How is the mechanism of easily fatique in atrial septal defect?

If there was a : Large shunt resulting in right heart strain and increased pulmonary pressures.

Pulmonary Congestion: With advancing age and decreased left ventricular compliance, there is an increase in left to right shunt across the defect with increase in pulmonary congestion (increased blood flow to the lungs) and this may cause symptoms. Also atrial arrhythmias, right ventricular volume overload, and pulmonary hypertension may contribute to symptoms, usually in advanced age.

Depends: This depends on the pressure in your pulmonary arteries. A simple vsd in a young patient should not contribute to fatigue. However, if unrepaired, a vsd can lead to increased pulmonary artery pressures which can lead to fatigue by increasing the workload on your heart. Please discuss this with your physician(s).