If all pacemakers fail, what is the last rate the heart can manage?

If all pacemakers fail, what is the last rate the heart can manage?

• A. 40-60 bpm
• B. 60-80 bpm
• C. 20-40 bpm
• D. 10-20 bpm

Answer: (C)

When evaluating the heart's intrinsic pacemaking ability, it's essential to understand the hierarchy of pacemakers. The heart primarily relies on the sinoatrial (SA) node for normal rhythm, but if it fails, the atrioventricular (AV) node can take over as a secondary pacemaker. If both of these fail, the bundle branches and Purkinje fibers can generate electrical impulses as a last resort.

The intrinsic rate of the SA node averages between 60 to 100 beats per minute (bpm), while the AV node usually paces the heart at about 40 to 60 bpm. However, if both of these pacemakers become nonfunctional, the lowest intrinsic rate of heart rhythm comes from the ventricular pacemakers (the bundle branches and Purkinje fibers), which can autonomously generate impulses at a much slower rate of 20 to 40 bpm. This is why the last viable heart rate that can be managed when all higher pacemakers fail is typically in the range of 20-40 bpm. Therefore, this option accurately reflects the heart's survival mechanism in critical scenarios of complete pacemaker failure.