Congenital heart disease presenting in the newborn — formative SAQs

SAQ 1 — The day-three collapsed neonate (20 marks, ~15 minutes)

A term infant was well at discharge on day 2 with normal pulses and a passed pulse-oximetry screen. He returns on day 3 with poor feeding, mottled cool peripheries, tachypnoea, weak femoral pulses and a capillary gas showing a metabolic acidosis. [6]

Questions

1. State the most likely diagnosis category and why the timing and signs point to it. (4 marks) [6]
2. Give your immediate first-hour management, naming the key drug, its dose and the principle that governs its timing. (6 marks) [9]
3. Outline the bedside assessment and first-line investigations that confirm the direction of the diagnosis, and state why the definitive test does not delay treatment. (5 marks) [9]
4. Explain to the parents, in plain language, why a baby who looked well and passed screening can still collapse on day three. (5 marks) [7]

Model answer (must-hit)

1. This is a duct-dependent obstructive lesion until proven otherwise — classically coarctation of the aorta or interrupted aortic arch. The baby was well while the ductus arteriosus supplied the lower body; as the duct functionally closes at 24 to 48 hours, systemic flow falls and the baby shocks with weak femoral pulses and metabolic acidosis. The weak femoral pulses and the day-three timing are the decisive clues. [6]
2. Stabilise the ABCs, establish access, correct hypoglycaemia, hypocalcaemia and acidosis, give empiric antibiotics to cover sepsis, and start prostaglandin E1 (alprostadil) 0.01 to 0.05 mcg/kg/min intravenously to reopen the duct. The governing principle is that prostaglandin is started the moment a duct-dependent lesion is suspected — before the echocardiogram — because the echo confirms but does not treat. Anticipate prostaglandin-induced apnoea and have intubation equipment ready, with a low threshold for elective intubation before retrieval. [9]
3. Assess colour, palpate right brachial and femoral pulses simultaneously (brachio-femoral delay or absent femorals), measure four-limb blood pressure (a gradient suggests coarctation), examine for hepatomegaly and a murmur, and run pre/post-ductal oximetry. Send a capillary gas with lactate, glucose, ionised calcium, cultures and a chest radiograph. The definitive test — echocardiography — defines the anatomy but is obtained in parallel; it never delays prostaglandin, because the diagnosis is clinical at the point of action. [9]
4. The ductus arteriosus is a fetal blood vessel that was keeping the lower body supplied; it closes over the first day or two. While it was open the baby looked well and passed the oxygen screen, because blood was still reaching the body. As it closed, the body lost that blood supply and the baby became sick. Screening is designed to catch most babies before this happens, but it cannot catch every lesion, which is why we watch for warning signs after going home. [7]

SAQ 2 — The cyanotic newborn with a positive screen (20 marks, ~15 minutes)

A routine pulse-oximetry screen at 24 hours shows saturations of 88 percent in the right hand and 87 percent in the foot in an asymptomatic term newborn. The chest radiograph shows an "egg on a string" mediastinum with increased pulmonary markings. [2]

Questions

1. Interpret the screen against the standard thresholds and state the immediate action. (4 marks) [4]
2. Give the most likely diagnosis, its mechanism, and the radiograph sign. (4 marks) [10]
3. Describe how you would use the hyperoxia test to support the diagnosis, including its limitation. (5 marks) [2]
4. Discuss the role and limits of antenatal diagnosis and pulse-oximetry screening for this lesion, citing the evidence. (7 marks) [2] [10]

Model answer (must-hit)

1. A saturation below 90 percent in any limb is a positive screen requiring urgent paediatric cardiology assessment, irrespective of how well the baby looks. The action is to assess the baby, start oxygen and prostaglandin if a duct-dependent lesion is suspected, and arrange urgent echocardiography and cardiology referral — not to repeat or discharge. [4]
2. The most likely diagnosis is d-transposition of the great arteries (d-TGA) — the commonest cause of neonatal cyanosis. The aorta arises from the right ventricle and the pulmonary artery from the left, so the circulations run in parallel and survival depends on mixing at the atrial or ductal level. The radiograph sign is the "egg on a string" with a narrow mediastinum and increased pulmonary flow. [10]
3. The hyperoxia test places the infant in 100 percent oxygen for about 10 minutes; in cyanotic CHD the arterial oxygen tension fails to rise substantially (saturations stay low), supporting a right-to-left shunt. The limitation is that persistent pulmonary hypertension of the newborn can behave similarly, so the test is interpreted alongside the work of breathing, the radiograph, pre/post-ductal difference and the echo. [2]
4. Antenatal diagnosis detects about half to two-thirds of critical lesions overall, but detection of d-TGA, though improved over 20 years, remains imperfect — so a normal antenatal scan does not exclude it. Pulse-oximetry screening adds sensitivity: the Lancet meta-analysis reported about 76 percent sensitivity and about 99 percent specificity, with a low false-positive rate after 24 hours, and it reduces the number of duct-dependent lesions first diagnosed after collapse. But it misses about a quarter of critical lesions, so it supplements rather than replaces clinical assessment and a clear safety-net. [2] [10]