects, which are associated with high flow, will be associated with more problems. Small to moderate VSDs are usually well tolerated in pregnancy and generally behave as well as repaired defects. The main aim is to identify pulmonary hypertension or even Eisenmenger’s, which will be associated with increased risk of complications.

Atrioseptal defects (ASD)

If previously undiagnosed, these patients present with an enlarged heart and murmurs in pregnancy, usually secundum ASDs. Primum ASDs are associated with other endocardial cushion defects with involvement of atrioventricular valves and earlier presentation. These patients may develop arrhythmias. Pregnancy is usually tolerated well in unoperated patients unless they have developed pulmonary hypertension.

Atrioventriculoseptal defects (AVSD)

Of complete AVSD, 75% occur in patients with Down’s syndrome whilst 90% of partial AVSD occur in non-Down’s patients. Patients with partial AVSD present in a manner similar to secundum VSD. Complete AVSD act essentially as univentricular hearts and are associated with heart failure.

These patients have a high risk during pregnancy due to the problems of cardiac failure risk, but particularly due to a tendency to pulmonary hypertension and Eisenmenger’s.