ECG Quest > Basics > Axis


The cardiac axis is just one way of summarising the overall direction of ventricular depolarisation.

You might have heard of other types of axis like the X and Y axis (in maths) or the Earth’s axis (in geography). In electrocardiography, the axis is just another imaginary line.

We can calculate the direction of an axis by using several limb leads from the 12-lead ECG (I, II, III, aVR, aVL, aVF). We can also calculate different axes for the atria or the ventricles of the heart. There are a range of directions that can be normal for each axis.

There are a lot of myths and misconceptions about the cardiac axis. Sometimes people think that the axis is the middle of the heart, or that it always lines up with the ventricular septum, but that’s not true. The axis can be in a completely different direction to the septum or anatomical middle of the heart. For example, in Ventricular Tachycardia (VT) the main cardiac axis can be rotated 180 degrees from normal, but that doesn’t mean that the heart has flipped upside down! It just means that depolarisation (electrical activation) is flowing from the apex of the heart back up towards the right shoulder because the arrhythmia has started in the ventricles and works its way back up through the heart from there. Remember, the axis is about electricity, not anatomy!

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