How is cardiac arrest defined?

Cardiac arrest is defined as the condition in which the heart has stopped beating completely. This cessation of cardiac activity results in the inability of the heart to pump blood effectively, depriving vital organs, including the brain, of oxygen and nutrients. This is a critical medical emergency requiring immediate intervention, typically through cardiopulmonary resuscitation (CPR) and defibrillation if indicated.

In contrast, the other options describe various states of heart function that do not equate to cardiac arrest. A slow heartbeat might indicate bradycardia but does not necessarily mean that the heart has stopped. An irregular heartbeat could mean conditions such as arrhythmia, where the heart may still be beating but not effectively. Similarly, a racing heart is a state of tachycardia, which indicates a fast heartbeat but, again, does not imply that the heart is not functioning. Thus, the definition provided by this choice is crucial for identifying and responding to a cardiac emergency effectively.