How to Identify Left Axis Deviation on ECG
Left axis deviation (LAD) in adults is identified when the mean frontal plane QRS axis is less than -30°, with moderate LAD ranging from -30° to -45° and marked LAD ranging from -45° to -90°. 1
Determining the QRS Axis
To identify LAD, you must first calculate the mean frontal plane QRS axis using the limb leads in the hexaxial reference system 2:
- Examine leads I and aVF to quickly determine the quadrant of the axis
- Lead I positive + aVF positive = normal axis (0° to +90°)
- Lead I positive + aVF negative = left axis deviation (0° to -90°)
- Lead I negative + aVF positive = right axis deviation (+90° to +180°)
- Lead I negative + aVF negative = extreme axis deviation (-90° to -180°) 2
For precise axis calculation, identify the lead with the most isoelectric (equiphasic) QRS complex—the axis is perpendicular to this lead 2. Alternatively, use the lead with the tallest net QRS deflection, as the axis points toward this lead 1.
Specific Criteria for Left Axis Deviation
In Adults
- Normal QRS axis range: -30° to +90° 1, 3
- Left axis deviation: Any axis less than -30° 3
- Moderate LAD: -30° to -45° 1, 3
- Marked LAD: -45° to -90° (often indicates left anterior fascicular block) 1, 3
In Pediatric Populations
The criteria differ substantially in children due to age-related rightward axis at birth that shifts leftward with development 1:
- Neonates: Normal axis 60° to 190°; LAD is distinctly abnormal 1, 2
- Ages 1-5 years: Normal axis 10° to 110° 1, 2
- Ages 5-8 years: Normal axis may extend to 140° 1
- Ages 8-16 years: Normal axis range extends to 120° 1
In children without known heart disease, LAD warrants echocardiography when accompanied by: QRS axis ≤-42°, ECG evidence of chamber enlargement or hypertrophy, or abnormal cardiac physical examination findings 4. Isolated LAD in asymptomatic children may not require further cardiovascular investigation 4.
Associated ECG Findings with Left Anterior Fascicular Block
When marked LAD (-45° to -90°) is present, evaluate for left anterior fascicular block using these additional criteria 1:
- qR pattern in lead aVL 1, 3
- R-peak time in lead aVL ≥45 ms 1, 3
- QRS duration <120 ms (distinguishes from complete left bundle branch block) 1, 3
Clinical Context and Pitfalls
Age-related considerations: The QRS axis naturally shifts leftward with increasing age, making mild LAD more common in older adults and potentially a longstanding normal variant 3. Failing to account for age-related changes leads to overdiagnosis 3.
Body habitus influences axis position and must be considered in interpretation 2. Obese patients or those with elevated diaphragms may have leftward axis shifts without pathology 2.
Common causes of LAD in adults include left ventricular hypertrophy, left anterior fascicular block, inferior myocardial infarction, and occasionally aortic valve disease 3, 5. In complete left bundle branch block patients, the presence of LAD has 41.9% sensitivity and 91.6% specificity for organic heart disease, particularly aortic valve disease 5.
Technical errors to exclude: Verify proper electrode placement, as limb lead reversal can create artifactual axis deviation 2. Do not rely solely on automated ECG interpretations, especially at axis range boundaries 2.
Avoid overdiagnosis: LAD is an ECG finding, not a diagnosis—it requires clinical correlation with symptoms, physical examination findings, and other ECG abnormalities 3. The presence of LAD alone has limited diagnostic value 3.