ECG Axis Interpretation

Overview

• Cardiac Axis: Represents depolarization vectors of cardiac myocytes, primarily reflecting the ventricular axis.
• Normal Axis: Directed downward and to the left, QRS axis between -30 and +90.
• Abnormal Axis Deviations:
  • Left Axis Deviation (LAD): QRS axis < -30.
  • Right Axis Deviation (RAD): QRS axis > +90.
  • Extreme Axis Deviation: QRS axis between -90 and 180 (Northwest Axis).

Pediatric ECG

• At Birth: Cardiac axis lies between +30 to +190.
• Age Progression: Axis moves leftward as the child grows.

Methods of ECG Axis Interpretation

1. Quadrant Method

• Leads: Use Lead I and Lead aVF.
• Interpretation:
  • Positive QRS in Lead I aligns the axis with Lead I.
  • Positive QRS in Lead aVF aligns the axis with Lead aVF.
  • Both positive = Normal Axis (0 to +90).

2. Three Lead Analysis

• Leads: Includes Lead I, II, and aVF.
• Interpretation:
  • Positive QRS in Lead I and II = Normal Axis (-30 to +90).
  • Helps differentiate physiological from pathological LAD.
  • Can use Lead III or aVF in analysis.

3. Isoelectric Lead Analysis

• Principles:
  • Positive QRS: Axis = Roughly same direction.
  • Negative QRS: Axis = Opposite direction.
  • Isoelectric QRS: Axis = 90° to the lead.
• Steps:
  1. Find isoelectric lead (zero net amplitude).
  2. Identify positive leads.
  3. Calculate QRS axis at 90° to isoelectric lead in direction of positive leads.

Examples

Example 1

• Leads: I (positive), II (positive), aVF (positive).
• Axis: Between 0 and +90 (normal axis).
• Isoelectric Lead: aVL at -30.
  • QRS axis = +60 (normal).

Example 2

• Leads: I (negative), II (equiphasic), aVF (positive).
• Axis: RAD between +90 and +180.
• Isoelectric Lead: II at +60.
  • QRS axis = +150.

Example 3

• Leads: I (positive), II (equiphasic), aVF (negative).
• Axis: Borderline LAD.
• Isoelectric Lead: II at +60.
  • QRS axis = -30.

Example 4

• Leads: I, II, aVF (all negative).
• Axis: Extreme Axis Deviation.
• Isoelectric Lead: aVL.
  • QRS axis = -120.

Example 5

• Leads: I (isoelectric), aVF (positive).
• Axis: +90 (vertical axis).

Causes of Axis Deviation

Right Axis Deviation (RAD)

• Right ventricular hypertrophy, acute right ventricular strain, lateral STEMI, chronic lung disease, hyperkalaemia, sodium-channel blockade, etc.

Left Axis Deviation (LAD)

• Left ventricular hypertrophy, left bundle branch block, inferior MI, ventricular pacing, Wolff-Parkinson-White Syndrome, etc.

Extreme Axis Deviation

• Ventricular rhythms, hyperkalaemia, severe right ventricular hypertrophy.

Further Reading

• Online Courses: ECG Mastery series from Medmastery.
• Textbooks: ECG Core Curriculum, Emergency ECGs: Case-Based Review, Marriotts Practical Electrocardiography, and others for comprehensive ECG understanding.

Authors

• Mike Cadogan: Emergency physician, co-founder of LITFL.
• Robert Buttner: Emergency Medicine Advanced Trainee, co-creator of the LITFL ECG Library.