Truncus arteriosus2

Truncal valve
The truncal valve has a variable number of leaflets and vari¬able morphology. There may be as few as two or as many as six leaflets though it is very rare to find more than four well-formed leaflets.2 Often one or two leaflets are poorly formed with inadequate commissural support which can result in important regurgitation.4 The poorly formed leaflets tend to be somewhat myxomatous and thickened and can hold sutures even in the neonatal period unlike normal semilunar valve tissue. It is rare for a truncal valve to be structurally stenotic.
Ventricular septal defect
The VSD in truncus arteriosus can be described as 'subarterial'. It lies immediately under the truncus and is usually separated from the tricuspid valve by the posterior limb of the septal band so that the risk of complete heart block at the time of sur¬gical closure is small.5 On occasion the truncal valve appears to lie predominantly over the right ventricle and the VSD can appear small so that there is a chance of restriction to left ventricular outflow at VSD level following repair.
Coronary arteries
Although the distal branching of the coronary arteries is usually normal it is not uncommon for one or both ostia to be abnormally positioned.6,7 The left ostium for example may be extremely close to the takeoff of the right or more commonly the left pulmonary artery.2-4 As with tetralogy there may be an anomalous anterior descending coronary artery which arises from the right coronary artery and passes across the infundibulum of the right ventricle.

ASSOCIATED ANOMALIES
A right aortic arch is present in approximately 25% of patients and an aberrant subclavian artery is seen in 5-10%.8 As noted above under embryology, a ductus is rarely present if the aortic arch is intact. However, in 10-15% of patients the aorta is interrupted, either between the left carotid and left subclavian or beyond the left subclavian9,10 and the descending aorta gives the appearance of being a direct con¬tinuation of the main pulmonary artery. The observation has been made by us and others (BG Barratt-Boyes, personal communication) that the ductus under these circumstances rarely closes and rarely requires a prostaglandin infusion to maintain patency.

PATHOPHYSIOLOGY
So long as pulmonary resistance remains at the high levels of the neonate the child with truncus arteriosus will have relatively balanced pulmonary and systemic circulations and may be free of symptoms. However, as pulmonary resistance falls in the first days and weeks of life there will be an increas¬ing amount of pulmonary blood flow and the child is likely to develop signs of congestive heart failure. Heart failure can be quite pronounced in the child with truncus because

pulmonary blood flow occurs not only during systole but also during diastole. There is often retrograde flow in the abdominal aorta during diastole stealing blood flow from the hepatic, renal and mesenteric circulation.
Retrograde flow is further exacerbated by the presence of truncal valve regurgitation. Generally a gradient across the truncal valve is a reflection of massively increased total flow across the valve secondary to both high pulmonary blood flow in combination with the systemic blood flow passing across the valve as well as the regurgitant fraction passing across the valve. After repair including repair of the regurgitant valve the gradient is usually eliminated.
Both right ventricular and left ventricular blood are ejected through the truncal valve resulting in at least a mild degree of cyanosis.