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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 heart axis.
Especially a change of the heart axis or an extreme deviation can be an indication of pathology.
*Positive (the average of the QRS surface above the baseline) QRS deflection in lead I: the electrical activity is directed to the left (of the patient)
*Positive QRS deflection in lead AVF: the electrical activity is directed down.
This indicates a normal heart axis. Usually, these two leads are enough to diagnose a normal heart axis!
The largest vector in the heart is from the AV-node in the direction of
the ventricular depolarization. Under normal circumstances, this is directed left and down.(towards leads I and AVF). The position of the QRS vector is given in degrees. See the figure , the middle of the figure is the AV-node. A horizontal line towards the left arm is defined as 0 degrees.
A normal heart axis is between -30 and +90 degrees.
The interpretation of the electrical
heartaxis has a few rules of thumb:
* First, when a positive depolarization wave moves towards a positive electrode, a positive, upwards deflection is registered on the ECG.