If the sinoatrial node fails, which conduction system component may assume pacemaker activity?

• A. Sinoatrial node
• B. Bundle of His
• C. Atrioventricular node
• D. Purkinje fibers

Answer: (C)

When the primary pacemaker stops, the heart relies on a backup automaticity source in the conduction system. The atrioventricular node commonly assumes pacemaker activity as the secondary pacemaker because it can generate impulses on its own, though at a slower intrinsic rate than the SA node—about 40–60 beats per minute. This slower rate still maintains a rhythm and keeps the ventricles moving, albeit with a longer PR interval due to its position in the conduction pathway.

The bundle of His and Purkinje fibers are primarily conduction pathways; they have little automaticity under normal conditions. Purkinje fibers can take over only if both the SA node and AV node fail, producing a ventricular escape rhythm around 20–40 bpm with wide QRS complexes—less reliable for perfusion. So, the AV node is the usual backup pacemaker when the SA node fails.