The amplitude (height) of an ECG R wave is proportional to how parallel the wave front of depolarization is to that lead.
When a wave of depolarization is moving toward the positive pole, the more directly parallel the wave front is to the lead,
the taller the R wave will be. The more perpendicular the wave of depolarization is, the smaller the resulting R wave will
be. A wave front of depolarization that is traveling 90 degrees perpendicular to a lead will result in an isoelectric QRS
complex. Isoelectric means that the positive deflection of the R wave and the negative deflection of the Q and S waves are
equivalent. Stated mathematically, the positive and negative deflections of the QRS complex sum to zero.
The mean electrical axis
The mean electrical axis is the principal vector of ventricular depolarization.3 It represents the sum of all of the waves of depolarization that are occurring simultaneously. The mean electrical axis
normally points toward the left ventricle, as this is the larger of the two ventricles. Deviations of the mean electrical
axis result from either right ventricular hypertrophy or a block in the intraventricular conduction system. An intraventricular
conduction block will result in deviation of the mean electrical axis toward the ventricle that is expected to be depolarized
by the bundle branch or fascicle that is blocked.
The hexaxial lead system is arranged around the heart in essentially a short-axis plane, which is referred to as the frontal plane. In this plane, the left ventricular apex is oriented generally toward the positive pole of lead II. The circumference of
the frontal plane is labeled in degrees (0 to +180 and 0 to -180). The normal range of the mean electrical axis in dogs is
+40 to +100 degrees and in cats is 0 to +160 degrees (Figure 1). In dogs, a right axis shift is defined as a mean electrical axis between +100 and -90 degrees. In cats, a right axis shift
occurs when the mean electrical axis is between +160 and -90 degrees. A left axis shift occurs in dogs when the mean electrical
axis is between +40 and -90 degrees and in cats when the mean electrical axis is between 0 and -90 degrees.
To determine the mean electrical axis, you must obtain a six-lead ECG from a patient positioned in right lateral recumbency
with the limbs perpendicular to the long axis of the patient's body. Four basic methods exist for estimating the mean electrical
axis. The first two methods are considered more accurate, while the second two are reasonable approximations.
1. Find an isoelectric lead—Identify a lead in which the positive and negative deflections of the QRS complex are equal. The
lead that is perpendicular to the isoelectric lead is the lead that will identify the mean electrical axis (Figure 2). If the QRS complex is predominantly positive in the perpendicular lead, then the mean electrical axis is directed toward
the positive pole. If the QRS complex is negative, then the mean electrical axis is directed away from the positive pole.
If no lead is isoelectric, use one of the other methods. If two leads are isoelectric, select the lead that has the smaller
QRS complex deflections to determine the mean electrical axis. In the unusual instance that all of the leads are isoelectric,
the mean electrical axis cannot be determined because it lies outside of the frontal plane of the hexaxial lead system.
Figure 2. The mean electrical axis can be easily identified when an isoelectric lead is present. Examination of this ECG
indicates that lead aVL is isoelectric. Lead II is perpendicular to aVL and is predominantly positive. Therefore, the mean electrical axis is normal at +60 degrees.