Understanding Atrial Septal Defect Findings on EKG
An atrial septal defect (ASD) cannot be directly visualized on an EKG—the EKG shows indirect electrical manifestations of right ventricular volume overload caused by the left-to-right shunt through the ASD. 1
Key EKG Findings by ASD Type
Secundum ASD (Most Common in Adults)
The classic EKG pattern includes:
- Right axis deviation 1
- Incomplete right bundle branch block (RBBB) - present in approximately 56% of ASD patients 1, 2
- Right atrial enlargement 1
- Crochetage R wave (notching in the R wave of inferior leads II, III, aVF) - present in 57% of ASD patients with 92% specificity 2
- Defective T waves - present in 48% of ASD patients with 97% specificity 2
Primum ASD
The distinctive pattern differs significantly:
- Superior left-axis deviation (leftward axis) 1
- Right ventricular conduction delay 1
- This combination reflects the anatomic position of conduction bundles and should not be confused with bifascicular block 1
Sinus Venosus ASD
- Abnormal P-wave axis (superiorly directed) 1
- Sinus node dysfunction or low atrial rhythm 1
- Atrial arrhythmias 1
Clinical Significance of EKG Findings
The EKG has limited sensitivity as a screening tool for ASD—only 57% of children with hemodynamically significant ASDs and right ventricular enlargement meet EKG criteria for RV hypertrophy. 3 However, sensitivity improves to 70% in younger patients and 80% for the largest defects 3.
The specificity of EKG findings is more reliable:
- Incomplete RBBB: 95% specificity 2
- Defective T waves: 97% specificity 2
- Crochetage R wave: 92% specificity 2
What the EKG Actually Indicates
The EKG findings reflect right ventricular volume overload from chronic left-to-right shunting, not the defect itself 1, 4. The pathophysiology involves:
- Increased pulmonary blood flow causing RV dilation 4
- Altered ventricular depolarization patterns due to RV enlargement 1
- Conduction system distortion from chamber enlargement 1
Critical Diagnostic Pitfall
Any adult presenting with atrial arrhythmia and a dilated RV on imaging should be investigated for an atrial level shunt lesion, even if the EKG is not classic for ASD. 1 The gradual onset of symptoms and subtlety of physical findings often lead to late diagnosis 1.
Next Steps When ASD is Suspected
When EKG suggests possible ASD:
- Transthoracic echocardiography is the primary diagnostic modality to confirm the diagnosis, demonstrate shunting, and assess RV volume overload 1, 4
- Patients with unexplained RV volume overload should be referred to an adult congenital heart disease center for further evaluation 1
- TEE may be necessary in adults with poor transthoracic windows or to evaluate for sinus venosus defects 1, 5
The EKG alone cannot diagnose or exclude ASD—it serves only as a clue prompting further imaging evaluation. 3, 2