ECG deflections, frontal cardiac axis and chest leads
THE CONTEXT
ECG polarity depends on the direction of depolarisation relative to an electrode; the frontal cardiac axis describes the average ventricular depolarisation vector.
Direction of depolarisation
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Depolarisation moving towards a positive electrode produces a positive ECG deflection; movement in the opposite direction produces a negative deflection.
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When the depolarisation vector is at right angles to a lead, the deflection is equally negative and positive, described as isoelectric.
Frontal cardiac axis
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The frontal cardiac axis is the average vector of ventricular depolarisation.
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The normal cardiac axis lies between −30° and +90°.
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In the illustrated example, an isoelectric QRS complex in aVL, a negative QRS complex in aVR and the strongest positive QRS complex in lead II indicate an axis of 60°.
Chest leads
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The six chest leads, V1–V6, are derived from electrodes on the anterior and lateral left chest over the heart.
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Each chest lead records between its positive chest electrode and the negative central terminal.
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V1 and V2 lie approximately over the right ventricle; V3 and V4 lie approximately over the interventricular septum.