Cardiac surgery
Introduction and cardiopulmonary bypass
In keeping with the move towards minimal access in other areas, coronary artery bypass grafting and various valvular procedures can now be performed ‘off-pump’ via small incisions on the beating heart without bypass. However, most cardiac surgery is still carried out on bypass, with a bloodless field created by clamping the ascending aorta just proximal to where the aortic cannula returns arterial blood from the bypass machine (see Fig. 44.1). Without coronary artery blood flow, myocardial ischaemic damage has to be mitigated by reducing metabolic demands by inducing diastolic arrest with a high-potassium cardioplegic solution (16 mmol/L KCl) (Table 44.1) and cooling (to between 4 and 12°C).
Table 44.1
Constituents of a typical infusate for cold cardioplegia (St Thomas’s solution)
| Constituent | Quantity |
| Sodium chloride | 110.0 mmol/L |
| Potassium chloride | 16.0 mmol/L |
| Magnesium chloride | 16.0 mmol/L |
| Calcium chloride | 1.2 mmol/L |
| Sodium bicarbonate | 10.0 mmol/L |
| Procaine | 16.0 mmol/L |
Fig. 44.1 The standard circuit for cardiopulmonary bypass
1. Systemic venous blood is siphoned into a reservoir by gravity from the right atrium (or from the superior and inferior vena cavae) via a venous cannula
2. Venous blood from the reservoir enters a roller pump
3. Blood is pumped under pressure through a membrane oxygenator
4. Oxygenated blood then passes through a heat exchanger to cool or rewarm the blood as necessary
5. The oxygenated and temperature-controlled blood finally passes through a filter and is returned to the systemic arterial circulation, usually via the ascending aorta by a roller pump
• Activation of the clotting cascade could occur, causing intravascular coagulation. This is prevented by anticoagulating with high-dose heparin (300 IU/kg) which is later reversed with protamine sulphate on coming off bypass
• Clotting factors and platelets are consumed in the extracorporeal circuit (mainly within the oxygenator) leading to a bleeding diathesis. Blood products may be needed to reverse this
• Activation of the complement cascade and other inflammatory mediators may result in systemic inflammatory response syndrome (SIRS) after bypass
Congenital cardiac disease
Types of congenital heart disease
Cyanotic heart disease
• Tetralogy of Fallot—the four features are ventricular septal defect (VSD), pulmonary stenosis, right ventricular hypertrophy and an aorta which overrides the ventricular septum, receiving blood from both ventricles
• Transposition of the great arteries—the pulmonary artery arises from the left ventricle and the aorta from the right ventricle
• Tricuspid atresia—absence of a functional tricuspid valve
• Truncus arteriosus—the pulmonary artery and aorta fail to develop separately
• Total anomalous pulmonary venous drainage—pulmonary venous blood drains into the right side of the heart
• Eisenmenger’s syndrome—increased pulmonary blood flow caused by a pre-existing left-to-right shunt (see next section) causes severe pulmonary hypertension later in life, which result in spontaneous reversal of the shunt so flow reverses to become right-to-left
Acyanotic heart disease
Acyanotic congenital heart disease may involve:
• A shunt from left to right sides of the heart (e.g. via an atrial or ventricular septal defect) or a ductus arteriosus which persists in its antenatal patent state (PDA)
• Failed or incomplete embryological development of parts of the heart or great vessels without shunting, e.g. coarctation of the aorta
Management of congenital heart disease
Palliating congenital cardiac disorders
When pulmonary blood flow is reduced (as in tricuspid atresia, tetralogy of Fallot or pulmonary artery stenosis), palliation aims to increase pulmonary flow by creating a shunt between the systemic arterial circulation and the pulmonary artery (see Fig. 44.2).
