If a victim has a pulse but is not breathing, which first aid measure should be taken first?

When a victim has a pulse but is not breathing, the first action to take is to open the victim's airway. This step is crucial because it allows you to access the airway and ensure that it is clear of any obstructions that could prevent breathing. If the airway is blocked, the victim will not be able to breathe, even though a pulse is present.

By positioning the head correctly—in most cases, using the head-tilt-chin-lift maneuver—you help facilitate airflow into the lungs. If the airway is open and the victim still does not breathe, rescue breaths may be administered. This step prioritizes the immediate need to provide oxygen and supports the vital function of the victim's pulse.

Other options involve actions that may not be required or relevant when a pulse is present. For instance, performing chest compressions is indicated only when there is no pulse, and calling emergency medical services, while always important, does not directly address the immediate need for airway management. Therefore, opening the airway is the most critical and appropriate first step when faced with a victim who has a pulse but is not breathing.