Atrial septal defect (ASD), ostium secundum type, is the most common congenital heart anomaly. Patients present with a significant left to right shunt. ECG patterns in patients with this condition include: 

• Frontal QRS +60° 
• Incomplete RBBB (r’ > r) with a slight clockwise rotation (in the presence of significant RVH)  
• Negative T waves in V₂ (possibly in V₂ through V₅)

Patients with an ASD who have only a small left to right shunt and a patent foramen ovale tend to have unremarkable ECG findings.

The differential ECG diagnosis of ASD with ostium secundum includes chronic, acute, and subacute pulmonary embolism. These can be ruled out with 2-D echocardiography, echo/Doppler, D-dimer, and ventilation/perfusion studies. 

Patients with valvular pulmonary stenosis have ECG characteristics similar to those seen with ASDs, but the ECG in the former case shows an rSr’ complex instead of a tall singular R wave in V₁.