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 meaning being tested is how the heart’s depolarization direction is described. The mean QRS axis represents the overall direction of ventricular depolarization, i.e., the combined ventricular vector. Because the left ventricle has much more muscle mass, its depolarization vector dominates and points toward the left and slightly downward toward the apex. Therefore the net axis on the ECG is directed leftward, which is captured by the mean QRS axis. The P wave axis is about atrial depolarization, not ventricles, and while some sources use “apex axis” informally, the standard term for this overall ventricular direction is the mean QRS axis.