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A 6 month old boy with trisomy 21 was referred for preoperative cardiac catheterisation of a ventricular septal defect, an ostium secundum atrial septal defect, a right aortic arch, and a patent ductus arteriosus. On physical examination there was no cyanosis, a grade 3/6 systolic murmur was heard at the left sternal border with a loud single second sound. Blood pressure on the right and left arm was 80/50 mm Hg.
Cardiac catheterisation revealed a left to right shunt with a Qp:Qs of 2.3:1 and a raised Rp/Rs of 0.27, as well as systemic blood pressure in the right ventricle and pulmonary arteries. A left ventriculography and an aortogram demonstrated a right aortic arch with only two branches arising from it, the first being the right common carotid artery followed by the right subclavian artery (fig 1). A small collateral vessel originated from the right sided descending aorta and extended obliquely upward to the left with a late slight opacification of the left subclavian artery that communicated through a left patent ductus arteriosus with the pulmonary artery. A right ventriculography and selective pulmonary angiography showed a left patent ductus arteriosus filling a left innominate artery and eventually the left common carotid and left subclavian arteries (fig 2).
The patient underwent patch closure of the ventricular septal defect and direct closure of the atrial septal defect via a right atriotomy. The left patent ductus arteriosus was divided and the left innominate artery was anastomosed with the ascending aorta by interposition of an 8 mm polytetrafluoroethylene tube. At follow up six months later the patient was asymptomatic with identical blood pressures on both arms.
Right aortic arch with isolation of the left innominate artery is a very rare congenital anomaly in which the innominate artery loses its attachment to the aorta and is connected to the pulmonary artery via a left ductus arteriosus.1-4 The embryological development of this anomaly can be explained by interruption at two levels in the hypothetical double aortic arch model proposed by Edwards5: one in the left posterior arch distal to the left ductus arteriosus and the other in the left anterior arch proximal to the left common carotid artery.4 Bilateral ductus arteriosus and atresia of the innominate artery or atresia of the left common carotid and subclavian arteries may be present.4Usually the blood supply to the isolated arteries is provided via mediastinal collaterals and retrogradely via the circle of Willis and the left vertebral and carotid arteries.2 ,3 The haemodynamic consequences depend on whether the left sided ductus arteriosus is patent or closed and, to some extent, on associated intracardiac anomalies. A closed ductus arteriosus predisposes to a congenital left subclavian steal phenomenon.2 A patent ductus arteriosus may lead to a pulmonary steal phenomenon from the carotid and vertebral arteries causing an extracardiac left to right shunt.3 Clinically, the left radial and carotid pulses are weak and the blood pressure in the left arm is diminished.4 Symptoms of cerebral ischaemia have also been described.2
In our patient, systemic blood pressure in the pulmonary arteries prevented a significant pulmonary steal phenomenon via the patent ductus arteriosus and symptoms of reduced perfusion in the left arm. Aortography provides clear identification of these abnormal vascular structures. Usually the isolated arteries are opacified in late films via collateral pathways.3 ,4 Direct opacification of the left innominate artery via a patent ductus arteriosus during pulmonary angiography, as seen in our patient, results from increased pulmonary vascular resistance and pressure.