The electrical axis (EA) may be defined as a vector originating in the center of Einthoven's equilateral triangle.3!3 A vector is a mathematical value expressed as an arrow that has magnitude, sense, and direction. On the other hand, scalar values only have magnitude. When applied to the EA of the QRS complexes, the vector that represents it also gives the direction of the activation process as projected in the plane of the limb leads. Its length represents the manifest potential of the dipole in the center of the triangle. These general considerations apply either to the instantaneous EA (the vector indicating the direction of the impulse at the instant at which it is determined) or to the mean EA (which is the resultant of all instantaneous electrical axes). Although the term EA can be used in reference to any of the major components of the ECG (P, T, or QRS), it is generally applied to the QRS. There are many methods for determining the mean EA. The one recommended by electrocardiographers of the classical school consists of calculating the net areas enclosed by the QRS complex in leads I, II, and IH.3,6,7,12,13 The net area is the absolute sum of the positive and negative areas of the QRS complex in the corresponding lead. One of the drawbacks of this method is that the absolute values of the net area cannot be determined accurately by inspection. Since the absolute magnitude of the EA is not of fundamental clinical importance, it has been recommended that arbitrary units be used. When this is done, the results can be counterchecked by using Einthoven's law. For example, if in a given case lead I is +4 units, lead II is +2 units, and lead III is -2 units, the calculation is accurate because the sum of leads I and III (+4 plus -2) must always equal lead II (+2). After having determined the net area, the results are plotted on the sides of the triangle, and perpendiculars are dropped from two or all three leads. The perpendiculars will meet at a point away from the center of the triangle. A line drawn from the latter to the former defines the mean EA. A simpler, though less precise, method of calculating the quadrant (or parts of a quadrant) in which the EA is located consists of using the maximal QRS deflection in leads I and aVF and, when necessary, lead II. This method is inexact from the mathematical viewpoint but has the value of simplicity.1516
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