Mean electrical QRS axis

Calculating axis

• Measure net voltage (positive minus negative) of two limb leads.
• Locate these values on the positive or negative portion of the hexaxial reference diagram
• Draw perpendiculars to these points
• Crossing point marks the mean axis

Quickly estimate axis

• Normal QRS axis
1. Normal axis = -30 to +90 degrees
2. Left axis deviation is left of -30 degrees
3. Right axis deviation is right of +90 degrees
• Axis is abnormal if…
1. Lead II is more negative than positive
2. Lead III is more positive than lead II
• Select lead nearest to being equiphasic
1. Axis is 90 degrees to this lead
2. If lead to left of this lead is positive, then axis is left of the equiphasic lead
3. If lead to right of this lead is positive, then axis is right of the equiphasic lead