Cerebral blood flow and blood volume in response to O2 and CO2 changes in normal humans.

Changes in cerebral blood volume (CBV) after head injury may be an important determinant of intracranial pressure (ICP). To determine the normal response of CBV to hypoxemia, hypercapnia, and hypocapnia, eight normal subjects (5 males and 3 females; ages 25 to 43) were studied under these conditions. Cerebral blood volume was measured using an external collimated gamma detector to determine 99m-Tc-labeled red blood cell (RBC) activity in the intracranial vascular pool, and cerebral blood flow (CBF) was determined by internal carotid artery duplex scanning. Hypocapnia (Paco2 = 26.0 +/- 1.7 mm Hg, mean +/- SE) was achieved by hyperventilation, hypercapnia (Paco2 = 47.8 +/- 1.5 mm Hg) was achieved by inhalation of 6% CO2, and hypoxemia (Pao2 = 38.1 +/- 1.1 mm Hg, O2 saturation = 76.7 +/- 2.0%) was achieved by inhalation of 10% O2. Changes in CBF and CBV were determined by comparing the values in each condition to the immediately preceding period of normoxia and normocapnia. For conditions of hypocapnia, hypercapnia, and hypoxemia, the percentage of change in CBV was: -7.2 +/- 0.01, 12.8 +/- 0.01, and 5.2 +/- 0.03, respectively. The simultaneous percentage of change in CBF for the same conditions was -30.7 +/- 4.0, 29.5 +/- 9.2, and 18.4 +/- 6.9, respectively. For all conditions, changes in CBF were greater than changes in CBV; however, this was most pronounced during hypocapnia induced by hyperventilation. Because the change in CBV reflects the potential change in ICP in response to treatment, therapeutic hyperventilation may impair CBF to a greater degree than it reduces ICP.(ABSTRACT TRUNCATED AT 250 WORDS)