What is the hallmark feature of a third-degree AV block?

The hallmark feature of a third-degree AV block is complete dissociation between atrial and ventricular activity. In this condition, the electrical impulses generated by the atria do not conduct through the atrioventricular (AV) node to the ventricles. As a result, the atria and ventricles beat independently of each other, resulting in a complete dissociation.

This dissociation can typically be observed on an electrocardiogram (ECG) where the P waves (representing atrial depolarization) occur at a different rate than the QRS complexes (representing ventricular depolarization). This independence is what distinctly characterizes third-degree AV block and often leads to significant bradycardia as the ventricles may rely on an intrinsic escape rhythm, which is usually slower than the atrial rate.

Understanding this feature is crucial for identifying third-degree AV block on an ECG, which is linked to potential clinical implications necessitating evaluation and often intervention. The other choices do not accurately represent this specific pathology or present interpretations more characteristic of other arrhythmias.