Final answer:
The first action a CET should take when a patient complains of dizziness during a stress test is to obtain vital signs to assess their physiological state.
Step-by-step explanation:
During a stress test, if a patient complains of dizziness, the first action that a Certified Exercise Technologist (CET) should take is to obtain vital signs. This is the immediate step to assess the patient's current physiological state, including heart rate, blood pressure, respiratory rate, and oxygen saturation. This information is crucial for determining the severity of the patient’s condition and for guiding further actions, such as notifying the physician or providing interventions like administering oxygen if medically necessary.