When might a burst of parasympathetic activity lead to a ventricular escape beat?

When might a burst of parasympathetic activity lead to a ventricular escape beat?

• A. When the atrial foci are overactive.
• B. When the ventricular foci are inhibited by electrical signals.
• C. When there is inhibition of the SA node and junctional foci, allowing ventricular foci to fire.
• D. When the cardiac rhythm is too fast.

Answer: (C)

A burst of parasympathetic activity can lead to a ventricular escape beat when there is inhibition of both the SA node and junctional foci, which allows the ventricular foci to take over and fire. The parasympathetic nervous system primarily exerts a slowing effect on the heart rate, often reducing the firing rate of the SA node, which is responsible for initiating normal heartbeats. When this inhibition is strong enough, it can cause the SA node to stop firing completely, and if the junctional foci (the secondary pacemakers) are also inhibited or fail to pace the heart adequately, there is a gap in the heart's electrical signals. During this gap, the ventricular foci can respond by generating an escape beat to maintain some activity in the heart, thereby preventing a complete cardiac standstill.

This mechanism highlights the heart's intrinsic ability to maintain rhythm through various pacemaker sites under altered physiological conditions, like heightened vagal tone, which can occur during rest or in certain pathological states. Understanding this process is crucial for interpreting EKGs, particularly in the presence of bradycardia or pauses in normal rhythm.