Frontal plane axis assessment uses a circle centered at which anatomical point as a reference?

Frontal plane axis assessment uses a circle centered at which anatomical point as a reference?

• A. AV node
• B. SA node
• C. Right atrium
• D. Interventricular septum

Answer: (A)

Frontal plane axis assessment relies on the hexaxial reference system, a circular diagram that represents the direction of the mean ventricular depolarization in the frontal plane. The circle is centered at the AV node, which provides a stable, convenient origin from which the six limb-lead directions are laid out around the heart. By projecting the average QRS vector onto these lead directions, you gauge where the axis points: a vector toward a given lead makes that lead more positive, while one directed away makes it negative. This center choice (the AV node) gives a consistent framework for estimating the axis across patients. The atrial pacemaker location (SA node) or the right atrium aren’t used as the reference center for this ventricular depolarization axis, and the interventricular septum, while part of conduction, isn’t the central reference point for the hexaxial circle.