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When you average all electrical signals from the heart, you can indicate the direction of the average electrical depolarization with an arrow (vector). This is the heartaxis. Especially a change of the heartaxis or an extreme deviation can be an indication for pathology.
Largest QRS deflection in lead I: the electrical activity is directed to the left (of the patient)* Largest QRS deflection in lead AVF: the electrical activity is directed down.
This indicates a normal heartaxis. Usually, these two leads are enough to diagnose a normal heartaxis!