What is left axis deviation on an electrocardiogram (EKG)?

Left Axis Deviation on Electrocardiogram (ECG)

Left axis deviation (LAD) on an electrocardiogram is defined as a mean frontal plane QRS axis of -30° or more leftward in adults, representing an abnormal electrical activation pattern of the ventricles. 1

Definition and Classification

• The American Heart Association (AHA), American College of Cardiology Foundation (ACCF), and Heart Rhythm Society (HRS) classify LAD as:

  • Moderate left axis deviation: -30° to -45°
  • Marked left axis deviation: -45° to -90° 1

• Normal QRS axis ranges vary 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

Most Common Cause: Left Anterior Fascicular Block (LAFB)

LAFB is one of the most common causes of LAD 2 and has specific diagnostic criteria:

1. Frontal plane axis between -45° and -90°
2. qR pattern in lead aVL
3. R-peak time in lead aVL of ≥45 ms
4. QRS duration <120 ms 3, 1

It's important to note that LAD alone should not be considered synonymous with LAFB. The delayed inscription of the intrinsicoid deflection in aVL is a useful supplemental criterion for diagnosis 4.

Other Causes and Associated Conditions

LAD can be associated with:

• Left ventricular hypertrophy (LVH)
• Congenital heart defects (particularly in children):
  • Atrioventricular septal defects
  • Tricuspid atresia
  • Endocardial cushion defects 3, 1
• Myocardial dysfunction
• Advanced conduction disease 5

Clinical Significance

LAD is one of the most commonly encountered ECG abnormalities and should alert clinicians to the possibility of underlying structural heart disease 2. The clinical significance varies:

• In adults with left bundle branch block, LAD is associated with:

  • Greater incidence of myocardial dysfunction
  • More advanced conduction disease
  • Greater cardiovascular mortality compared to those with normal axis 5

• In children without previously known heart disease:

  • LAD discovered in isolation in asymptomatic children may not necessitate further investigation
  • Further evaluation is warranted when LAD is accompanied by:
    • ECG evidence of chamber enlargement or hypertrophy
    • QRS axis ≤-42°
    • Abnormal cardiac physical examination findings 6

Diagnostic Approach

When LAD is detected on ECG:

1. Review for other ECG abnormalities, particularly:

   • Chamber enlargement or hypertrophy
   • Conduction disturbances
   • QRS duration

2. Consider echocardiography when:

   • LAD is marked (≤-45°)
   • Other ECG abnormalities are present
   • Abnormal cardiac physical examination findings exist
   • Clinical symptoms suggest heart disease 1, 6

3. Focus evaluation on:

   • Ventricular size and function
   • Presence of congenital heart defects
   • Evidence of pulmonary hypertension 1

Limitations in Interpretation

• Body position, lead placement, and body habitus can influence QRS axis determination
• LAD may have different significance in different age groups
• The correlation between LAD and LAFB is not perfect; not all patients with LAD have LAFB 4, 7

Remember that LAD is an ECG finding that requires clinical correlation and may indicate underlying cardiac pathology, especially when accompanied by other abnormal findings.