The electrocardiographic axis on the ECG, usually refers to the QRS electrocardiographic axis on the frontal plane, commonly used as the maximum vector of the QRS wave group, is represented on the frontal plane by the angle made with the I lead, representing the size and direction of the ventricular depolarization, with the normal electrocardiographic axis pointing downward to the left, at 0-90 degrees. The leftward bias of the ECG axis refers to the main wave of the QRS wave group in lead I upward and the main wave of the aVF lead downward. If the coordinate method is used to measure the ECG axis, an ECG axis at 0-30 degrees is mild left deviation, and -30 – 90 degrees is left deviation of the electrical axis, which is commonly seen in transverse heart or diaphragm elevation, left heart shift, left ventricular hypertrophy, left bundle branch block, left anterior branch block, and right ventricular infarction.