Cardiac Axis & Conduction Summary

Cardiac Axis Classification

  • Normal: -30° to +90°
  • LAD: -30° to -90° (LAFB, LVH, inferior MI)
  • RAD: +90° to +180° (LPFB, RVH, lateral MI)
  • Extreme: -90° to -180° (ventricular rhythms)

Bundle Branch Blocks

  • RBBB: rSR' in V1, wide S in V6
  • LBBB: Broad R in V6, deep S in V1
  • New LBBB + chest pain = STEMI equivalent

Fascicular Blocks

  • LAFB: LAD (-45° to -90°), qR in I/aVL
  • LPFB: RAD (>+90°), qR in II/III/aVF
  • Isolated LPFB is rare

Bifascicular Blocks

  • RBBB + LAFB: Most common
  • RBBB + LPFB: Less common
  • Risk of complete heart block

Emergency Recognition

  • New LBBB with chest pain → STEMI protocol
  • Bifascicular block + syncope → Pacemaker evaluation
  • Complete trifascicular → Complete heart block

Axis Determination

  • Quadrant method: Leads I & aVF
  • Three-lead method: I, II, aVF
  • Isoelectric lead = perpendicular to axis

Key ECG Features

  • QRS duration ≥120ms for BBB
  • Discordant ST-T changes in BBB
  • Absent Q waves in LBBB
  • Delayed R wave peak in LAFB

Cardiac Axis & Conduction Abnormalities

Comprehensive guide to ECG interpretation of cardiac axis and conduction system disorders

Cardiac Axis Interpretation

The QRS axis represents the overall direction of ventricular depolarization. Normally directed downward and slightly left due to dominant left ventricular muscle mass.

Axis Classification

Classification QRS Axis Range Clinical Significance
Normal Axis -30° to +90° Normal ventricular activation
Left Axis Deviation (LAD) -30° to -90° Left anterior fascicular block, LVH, inferior MI
Right Axis Deviation (RAD) +90° to +180° Left posterior fascicular block, RVH, lateral MI
Extreme Axis ("Northwest") -90° to -180° Ventricular rhythms, lead reversal, severe RVH

Methods of Axis Determination

Quadrant Method (Leads I & aVF)

  1. Determine QRS polarity in Lead I (+/-/isoelectric)
  2. Determine QRS polarity in Lead aVF
  3. Use combination to identify quadrant:
    • Both positive: Normal axis (0° to +90°)
    • I+, aVF-: LAD (-30° to -90°)
    • I-, aVF+: RAD (+90° to +180°)
    • Both negative: Extreme axis (-90° to -180°)

Three-Lead Analysis (I, II, III/aVF)

  1. Assess QRS in Leads I, II, and aVF
  2. Most positive lead points toward axis
  3. Isoelectric lead is perpendicular to axis
  4. Example: Positive I & II = Normal axis

Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

  • QRS duration ≥120ms
  • V1/V2: rSR' pattern ("M-shaped")
  • V6: Wide slurred S wave
  • Secondary ST-T changes opposite to QRS
  • Normal axis typically maintained

Left Bundle Branch Block (LBBB)

  • QRS duration ≥120ms
  • V6: Broad notched/slurred R wave
  • V1: Deep S wave or QS complex
  • Absent Q waves in lateral leads
  • ST-T changes discordant to QRS

Clinical Significance

New LBBB with chest pain is STEMI equivalent until proven otherwise. RBBB is often benign but may indicate RV strain or conduction system disease.

Fascicular Blocks

Left Anterior Fascicular Block (LAFB)

  • Axis -45° to -90°
  • qR pattern in I/aVL
  • rS pattern in II/III/aVF
  • QRS <120ms (unless with RBBB)
  • Delayed R wave peak in aVL (>45ms)

Left Posterior Fascicular Block (LPFB)

  • Axis +90° to +180°
  • rS pattern in I/aVL
  • qR pattern in II/III/aVF
  • QRS <120ms (unless with RBBB)
  • Must exclude other causes of RAD

Isolated LPFB is rare - always consider RVH, lateral MI, and dextrocardia as alternative causes of RAD.

Bifascicular & Trifascicular Blocks

Bifascicular block with alternating BBB or syncope may require pacemaker implantation due to risk of complete heart block.

Definitions

Type Components ECG Findings
Bifascicular Block RBBB + LAFB (most common) RBBB pattern + LAD (-45° to -90°)
RBBB + LPFB RBBB pattern + RAD (>+90°)
Trifascicular Block Bifascicular + 1° AV block RBBB + fascicular block + PR>200ms
Complete Trifascicular Bifascicular + 3° AV block RBBB + fascicular block + complete HB

Clinical Examples

Complete Trifascicular Block
  • RBBB pattern (rsR' in V1, wide S in V6)
  • Left axis deviation (LAFB)
  • Third-degree AV block (complete dissociation)
Bifascicular Block (RBBB+LAFB)
  • rsR' in V1-V2 with appropriate discordance
  • Deep S waves in lateral leads (V4-V6)
  • Left axis deviation (-45° to -90°)
  • Normal PR interval unless incomplete trifascicular