If the junctional automaticity foci fail, which automaticity foci will start pacemaking at what rate?

If the junctional automaticity foci fail, which automaticity foci will start pacemaking at what rate?

• A. Atrial foci at 40-60 bpm
• B. Ventricular foci at 20-40 bpm
• C. AV foci at 30-50 bpm
• D. Purkinje fibers at 60-80 bpm

Answer: (B)

The scenario of junctional automaticity foci failure indicates that the conduction system has a fallback mechanism where another automatic pacemaker takes charge. In this situation, with the junctional foci no longer able to initiate impulses, the next tier of automatic foci becomes active.

The ventricular foci, which can initiate impulses at a rate of 20-40 beats per minute, are typically the least efficient pacemakers. They take over when the higher-level foci, like the atrial foci and junctional foci, are non-functional. While they have a slower intrinsic firing rate compared to other pacemaking sites, they are crucial in maintaining some level of cardiac activity during failures of the higher pacemaking centers.

Understanding the hierarchy of pacemakers reinforces the concept that the heart has mechanisms in place to maintain rhythm, albeit at lower rates as you progress down the hierarchy from the sinoatrial (SA) node through the junctional and finally to the ventricular foci. This concept is fundamental in EKG interpretation and understanding arrhythmias.