carotid sinus massage

The real-world cousin of the Vulcan nerve pinch, carotid sinus massage is used diagnostically to evaluate vagus nerve function and therapeutically to control tachyarrhythmias. The carotid sinus is found at the bifurcation of the common carotid artery (just lateral to the thyroid cartilage of the larynx), and contains baroreceptors which normally sense blood pressure (together with similar receptors in the aortic arch). In normal people, massage of said sinus results in bradycardia (and hypotension)—the baroreceptor reflex, which normally acts to correct rapid changes in blood pressure (eg the drop associated with standing suddenly, in which case the reflex acts to increase the blood pressure).

It is potentially quite dangerous, and should not be attempted outside the hospital setting. Acutely, bradycardia, hypotension and heart block may occur; in the presence of atheromatous vascular disease, plaque fracture and embolism may result in stroke.

May also be performed unintentionally by males while shaving. The pressure of the razor blade over the carotid sinus under certain circumstances can trigger the same response, sometimes resulting in a faint.

I know this because it happened to me. One second I was shaving in the mirror, the next I was lying on the floor, blinking and wondering what had just happened. I was terrified I had a brain tumor, and went straight to the doctor. He calmly explained to me what I had experienced. No harm done. Apparently what concerns doctors most about the phenomenon is that people often suffer concussions when they fall.

j3z_ mentions standing suddenly, and that is exactly what this was like. Your vision just goes black. I have only felt like I was going to pass out when standing up, however. This time, I really did lose consciousness. Interesting analogy, though.