What is Truncus Arteriosus ?
Truncus arteriosus is a rare complex birth defect of the heart in which only one artery leaves the heart, instead of the normal pattern where two great arteries – the aorta and the pulmonary artery – carry blood away from the heart.
In truncus arteriosus, a single large arterial trunk arises in common from both the right and left ventricles. It then divides into the aorta and pulmonary artery, after first giving off coronary artery branches.
The method of branching allows the classification of truncus arteriosus into three types.
- Type 1 : The truncus gives off a main pulmonary artery and then continues as the aorta. The main pulmonary artery then divides into two branch pulmonary arteries – right and left.
- Type 2 : The right and left branch pulmonary arteries arise as separate branches, but close to each other, directly from the truncus arteriosus. Beyond these branches, the truncus continues as the aorta.
- Type 3 : The right and left pulmonary arteries arise separately from the truncus, but widely separated from each other – one on each side, right and left, of the truncus. This type resembles type 2 in all other respects.
In all three types, the coronary arteries are the first branches of the common truncus.
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 Atrial Septal Defect (ASD) and Ventricular Septal Defect (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 ?
The natural history (without treatment) of this condition is very bad.
Nearly one half of patients succumb to the disease within one month of life. Only one in ten survive to their first birthday. Even among these few survivors, long term outcome is not promising. Many develop pulmonary vascular obstructive disease and fall a victim to pulmonary hypertension or rarely infective endocarditis.
Should truncus arteriosus be operated ?
The poor natural history of this condition makes surgery mandatory in almost all cases as soon as the diagnosis is made. Operation is preferred at the earliest date to minimise the risks caused by high lung blood flow for a prolonged period.
What are the treatment options for truncus ?
Being such a complex condition with serious consequences even at a young age, the only treatment currently available today is early operation. Many centers are now able to operate on these patients even within the first month of life with good results.
What is the operative repair for truncus arteriosus ?
In its simplest form, the repair involves
- separating the pulmonary arteries from the truncus
- establishing a connection between the right ventricle and pulmonary arteries
- closing the VSD
This requires an open heart operation, and sometimes involves the use of a technique called “Total Circulatory Arrest“.
The patient is hooked up to the heart lung machine and the heart beat stopped. The first step in the operation is to disconnect the pulmonary artery (or its branches) from the common arterial trunk. The hole left in the truncus is closed, either directly by sutures, or if it is large, by using a patch made of fabric (Dacron, PTFE).
The right ventricle is then opened and the VSD visualised. The VSD is commonly located right below the truncal valve, and it is repaired in the usual manner with a patch of fabric held in place with sutures of polypropylene.
The connection between the right ventricle and pulmonary artery is then established using a valved conduit.
What is a valved conduit ?
A conduit is a tube used to connect the ventricle and pulmonary artery. It may be made of fabric like Dacron or PTFE, or may be a homograft (aortic or pulmonary), or sometimes even pericardium fashioned into a hollow tube. A homograft conduit will have a valve inside it. A fabric or pericardium conduit can be modified to have either a prosthetic valve or a “monocusp” valve inside it.
The conduit is connected by sutures on one side to the pulmonary artery (or its branches) and on the other end to the opening in the right ventricle. Blood can now flow across the conduit from the right ventricle into the lungs.
What are the risks of operation ?
Repair of truncus arteriosus is a risky operation since it is done on very small children with a complex heart problem. But with growing experience, results have improved tremendously, and specialized centers have an acceptably small risk of mortality and morbidity.
Problems might arise due to
- leaky truncal valves, which might need to be repaired or rarely even replaced by mechanical prosthetic valves.
- abnormal coronary arteries.
- associated conditions like interrupted aortic arch, which can be corrected at the same procedure but increase the risk slightly.
After surgery, a complication that often occurs is a condition called Pulmonary Hypertensive Crisis. In this condition, there are sudden episodes of severely increased lung blood pressure. These must be anticipated and treated vigorously and might rarely be fatal.