What are the EKG (electrocardiogram) characteristics of ostium primum atrial septal defect (ASD)?

EKG Characteristics of Ostium Primum Atrial Septal Defect

The most distinctive EKG characteristic of ostium primum ASD is superior left-axis deviation with a counterclockwise loop in the frontal plane, accompanied by an incomplete right bundle branch block pattern. 1

Primary EKG Features

• Superior left-axis deviation: The hallmark finding that distinguishes primum ASD from other types of ASDs 1
• Incomplete right bundle branch block: Typically seen as an rSR' pattern in lead V1 with QRS duration <120 ms 1, 2
• Right ventricular hypertrophy: Due to right ventricular volume overload 1
• Right atrial enlargement: Manifested as P-wave ≥2.5 mm in leads II, III, or aVF 2

Important Distinctions from Other ASDs

• Secundum ASDs typically show right-axis deviation, whereas primum ASDs show superior left-axis deviation 1
• The superior left axis in primum ASD is due to the anatomic position of the conduction bundles and should not be confused with bifascicular block 1
• Sinus venosus defects may present with abnormal P-wave axis (especially superiorly located defects) 1

Additional EKG Findings

• First-degree AV block may be present 1
• Left atrial enlargement may be seen if significant left AV valve regurgitation exists 1
• In older patients, atrial arrhythmias (flutter/fibrillation) may develop 1

Clinical Correlation

The EKG findings reflect the underlying pathophysiology:

• RV volume overload leads to right ventricular hypertrophy and incomplete right bundle branch block
• The anatomical location of the defect near the AV node and conduction system explains the superior left axis deviation
• As the disease progresses, right atrial enlargement becomes more prominent

Common Pitfalls to Avoid

1. Misdiagnosis: Don't confuse the superior left axis deviation of primum ASD with left anterior fascicular block 3
2. Incomplete evaluation: The absence of classic EKG findings doesn't exclude the diagnosis - echocardiography is essential for confirmation 1
3. Late recognition: The gradual onset of symptoms and subtle physical findings often lead to delayed diagnosis 1
4. Underestimating significance: Rapid decompensation due to arrhythmias or AV block occurs more frequently with primum than secundum ASDs, approaching 80% by age 45 4

Progression with Age

• In younger patients, the EKG may show only mild changes
• With increasing age, more pronounced findings develop, including:
  • More significant right ventricular hypertrophy
  • Higher incidence of atrial arrhythmias
  • Potential development of complete heart block in familial cases 1

The EKG findings of ostium primum ASD should prompt further evaluation with echocardiography, which remains the gold standard for diagnosis and assessment of associated abnormalities such as cleft mitral valve and mitral regurgitation.