If the SA node fails, which part typically assumes pacing, acting as a secondary pacemaker?

If the SA node fails, which part typically assumes pacing, acting as a secondary pacemaker?

• A. AV node
• B. SA node
• C. Purkinje fibers
• D. Bundle of His

Answer: (A)

The main idea is that the heart has backup pacing built into its conduction system. The SA node is the fastest natural pacemaker, but when it fails, a secondary pacemaker takes over to keep the rhythm going. The AV node has intrinsic automaticity and can generate impulses at about 40–60 beats per minute, slower than the SA node but enough to maintain a heartbeat. These impulses can propagate to the ventricles, producing a regular rhythm that is slower than normal. If the AV node also fails, the Purkinje fibers can take over, but they pace the heart even more slowly (around 20–40 bpm) and often result in a much slower, less reliable rhythm. The Bundle of His is simply part of the conduction pathway rather than an automatic pacemaker.

The main idea is that the heart has backup pacing built into its conduction system. The SA node is the fastest natural pacemaker, but when it fails, a secondary pacemaker takes over to keep the rhythm going. The AV node has intrinsic automaticity and can generate impulses at about 40–60 beats per minute, slower than the SA node but enough to maintain a heartbeat. These impulses can propagate to the ventricles, producing a regular rhythm that is slower than normal. If the AV node also fails, the Purkinje fibers can take over, but they pace the heart even more slowly (around 20–40 bpm) and often result in a much slower, less reliable rhythm. The Bundle of His is simply part of the conduction pathway rather than an automatic pacemaker.