What are the causes of left axis deviation on an electrocardiogram?

Causes of Left Axis Deviation on ECG

Left axis deviation (LAD), defined as a mean frontal plane QRS axis between -30° and -90°, is most commonly caused by left anterior fascicular block, left ventricular hypertrophy, and age-related changes, with additional causes including congenital heart defects and cardiomyopathies. 1, 2

Primary Causes

Left Anterior Fascicular Block (LAFB)

• LAFB is the most common pathological cause of LAD, particularly when marked LAD (-45° to -90°) is present 1
• Diagnostic criteria include:
  • Frontal plane axis between -45° and -90° 1
  • qR pattern in lead aVL 1
  • R-peak time in lead aVL of 45 ms or more 1
  • QRS duration less than 120 ms 1
• The QRS vector shifts in a posterior and superior direction, resulting in larger R waves in leads I and aVL 1

Left Ventricular Hypertrophy (LVH)

• LAD may be associated with LVH, though it should be used as a supporting criterion rather than a diagnostic criterion 1
• The mechanism is unclear—LAD may result from hypertrophy itself, a degree of left anterior fascicular block, or age-related factors 1
• LAD is more reliable when combined with non-voltage criteria for LVH 2

Congenital Heart Defects

• Complete atrioventricular septal defect causes LAD due to abnormal location of the conduction system 1, 2
• Tricuspid atresia and other defects with underdevelopment of the right ventricle produce leftward QRS-axis shifts 1
• In pediatric patients, isolated LAD in the context of congenital defects requires specific age-adjusted interpretation 1, 2

Cardiomyopathies

• Various cardiomyopathies can produce LAD through alterations in ventricular activation patterns 2
• In athletes, LAD may indicate pathological LV hypertrophy rather than physiological adaptation and warrants echocardiographic evaluation 2

Age-Related and Physiological Factors

Normal Age Variation

• The QRS axis shifts progressively leftward with increasing age, even in the absence of structural heart disease 1
• In adults, moderate LAD (-30° to -45°) may represent normal aging rather than pathology 1
• Body habitus also influences axis position 1

Pediatric Considerations

• Neonates normally have a rightward axis (60° to 190°) that shifts leftward throughout childhood 1
• By ages 1-5 years, the normal range is 10° to 110°; by ages 5-8 years, it extends to 140° 1
• Age-appropriate interpretation is critical to avoid misdiagnosis 2

Clinical Approach Algorithm

Step 1: Confirm True LAD

• Verify QRS axis is between -30° and -90° 1
• Rule out technical errors (electrode misplacement, lead inversion) 2
• Distinguish moderate (-30° to -45°) from marked (-45° to -90°) LAD 1

Step 2: Assess for LAFB

• Look for qR pattern in aVL with R-peak time ≥45 ms 1
• Confirm QRS duration <120 ms (excludes bundle branch block) 1
• Check for deeper S waves in leads V5 and V6 1

Step 3: Evaluate for LVH

• Assess QRS voltage criteria in precordial leads 1
• Look for ST-T wave abnormalities (strain pattern) 1
• Consider P-wave abnormalities suggesting left atrial enlargement 1

Step 4: Consider Clinical Context

• Obtain detailed history focusing on hypertension, family history of cardiomyopathy or sudden cardiac death 2
• Perform thorough cardiac physical examination 2
• In pediatric patients, assess for symptoms and family history of congenital heart disease 1, 2

Step 5: Determine Need for Further Testing

• Echocardiography is indicated for: patients with symptoms suggestive of cardiovascular disease, family history of cardiomyopathy or sudden cardiac death, or athletes with LAD 2
• Exercise testing may be warranted to evaluate exercise capacity and detect ischemia 2
• In older patients with risk factors, consider evaluation for coronary artery disease 2

Important Caveats

• LAD alone should not be considered synonymous with LAFB—additional criteria must be met 1
• Approximately 60% of patients with LAD may have no other cardiovascular abnormalities 2, 3
• Old myocardial infarction patterns (anterior, inferior, lateral) are not obscured by associated anterior fascicular block 4
• Automated ECG interpretations can be unreliable at axis range limits—clinical correlation is essential 2
• In the presence of left anterior fascicular block, R-wave amplitude in leads I and aVL are not reliable criteria for LVH 1