Which electrocardiogram (ECG) lead is preferred for diagnosing left axis deviation, lead 2 or lead 3?

Diagnosing Left Axis Deviation: Lead II vs Lead III

For diagnosing left axis deviation (LAD), lead II is preferred over lead III because it provides more accurate assessment of the frontal plane axis and better correlates with the diagnostic criteria for LAD. 1

Understanding Left Axis Deviation

Left axis deviation is defined as a mean frontal plane QRS axis that is more negative than -30° 1. According to AHA/ACCF/HRS guidelines, LAD can be categorized as:

• Moderate LAD: Between -30° and -45°
• Marked LAD: Between -45° and -90° (often associated with left anterior fascicular block)

Diagnostic Approach Using Lead II

When evaluating for LAD, the following approach using lead II is recommended:

1. Primary Assessment: Examine lead II first

   • In LAD, lead II typically shows small R waves or may become negative (rS or QS pattern)
   • When the QRS axis becomes more negative than -30°, lead II transitions from positive to negative

2. Confirmatory Findings:

   • Lead I: Positive deflection (upright R wave)
   • Lead aVF: Negative deflection (predominant S wave)
   • Lead aVL: Positive deflection (predominant R wave)

3. Additional Criteria for Left Anterior Fascicular Block 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

Why Lead II is Preferred Over Lead III

1. More Reliable Orientation: Lead II has a more consistent orientation at +60° in the frontal plane, making it a more stable reference point for axis determination 1

2. Better Correlation with Axis Calculation: When calculating the frontal plane axis, lead II provides more consistent information than lead III, which is more variable in its relationship to the axis 1

3. Transition Point: Lead II represents the critical transition point for LAD diagnosis - when the axis shifts beyond -30°, lead II changes from positive to negative 1

4. Higher Diagnostic Accuracy: Research has shown that using lead II for LAD assessment correlates better with other diagnostic criteria for left anterior fascicular block 2

Clinical Pearls and Pitfalls

• Age Considerations: Normal QRS axis is age-dependent, with a gradual leftward shift occurring with advancing age 3
• Pediatric Patients: In children, a QRS axis ≤-42° is more strongly associated with underlying heart disease 4
• False Positives: LAD alone should not be considered synonymous with left anterior fascicular block; additional criteria should be evaluated 2
• Lead Placement Errors: Transposition of limb leads can create false LAD patterns; ensure proper electrode placement 1
• Associated Findings: When LAD is accompanied by ECG evidence of chamber enlargement or hypertrophy, further cardiovascular investigation is warranted 4

Differential Diagnosis of LAD

LAD may be associated with various conditions including:

• Left anterior fascicular block
• Left ventricular hypertrophy
• Inferior wall myocardial infarction
• Congenital heart disease (especially ostium primum atrial septal defect)
• Cardiomyopathies
• Normal variant in some elderly individuals

Remember that while lead II is preferred for initial assessment, a comprehensive evaluation of the 12-lead ECG is essential for accurate diagnosis of LAD and its underlying causes.