Which statement about the AV node as a backup pacemaker is true?

• A. It cannot function as a backup pacemaker.
• B. It functions as a backup pacemaker only during exercise.
• C. It has a maximum rate similar to the SA node and cannot serve as backup.
• D. It can function as a backup pacemaker if the sinus node fails.

Answer: (D)

The AV node can act as a backup pacemaker when the sinus node fails, which is why this statement is true. The heart has subsidiary pacemaker sites, and the AV node can take over automaticity to sustain a rhythm when the primary pacemaker (the SA node) isn’t driving the heart. Its intrinsic rate is slower, about 40–60 beats per minute, and it paces the ventricles via the His-Purkinje system. This slower but reliable escape rhythm helps prevent asystole when sinus node failure or conduction block occurs.

That’s why other ideas don’t fit: the AV node certainly can function as a backup pacemaker; it does not require exercise to take over, so its backup role isn’t limited to during activity; and its maximum pacing rate is not similar to the SA node—it's slower—so it can serve as a backup even though it won’t pace as fast as the SA node.