Cardiovascular Physiology Concepts
                                    Richard E. Klabunde, Ph.D.

Ventricular Depolarization and the Mean Electrical Axis

Sequence of Ventricular Depolarization

The mean electrical axis is the average of all the instantaneous mean electrical vectors occurring sequentially during depolarization of the ventricles.  The figure to the right depicts the sequence of depolarization within the ventricles.  The septum and free left and right ventricular walls are shown.  In this model, each of the four vectors is depicted as originating from the top of the interventricular septum just below the AV node.  The electrode placement represents lead II.  During ventricular activation, impulses are first conducted down the left and right bundle branches on either side the septum. This causes the septum to depolarize from left-to-right as depicted by vector 1 (Panel A).  This vector is heading slightly away from the positive electrode (to the right of a line perpendicular to the lead axis) and therefore will record a small negative deflection (Q wave of the QRS).  About 20 milliseconds later, the mean electrical vector points downward toward the apex (vector 2), and is heading toward the positive electrode (Panel B).  This will produce a very tall positive deflection (R wave of the QRS).  After another 20 milliseconds later, the mean vector is pointing toward the left arm and anterior chest as the free wall of the ventricle depolarizes from the endocardial to the epicardial surface (vector 3, Panel C).  This vector will record a small positive voltage in lead II.  Finally, the last regions to depolarize will result in vector 4 (Panel D), which will cause a slight negative deflection (S wave) of the QRS.

The shape of the QRS complex is different for each of the limb leads because each of the leads will "see" the sequence of depolarization vectors from a different perspective (see axial reference system).  The animated figure to the right shows how the QRS complex appears for leads aV_F and aV_L. The positive electrodes for these two leads are at +90º  and -30º, respectively. In this illustration, the mean electrical axis (see below) is about +45º.  Note that aV_F shows a large net positive QRS. There is no Q wave because septal depolarization is not directed away from the lead (see ECG rules). The R wave is very positive because early ventricular depolarization is largely directed toward this lead. The S wave is also present because the terminal depolarization of the upper wall of the left ventricle is directed away from aV_F. In contrast, aV_L shows an initial Q wave (septal depolarization is directed away from the lead) followed by a moderately positive R wave.

The Mean Electrical Axis

In the above illustration, the mean electrical axis will be the sum of all of the mean electrical vectors.  The mean electrical axis is depicted by the red arrow in the figure above and in the figure to the right, which is the same figure superimposed upon the axial reference system.  In this example, the mean electrical axis is approximately +30º.  The mean electrical axis for the heart normally lies between -30 and +90º. (Some textbooks say the normal range is 0 to +90º.) Less than -30º (or less than 0º) is termed a left axis deviation and greater than +90º is termed a right axis deviation.  Axis deviations can be caused by increased cardiac muscle mass (e.g., left ventricular hypertrophy), changes in the sequence of ventricular activation (e.g., conduction defects), or because of ventricular regions being incapable of being activated (e.g., infarcted tissue).  

The mean electrical axis corresponds to the axis that is perpendicular to the lead axis that has the smallest net QRS amplitude (net amplitude = positive minus negative deflection voltages of QRS complex).  In the above figure, lead III would have the smallest net amplitude (the ECG would be biphasic with equal positive and negative deflections).  The mean electrical axis, therefore, is perpendicular to lead III, which is 120º minus 90º, or approximately +30º in this example.  Leads I and II will have equally positive QRS deflections.  Lead aV_R would have the greatest negative deflection. 

To determine the mean electrical axis from the ECG, find the lead axis that has a biphasic (equally positive and negative QRS deflections - i.e., no net deflection), then find the lead axis that is perpendicular (90º) to the biphasic lead and that has a positive net deflection.  In the six limb leads in the example below, aV_L is biphasic.  The positive perpendicular axis to aV_L is +60º. Therefore, the mean electrical axis is +60º, which is normal.

RK Revised 04/06/07

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.

© 1999-2008 Richard E. Klabunde, all rights reserved.