What are the other features of a truncus arteriosus ?

In addition to the main abnormalities of branching patterns of the great arteries of the body, there are some special features of truncus arteriosus:

• The common truncus is guarded by a single semi-lunar valve, with varying number of cusps. Most patients have three or four cusps. Sometimes, this valve - called the truncal valve - is leaky (regurgitation).
• A Ventricular Septal Defect (VSD) is almost always present and located just below the origin of the truncus from the ventricles.

What are the associated anomalies with truncus arteriosus ?

Common associations with a truncus arteriosus are coarctation of the aorta, interrupted aortic arch and abnormalities of the coronary arteries. Other birth defects may also be found in addition, but the incidence is lower.

What happens in a truncus arteriosus ?

The main functional alteration in truncus arteriosus is in the pathway of blood flow out of the heart. Normally, pure blood from the left ventricle will pass into the aorta and to the different organs of the body, while impure blood will flow from the right ventricle into the pulmonary artery and then to the lungs for purification. In truncus, blood from both ventricles passes into the common arterial trunk.

From the common trunk, blood flow becomes divided. One part flows into the lungs through the pulmonary artery branches. The remainder flows into the aorta beyond them. The harmful effects of truncus arteriosus depend on what proportion of this blood flow reaches the lungs.

What then determines this proportion ? As we have discussed in many other conditions like ASD and VSD, the most important factor that determines blood flow through an arterial system is the resistance it offers. The blood vessels in the lungs are thin walled and distensible. So they offer low resistance to blood flow. In contrast, the aorta and its branches are thicker walled and offer higher resistance. So blood in the common arterial trunk flows preferentially into the lungs.

What are the effects of this increased lung blood flow ?

These effects are again similar to those seen in conditions like ASD and VSD. A larger volume of blood flows through the lungs and into the left ventricle, making it work harder than normal. After a while, the left ventricle is unable to cope with the increased work load, and it "fails". This condition, called heart failure, is manifested in the child by breathlessness, difficulty in feeding, frequent chest colds, constant crying and failure to thrive.

If the condition is left untreated, over time the blood vessels in the lungs become thick and hard walled - a condition called Pulmonary Vascular Obstructive Disease. This results in a very high blood pressure inside the lungs - Pulmonary Hypertension. At this stage, corrective surgery is no longer feasible.

What happens if truncus arteriosus is not treated ?... Read on.

 

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