A 3-day-old neonate born at term presents with severe cyanosis (SpO₂ 65% on room air) and respiratory distress. Clinical examination reveals a single loud S2 and an ejection systolic murmur at the left upper sternal border. Chest X-ray shows an 'egg-on-string' appearance with pulmonary oligemia. Which investigation is most appropriate to confirm the diagnosis and guide immediate management?

Explanation

## Clinical Context The presentation is classic for **Transposition of the Great Arteries (TGA)**, a cyanotic congenital heart defect requiring urgent intervention. The 'egg-on-string' sign on CXR (narrow mediastinum due to anteroposterior arrangement of aorta and pulmonary artery) is pathognomonic. ## Investigation of Choice: Transthoracic Echocardiography **Key Point:** Transthoracic echocardiography is the first-line, non-invasive imaging modality for diagnosis and hemodynamic assessment of cyanotic congenital heart disease in neonates. ### Why Echocardiography is Optimal | Feature | Advantage | |---------|----------| | **Speed** | Can be performed at bedside in minutes; critical in unstable neonates | | **Safety** | Non-invasive, no radiation, no sedation required | | **Diagnostic accuracy** | Directly visualizes atrial and ventricular relationships, great artery origins, and septal defects | | **Hemodynamic data** | Assesses shunt direction via color Doppler; identifies PDA patency (crucial for TGA survival) | | **Guides therapy** | Allows immediate decision on prostaglandin E₁ infusion and need for balloon atrial septostomy (Rashkind procedure) | **High-Yield:** In TGA, the aorta arises from the right ventricle and the pulmonary artery from the left ventricle. Echocardiography definitively shows this abnormal ventriculoarterial connection. ## Clinical Pearl **Key Point:** Neonates with TGA depend on a patent foramen ovale (PFO) or patent ductus arteriosus (PDA) for mixing of oxygenated and deoxygenated blood. Prostaglandin E₁ is started immediately to keep the ductus venosus patent while awaiting echocardiographic confirmation and definitive surgical repair (arterial switch operation). ## Why Echocardiography Precedes Catheterization 1. **Diagnostic confirmation** — echocardiography provides all necessary anatomic and hemodynamic data in acute presentation 2. **Avoids delay** — catheterization is reserved for complex anatomy or when echocardiography is inconclusive 3. **Reduces risk** — avoids invasive procedure in a critically ill, hypoxic neonate unless absolutely necessary