Introduction
Tetralogy of Fallot (ToF) is the commonest cyanotic heart condition and accounts for 5 per cent of all congenital heart diseases. The four components are a ventricular septal defect (VSD), overriding aorta, right ventricular outflow tract (RVOT) obstruction and right ventricular hypertrophy (RVH). Morphologically, this can be characterized by an antero-cephalad deviation of the outflow septum that results in a large peri-membranous VSD that is overridden by the aorta and multi-level stenosis of the deviated RVOT. The RVH is the natural response to this outflow tract obstruction (Figure 11.1).
Figure 11.1 (A) The anatomy of tetralogy of Fallot. There is a large ventricular septal defect and multi-level obstruction to the right ventricular outflow tract; this is of a variable degree of severity that consists of muscular obstruction below the valve, valvar stenosis and a small main pulmonary artery. The arrow shows the right-to-left flow across the VSD. (B) Surgical repair of tetralogy of Fallot showing the VSD closed with a patch and the RVOT enlarged with a combination of muscle resection and placement of a trans-annular patch (LV = left ventricle; RV = right ventricle; LA = left atrium; RA = right atrium; Ao = aorta; RVOT = right ventricular outflow tract obstruction).
The degree of right ventricular outflow tract (RVOT) obstruction is variable but involves the muscular infundibulum, the pulmonary valve and annulus and the main pulmonary artery and the branch origins. Beyond this, the pulmonary arteries and vasculature are usually normal.
ToF is also associated with right-sided aortic arch (15 per cent), bilateral superior venae cavae (SVCs; 10 per cent) and, as part of the family of cono-truncal anomalies, has an association with DiGeorge syndrome (15 per cent). It can also be associated with other non-cardiac conditions such as vertebral defects, anal atresia, cardiac defects, tracheo-oesophageal fistula, renal anomalies, and limb abnormalities (VACTERL) association and coloboma, heart defects, atresia choanae, growth retardation, genital abnormalities, and ear abnormalities (CHARGE) syndrome. There is a subgroup of (usually more severe) cases in which the aorta is more committed to the right ventricle than it is to the left: in this arrangement, the aorta has moved more anteriorly and is no longer in continuity with the mitral valve. This is a variant of double-outlet right ventricle (DORV) and can be more challenging to repair.
Clinical Presentation.
Time of presentation and symptoms will depend upon the degree of RVOT obstruction, which dictates the degree of cyanosis. Most neonates are acyanotic or only mildly cyanosed but become gradually more cyanosed as the RVH progresses and the degree of RVOT obstruction increases. A newborn who is severely cyanosed will often respond to conservative management using prostaglandin to maintain ductal patency and allow the high pulmonary vascular resistance of the newborn to subside. Infants and older children may develop cyanotic ‘spells’ that are caused by a temporary dynamic increase in the degree of muscular infundibular obstruction – these may respond to beta-blocker therapy that helps relax the infundibular myocardium. Children may remain symptom-free for many years if the degree of RVOT obstruction is mild, but the natural history is one of gradual but progressive cyanosis. Heart failure is very rare since the RVOT obstruction protects the lungs from over-circulation despite the large VSD.
Investigation.
CXR shows a characteristic boot-shaped heart representing the prominent right ventricle but relatively small main pulmonary arteries (Figure 11.2). ECG will show RVH occasionally with right bundle branch block (RBBB). Echocardiography is the mainstay of investigation (Figure 11.3) and will usually be able to demonstrate all features of the anatomy. Cardiac catheter or CT angiography is sometimes required to delineate the branch pulmonary artery anatomy or to confirm coronary pattern.
Figure 11.2 CXR in tetralogy of Fallot showing the classic ‘boot-shaped heart’, which reflects the hypertrophied right ventricle and relatively small main pulmonary artery.
Figure 11.3 Echocardiographic still images of tetralogy of Fallot showing turbulent flow in the long, narrowed right ventricular infundibulum. The lower two images (A and B) show the composite hypoplasia of small infundibulum, valve annulus, and main and branch pulmonary arteries.
Management.
The condition is usually relatively stable with a natural history of gradually progressive cyanosis as the RVOT obstruction progresses. Occasionally, severe cases present as newborns or small infants with severe cyanosis requiring inpatient treatment, rehydration (if water depleted) and the use of prostaglandin in severe cases. Newborns may settle with this treatment alone as the pulmonary vascular resistance falls over the first days of life and saturations improve. Persistently cyanosed patients require early intervention – this has traditionally been the use of a Blalock-Taussig shunt (see Chapter 4), but primary repair is also feasible in appropriate cases. An alternative is to place a RVOT stent or a small outflow tract patch to improve pulmonary blood flow. The stent is attractive in that it avoids surgery, but it does require considerable skill to deploy in a small baby.
Older infants and children may gradually become more cyanosed but may also develop hypercyanotic ‘spells’ often triggered by physical exertion or coughing. These are caused by spasm of the infundibular muscle and respond to increasing preload on the heart (i.e. giving volume, although older patients learn strategies to increase their own preload with manoeuvres such as squatting) or the use of beta-blockers to relieve infundibular spasm. Treatment of ToF is usually supportive therapies such as these until the child is referred for elective repair.
Surgical Repair.
Anatomical correction is achieved by closing the VSD and relieving any outflow tract obstruction. Performed under moderate hypothermia with bicaval cannulation, the pulmonary arteries should be clearly defined such that any areas of stenosis (usually at their origins) can be addressed. Any residual patent ductus arteriosus (PDA) is ligated. The VSD can usually be closed trans-atrially with a prosthetic patch, working through the tricuspid valve. Relief of the RVOT obstruction needs careful consideration: obstructive septo-parietal muscle bars in the infundibulum can be divided, and if the cusps of the pulmonary valve are fused, they can be opened with a valvotomy, but if the annulus of the valve is too small, then the incision in the main pulmonary artery needs to be extended across the annulus and a short distance into the infundibulum to allow the outflow tract to open out adequately. The aim should be to ensure that the outflow tract reaches the predicted size for the patients’ height/weight (Kirklin tables) or at least within a Z-score of −2. The outflow tract is then repaired with a prosthetic patch – if the annulus is crossed, then this is referred to as a ‘trans-annular patch’ and will have inevitably rendered the pulmonary valve incompetent (Figure 11.4). Various techniques have been used to re-establish a competent valve in these situations, most commonly by creating a mono-cusp outflow tract patch in two layers or with a mono-cusp patch created from an aortic homograft. These techniques are effective in reducing the amount of pulmonary regurgitation (PR) immediately postoperatively, but most go onto develop significant PR over the following years.
