Axis Deviation in ECG: Significance of Right and Left Axis Deviation
Axis deviation on an electrocardiogram (ECG) is a critical diagnostic finding that can indicate underlying cardiac pathology, with right axis deviation (RAD) and left axis deviation (LAD) each carrying distinct clinical significance for patient morbidity and mortality.
Understanding Cardiac Axis
The cardiac axis refers to the overall direction of electrical depolarization through the ventricles, represented as a vector in the frontal plane of the ECG.
- Normal axis: +30° to +90° in adults 1
- Left axis deviation (LAD): Mean frontal plane QRS axis of -30° or more leftward 1
- Right axis deviation (RAD): Mean frontal plane QRS axis of +91° to +180° 1
- Extreme right axis deviation: +181° to -91° 1
Practical Axis Determination
A systematic approach to determining axis 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°) |
Significance of Right Axis Deviation (RAD)
RAD is a significant finding that may indicate:
Pulmonary Hypertension: RAD is seen in 79% of patients with idiopathic pulmonary arterial hypertension (IPAH) 2. It represents right ventricular pressure overload.
Right Ventricular Hypertrophy: Often accompanies RAD in conditions causing chronic right ventricular pressure overload 2.
Pulmonary Embolism: Though not always present, RAD can be seen in acute pulmonary embolism, reflecting right heart strain 3.
Congenital Heart Disease: Particularly those with right ventricular volume or pressure overload 1.
Normal Variant: In some cases, particularly in children and young adults, RAD may be a normal finding 4.
Diagnostic Value in Pulmonary Hypertension
While RAD is common in pulmonary hypertension, its sensitivity as a screening tool is limited:
- Sensitivity of RAD for pulmonary hypertension: 73% 2
- Specificity: 70% 2
- 8 of 61 patients with severe pulmonary arterial hypertension had normal ECG findings despite significant disease 2
Significance of Left Axis Deviation (LAD)
LAD carries important clinical implications:
Left Anterior Fascicular Block: Most common cause of LAD, representing conduction system disease 1.
Structural Heart Disease: LAD should alert clinicians to possible underlying structural heart disease 1.
Coronary Artery Disease: Significantly higher incidence of coronary artery disease in patients with LAD, especially when combined with right bundle branch block 5.
Congenital Heart Defects: Particularly ostium primum atrial septal defects 6.
Pediatric Significance: In children, isolated LAD without other ECG abnormalities may not require further investigation, but LAD with a QRS axis ≤-42°, chamber enlargement, or abnormal cardiac exam findings warrants echocardiography 7.
Comparative Clinical Implications
| Feature | Right Axis Deviation | Left Axis Deviation |
|---|---|---|
| Primary associations | Pulmonary hypertension, RV hypertrophy | Left anterior fascicular block, structural heart disease |
| Sensitivity for disease | 73% for pulmonary hypertension [2] | High for coronary artery disease when with RBBB [5] |
| Prognostic value | Associated with increased mortality in PAH [2] | Associated with higher risk of cardiac disease [5] |
| Normal variant | Can be normal in children and athletes [1] | Rarely a normal variant in adults |
Common Pitfalls and Caveats
Normal ECG Does Not Exclude Disease: A normal ECG does not exclude significant pulmonary hypertension or other cardiac pathology 2.
Age-Specific Norms: Normal axis ranges vary significantly by age, particularly in children 1.
Isolated Finding vs. Pattern: Axis deviation should be interpreted in the context of other ECG findings and clinical presentation 1.
Pulmonary Embolism Presentation: In acute pulmonary embolism, LAD can be seen almost as frequently as RAD (7% vs. 7%) 3, contrary to traditional teaching.
Prognostic Implications: In pulmonary hypertension, certain ECG features like P-wave amplitude ≥0.25 mV in lead II carry prognostic significance, associated with a 2.8-fold greater risk of death over 6 years 2.
Management Approach
Management should focus on the underlying cause rather than the axis deviation itself:
For RAD: Evaluate for pulmonary hypertension, congenital heart disease, or pulmonary embolism with appropriate imaging (echocardiography, CT pulmonary angiography) 1.
For LAD: Consider echocardiography to evaluate for structural heart disease, particularly when accompanied by other ECG abnormalities or abnormal cardiac exam findings 1, 7.
For both: Interpret in clinical context, considering patient age, symptoms, and other clinical factors 1.