In a newborn or young infant with systemic obstruction or left‑sided heart failure, how can we clinically differentiate coarctation of the aorta, interrupted aortic arch, hypoplastic left heart syndrome, and critical aortic stenosis?

Clinical Differentiation of Ductal-Dependent Left-Sided Obstructive Lesions in Neonates

In a newborn presenting with shock or heart failure, the key clinical differentiators are pulse examination (upper vs. lower extremity), precordial findings, and the presence or absence of a murmur—with echocardiography being the definitive diagnostic test that must be obtained urgently before administering oxygen. 1

Initial Clinical Assessment: The Critical Physical Examination

Pulse Examination (The Most Discriminating Physical Finding)

Upper and lower extremity pulse assessment is the single most important clinical maneuver:

• Coarctation of the aorta: Bounding upper extremity (especially right arm) pulses with weak or absent femoral pulses; blood pressure gradient >10-20 mmHg between arms and legs 1
• Interrupted aortic arch: Absent or severely diminished pulses in all extremities once the ductus closes; may have differential pulses depending on interruption type 1
• Hypoplastic left heart syndrome: Weak pulses in all four extremities; no differential between upper and lower extremities; pulses may only be palpable in carotid arteries in severe cases 1, 2
• Critical aortic stenosis: Weak but equal pulses in all extremities; no upper-to-lower extremity gradient 3

Cardiac Auscultation Findings

The presence and characteristics of murmurs help narrow the diagnosis:

• Critical aortic stenosis: Harsh systolic ejection murmur at right upper sternal border radiating to carotids; late-peaking murmur indicates greater severity; may have palpable thrill; diminished or absent A2 3
• Coarctation: Systolic ejection murmur heard best in left infraclavicular area and left back; continuous murmur if significant collaterals develop (rare in neonates) 1
• Hypoplastic left heart: Often NO murmur or only a non-specific soft murmur; loud single S2 due to aortic atresia 2
• Interrupted aortic arch: Usually no specific murmur; may have associated VSD murmur 1

Precordial Activity and Heart Failure Signs

Assess for signs of ventricular dysfunction and systemic hypoperfusion:

• Respiratory rate >50 breaths/min in sleeping infant indicates heart failure 4
• Hepatomegaly, diaphoresis with feeding, and mottled skin suggest low cardiac output 1
• Increased precordial activity may be present in all conditions but is less prominent in hypoplastic left heart 1
• Cyanosis is typically mild or absent initially but develops as the ductus closes and mixing decreases 3

Echocardiographic Differentiation (The Definitive Test)

Transthoracic echocardiography must be performed immediately and will definitively categorize the lesion: 1, 5

Key Echocardiographic Measurements

Critical aortic stenosis:

• Normal or near-normal LV size (may be borderline small)
• Thickened, doming aortic valve with restricted opening
• Mean Doppler gradient ≥40 mmHg defines severe stenosis
• LV systolic dysfunction may be present
• Normal mitral valve and ascending aorta size 3, 6

Hypoplastic left heart syndrome:

• Severely hypoplastic or atretic mitral valve
• Tiny, non-apex-forming LV
• Aortic atresia or severe hypoplasia
• Hypoplastic ascending aorta (typically <3 mm)
• LV:RV diastolic longitudinal ratio <0.75 and mitral/tricuspid ratio <0.8 indicate univentricular pathway 2, 6

Coarctation of the aorta:

• Discrete narrowing in juxtaductal region (just distal to left subclavian artery)
• May have transverse arch hypoplasia (arch index <0.25 requires extended repair)
• Normal LV size and function unless severe or associated lesions
• Turbulent jet visible on color Doppler at coarctation site
• Assess for bicuspid aortic valve (present in 30-40% of cases) 1, 5, 7

Interrupted aortic arch:

• Complete discontinuity of aortic arch (most commonly between left carotid and left subclavian)
• Ductus arteriosus supplies descending aorta
• Frequently associated with VSD (>90% of cases)
• May have posterior malalignment of infundibular septum 1

Critical Management Principles Before Diagnosis

Prostaglandin E1 Administration

Start PGE1 immediately in any neonate with suspected ductal-dependent systemic circulation: 1

• Indications: Shock, severe heart failure, or differential pulses in a neonate
• Dose: 0.01-0.05 mcg/kg/min IV infusion
• Applies to: Coarctation, interrupted aortic arch, hypoplastic left heart, and critical aortic stenosis with LV dysfunction
• Do NOT delay PGE1 while awaiting echocardiography if clinical suspicion is high 1

Oxygen Administration: A Critical Pitfall

Withhold supplemental oxygen until echocardiographic diagnosis is established: 1

• Oxygen causes pulmonary vasodilation and systemic vasoconstriction, which worsens systemic output in ductal-dependent lesions
• Oxygen promotes ductal constriction, potentially causing cardiovascular collapse in hypoplastic left heart or interrupted aortic arch
• Exception: If severe hypoxemia (SpO2 <70%) threatens end-organ perfusion, use minimal oxygen to maintain SpO2 75-85% 1

Diuretics: Use With Caution

Furosemide may be given for pulmonary edema but use cautiously: 1

• Acute preload reduction can cause hypotension in ductal-dependent lesions
• Dose: 0.5-1 mg/kg IV
• Monitor blood pressure closely after administration 1

Algorithmic Approach to Differentiation

Step 1: Assess pulses and blood pressures in all four extremities

• Differential pulses → Coarctation or interrupted aortic arch
• Globally weak pulses → Hypoplastic left heart or critical aortic stenosis

Step 2: Cardiac auscultation

• Harsh ejection murmur at RUSB → Critical aortic stenosis
• No murmur + single loud S2 → Hypoplastic left heart
• Left-sided ejection murmur → Coarctation

Step 3: Immediate echocardiography

• Defines anatomy definitively
• Guides surgical vs. catheter intervention vs. transplant pathway

Step 4: Initiate PGE1 and withhold oxygen

• Start before echo if high clinical suspicion
• Stabilizes patient for definitive diagnosis and intervention

Common Pitfalls and How to Avoid Them

Pitfall 1: Administering oxygen before diagnosis

• Oxygen can precipitate cardiovascular collapse in hypoplastic left heart by constricting the ductus 1
• Solution: Maintain room air or minimal oxygen (SpO2 75-85%) until anatomy is defined

Pitfall 2: Missing the pulse examination

• Failure to check all four extremities and both carotids misses the diagnosis 1, 4
• Solution: Make four-extremity pulse and BP assessment mandatory in every neonate with shock or heart failure

Pitfall 3: Delaying PGE1 while awaiting echocardiography

• Ductal closure causes irreversible end-organ damage and death 1, 2
• Solution: Start PGE1 empirically if clinical suspicion is high; it can be stopped if echo rules out ductal-dependent lesion

Pitfall 4: Assuming a murmur is always present

• Hypoplastic left heart often has NO murmur, leading to missed diagnosis 2
• Solution: Maintain high suspicion in any neonate with shock, even without a murmur

Pitfall 5: Confusing coarctation with critical aortic stenosis

• Both can present with heart failure, but pulse differential is key 1, 3
• Solution: Always check upper vs. lower extremity pulses and blood pressures

Nuances in Borderline Cases

Some neonates present with overlapping features that challenge biventricular vs. univentricular decision-making: 6

• LV:RV diastolic longitudinal ratio >0.75 and mitral/tricuspid ratio >0.8 predict successful biventricular repair even with small mitral valve 6
• Apex-forming LV on echo suggests biventricular potential; non-apex-forming LV indicates univentricular pathway 6
• Transverse arch hypoplasia with coarctation (arch index <0.25) may require extended arch reconstruction rather than simple coarctation repair 7
• Critical aortic stenosis with borderline LV may show substantial LV growth after ductal patency is maintained, allowing delayed decision on surgical pathway 8