Determination of the cardiac axis on ECG

What it is

The mean axis indicates the direction of the mean vector of the heart. Determining the axis is step 3 of reading the ECG, and it is read off two leads rather than from the whole ECG: look at the QRS complex in lead I and aVF.

The table of the two leads

Take the sign of the QRS in each of the two leads:

• Lead I positive, lead aVF positive: normal
• Lead I positive, lead aVF negative: left axis deviation
• Lead I negative, lead aVF positive: right axis deviation
• Lead I negative, lead aVF negative: extreme axis deviation

Each row is drawn beside the table as the QRS complex itself, so the row is read as a shape and not only as a sign: an upright QRS in I with an upright QRS in aVF, an upright I with a downward aVF, a downward I with an upright aVF, and both downward.

The normal range, and the circle behind the table

Normal is -30 to 105. The table sits beside a circle of the frontal leads which gives every value the table is naming: -90 at the top, aVR at -150, aVL at -30, 180 at the left, lead I at 0, lead II at +60, aVF at +90 and lead III at +120. The circle is divided into four and the four are named the same way as the rows: Extreme AXIS Deviation, LAD, NORMAL QRS AXIS and RAD.