The QRS axis is the overall direction of ventricular depolarisation. It is also known as the Cardiac Axis.

You can think of the axis like drawing a single arrow on the front of the chest. If the heart is normal, the ventricles are often depolarised from top to bottom and right to left. We could make it a lot more complex by describing how depolarisation is composed of many individual vectors as it moves down the bundle branches, across the septum and through the right and left ventricular walls… but the axis neatly simplifies this down to just one overall direction. The axis is calculated from the limb leads, most often using leads I, II and aVF. There are several different methods (see below).

There are many causes of axis deviation. In general, the axis will rotate towards hypertrophy or conduction blocks and away from infarction.

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This quiz draws from a pool of 344 FOAM ECGs, each reused according to the terms of their original Creative Commons licences.