For which of the following cardiac rhythms is a defibrillator indicated?

The use of a defibrillator is indicated primarily for pulseless ventricular tachycardia. This condition involves a fast, ineffective heart rhythm that does not allow the heart to pump blood, leading to inadequate circulation and potential loss of consciousness or death. A defibrillator delivers a controlled electrical shock to the heart in an attempt to restore a normal rhythm. By doing so, it helps to "reset" the heart's electrical activity and can re-establish effective cardiac function.

In contrast, the other conditions listed do not typically require defibrillation. Atrial tachycardia, for instance, usually involves a rapid heartbeat but does not necessarily mean there is no pulse or blood flow. Second-degree block and sinus bradycardia represent slower heart rhythms that often have a pulse and may require other interventions, such as medications or pacing, rather than defibrillation. Understanding these differences is crucial for effective emergency response in cardiac care.