QRS Axis

There are many ways to determine QRS axis. The one described below combines simplicity and efficiency.

The limits of normal and abnormal QRS axis are summarized in the diagram to the right.
QRS axis is the direction in which the mean QRS current flows. The normal axis points mostly downward and to the left because the more muscular left ventricle generates a stronger depolarizing current that overwhelms that generated by the less bulky right. Although both right axis deviation (RAD) and left axis deviation (LAD) are not necessarily associated with organic heart disease, they are seen in a number of settings and their presence can provide added evidence to support a clinical diagnosis.
RAD is seen in right ventricular (RV) hypertrophy and in infarction involving the left ventricle (LV). Right ventricular muscle bulk is relatively larger than that of the left in both conditions and generates a stronger depolarizing current in its direction.
LAD is seen, but not always, in patients with left ventricular hypertrophy. More commonly the QRS axis is horizontal in this condition.

It is only necessary to examine the QRS complexes in leads I and II to determine whether the QRS axis in normal or deviated to the left or the right; a precise calculation of the QRS axis is not required in clinical interpretation of the ECG.

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It has been explained in a previous section that a current flowing in the direction of a recording electrode (an ECG lead) registers a positive deflection and a current flowing away registers a negative deflection.

Therefore, the QRS in lead I would be positive if the QRS current flows in the direction of lead I and negative if away. Similarly the QRS in lead II would be positive if the QRS axis points in the direction of lead II and negative if away.

By overlapping the two circles representing leads I and II, it can be seen that the QRS axis is between +90o and -30o and normal if the QRS is positive both in lead I and lead II.
QRS axis is between -30o and -90o or deviated to the left (left axis deviation or LAD) if the QRS is positive in lead I but negative in lead II.
QRS axis is between +90o and +150o or deviated to the right (right axis deviation or RAD) if the QRS is negative in lead I but positive in lead II.

On occasions the QRS complexes in all 6 limb leads are biphasic, neither positive nor negative. In these instances the QRS axis is said to be indeterminate.

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In summary:

Normal QRS axis.
Left axis deviation
Right axis deviation

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