What type of axis deviation is indicated by a positive QRS in Lead I and a negative QRS in aVF on a 12-lead (twelve-lead) electrocardiogram (ECG)?

Cardiac Axis Determination on ECG

A 12-lead ECG with a positive QRS in Lead I and a negative QRS in aVF indicates left axis deviation (LAD).

Understanding Cardiac Axis

The cardiac axis represents the overall direction of ventricular depolarization in the frontal plane. It is determined by analyzing the QRS complexes in the limb leads, particularly leads I and aVF which are perpendicular to each other in the hexaxial reference system.

Axis Quadrants and Determination

• Normal axis: +30° to +90° (positive QRS in both leads I and aVF)
• Left axis deviation: -30° to -90° (positive QRS in lead I, negative QRS in aVF)
• Right axis deviation: +90° to +180° (negative QRS in lead I, positive QRS in aVF)
• Extreme right axis deviation: +180° to -90° (negative QRS in both leads I and aVF)

Clinical Significance of Left Axis Deviation

Left axis deviation may indicate:

1. Left anterior fascicular block (LAFB) - The most common cause of LAD with specific criteria 1:

   • Frontal plane axis between -45° and -90°
   • qR pattern in lead aVL
   • R-peak time in lead aVL of 45 ms or more
   • QRS duration less than 120 ms

2. Other potential causes:

   • Left ventricular hypertrophy
   • Inferior myocardial infarction
   • Congenital heart defects (atrioventricular septal defects, endocardial cushion defects)
   • Ventricular pre-excitation syndromes
   • Hyperkalemia

Diagnostic Algorithm for Left Axis Deviation

When LAD is identified:

1. Measure QRS duration:

   • If <120 ms: Consider LAFB or other causes
   • If ≥120 ms: Consider LBBB or other conduction abnormalities

2. Look for additional ECG findings:

   • qR pattern in aVL (supports LAFB)
   • Signs of LVH (supports hypertrophy as cause)
   • Q waves in inferior leads (may indicate inferior MI)
   • Delta waves (suggests pre-excitation)

3. Consider clinical context:

   • Age (LAD is more common in elderly patients)
   • Presence of cardiovascular disease
   • Symptoms of heart failure or coronary artery disease

Clinical Implications

According to the American College of Cardiology, LAD can be associated with structural heart disease and requires further evaluation 2. When LAD is detected, consider:

• Echocardiography to evaluate ventricular size and function
• Assessment for congenital heart defects
• Evaluation for evidence of pulmonary hypertension

Common Pitfalls in Axis Determination

• Misplacement of ECG leads can affect axis determination
• Presence of other conduction abnormalities may complicate interpretation
• In athletes, borderline LAD (between -30° and -90°) may be a normal variant 1
• In children, normal axis ranges differ from adults, with more rightward normal axes

Summary

A positive QRS in lead I with a negative QRS in aVF definitively indicates left axis deviation. This finding should prompt consideration of underlying causes, particularly left anterior fascicular block if QRS duration is normal, or other structural heart diseases that may require further evaluation.