Both positive is normal; leads pointing away from each other indicate deviation
Mnemonic
“Leaving means Left, Reaching means Right”, using leads I and aVF:
| Lead I | Lead aVF | Axis |
|---|---|---|
| Positive | Positive | Normal |
| Positive | Negative | Left axis deviation |
| Negative | Positive | Right axis deviation |
| Negative | Negative | Extreme (northwest) |
Or the thumbs method: if the QRS in I and aVF both point up, the axis is normal; if they point away from each other, the axis is left; if they point towards each other, it is right.
Left axis deviation causes: left anterior fascicular block, inferior infarction, left ventricular hypertrophy, Wolff-Parkinson-White.
Right axis deviation causes: right ventricular hypertrophy, pulmonary embolism, chronic lung disease, left posterior fascicular block, lateral infarction, and it is normal in tall thin people and children.
Expansion
Look at the net QRS deflection in lead I and aVF:
| Lead I | aVF | Axis |
|---|---|---|
| Positive | Positive | Normal (-30 to +90 degrees) |
| Positive | Negative | Left axis deviation |
| Negative | Positive | Right axis deviation |
| Negative | Negative | Extreme axis (northwest) |
Causes of left axis deviation: left anterior fascicular block, inferior myocardial infarction, left ventricular hypertrophy, Wolff-Parkinson-White syndrome.
Causes of right axis deviation: right ventricular hypertrophy, pulmonary embolism, chronic lung disease, left posterior fascicular block, lateral infarction, and normally in tall thin people and children.
The axis can also be estimated by finding the isoelectric lead; the axis lies perpendicular to it.