Triangles of the Neck

A 72-year-old man with a history of hypertension and smoking presents to the outpatient clinic. During physical examination, the physician gently palpates the region marked **D** in the diagram (Carotid Triangle) to assess the carotid pulse. Immediately after palpation, the patient experiences sudden onset dizziness, bradycardia (HR 42/min), and transient loss of consciousness lasting 10 seconds. Which of the following best explains the pathophysiology of this clinical event?