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

Left Axis Deviation on ECG: Definition, Causes, and Clinical Significance

Left axis deviation (LAD) on an electrocardiogram (ECG) 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 Criteria

According to the American Heart Association (AHA), American College of Cardiology Foundation (ACCF), and Heart Rhythm Society (HRS) guidelines:

• In adults, normal QRS axis ranges from +30° to +90°
• Left axis deviation is defined as axis ≤ -30°
• Moderate left axis deviation: -30° to -45°
• Marked left axis deviation: -45° to -90° 1

Age-specific normal ranges are important to consider:

• Neonates: normal axis is +30° to +190° (left axis deviation if < +30°)
• Children 1 month to 1 year: normal axis is +10° to +120° (left axis deviation if < +10°)
• Children 1-5 years: normal axis is +5° to +100° (left axis deviation if < +5°)
• Children 5-8 years: normal axis is 0° to +140° (left axis deviation if < 0°)
• Children 8-16 years: normal axis is 0° to +120° (left axis deviation if < 0°) 1, 2

Common Causes of Left Axis Deviation

Left Anterior Fascicular Block (LAFB)

The most common cause of marked left axis deviation, with specific criteria:

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

Other Common Causes

• Left ventricular hypertrophy (LVH) 1
• Inferior myocardial infarction
• Wolff-Parkinson-White syndrome
• Artificial pacemaker rhythm
• Hyperkalemia
• Emphysema (due to low diaphragm position)
• Mechanical shifts (obesity, pregnancy)

Congenital Heart Disease

In pediatric patients, LAD may indicate:

• Atrioventricular septal defects
• Tricuspid atresia
• Endocardial cushion defects
• Other congenital anomalies with abnormal conduction system location 1, 3

Clinical Significance

LAD is not a diagnosis but a finding that requires clinical correlation:

• In asymptomatic adults without other ECG abnormalities, isolated LAD may be a normal variant or reflect age-related changes in the conduction system 4

• When LAD occurs with left bundle branch block (LBBB), it suggests more advanced conduction disease and is associated with:

  • Greater incidence of myocardial dysfunction
  • Higher cardiovascular mortality
  • Increased risk of developing complete heart block 5

• LAD supports the diagnosis of LVH when present with other voltage criteria for LVH 1

• In children, LAD may be a normal variant but can also indicate congenital heart defects, especially when extreme 3

Evaluation Approach

When LAD is detected:

1. Review patient history for cardiac risk factors, symptoms of heart disease, and congenital anomalies

2. Examine other ECG features:

   • QRS duration (normal or prolonged)
   • Associated bundle branch blocks
   • Evidence of chamber enlargement
   • ST-T wave abnormalities

3. Consider further evaluation based on clinical context:

   • Echocardiography if suspected structural heart disease
   • Cardiac monitoring if conduction system disease suspected
   • Electrophysiology studies in selected cases

Important Considerations

• The term "left anterior hemiblock" should be reserved for cases where damage to the conduction system is confirmed; "left axis deviation" is more appropriate in other cases 3

• LAD alone is not synonymous with left anterior fascicular block; additional criteria such as delayed intrinsicoid deflection in aVL should be considered 6

• Body position, lead placement, and body habitus can influence QRS axis determination 2

• In patients with left bundle branch block, the presence of LAD may indicate a worse prognosis 5