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A defining characteristic of third-degree heart block, also known as complete heart block, is that P waves are present without accompanying QRS complexes. This occurs because the electrical signals from the atria (which generate the P waves) are not being conducted through the AV node to the ventricles, leading to a dissociation between the atrial and ventricular rhythms. In third-degree heart block, the atria can contract independently of the ventricles due to this lack of communication.

The presence of P waves without QRS complexes indicates that while the atria are still generating electrical impulses, these impulses are not leading to ventricular contraction. This dissociation can result in a slower ventricular rate, often necessitating interventions such as pacemaker placement if symptoms arise.

In contrast, the other options represent conditions that do not characterize third-degree heart block. In particular, the normal function of the AV node would indicate a different type of heart block, while rapid heart rates are typically more associated with other cardiac arrhythmias rather than the slower rhythms seen in complete heart block.