What causes left axis deviation on a 12-lead (twelve-lead) electrocardiogram (EKG)?

Causes of Left Axis Deviation on 12-Lead EKG

Left axis deviation (LAD) on a 12-lead EKG is most commonly caused by left anterior fascicular block, cardiomyopathies, congenital heart defects, complete atrioventricular septal defect, and left ventricular hypertrophy with non-voltage criteria. 1

Definition of Left Axis Deviation

• LAD is defined as a mean frontal plane QRS axis between -30° and -90° in adults 1
• Moderate LAD ranges from -30° to -45° while marked LAD ranges from -45° to -90° 1, 2
• Normal QRS axis in adults is considered to be between +30° and +90° 2

Common Pathological Causes of Left Axis Deviation

Conduction System Abnormalities

• Left anterior fascicular block (LAFB) - the most common cause of LAD 1, 3
  • Requires both LAD and delayed intrinsicoid deflection in lead aVL 4
  • Not all LAD represents LAFB - the correlation between LAD and delayed intrinsicoid deflection varies with the degree of axis deviation 4

Structural Heart Disease

• Left ventricular hypertrophy (LVH) with non-voltage criteria 1, 5
  • LAD is often associated with pathological LVH rather than physiological adaptation 1
  • When LAD is present with LVH, it suggests more severe hypertrophy 5
• Cardiomyopathies (hypertrophic, dilated, etc.) 1
• Congenital heart defects, particularly complete atrioventricular septal defect 1
• Coronary artery disease, especially in patients with left bundle branch block 6

Age-Related Changes

• Natural leftward shift of QRS axis occurs with increasing age 2, 7
• In early life, the axis is determined by relative ventricular weights 7
• After reaching adult ventricular weight ratios, there is a period of axis stability followed by gradual leftward drift governed by left anterior fascicular conduction 7

Clinical Significance and Evaluation

• Isolated LAD has limited prognostic value but should prompt evaluation for underlying heart disease 2
• LAD in combination with other ECG abnormalities warrants more thorough cardiovascular evaluation 2
• In athletes, LAD is considered a "borderline" ECG finding that may indicate pathological LVH rather than physiological adaptation 1
• Echocardiography is recommended in patients with LAD to exclude underlying structural heart disease 1

Prognostic Implications

• Among patients with left bundle branch block, those with LAD have:
  • Greater incidence of myocardial dysfunction 6
  • More advanced conduction disease 6
  • Greater cardiovascular mortality compared to those with normal axis 6

Diagnostic Approach

• Determine the exact QRS axis and look for additional ECG abnormalities 1
• Evaluate for symptoms and perform thorough cardiac physical examination 1
• Assess for family history of cardiac disease or sudden cardiac death 1
• Consider echocardiography as the primary imaging modality 1

When LAD is identified on an EKG, it should prompt consideration of underlying cardiac pathology rather than being dismissed as an isolated finding, particularly when accompanied by other ECG abnormalities or clinical symptoms.