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Why there is Fixed split in ASD

Split is fixed in ASD because of the following reasons.
1.Septal defect produce equalisation of pressure between the atria during the respiratory cycle.
2.The hangout interval is already widened. Ti cannot be further increased during inspiration.
What is the prerequisite before diagnosing wide fixed split 
Before diagnosing the fixed split always examine the patient in the standing or sitting position, because the split tend to be narrow on standing due to shortening of right ventricle ejection time more than left ventricle.