The axis of the ECG is the major direction of the overall electrical activity of the heart. It can be normal, leftward (left axis deviation or LAD), rightward (right axis deviation or RAD), or indeterminate (northwest axis). The QRS axis is the most important to determine, however the P wave or T wave axis can also be measured.
To determine the QRS axis, the limb leads need to be examined (not the precordial leads). The depiction of the standard leads and their relationship to the cardiac axis is below.
Note that lead I is at zero degrees, lead II is at +60 degrees, and lead III is at +120 degrees. Lead aVL (L for left arm) is at -30 degrees, lead aVF (F for foot) is at +90 degrees, and the negative of lead aVR (R for right arm) is at +30 degrees. The positive of lead aVR is actually at -150 degrees.
Memorizing the above picture is crucial to accurately determining axis, however there are some shortcuts to quickly determine the axis as mentioned below.
The normal QRS axis should be between - 30 and +90 degrees. Left axis deviation is defined as the major QRS vector falling between -30 and -90 degrees. Right axis deviation occurs with the QRS axis is between +90 and +180 degrees. Indeterminate axis is between +/- 180 and -90 degrees. This is summarized in the image below:
LAD = Left Axis Deviation
RAD = Right Axis Deviation
NW = northwest axis or indeterminate axis
The fastest, non-specific method to determine the QRS axis is to find the major direction of the QRS complex (positive or negative) in leads I and aVF.
Normal QRS Axis
If the QRS complex is upright (positive) in both lead I and lead aVF, then the axis is normal. The below image demonstrates this example with the electrical vector heading towards the positive of lead I and the positive of lead aVF as indicated by the arrows. The QRS axis is thus between these two arrows which falls within the normal range.
Left Axis Deviation
If the QRS is upright in lead I (positive) and downward in lead aVF (negative), then the axis is between 0 and -90 degrees. However recall that left axis deviation is defined as between -30 and -90 and thus this scenario is not always technically left axis deviation. In this scenario, the QRS axis could fall between 0 and -30 which is within normal limits. To further distinguish normal from left axis deviation in this setting, look at lead II. If lead II is downward (negative), then the axis is more towards -120 and left axis deviation is present. If the QRS complex in lead II is upright (positive), then the axis is more towards +60 degrees and the QRS axis is normal. Below is an example of left axis deviation to help visualize the above explanation:
Right Axis Deviation
If the QRS is predominantly negative in leads I and positive in lead aVF, then the axis is rightward (righ axis deviation).
If the QRS is downward (negative) in lead I and downward (negative) in lead aVF, then the axis is indeterminate.