Duct-dependent congenital heart disease: Viva

Branch 1: Recognising the time-critical problem

The candidate should immediately recognise that this baby has cyanosis unresponsive to 100 percent oxygen in a neonate who was well at discharge and collapsed on day 4. This is the classic presentation of duct-dependent congenital heart disease, specifically a duct-dependent pulmonary circulation where blood cannot reach the lungs once the ductus arteriosus has closed. The saturation of 70 percent that does not improve with oxygen is the key discriminator from a pulmonary cause, and the absence of a murmur does not exclude critical congenital heart disease because many severe obstructive lesions generate no turbulent flow. [1]

The candidate should state that the single most urgent intervention is to start prostaglandin E1 (alprostadil) at 0.01 to 0.05 micrograms per kilogram per minute intravenously, without waiting for the echocardiogram. The rationale is that the closed ductus arteriosus was the only route for blood to reach the lungs, and reopening it restores pulmonary blood flow and improves the saturation. The first priorities are airway, breathing, and circulation, an intravenous line, and a bedside glucose, followed by the prostaglandin infusion the moment the suspicion is raised. [1]

Branch 2: The two-pathway split and investigations

The examiner will probe the candidate's understanding of the classification. The candidate should explain that duct-dependent lesions split into two pathways based on which circulation fails when the duct closes. Duct-dependent pulmonary circulation — including pulmonary atresia, critical pulmonary stenosis, severe tetralogy of Fallot, tricuspid atresia, and Ebstein anomaly — presents with cyanosis because blood cannot reach the lungs. Duct-dependent systemic circulation — including hypoplastic left heart syndrome, critical coarctation, interrupted aortic arch, critical aortic stenosis, and mitral atresia — presents with shock because blood cannot reach the body. This baby's cyanosis without shock points to the pulmonary pathway. [1]

The candidate should outline the parallel investigations: a venous or capillary gas to assess the acid-base status, a bedside glucose, four-limb blood pressures to check for a coexisting coarctation component, a chest X-ray to assess pulmonary vascularity and heart size, and an echocardiogram to confirm the anatomy. The hyperoxia test can be mentioned as the classical bedside discriminator: a PaO2 below 100 millimetres of mercury after 10 minutes in 100 percent oxygen indicates a fixed right-to-left shunt. The echocardiogram is definitive but must not delay the prostaglandin. [1]

The candidate should also address the screening question. The examiner may ask why this baby was not detected earlier. The answer is that pulse oximetry screening, which is performed after 24 hours of age by measuring the right hand and either foot, detects about 75 percent of critical congenital heart defects. This baby may have passed the screen if the duct was still open and providing enough flow, or the screen may not have been performed. The Thangaratinam meta-analysis in the Lancet established the sensitivity and specificity of the screening programme. [3]

Branch 3: Prostaglandin therapy, side effects, and transfer

The candidate should give the prostaglandin E1 dose precisely: 0.01 micrograms per kilogram per minute starting, titrated up to 0.05 micrograms per kilogram per minute until the saturation improves, using the minimum effective dose to minimise side effects. The infusion works within minutes to hours by relaxing the smooth muscle of the ductus arteriosus. [2]

The examiner will test the candidate's knowledge of side effects. The candidate should list apnoea in approximately 10 percent of treated neonates, hypotension, fever, flushing, and diarrhoea, and explain that the pyrexia can mask a fever from sepsis. The correct response to apnoea is to intubate and ventilate while continuing the prostaglandin, not to stop the infusion. For transfer, the candidate should describe electively intubating before the journey if the baby is on prostaglandin, using a dedicated neonatal transport team, continuing the infusion throughout, and monitoring continuously. The destination is a cardiac surgical centre where the echocardiogram will define the anatomy and the surgeons will plan definitive repair or staged palliation. The prostaglandin continues until surgery. [2]