Electrical Axis and Axis Deviation

Mean QRS Axis: A Working Definition

The depolarization stimulus spreads through the ventricles in different directions from one instant to the next. The overall direction of the QRS complex, or mean QRS electrical axis, can also be described. If you draw an arrow to represent the overall, or mean, direction in which the QRS complex is pointed in the frontal plane of the body, you are representing the electrical axis of the QRS complex. The term mean QRS axis therefore indicates the general direction in the frontal plane toward which the QRS complex vector is predominantly pointed.

Because the QRS axis is defined with respect to the frontal plane, the reference is only to the six extremity leads. Therefore the scale of reference used to measure the mean QRS axis is the diagram of the frontal plane leads (described in Chapter 4 and depicted again in Fig. 6.1 ). Einthoven’s triangle can be readily converted into a triaxial (three-axis) lead diagram by sliding the axes of the three standard limb leads (I, II, and III) so they intersect at a central point ( Fig. 6.1A ). Similarly, the axes of the three augmented limb leads (aVR, aVL, and aVF) also form a triaxial lead diagram ( Fig. 6.1B ). These two triaxial lead diagrams can be geometrically overlapped to produce a hexaxial (six-axis) lead diagram ( Fig. 6.1C ). We use this diagram to determine the mean QRS axis and describe axis deviation.

As noted in Chapter 4 , each lead has a positive and negative pole (see Fig. 6.1C ). As a wave of depolarization spreads toward the positive pole, an upward (positive) deflection occurs. Conversely, as a depolarizing wave spreads toward the negative pole, a downward (negative) deflection is inscribed.

Finally, we need a reference system from which to calculate the mean QRS axis. By convention, the positive pole of lead I is located at 0 degrees. All points below the lead I axis are positive, and all points above that axis are negative ( Fig. 6.2 ). Thus, toward the positive pole of lead aVL (–30 degrees), the axis becomes negative. Downward toward the positive poles of leads II, III, and aVF, the scale becomes more positive (lead II at +60 degrees, lead aVF at +90 degrees, and lead III at +120 degrees).

The complete hexaxial diagram used to measure the QRS axis is shown in Fig. 6.2 . By convention again, an electrical axis that points toward lead aVL is termed horizontal or leftward. An axis that points toward leads II, III, and aVF is termed vertical, rightward, or sometimes inferior ).