The mean frontal QRS axis is extremely helpful for many ECG diagnoses, such as ventricular hypertrophy or fascicular blocks. During depolarization of the ventricles (normal up to 0.10 seconds), the electrical stimulus travels in many different directions at different points in time. The mean QRS axis is the average (or general direction) of the majority of these electrical forces in the frontal plane. The normal mean QRS axis is from right to left, and from the right shoulder (or superior) towards the left hip (or inferior) but there is a broad range of normality within these general confines.
Axis values
The normal adult axis usually ranges between -30° to +90°, although young adults may have an axis up to 105°. A left axis is -30° to -90°. A right axis is +90° to +180°. A extreme right axis is -90° to -180°.
In patients with a positive lead I, the axis can either be normal or left. Note the polarity of lead II to make this determination. If lead I and II are both positive, there is a normal axis, betweeen -30° to +90°. If lead I is positive and lead II is negative, there is a left axis, between -30° to -90°.
In patients with a negative lead I, the axis can be right or extreme right. Note the polarity of lead aVF to make this determination. If lead I is negative and lead aVF is positive, there is a right axis, between +90° and +180°. If leads I and aVF are both negative, the axis is extreme right, between -180° and -90°.
Axis table
Here is a table showing whether the axis is normal, left or right. Note that leads I and II are used to differentiate normal from left axis and leads I and aVF to differentiate normal from right axis.
Example of normal axis
Note that leads I and II are positive, therefore, the QRS axis is normal.
Example of left axis
Note that lead I is positive and lead II is negative, therefore, the QRS axis is left.
Example of right axis
Note that lead I is negative and lead aVF is positive, therefore, the QRS axis is right.
Calculating exact axis
Determining between normal and abnormal axis deviation can be helpful in assessing a variety of suspected cardiac conditions. An exact numerical determination of the frontal plane QRS axis can be important. In order to calculate axis, remember the hexaxial frontal plane reference system and the degrees assigned to the various limb leads. The mean axis is usually oriented in the general direction of the lead with the tallest R wave, thus, if the R wave in lead II is the tallest one, the QRS axis is roughly in the direction of lead II or +50°. Conversely, the QRS axis is perpendicular to the isoelectric lead, that is, the lead with the R/S ratio closest to 1. The use of the isoelectric lead to determine QRS axis is the most accurate method and will be discussed in the following section.
Isoelectric method
The most accurate QRS axis determination method is the isoelectric method. First, determine which of the six frontal leads is the most isoelectric, that is, the lead where the sum of the positive and negative deflections is closest to zero. The major direction of depolarization, the QRS axis, is then perpendicular to this isoelectric lead. Next, determine the polarity of the lead perpendicular to the isoelectric lead. In this example, lead aVL is the most isoelectric lead, thus, the axis perpendicular to lead aVL is either +60° or -120°, the positive or negative of lead II. Since lead II is positive, the axis is, therefore, +60°. Lead II, incidentally, in this example was also the lead with the largest R wave.
In the example above, we can see that lead III is the most isoelectric lead. Since the perpendicular to lead III is aVR, the mean QRS axis is either +30° or -150°. The axis must be +30° for two good reasons: first, we already know that the axis is normal, since leads I and II are both positive, therefore, the axis must be 30° rather than -150°; second, since aVR is negative, the axis must be towards the negative of aVR, which is +30° rather than -150°.