What should you do upon reaching an adult who has collapsed and see that a bystander has started CPR?

When arriving at the scene of an adult who has collapsed and observing that a bystander has already initiated CPR, the most appropriate action is to deliver the first shock as soon as possible if an Automated External Defibrillator (AED) is available. This decision is based on the critical importance of early defibrillation in cases of cardiac arrest, particularly if the patient is in a shockable rhythm, such as ventricular fibrillation or pulseless ventricular tachycardia.

Using the AED promptly allows for the treatment of potentially reversible cardiac dysrhythmias, improving the likelihood of survival and favorable neurological outcomes. This intervention should occur as soon as the AED is attached and analyzes the heart rhythm. If a shock is advised, delivering it quickly can significantly enhance the chance of restoring a viable cardiac rhythm.

While monitoring the patient's pulse, waiting before using the AED, and administering oxygen are relevant actions in different contexts, in the case of a witnessed cardiac arrest with CPR already in progress, immediate defibrillation is the priority. Quickly addressing the heart's electrical activity with the AED takes precedence over other interventions to maximize the patient's chances of survival.