What does the axis on an electrocardiogram (EKG) indicate?

What the Cardiac Axis on EKG Indicates

The cardiac axis represents the mean direction of electrical depolarization through the ventricles during the cardiac cycle, expressed as an angle in the frontal plane, and serves as a fundamental ECG parameter for detecting ventricular hypertrophy, conduction abnormalities, and structural heart disease. 1

Core Concept

The axis originates from the 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. 1 This vector represents the sum of all dipole vectors located along the instantaneous depolarization wavefront as it spreads through the ventricular myocardium. 2

Normal Values and What Deviations Indicate

In Adults

• Normal adult QRS axis ranges from +30° to +90° 1, 3
• Left axis deviation (less than -30°) indicates potential:
  • Left ventricular hypertrophy 1, 4
  • Left anterior fascicular block (especially when marked deviation of -45° to -90° is present) 1, 4
  • Normal variant with increasing age 1, 4
• Right axis deviation (exceeding +90°) suggests:
  • Right ventricular hypertrophy 1
  • Left posterior fascicular block (when marked deviation of +120° to +180°) 3

Age-Related Variations

The axis naturally shifts leftward with increasing age, making mild left axis deviation more common in older adults. 1, 3 In pediatric populations, the cardiac axis varies dramatically:

• Neonates (0-7 days): 60° to 190°, reflecting right ventricular dominance 1, 4
• Ages 1-5 years: 10° to 110° 1, 3
• Ages 5-8 years: May extend to 140° 1, 3
• Ages 8-16 years: Extends to 120° 1

Clinical Utility

The axis serves multiple diagnostic purposes:

• Detecting ventricular hypertrophy: Right ventricular hypertrophy causes displacement of the QRS vector toward the right and anteriorly, while left ventricular hypertrophy shifts it leftward 1
• Identifying conduction abnormalities: Bundle branch blocks and fascicular blocks deviate the axis significantly from normal 1
• Assessing structural heart disease: The presence of axis deviation should alert clinicians to the possibility of underlying structural heart disease 5

Critical Pitfalls to Avoid

• Never interpret axis deviation without clinical context—axis deviation alone rarely establishes a diagnosis 1
• Always consider age-related changes in QRS axis, particularly in pediatric and elderly populations 1, 3
• Do not overlook secondary repolarization abnormalities that accompany axis deviations due to conduction abnormalities 1
• Avoid misinterpreting COPD patterns as right ventricular hypertrophy—RVH is only suggested in COPD if R-wave amplitude in V1 is relatively increased 1
• Always compare with previous ECG tracings to determine if axis deviation is new or longstanding 6

Factors That Affect the Axis

Body position and geometry affect the lead vector direction and strength, depending on body geometry and varying electrical impedances of torso tissues. 1 Pathological conditions such as ventricular hypertrophy, bundle branch blocks, and fascicular blocks can deviate the axis significantly from normal. 1