ECG Tutorial

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The Cardiac Axis

Six ECG leads (I, II, III, aVR, aVL, aVF) examine the flow of depolarisation and repolarisation through the heart in the frontal plane. In this plane, depolarising forces of differing magnitudes are moving through the ventricles in many different directions during the normal process of ventricular depolarisation. If we sum together all of the depolarisation vectors occurring in the frontal plane throughout the duration of ventricular depolarisation, we can generate an overall vector which has a magnitude and direction. The direction of this total frontal qrs vector, defined by the angle (alpha) it makes with lead I, is the cardiac axis.