What indicates a third-degree AV block?

A third-degree AV block, also known as complete heart block, is characterized by a complete dissociation between atrial and ventricular activity. This means that the electrical impulses from the atria (represented by P waves) do not effectively conduct to the ventricles (represented by QRS complexes). In this scenario, the P waves will appear at a regular rate and will be independent of the QRS complexes, indicating that the atria and ventricles are functioning independently of each other.

The presence of P waves that do not correlate with QRS complexes is a hallmark of this block. The atrial rhythm is maintained, but the signal does not reach the ventricles, leading to their own autonomous rhythm, typically slower than the atrial rate. This clear dissociation is what makes option A the correct choice for identifying a third-degree AV block.

The other choices, while reflecting aspects of cardiac activity, do not encapsulate the definitive feature of a third-degree AV block as closely as option A. Therefore, they would not accurately describe this condition.