Annex A > Chapter 3 - Developments in the UK, in the Diagnosis and Treatment of Congenital Heart Abnormalities in Children, 1984-1995 > The specific heart abnormalities and procedures referred to in the Inquiry > Truncus Arteriosus
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Truncus Arteriosus
The problem
55 Truncus Arteriosus is very rare, occurring in fewer than 1% of congenital heart abnormalities, but it has provoked a great deal of interest. In order to understand the nature of the abnormality it is helpful to recall that in the normal heart the aorta arises from the left ventricle and the pulmonary artery from the right. In Truncus Arteriosus, a single large arterial trunk arises from both ventricles, and there is a large VSD just below the trunk. Soon after its origin from the ventricles, the arterial trunk gives rise to large pulmonary arteries and then it continues as the aorta. Both ventricles pump blood simultaneously into the trunk which consequently receives both deoxygenated blood from the right ventricle and oxygenated blood from the left. If this blood were evenly mixed, one would expect that the relatively high contribution of deoxygenated blood would cause obvious blueness or cyanosis (see para 35). Indeed, careful measurement of the blood oxygen content does reveal some degree of deoxygenation but this is usually very mild.
56 The more important effect of the abnormality is that there is the usual tendency for blood flow to take the path of least resistance, i.e. to flow preferentially into the pulmonary arteries and to the lungs. Blood flow to the lungs tends to be torrential with a large return of oxygenated blood to the left heart. Therefore the flow of oxygenated blood from the left ventricle into the common trunk is many times more than that from the right ventricle. Consequently, the effect of the deoxygenated blood flowing to the aorta is considerably reduced, accounting for the relative lack of cyanosis.
57 The presenting effects on the baby are similar to those seen when there is a large VSD. The increased volume of blood flowing to the lungs and returning to the left side of the heart causes heart failure in the same way as described for VSD. The heart failure tends to develop much more rapidly and with greater severity with the result that 70% of babies with Truncus Arteriosus do not survive beyond the age of 3 months if left untreated.
58 Not uncommonly, there is another related problem involving the valve between the arterial trunk and the two ventricles. The valve may open well but may not close fully, with the consequence that blood that has been pumped from the ventricles into the arterial trunk may leak back into the ventricles. This adds to the amount of work done by the ventricles and hastens the development of heart failure.
59 Another associated abnormality that may accompany Truncus Arteriosus is known as `interrupted aortic arch', which is an extreme form of Coarctation of the Aorta. Babies with this combination usually present during the first week of life and their general condition may deteriorate quite rapidly without intensive treatment and early surgery.
Diagnosis
60 The baby usually presents during the first few weeks of life with evidence of severe heart failure. The diagnosis can usually be made quite accurately by echocardiography alone. At least this has been true since the early 1990s when colour-flow Doppler mapping of blood flow became routinely available. During the 1980s it was usually possible to diagnose the presence of Truncus Arteriosus with echocardiography but the precise nature of the origin of the pulmonary arteries from the arterial trunk could not always be defined. In some patients there is the additional problem of an associated interrupted aortic arch which may be very difficult to diagnose. In those cases, both then and today, additional investigation by cardiac catheterisation may need to be undertaken.
Management
61 Because the newborn infant with Truncus Arteriosus is usually so ill, the standard drugs for the treatment of heart failure are of limited value. The baby often, but not always, requires early treatment in the ICU with the support of a ventilator. Surgical correction is usually undertaken in babies who are less than 6 weeks old. The operation is a major open-heart procedure and is complex. Essentially the VSD is closed in such a way that all the blood flow from the left ventricle is directed into the common arterial trunk. The pulmonary arteries are separated from the trunk, which is then reconstituted as a single large artery, and it becomes the new aorta. The pulmonary arteries are connected to a tube or conduit, if possible a piece of donated human aorta or pig's aorta, and the conduit is connected to a surgically created opening in the right ventricle. In some cases a conduit with a human or pig's valve is used. In this way the right ventricle pumps blood directly to the pulmonary arteries while the left ventricle pumps to the aorta. If the truncal valve is leaking it may need to be repaired and this can be a significant complicating factor in determining outcome.
62 Because Truncus Arteriosus is such a rare condition, very few operations are done in any one centre each year. It is therefore difficult to assess with certainty the mortality rates across the UK during the period covered by the Inquiry's Terms of Reference. In the mid-1980s corrective operations on babies under the age of 1 year were considered to carry a very high risk of death. The results improved during the late 1980s to the early 1990s but by 1995 the operation still carried a very high risk, even in those cases in which there were no associated complicating problems.
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