Global cerebral blood flow decreases during pain

Positron emission tomography studies have identified a common set of brain regions activated by pain. No studies, however, have quantitatively examined pain-induced CBF changes. To better characterize CBF during pain, 14 subjects received positron emission tomography scans during rest, during capsaicin-evoked pain (250 mu g, intradermal injection), and during innocuous vibration. Using the (H2O)-O-15 intravenous bolus method with arterial blood sampling, global CBF changes were assessed quantitatively. Painful stimulation produced a 22.8% decrease in global CBF from resting levels (P < 0.0005). This decrease was not accounted for by arterial PCO2, or heart rate changes. Although the exact mechanism remains to be determined, this pain-induced global decrease represents a previously unidentified response of CBF.