Frontal plane axis with Lead I positive and Lead aVF negative.

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Interpreting the frontal QRS axis hinges on where the mean depolarization vector points in the frontal plane, assessed by the polarity of the QRS complexes in limb leads, especially Lead I (0 degrees) and Lead aVF (+90 degrees). If the QRS is positive in Lead I, the axis lies toward the left side; if it’s negative in Lead aVF, the axis is not directed downward. The combination of a positive Lead I and a negative Lead aVF indicates the depolarization vector is directed leftward and away from the inferior electrode, placing the mean axis in the left half of the frontal plane—left axis deviation (roughly -30 to -90 degrees). The other patterns don’t fit this vector direction: a normal axis would have a positive QRS in aVF as well, a right axis deviation would show Lead I negative (and aVF positive), and extreme axis deviation would produce negative QRS in both leads.