Final answer:
In the event of a witnessed cardiac arrest, the emergency response system should be activated before initiating CPR.
Step-by-step explanation:
The response to a cardiac arrest witnessed by a responder is critical for the patient's survival. The emergency response system should be activated before initiating CPR (cardiopulmonary resuscitation). Automated external defibrillators (AED) found in many public places are designed to be used by laypersons and can diagnose the patient's heart rhythm automatically before delivering an appropriate shock. Performing CPR is advised in many situations before using an AED to maintain blood flow to vital organs until defibrillation can occur.