What should you do if you notice a casualty is not breathing?

Performing CPR immediately when a casualty is not breathing is crucial because it helps to maintain blood flow to vital organs, particularly the brain and heart. This action can help prevent irreversible damage due to oxygen deprivation and can significantly increase the chances of survival until emergency services arrive.

Time is of the essence in such situations, as brain cells can begin to die within minutes without oxygen. Initiating CPR can effectively sustain a minimal level of circulation, buying valuable time for more advanced medical interventions to take place.

In contrast, the other options do not prioritize immediate action that could save the casualty's life. Leaving the individual to seek help does not provide any immediate assistance and could lead to deterioration of their condition. Checking for a pulse is important, but if breathing has ceased, the priority should be to start CPR without delay. Waiting for emergency services to arrive without taking action would also greatly reduce the casualty's chances of recovery. Thus, starting CPR is the most effective and life-saving response when a person is not breathing.