To determine the QRS axis, which two leads are examined, and what indicates a normal axis when both are positive?

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Multiple Choice

To determine the QRS axis, which two leads are examined, and what indicates a normal axis when both are positive?

The main idea is to determine the overall direction of ventricular depolarization in the frontal plane by looking at the polarity of the QRS complexes in two key leads: Lead I and Lead aVF. If the QRS is positive in both leads, the depolarization vector has components toward the left (positive in Lead I) and toward the feet (positive in Lead aVF). This places the resultant axis in the quadrant between 0° and +90°, which corresponds to the normal axis range (roughly -30° to +90°). So examining Lead I and Lead aVF and finding both positive indicates a normal QRS axis. If one lead were negative, that would suggest left or right axis deviation depending on which lead is negative, and if both were negative, the axis would be indeterminate or extreme.

The main idea is to determine the overall direction of ventricular depolarization in the frontal plane by looking at the polarity of the QRS complexes in two key leads: Lead I and Lead aVF. If the QRS is positive in both leads, the depolarization vector has components toward the left (positive in Lead I) and toward the feet (positive in Lead aVF). This places the resultant axis in the quadrant between 0° and +90°, which corresponds to the normal axis range (roughly -30° to +90°). So examining Lead I and Lead aVF and finding both positive indicates a normal QRS axis. If one lead were negative, that would suggest left or right axis deviation depending on which lead is negative, and if both were negative, the axis would be indeterminate or extreme.

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