Final answer:
The priority nursing action is to elevate the head of the bed.
Step-by-step explanation:
The priority nursing action to take in a patient with increased intracranial pressure (ICP) of 18 mm Hg after a head injury is to elevate the head of the bed. Elevating the head of the bed helps to promote venous drainage and reduce cerebral blood volume, which can help decrease ICP. The other options, such as administering mannitol, administering sedatives, or initiating hyperventilation, may be used in specific situations but are not the priority nursing action in this case.