If the SA Node fails, what is the normal firing rate of the AV Node?

The AV Node serves as a backup pacemaker when the SA Node, which is the primary pacemaker of the heart, fails to initiate impulses. Under normal physiological conditions, the AV Node can generate impulses at a rate of 40 to 60 beats per minute. This intrinsic firing rate is critical because it ensures that the heart continues to function even when the SA Node is not firing effectively.

This rate allows for adequate heart function and allows for some degree of blood flow, even in the event of SA Node dysfunction. It is important to note that while the heart ideally beats faster when driven by the SA Node (typically 60-100 bpm), this backup mechanism provided by the AV Node is crucial for sustaining life in situations where higher pacemaker function is impaired.

The other options do not accurately reflect the normal firing rate of the AV Node. For instance, rates of 30-50 bpm indicate a much slower pace, which is typically associated with other pacemaker sites (like the bundle of His or Purkinje fibers). A rate of 60-80 bpm suggests a faster intrinsic rate than the AV Node can achieve, while 70-100 bpm exceeds the normal range for this particular pacemaker site.