Final answer:
The first intervention a nurse should perform when a client expresses difficulty breathing but is not in visible distress is to assess vital signs.
Step-by-step explanation:
When a hospital client has illuminated the call light and expresses they cannot catch their breath but appears to be in no visible distress, the first step a nurse should take is to assess vital signs. This includes checking the patient's heart rate, respiratory rate, oxygen saturation, temperature, and blood pressure. This assessment can help determine the appropriate intervention, whether it might be administering oxygen, elevating the legs, or, if the situation becomes critical, performing CPR. Since the client is not currently in visible distress, it is not appropriate to begin with chest compressions or more invasive measures without assessing the situation thoroughly. If abnormal signs are observed, such as elevated heart rate or low oxygen saturation, the nurse may then provide supplementary oxygen or other necessary actions based on the assessment.