The Importance of Axis in ECG Interpretation
The cardiac axis on ECG represents the overall direction of electrical depolarization through the ventricles and is crucial for identifying underlying cardiac pathologies that affect morbidity and mortality, including pulmonary hypertension, structural heart disease, and conduction abnormalities. 1
Understanding Cardiac Axis
The standard 12-lead ECG records potential differences between prescribed sites on the body surface that vary during the cardiac cycle, reflecting differences in transmembrane voltages in myocardial cells during depolarization and repolarization. The ECG can be conceptualized as originating from a stationary, time-dependent single-dipole source represented by a vector (the heart vector). 2
Normal Axis Ranges by Age
- Adults: +30° to +90°
- Children 8-16 years: up to +120°
- Children 5-8 years: up to +140°
- Children 1-5 years: +10° to +110°
- Neonates: +30° to +190° 1
Clinical Significance of Axis Deviation
Right Axis Deviation (RAD)
- Present in 79% of patients with idiopathic pulmonary arterial hypertension
- Sensitivity of 73% and specificity of 70% for pulmonary hypertension
- Associated with increased mortality in pulmonary arterial hypertension
- Often accompanies right ventricular hypertrophy in conditions causing chronic right ventricular pressure overload 1
Left Axis Deviation (LAD)
- Defined as mean frontal plane QRS axis of -30° or more leftward in adults
- Often associated with left anterior fascicular block (axis between -45° and -90°)
- Should alert clinicians to the possibility of underlying structural heart disease 1, 3
- Associated with a higher incidence of coronary artery disease 4
Systematic Approach to Axis Determination
The American College of Cardiology recommends this approach using leads I and aVF:
| Lead I | Lead aVF | Axis Interpretation |
|---|---|---|
| Positive | Positive | Normal axis (0° to +90°) |
| Positive | Negative | Left axis deviation (-1° to -90°) |
| Negative | Positive | Right axis deviation (+91° to +180°) |
| Negative | Negative | Extreme right axis deviation (+181° to -91°) [1] |
Diagnostic and Prognostic Implications
When to Consider Further Evaluation
Patients with axis deviation may warrant additional workup (such as echocardiography) when:
- QRS axis is significantly deviated (≤-42° for LAD)
- ECG shows chamber enlargement or hypertrophy
- Abnormal cardiac physical examination findings are present 5
- RAD is present, particularly when evaluating for pulmonary hypertension 1
Limitations
- A normal ECG (including normal axis) does not exclude significant pulmonary hypertension or other cardiac pathology
- ECG has limited sensitivity (73%) for detecting pulmonary hypertension 1
Clinical Applications
Screening for Cardiac Disease: Axis deviation can serve as an early indicator of underlying cardiac pathology, prompting further investigation 6
Monitoring Disease Progression: Serial ECGs can track changes in axis that may reflect worsening of conditions like pulmonary hypertension or ventricular hypertrophy 1
Evaluating Treatment Response: Changes in axis may reflect improvement or deterioration in response to treatment of underlying conditions 1
Risk Stratification: Certain axis deviations (particularly RAD in pulmonary hypertension) are associated with increased mortality 1
Common Pitfalls in Axis Interpretation
- Ignoring age-specific normal ranges: What's abnormal in adults may be normal in children
- Overlooking secondary repolarization abnormalities: Changes in the sequence of depolarization can alter the repolarization sequence 2
- Failing to correlate with clinical findings: Axis deviation should be interpreted in the context of the patient's clinical presentation
- Dismissing isolated LAD: Even when found in isolation, significant axis deviation warrants consideration of underlying pathology 3
By understanding and correctly interpreting cardiac axis on ECG, clinicians can identify potentially life-threatening conditions early, leading to appropriate interventions that can significantly impact patient outcomes.