C. Electrocardigraphy
In all forms of AVSD, there is extreme left axis deviation with a counterclockwise loop in the frontal plane. The ECG is an important diagnostic tool. Only 5% of isolated VSDs have this ECG abnormality. First-degree heart block occurs in over 50% of patients. Ight, left, or combined ventricular hypertrophy is present depending on the particular defect and the presence or absence of pulmonary hypertension.