Final answer:
The priority nursing action is to administer naloxone to reverse the respiratory depression caused by hydromorphone.
Step-by-step explanation:
The priority nursing action in this situation is to administer naloxone 0.4 mg IV. Naloxone is an opioid antagonist that can reverse the respiratory depression caused by the hydromorphone, which is likely the cause of the client's difficulty in arousing and low respiration rate.
Elevating the head of the bed (option A) can help improve oxygenation and perfusion, but it will not address the client's respiratory depression. Assessing breath sounds (option C) and checking vital signs and pulse oximetry (option D) are important, but they can be done after administering naloxone to address the immediate concern of respiratory depression and difficulty in arousing.