Neonatal respiratory distress — diagnostic approach — formative SAQs

SAQ 1 — Differentiating RDS, TTN and MAS in the first hour (20 marks, ~15 minutes)

Three newborns are admitted to the neonatal unit within the same hour: a 28-week preterm infant with grunting and rising oxygen need; a term infant born by elective caesarean with mild tachypnoea at 2 hours; and a term post-dates infant born through meconium-stained liquor with coarse respiratory distress. [1]

Questions

1. For each infant, state the most likely diagnosis and the single most discriminating risk factor or feature. (6 marks) [1]
2. Describe the expected chest X-ray pattern for each infant and explain how it confirms the mechanism. (6 marks) [5]
3. Outline the first-line management for each infant, naming the threshold for surfactant in the preterm infant and the evidence supporting it. (5 marks) [1] [3]
4. State one critical differential that must be excluded in the term infant whose saturations do not improve with oxygen, and the immediate action. (3 marks) [5]

Model answer (must-hit)

1. Preterm infant: respiratory distress syndrome (RDS), discriminated by gestation below 34 weeks and surfactant deficiency. Term caesarean infant: transient tachypnoea of the newborn (TTN), discriminated by elective caesarean before labour (bypassed catecholamine-driven lung liquid clearance). Post-dates infant: meconium aspiration syndrome (MAS), discriminated by meconium-stained liquor. [1]
2. RDS: symmetric reticulogranular (ground-glass) pattern with air bronchograms and low lung volumes, reflecting diffuse alveolar collapse. TTN: perihilar streaking, fluid in the minor fissure, and mild hyperinflation, reflecting retained fetal lung liquid. MAS: coarse patchy infiltrates with hyperinflation and possible pneumothorax, reflecting airway obstruction (ball-valve) and chemical pneumonitis. [5]
3. RDS: start CPAP 5–6 cm H₂O; give exogenous (animal-derived) surfactant by a less invasive technique (LISA/INSURE) when FiO₂ exceeds 0.30, supported by the 2022 European Consensus Guidelines and the SUPPORT trial showing CPAP-first is non-inferior to prophylactic intubation. TTN: supportive oxygen titrated to target, minimal handling, expect resolution in 24–72 hours. MAS: ventilation for the depressed infant, no routine tracheal suctioning, consider surfactant for severe disease and inhaled nitric oxide for PPHN. [1] [3] [4]
4. A term infant with refractory hypoxaemia and a normal chest X-ray must be assumed to have a duct-dependent congenital heart lesion; check pre/post-ductal saturations, start an intravenous prostaglandin E1 infusion at 0.01–0.05 microgram/kg/min, and obtain urgent echocardiography. [5] [6]

SAQ 2 — The term infant with refractory cyanosis (20 marks, ~15 minutes)

A term infant born after an uneventful pregnancy is noted to be tachypnoeic at 6 hours of age. In 80% oxygen the pre-ductal saturation is 96% but the post-ductal (foot) saturation is 84%. The chest X-ray shows clear lung fields with slightly diminished pulmonary vascular markings. Femoral pulses are weak. [5]

Questions

1. Interpret the pre-ductal/post-ductal saturation difference and state its pathophysiological significance. (5 marks) [6]
2. Give the most likely diagnosis and the clinical features that support it, and state why RDS and TTN are unlikely. (5 marks) [5]
3. Outline the immediate management, including a named drug, dose, route and the rationale for starting it before echocardiography. (6 marks) [5] [6]
4. Describe how you would explain to the parents the need for urgent transfer to a cardiac centre. (4 marks) [6]

Model answer (must-hit)

1. A pre-ductal to post-ductal saturation difference greater than 10% (here 96% versus 84%) denotes differential cyanosis and indicates a right-to-left ductal shunt: deoxygenated pulmonary arterial blood bypasses the lungs through the ductus arteriosus into the descending aorta, desaturating the post-ductal circulation. This points to persistent pulmonary hypertension of the newborn or a duct-dependent congenital heart lesion rather than primary lung disease. [6]
2. The most likely diagnosis is a duct-dependent congenital heart lesion (for example, a critical left-heart obstruction or a cyanotic lesion), supported by refractory cyanosis, a clear chest X-ray with reduced pulmonary markings, weak femoral pulses, and the post-ductal desaturation. RDS is unlikely because the infant is term and the chest X-ray is clear; TTN is unlikely because gas exchange is profoundly deranged rather than mildly so and the X-ray lacks the fluid-in-fissure pattern. [5]
3. Start an intravenous prostaglandin E1 (alprostadil) infusion at 0.01–0.05 microgram/kg/min to maintain ductal patency and improve mixing or systemic perfusion, titrating to effect; monitor for apnoea and hypotension, which are common side effects. It is started before echocardiography because delaying ductal support risks circulatory collapse as the ductus closes, and the clinical picture of refractory cyanosis with a clear chest X-ray is sufficient to justify empiric PGE1. Arrange urgent echocardiography to confirm the anatomy. [5] [6]
4. Explain in plain language that the baby's heart needs a blood vessel that is closing, that a medicine can keep it open for now, but that definitive diagnosis and treatment require a specialist cardiac team; reassure that transfer is planned and coordinated, that a parent can usually accompany, and that the team will stay in contact. Document the discussion and consent. [6]