What is the differential diagnosis for a pediatric patient with a murmur audible in the right upper sternal border (RUSB), left lower sternal border (LLSB), and left upper sternal border (LUSB), considering Atrial Septal Defect (ASD) versus Patent Ductus Arteriosus (PDA)?

Differential Diagnosis: ASD vs PDA with Murmur at RUSB, LLSB, and LUSB

A murmur audible at the right upper sternal border (RUSB), left lower sternal border (LLSB), and left upper sternal border (LUSB) most likely represents an atrial septal defect (ASD) rather than patent ductus arteriosus (PDA), as this distribution reflects the systolic pulmonary flow murmur and fixed split S2 characteristic of ASD, whereas PDA produces a continuous "machinery" murmur localized to the left infraclavicular area and LUSB. 1

Key Distinguishing Clinical Features

Murmur Characteristics That Differentiate These Lesions

ASD produces a systolic ejection murmur (not continuous) best heard at the pulmonary area (RUSB/LUSB) due to increased flow across the pulmonary valve from right ventricular volume overload. 1, 2 The murmur may radiate to the LLSB, and with large shunts, a diastolic rumble across the tricuspid valve can be heard at the LLSB. 1

PDA produces a continuous "machinery-type" murmur heard best at the left infraclavicular area and LUSB that persists through systole and diastole. 3, 1 However, if pulmonary arterial hypertension develops, only a systolic component may be audible. 3

Critical Physical Examination Findings

For ASD:

• Fixed splitting of the second heart sound (pathognomonic when present) 1, 2
• Normal peripheral pulses without bounding quality 1
• Precordial lift indicating right ventricular volume overload 1
• Normal pulse pressure 1

For PDA:

• Bounding peripheral pulses with increased pulse amplitude 1, 4
• Wide pulse pressure when the PDA is moderate to large 3, 1
• Increased pulses 3

A critical pitfall: In PDA with severe pulmonary arterial hypertension and Eisenmenger physiology, the continuous murmur may disappear and only a systolic murmur remains. 3 Always check oxygen saturation in all four extremities—differential cyanosis (lower saturation in feet compared to right hand) indicates right-to-left shunting at the ductal level. 3, 1

Diagnostic Algorithm

Step 1: Characterize the Murmur Timing

• Systolic only → Favors ASD (or PDA with pulmonary hypertension) 1, 2
• Continuous (machinery-like) → Strongly suggests PDA 3, 1

Step 2: Assess the Second Heart Sound

• Fixed split S2 → Diagnostic of ASD 1, 2
• Normal or single S2 → Consider PDA or other diagnoses 3

Step 3: Evaluate Peripheral Pulses and Pulse Pressure

• Normal pulses, normal pulse pressure → Favors ASD 1
• Bounding pulses, wide pulse pressure → Strongly suggests PDA 3, 1

Step 4: Check for Differential Cyanosis

• Measure oxygen saturation in right hand and both feet 3, 1
• Lower saturation in lower extremities indicates PDA with Eisenmenger physiology 3, 1

Confirmatory Testing

Transthoracic echocardiography is the primary diagnostic modality for both conditions and should be performed immediately after clinical assessment. 1

For ASD:

• Visualize atrial septum from parasternal, apical, and subcostal views 1
• Color Doppler demonstrates left-to-right shunting across the atrial septum 1
• Right ventricular volume overload with dilated right atrium and right ventricle 1

For PDA:

• Direct visualization by color Doppler in parasternal short-axis view 3, 1
• Continuous wave Doppler measures transpulmonary gradient to estimate pulmonary artery pressure 3, 1
• Left ventricular volume overload with dilated left atrium and left ventricle (not right-sided chambers) 1

Electrocardiographic Differentiation

ASD typically shows:

• Right-axis deviation 1
• Incomplete right bundle-branch block (secundum ASD) 1
• Right atrial enlargement 1

PDA typically shows:

• Normal ECG if small 3, 1
• Left atrial enlargement and LV hypertrophy with moderate shunt 3, 1
• RV hypertrophy when pulmonary arterial hypertension develops 3, 1

Important Clinical Caveat

Both defects can coexist in the same patient, though this combination is infrequent. 5 If echocardiography confirms both lesions, transcatheter closure of the PDA before the ASD may be the preferred approach. 5

Cardiac catheterization is NOT indicated for uncomplicated ASD or PDA when noninvasive imaging is adequate. 3, 1 However, catheterization should be performed when pulmonary artery pressure appears elevated on echocardiography to measure pulmonary vascular resistance and assess operability. 1