Determining Axis & Vectors
The Mean Electrical Axis
The mean electrical axis of the QRS complex is the average of the total depolarization of the ventricles in the frontal plane. There is a correlation between the mean electrical axis of the QRS complex and the electrical activity of the heart. Therefore, in pathologic hypertrophy of either ventricle (ie. when there is more muscle being depolarized), the axis tends to shift in the direction of the hypertrophied ventricle.
Primary conduction abnormalities (such as left anterior hemiblock) can also cause an electrical axis shift. The normal electrical axis of the QRS complex (represented by the gray shaded area on the diagram above) is from -30 degrees to +110 degrees.
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Any value of the QRS mean electrical axis more negative than -30 degrees is considered a pathologic left axis deviation (LAD). This can be seen with left anterior hemiblock among other causes.
Any value of the QRS mean electrical axis more positive than 110 degrees is considered a pathologic right axis deviation (RAD). This can be seen with right ventricular hypertrophy or left posterior hemiblock among other causes.
A QRS mean electrical axis between -90 degrees and 180 degrees is not usually seen clinically.
