What does leftward axis deviation on an electrocardiogram (ECG) mean?

Leftward Axis Deviation on Electrocardiogram

Leftward axis deviation (LAD) on an electrocardiogram is defined as a QRS axis between -1° and -90° and is diagnosed when lead I shows a positive deflection while lead aVF shows a negative deflection. 1

Definition and Diagnosis

The American College of Cardiology provides a systematic approach to axis determination:

Lead I	Lead aVF	Axis Interpretation
Positive	Positive	Normal axis (0° to +90°)
Positive	Negative	Left axis deviation (-1° to -90°)
Negative	Positive	Right axis deviation (+91° to +180°)
Negative	Negative	Extreme right axis deviation (+181° to -91°)

It's important to note that normal QRS axis varies by age:

• Adults: +30° to +90°
• Children 8-16 years: up to +120°
• Children 5-8 years: up to +140°
• Children 1-5 years: +10° to +110°
• Neonates: +30° to +190° 1

Clinical Significance and Common Causes

LAD is a common ECG finding that should alert clinicians to the possibility of underlying structural heart disease 2. The most common causes include:

1. Left Anterior Fascicular Block (LAFB)

   • Most common cause of LAD 2
   • Additional diagnostic criteria beyond just LAD include:
     • QRS duration <120 ms
     • Small q waves in leads I and aVL
     • Small r waves in leads II, III, and aVF 1

2. Age-related changes

   • Normal aging process can cause a gradual leftward drift of the QRS axis 3
   • After reaching adult ventricular weight ratios, there is a period of axis stability followed by this gradual leftward shift 3

3. Structural heart disease

   • Left ventricular hypertrophy (LVH)
   • Cardiomyopathies
   • Valvular heart disease (particularly aortic stenosis)
   • Coronary artery disease 1, 2

4. Conduction system disease

   • Left bundle branch block with LAD indicates more advanced conduction disease 4
   • Higher risk of progression to higher-degree AV block 1

Clinical Implications

LAD has important prognostic implications:

• In patients with left bundle branch block, those with LAD have:

  • Greater incidence of myocardial dysfunction
  • More advanced conduction disease
  • Higher cardiovascular mortality compared to those with normal axis 4

• The presence of LAD should prompt evaluation for:

  • Underlying structural heart disease
  • Conduction system abnormalities
  • Risk factors for cardiovascular disease 1, 2

Diagnostic Pitfalls

• LAD alone is not synonymous with LAFB; additional criteria should be considered 5

• The correlation between the degree of LAD and delayed intrinsicoid deflection in aVL (another criterion for LAFB) varies:

  • 41% at -30 degrees
  • 69% at -45 degrees
  • 82% at -60 degrees
  • 100% at -75 degrees or greater 5

• When evaluating LAD, consider the patient's age, as normal axis is age-dependent 3

Management Approach

For patients with LAD on ECG:

1. Assess for symptoms and signs of underlying heart disease
2. Consider echocardiography to evaluate for structural abnormalities
3. For LAD associated with structural heart disease, focus treatment on the underlying condition (hypertension, aortic stenosis, etc.) 1
4. For LAD with conduction system disease, monitor for progression to higher-degree AV block and consider pacemaker implantation if symptomatic bradycardia or advanced AV block develops 1
5. Regular clinical assessment and serial echocardiography are necessary to monitor progression in patients with underlying structural heart disease 1