Understanding the Electrical Axis on ECG
What the Cardiac Axis Represents
The cardiac axis is the mean direction of electrical depolarization through the ventricles expressed as an angle in the frontal plane, serving as a fundamental ECG parameter for detecting ventricular hypertrophy, conduction abnormalities, and structural heart disease. 1
The electrical axis originates from Einthoven's concept that the heart's electrical activity can be represented as a time-dependent vector—the "heart vector"—which projects onto different ECG leads based on their spatial orientation. 2, 1 The standard 12-lead ECG records potential differences between body surface electrodes that reflect transmembrane voltage changes in myocardial cells during depolarization. 2
Normal Values and Clinical Ranges
Adult Population
- Normal adult QRS axis: +30° to +90° 1
- Left axis deviation (LAD): Any axis less than -30° 1
- Right axis deviation (RAD): Axis exceeding +90° 1
The axis naturally shifts leftward with increasing age, making mild left axis deviation more common in older adults. 1, 3 Research demonstrates that the mean frontal QRS axis shifts leftward with advancing age (statistically significant in men), increased weight, increased chest circumference, and increased obesity index. 3
Pediatric Variations
The cardiac axis varies dramatically with age in children: 1
- Neonates (0-7 days): 55° to 200° (reflecting right ventricular dominance)
- 1 month: Upper limit 160° or less
- Ages 1-5 years: 10° to 110°
- Ages 5-8 years: May extend to 140°
- Ages 8-16 years: Extends to 120°
How to Determine the Axis
Practical Method Using Key Leads
Use leads I and aVF as your primary reference points to determine the quadrant, then use secondary leads (II, III, aVR, aVL) to find the precise axial position within that quadrant. 4 When either lead I or aVF is isoelectric, only one key lead is needed. 4
Technical Considerations
Important caveat: Combining bipolar leads (I, II, III) with unipolar leads (aVR, aVL, aVF) requires correction factors because these lead types have different strengths. 5 The uncorrected formula EA = Arctan(aVF/I) produces systematically lower values than the corrected formula EA = ±Arctan((2×aVF)/(√3×I)), which accounts for the 2/√3 correction factor required when mixing lead types. 5
Clinical Significance of Axis Deviations
Left Axis Deviation
- Moderate LAD: -30° to -45° 1
- Marked LAD: -45° to -90° (often associated with left anterior fascicular block) 1
Common causes include: 1
- Left ventricular hypertrophy
- Left anterior fascicular block
- Normal variant with increasing age
Right Axis Deviation
Right ventricular hypertrophy causes displacement of the QRS vector toward the right and anteriorly. 1 Key patterns include right axis deviation with prominent anterior forces in right precordial leads, volume overload pattern, and pressure overload pattern. 1
Factors That Alter the Axis
Body position and geometry significantly affect lead vector direction and strength, depending on body geometry and varying electrical impedances of torso tissues. 2, 1 Pathological conditions such as ventricular hypertrophy, bundle branch blocks, and fascicular blocks can deviate the axis significantly from normal. 1
Critical Pitfalls to Avoid
Do not interpret axis deviation without clinical context—axis deviation alone rarely establishes a diagnosis. 1 Failing to consider age-related changes in QRS axis, particularly in pediatric and elderly populations, leads to misinterpretation. 1
In COPD patients, do not misinterpret patterns as right ventricular hypertrophy—RVH is only suggested in COPD if R-wave amplitude in V1 is relatively increased. 1
When evaluating axis deviations accompanied by conduction abnormalities, always consider secondary repolarization abnormalities. 1 These are ST-T changes that result from altered ventricular depolarization sequence rather than primary myocardial repolarization changes. 2, 6 In right bundle branch block, ST-T changes are directed opposite to the slow terminal component of the QRS complex. 2, 6
Understanding Primary vs. Secondary Changes
The distinction between primary and secondary repolarization abnormalities is clinically relevant because primary abnormalities indicate changes in ventricular myocyte repolarization characteristics, whereas secondary changes do not. 2, 6 Both can coexist—for example, in ventricular hypertrophy where changes in action potential shape occur independently of QRS-amplitude changes. 2