An extreme axis deviation means that the overall direction of ventricular depolarisation is so abnormal that we can’t work out whether to call it left or right deviation! This direction can be between -90 degrees (straight upwards) to 180 degrees (to the right). If the axis is extreme, Lead I and lead aVF will both be negative. Extreme axis deviation is usually due to a ventricular rhythm, but can also be due to hyperkalemia or very severe right ventricular hypertrophy.
See also: QRS axis
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This ECG is from an elderly woman who presented with chest pain on a background of a normal angiogram 3 months prior.
This ECG shows ventricular paced rhythm (likely biventricular) with concordant ST depression in III, aVF and V3. There is excessive discordant ST elevation in I and aVL. The cause was an acute left main occlusion due to suspected embolism.