Introduction

Truncus arteriosis is a congenital heart condition where there is a single arterial outlet from the heart, the so-called truncus, which then divides into the aorta and the pulmonary artery (PA). The condition makes up 3 per cent of all congenital heart defects. It is thought that during development of the heart, there is failure of separation of the PA from the aorta, which are both derived from the primitive truncal vessel. The PA can arise as a single opening from the trunk and then divides into left and right PAs (type 1 truncus in the Van Praagh classification; Figure 19.1). The left and right PAs may arise as two separate vessels (type 2). Rarely, a ductus arteriosus supplies the left PA, and the right PA alone arises from the truncus vessels (type 3). In some 60 per cent of cases, the aorta is to the left side of the trachea, and 30 per cent are located to the right side. In about 10 per cent of cases there is an interruption of the aortic arch between the left carotid arteries and the left subclavian arteries (type 4). Coarctation of the aorta is extremely rare. The coronary arteries usually arise in their usual positions from the sinuses of the truncal vessel above the truncal valve, but there can be a lot of variability in the origin of these vessels and their pathways to the myocardium. Truncus arteriosus is associated in the vast majority of patients with a large, unrestrictive sub-arterial ventricular septal defect (VSD). In the majority of cases, the truncal valve is tricuspid and neither stenotic nor incompetent. The truncal valve, however, shows great variability in morphology and can be bicuspid or quadri-cuspid and in these cases quite dysplastic. In particular, the quadri-cusp valve can be incompetent and stenotic. Management of the truncal valve in these situations is a surgical challenge. Rarely, the left ventricle in this condition is small, merging towards the hypoplastic left heart spectrum of conditions. These patients may require univentricular repair. As with other congenital heart conditions with aortic arch problems, DiGeorge syndrome is a common association. Therefore, irradiated blood products are recommended in managing these patients in an attempt to avoid graft-versus-host sensitization. In association with the DiGeorge syndrome, there may be problems with calcium metabolism, and calcium supplementation may be needed.

Figure 19.1 Anatomy of truncus arteriosus (RA = right atrium; LA = left atrium; RV = right ventricle; LV = left ventricle).

Diagnosis and Presentation

Diagnosis is invariably made by echocardiography alone without the need for invasive cardiac catheterization. If more information is needed to define the morphology of the aorta and PAs, then a CT scan or MRI may be advised. Often the diagnosis is made antenatally by maternal-fetal scans. The non-invasive investigations can define the underlying anatomy, the presence of the VSD, the single origin of the great vessel from the heart and the presence of aortic arch anomalies. The underlying ventricular function can also be assessed. It is important to define as clearly as possible the morphology and function of the truncal valve.

Clinically, these patients present with congestive cardiac failure. Whilst they may be quite stable for the first few days or week or two of life with high post-natal pulmonary vascular resistance (PVR), as the PVR falls, the left-to-right shunt through the VSD increases volume loading the heart and creating heart failure. Complicating this picture is the diastolic runoff to the low-resistance pulmonary vascular bed, so the diastolic pressure can be very low. This creates a steal of blood from the coronary arteries and can cause severe myocardial ischaemia with associated changes on ECG. These patients are susceptible to sudden cardiac arrest because of myocardial ischaemia, and even when resuscitated, this can be repeated. Clinically, then, these patients are in low cardiac output, congestive cardiac failure with high pulmonary blood flow because of the diastolic runoff to the pulmonary arteries and left-to-right shunt through the VSD. They are usually very tachypnoeic. They fail to thrive and do not gain weight. They are susceptible to sudden death. For this reason, these patients remain in hospital and are operated upon as soon as possible to correct the truncus arteriosus abnormalities.

Patients who are not diagnosed because they do not develop these symptoms may be diagnosed because of continuing high PVR, and they may go on to develop pulmonary vascular disease later in life and because of this become inoperable. This is a very rare situation in the developed countries.