General Principles:
The incidence of significant congenital or acquired heart disease in women presenting to obstetricians is approximately 0.5 to 2.0%. Cardiac output rises by 40-50% prior to the onset of labour and is further increased by the pain of labour and expulsive efforts by the mother in the second stage. The presence of significant cardiac disease can pose particular problems for both the patient and the anaesthetist.
Early, continuous epidural analgesia, cautiously administered using small incremental doses or an infusion, is recommended for most patients with cardiac disease (Chapter 83). Effective analgesia abolishes pain and, sometimes, the bearing down reflex, both of which can increase right heart pressures and induce tachycardia reducing the time available for cardiac filling and coronary perfusion. Monitoring for NYHA Class I and II patients (Table 108.1) should include continuous electrocardiography and pulse oximetry. In addition, invasive systemic and pulmonary arterial pressure monitoring should be considered for NHYA III and IV patients.
Epidural morphine and fentanyl have been shown to produce adequate, but incomplete, analgesia for first stage labour in patients with significant heart disease with minimal cardiovascular instability (1). The addition of low concentrations of bupivacaine will usually ensure satisfactory analgesia. In the second stage, a low subarachnoid or "saddle block" may be useful for delivery as an alternative to epidural block.
In compromised patients, the volume of crystalloid infused must be kept under tight control and titrated against the central venous or pulmonary capillary wedge (or pulmonary artery diastolic) pressure. Special care must be taken to avoid aortocaval compression and supine hypotension at all times (Chapter 28). Oxygen should be administered throughout labour (Chapter 87).
If hypotension secondary to a reduced systemic vascular resistance occurs, phenylephrine may be preferable to ephedrine. Phenylephrine exhibits less beta-agonist activity than ephedrine and has a lesser tendency to increase cardiac work and to cause tachycardia (Chapter 79). The potential cardiovascular benefits of the use of phenylephrine should be weighed against the possible detrimental effect of phenylephrine on utero-placental blood flow although recent work suggests that phenylephrine can, in fact, be used with safety (3).
In those patients with prosthetic cardiac valves or conduits, congenital malformations, mitral valve prolapse or other valvular disease, broad-spectrum antibiotic prophylaxis against subacute bacterial endocarditis should be commenced antepartum and should be continued for 3 to 4 days after delivery.
Anticoagulant therapy with heparin may have been prescribed in the latter half of pregnancy to patients with prosthetic valves, conduits, atrial fibrillation or a history of thrombo-embolism. Full-dose heparin should be stopped prior to labour or at least three hours before institution of a major regional block (Chapter 72).
General Principles: Specific Cardiac Conditions (Chapter 82 and Chapter 83):
Acquired Disease:
Rheumatic heart disease is probably the commonest (75%) form of heart disease encountered in pregnancy. Mitral stenosis in particular, may be associated with a relatively low, fixed, cardiac output and it is particularly important to avoid hypotension, hypervolaemia and tachycardia. The latter two, if uncontrolled, can lead to congestive cardiac failure.
Ischaemic heart disease and peripartum cardiomyopathy occur less commonly (Chapter 41). These mothers may also benefit from the judicious use of continuous epidural block and intensive monitoring.
Congenital heart disease:
Unless shunt reversal occurs, parturients with a left-to-right shunt (commonly an atrial or ventricular septal defect) usually experience labour and delivery without major problems.
If a right-to-left shunt is already present or develops as a result of shunt reversal, the outcome for mother and baby is, generally, poor. The anaesthetist should be aware of the specific problems of right-to-left shunting which relate to:
1. central cyanosis (which is relatively unresponsive to supplemental oxygen),
2. the effect of changing the balance between pulmonary and systemic vascular resistances - particularly the effect of pulmonary hypertension and/or systemic hypotension on this relationship,
3. the effect of a reduction in cardiac output on arterial oxygen saturation,
4. the potential for air embolism (particularly from intravenous lines), and
5. the impairment of platelet function which is a feature of cyanotic heart disease.
Shunt reversal is typically seen in Eisenmenger's syndrome and Ebstein's anomaly. Eisenmenger's (3% of congenital heart disease) consists of a right-to-left or bi-directional shunt at atrial, ventricular or aortopulmonary level, with cyanosis. The syndrome is associated with a high maternal (12-30%) and fetal (50%) mortality rate (2). Right-to-left shunting is also a feature of Fallot's Tetralogy which, however, is very rarely seen in the obstetric patient (Figure E.4).
References:
1. Macdonald R. Indications and contraindications for epidural blockade in obstetrics. In: Reynolds F, ed. Epidural and spinal blockade in obstetrics. London: Balliere Tindall,1990:19-32.
2. Rocke DA, Rout CC, Orlikowski CEP. Anesthesia and coexisting maternal disease; Part 1. Cardiac and hematologic disease. In: Norris MC, ed. Obstetric Anesthesia. Philadelphia: J.B.Lippincott, 1993: 447-473.