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 simultaneously with the velocity of blood flow through the left middle cerebral artery (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 ranged from 42.8 to 94.2 cm/s. The relationship of CBF to MCA blood-flow velocity over the imposed range of arterial Pco(2) was described analytically 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 velocity 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 were significantly smaller than corresponding percent changes in CBF at increased levels of arterial Pco(2) and larger than CBF changes at the lower arterial Pco(2). These observed relative changes are consistent with MCA vasodilation at the site of measurement during exposure to progressive hypercapnia and also during extreme hyperventilation hypocapnia.