The hallmark of tetralogy of Fallot is anterior and cephalad deviation of the conotruncal septum.

Pulmonic stenosis – is always significant and hence pulmonary artery pressure is normal or low, never high as in pure VSD. The obstruction is subpulmonic. If this is severe, distal obstructions are more commonly present. Usually pulmonary arteries are of adequate size to permit surgical repair. In pulmonary atresia, pulmonary artery size may be too less for repair.

Subpulmonic obstruction is due to both antero-cephalad deviation of the conotruncal septum and muscular hypertrophy of the deviated septum and the right ventricular free wall. In addition, there may be intracavitary obstruction due to hypertrophied muscle bundles.

The pulmonary valve is commonly small and stenotic and is usually unicuspid or bicuspid. Supravalvular discrete stenosis may occur. Branch ostial stenosis may occur, especially on the left. In pulmonary atresia, the pulmonary arteries are very small and are feeded by aortopulmonary collaterals.

Ventricular septal defect– This has fibrous continuity between tricuspid and aortic valve and hence is a true perimembranous defect, even though it lies in a subarterial location. It is non-restrictive by definition, though some defects may be restrictive due to tricuspid valve tissue.

Aortic override– This may vary from 15% to 95%. Override of more than 50% does not mean double outlet right ventricle- this needs presence of both subaortic conus (absent aorto-mitral continuity) and subpulmonary conus. Aortic override is not only due to the malalignment. It is also due to aortic dilation due to malseptation of the conotruncus and due to rotation of the aorta so that the right aortic sinus becomes more anterior (and left).

Coronary arteries

  1. 15% have a large conal branch or accessory left anterior descending artery.
  2. 5% have origin of left anterior descending artery from right coronary artery. This then crosses the right ventricular outflow tract- this is important surgically as the surgeon cannot approach through this area for repair.
  3. 4% have single origin of coronaries.

Coronary artery origin has to be found by echocardiography, angiography or MRI before surgery.

Aortic arch anomalies – These are more common if there is Catch 22. Right aortic arch is present in 25%. Aberrant origin of ipsilateral subclavian artery and origin of left subclavian artery from pulmonary artery are sometimes seen.

Aortopulmonary collaterals– Rabinovich type 1 (bronchial artery collaterals) collaterals are uncommonly present in Tetralogy of Fallot with pulmonic stenosis. Collaterals are much more common in Tetralogy of Fallot with pulmonary atresia.

Other anomalies– ASD is present in 83%. Left superior vena cava is found in 11%. Atrioventricular septal defect may be associated, especially in Down’s syndrome. Left sided anomalies are rare.