QRS Axis of <-30 Degrees Indicates Left Axis Deviation, Not Right Axis Deviation
No, a QRS axis less than –30 degrees indicates left axis deviation (LAD), not right axis deviation. This is a fundamental principle of ECG interpretation defined by major cardiology societies.
Standard Definitions of QRS Axis Deviation
Left axis deviation is defined as a mean frontal-plane QRS axis between –30° and –90° by the American College of Cardiology 1. This range is further subdivided into:
Right axis deviation occurs when the QRS axis is greater than +90° to +100° in adults 1, 2. The normal QRS axis in adults typically ranges from approximately –30° to +90° 1.
Age-Specific Considerations
Adults
In adult populations, any axis more negative than –30° represents left axis deviation and warrants evaluation for underlying causes such as left anterior fascicular block, left ventricular hypertrophy, or age-related conduction changes 1.
Neonates and Infants
The normal full-term neonate has a QRS axis between 55° and 200°, which would be considered right axis deviation in adults but is physiologically normal in newborns 3, 2. By 1 month of age, the normal upper limit falls to 160° or less 3, 2. What appears as right axis deviation in neonates (e.g., 120°) is a normal developmental finding 3.
Common Clinical Pitfall
A critical error in ECG interpretation is misidentifying the direction of axis deviation. An axis of <-30° shifts the electrical vector leftward and superiorly, producing:
This pattern is the opposite of right axis deviation, which produces dominant R waves in the inferior leads and deeper S waves in leads I and aVL.
Clinical Implications of True Left Axis Deviation (<-30°)
When you encounter a QRS axis <-30°, the most common pathological cause is left anterior fascicular block (LAFB), especially when the deviation is marked (–45° to –90°) 1. Additional diagnostic criteria for LAFB include:
- qR pattern in lead aVL 1
- R-peak time in aVL ≥45 ms 1
- QRS duration <120 ms (excludes bundle branch block) 1
Other causes of LAD include left ventricular hypertrophy, cardiomyopathies, congenital heart defects (particularly complete atrioventricular septal defect), and normal age-related leftward shift 1.
Extreme Right Axis Deviation (Actual Definition)
For completeness, extreme right axis deviation is defined as a QRS axis between +180° and +270° (or equivalently –90° to –180°), sometimes called a "northwest axis" 5, 6. This represents a distinctly different and ominous finding associated with extensive myocardial ischemia, ventricular fibrillation, cardiogenic shock, and high mortality 5.