Left axis deviation (LAD) is where the mean electrical axis as shown on the ECG lies between -30° and -90° in the coronal plane. This is shown in limb leads:
- Lead I shows a complex, weak signal
- Leads II, III and aVF become markedly negative
This is often seen in cases of left ventricular hypertrophy although the reason for this is not certain, but is more likely to be due to conduction problems rather (otherwise hypertrophy should produce an increase of the normal ECG- not a deviation). This is supported by the fact that left axis deviation is also found in patients with left anterior fascicular block and inferior MIs.