How do I compute axis deviation on an electrocardiogram (ECG)?

How to Compute Axis Deviation on ECG

To determine the QRS axis, examine leads I and aVF first: if both are positive, the axis is normal (0° to +90°); if lead I is positive and aVF is negative, there is left axis deviation; if lead I is negative and aVF is positive, there is right axis deviation. 1

Quick Quadrant Method

The fastest approach uses the two perpendicular limb leads to identify which quadrant contains the axis 1:

• Lead I positive + aVF positive = Normal axis (0° to +90°)
• Lead I positive + aVF negative = Left axis deviation (-90° to 0°)
• Lead I negative + aVF positive = Right axis deviation (+90° to +180°)
• Lead I negative + aVF negative = Extreme axis deviation (-90° to +180°)

Precise Axis Calculation

For more accurate measurement, use one of two methods 1:

Method 1 - Isoelectric Lead:

• Find the lead with the most isoelectric (equiphasic) QRS complex, where positive and negative deflections are equal 1
• The axis is perpendicular (90°) to this lead 1

Method 2 - Tallest Deflection:

• Identify the lead with the tallest net QRS deflection 1
• The axis points toward this lead 1

Normal Values and Deviation Definitions

Adults

• Normal axis: +30° to +90° 1, 2
• Left axis deviation: Less than -30° 1
  • Moderate LAD: -30° to -45° 1
  • Marked LAD: -45° to -90° (often indicates left anterior fascicular block) 1
• Right axis deviation: +90° to +180° 1

Pediatric Populations

The normal axis varies significantly with age 3:

• Neonates: 60° to 190° (extreme right axis 90° to 190°) 3
• Ages 1-5 years: 10° to 110° 3
• Ages 5-8 years: May extend to 140° 3
• Ages 8-16 years: Range extends to 120° 3

Clinical Context

Common pitfall: Axis deviation is a finding that requires clinical correlation, not a standalone diagnosis 1. An abnormal axis should prompt evaluation of the complete ECG for associated findings such as bundle branch blocks, fascicular blocks, or ventricular hypertrophy 4, 5.

Age consideration: The QRS axis naturally shifts leftward with increasing age in adults, making mild left axis deviation more common in older populations 1, 6.

When marked LAD (-45° to -90°) is present, evaluate for left anterior fascicular block using additional criteria: qR pattern in lead aVL, R-peak time in lead aVL ≥45 ms, and QRS duration <120 ms 3, 1.