The combined ventricular vector, which flows down to the left due to the left ventricle's larger muscle mass, is known as the _____ axis.

The combined ventricular vector, which flows down to the left due to the left ventricle's larger muscle mass, is known as the _____ axis.

• A. Mean QRS axis
• B. P wave axis
• C. QRS axis
• D. Apex axis

Answer: (A)

The mean QRS axis describes the net direction of ventricular depolarization in the frontal plane. Because the left ventricle has much greater muscle mass, its vector dominates, pulling the overall depolarization direction down and to the left. This makes the combined ventricular vector point toward the left side of the body, which is captured by the mean QRS axis. The P wave axis relates to atrial depolarization and does not reflect ventricular depolarization. The term apex axis isn’t the standard label for this global ventricular direction, so the appropriate description for this vector is the mean QRS axis.