A nurse finds an unresponsive client without a pulse. What is the first action?

Cardiac arrest requires immediate action. The current AHA Basic Life Support sequence (C-A-B): (1) Check for responsiveness; (2) Activate emergency response (call code blue, ask for AED if available); (3) Begin chest compressions immediately — 100-120 per minute, 2-2.4 inches deep on adults, allowing full recoil; (4) Open airway and provide ventilations after 30 compressions (in two-rescuer CPR, alternate). The change from A-B-C (Airway-Breathing-Circulation) to C-A-B (Compressions-Airway-Breathing) reflects evidence that early chest compressions are the single most important intervention. Don't waste time checking for breathing extensively or attempting ventilation first. The pulse check should be 5-10 seconds maximum. Continue CPR until: the client recovers, the code team takes over, you are physically exhausted, or the situation becomes unsafe. After arrest, monitor for return of spontaneous circulation (ROSC) and prepare for post-arrest care.