Upon hearing a ventilator's high-pressure alarm, the nurse should first:

Manually ventilating the client with a resuscitation bag is the most immediate and appropriate response upon hearing a high-pressure alarm from a ventilator. This alarm typically indicates a potential issue with airflow, which could signify that the patient is experiencing breathing difficulties, such as an obstructed airway, decreased lung compliance, or something else that is critically affecting their ability to breathe effectively.

By manually ventilating the patient, the nurse ensures that the patient continues to receive adequate oxygenation and ventilation until the underlying cause of the alarm can be determined and addressed. This is crucial, as any delay in supporting the patient's breathing can lead to serious complications, including respiratory failure.

Taking action on the ventilator settings or assessing vital signs are also important steps, but they should come after stabilizing the patient’s immediate needs. Calling for assistance might be necessary eventually, but the nurse’s first priority is to ensure the patient is adequately ventilated before seeking help.