If you determine that a victim is not breathing, your next step should be to:

When you determine that a victim is not breathing, immediately starting CPR is the appropriate action because it is critical to provide oxygenated blood to the brain and other vital organs as quickly as possible. The absence of breathing indicates a serious life-threatening situation, and initiating CPR helps prevent brain damage and increases the victim's chances of survival.

In the context of emergency response, the first step after recognizing that a victim is not breathing is to start with high-quality chest compressions. This is essential in maintaining circulation until advanced medical help arrives. While checking for a heartbeat, contacting EMS, or placing the victim in the recovery position are important actions in other circumstances, these steps do not directly address the immediate need for circulation in a non-breathing victim. Therefore, starting CPR is the most critical next step in this scenario to provide the necessary support until professional medical assistance can take over.