Truncus arteriosus and single-ventricle physiology — structured clinical encounter

A four-week-old term infant, born vaginally at 39 weeks after an uncomplicated pregnancy, was noticed to be mildly dusky from the first day of life but fed reasonably well until this week. Over the last five days the mother reports increasing tachypnoea, sweating with feeds and poor weight gain. In the emergency department the infant is tachypnoeic at a respiratory rate of 70 and mildly centrally cyanotic with an oxygen saturation of 88 percent in air. The pulses are bounding in all four limbs with a wide pulse pressure (systolic 95, diastolic 35 mmHg). The second heart sound is single and there is a loud systolic ejection murmur at the left sternal edge with an early diastolic murmur. The chest radiograph shows increased pulmonary markings and a slightly enlarged heart. The working diagnosis is truncus arteriosus.