Which ECG rhythm indicates a complete absence of electrical activity in the heart?

Asystole is characterized by a complete absence of electrical activity in the heart, which means there are no discernible waves or complexes on the ECG tracing. In asystole, the heart is essentially "flatlined," indicating that there is no coordinated electrical activity to stimulate heart contractions. This is a critical condition that requires immediate intervention, as it signifies that the heart is not pumping blood, resulting in the cessation of effective circulation.

In contrast, other rhythms such as atrial fibrillation and ventricular tachycardia still exhibit electrical activity, although it may be disorganized or excessively rapid, respectively. Atrial fibrillation involves irregular electrical signals in the atria, while ventricular tachycardia entails rapid firing from the ventricles, which can still produce some cardiac output. Sinus bradycardia, although slow, is a relatively normal rhythm that represents a functioning heart with heartbeats, just at a reduced rate. Therefore, the absence of any electrical activity distinctly identifies asystole as the correct answer to this question.