RELATIONSHIP OF XE-133 CEREBRAL BLOOD-FLOW TO MIDDLE CEREBRAL ARTERIAL FLOW VELOCITY IN MEN AT REST

Cerebral blood flow (CBF) was measured by Xe-133 clearance simultaneou sly with the velocity of blood flow through the left middle cerebral a rtery (MCA) over a wide range of arterial Pco(2) in eight normal men. Average arterial Pco(2), which was varied by giving 4% and 6% CO2 in O -2 and by controlled hyperventilation on O-2, ranged from 25.3 to 49.9 mm Hg. Corresponding average values of global CBF15 were 27.2 and 65. 0 ml 100 g min(-1), respectively, whereas MCA blood-flow velocity rang ed from 42.8 to 94.2 cm/s. The relationship of CBF to MCA blood-flow v elocity over the imposed range of arterial Pco(2) was described analyt ically by a parabola with the equation: CBF = 22.8 - 0.17 x velocity 0.006 x velocity(2) The observed data indicate that MCA blood-flow ve locity is a useful index of CBF response to change in arterial Pco(2) during O-2 breathing at rest. With respect to baseline values measured while breathing 100% O-2 spontaneously, percent changes in velocity w ere significantly smaller than corresponding percent changes in CBF at increased levels of arterial Pco(2) and larger than CBF changes at th e lower arterial Pco(2). These observed relative changes are consisten t with MCA vasodilation at the site of measurement during exposure to progressive hypercapnia and also during extreme hyperventilation hypoc apnia.