A 55-year-old man has been diagnosed with Stokes- Adams syndrome. Two minutes after the onset of syncopal attack, which of the following is the pacemaker of the hea?
**Core Concept**
Stokes-Adams syndrome is a condition characterized by episodes of syncope due to complete heart block, where the atria and ventricles beat independently. This results in a failure of the atria to stimulate the ventricles, leading to a pause in ventricular contraction.
**Why the Correct Answer is Right**
During a complete heart block, the sinoatrial (SA) node, the normal pacemaker of the heart, is unable to stimulate the ventricles. As a result, the ventricles rely on an ectopic focus, specifically the atrioventricular (AV) junction, to generate a new rhythm. The AV junction, including the AV node and the bundle of His, has a slower intrinsic rate than the SA node but is capable of taking over as the pacemaker in the absence of SA node activity.
**Why Each Wrong Option is Incorrect**
**Option A:** The SA node is the normal pacemaker of the heart, but in the context of Stokes-Adams syndrome, it is unable to stimulate the ventricles.
**Option B:** The interatrial septum is not a pacemaker structure in the heart.
**Option C:** The His-Purkinje system is a conduction system that facilitates the transmission of electrical impulses from the AV node to the ventricles, but it is not a pacemaker in itself.
**Clinical Pearl / High-Yield Fact**
In Stokes-Adams syndrome, the pause in ventricular contraction can be so long that it may lead to a decrease in cardiac output, resulting in syncope or even cardiac arrest. The AV junction's ability to take over as the pacemaker is a crucial compensatory mechanism in this condition.
**Correct Answer:** C. The AV junction.