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 …