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 a hexaxial reference system: a circular diagram where the six limb leads point in different frontal directions. The mean direction of ventricular depolarization—the QRS axis—is determined by projecting that overall vector onto this circle. The center of that reference circle represents the heart’s electrical “center,” which is anatomically near the AV node on the interatrial septum. Placing the origin there gives the limb-lead directions their proper angular relationship to the actual ventricular activation. The SA node sits in the high right atrium and isn’t the reference for this axis diagram, and the right atrium or interventricular septum aren’t used as the circle’s center. So the circle is centered near the AV node to reflect the central point of ventricular activation.

Frontal plane axis assessment relies on a hexaxial reference system: a circular diagram where the six limb leads point in different frontal directions. The mean direction of ventricular depolarization—the QRS axis—is determined by projecting that overall vector onto this circle. The center of that reference circle represents the heart’s electrical “center,” which is anatomically near the AV node on the interatrial septum. Placing the origin there gives the limb-lead directions their proper angular relationship to the actual ventricular activation.

The SA node sits in the high right atrium and isn’t the reference for this axis diagram, and the right atrium or interventricular septum aren’t used as the circle’s center. So the circle is centered near the AV node to reflect the central point of ventricular activation.