CEREBRAL BLOOD-FLOW AND BLOOD-VOLUME IN RESPONSE TO O-2 AND CO2 CHANGES IN NORMAL HUMANS

Changes in cerebral blood volume (CBV) after head injury may be an imp ortant determinant of intracranial pressure (ICP). To determine the no rmal response of CBV to hypoxemia, hypercapnia, and hypocapnia, eight normal subjects (5 males and 3 females; ages 25 to 43) were studied un der these conditions. Cerebral blood volume was measured using an exte rnal collimated gamma detector to determine 99m-Tc-labeled red blood c ell (RBC) activity in the intracranial vascular pool, and cerebral blo od flow (CBF) was determined by internal carotid artery duplex scannin g. Hypocapnia (PaCO2 = 26.0 +/- 1.7 mm Hg, mean +/- SE) was achieved b y hyperventilation, hypercapnia (PaCO2 = 47.8 +/- 1.5 mm Hg) was achie ved by inhalation of 6% CO2, and hypoxemia (PaO2 = 38.1 +/- 1.1 mm Hg, O-2 saturation = 76.7 +/- 2.0%) was achieved by inhalation of 10% O-2 . Changes in CBF and CBV were determined by comparing the values in ea ch condition to the immediately precedig period of normoxia and normoc apnia. For conditions of hypocapnia, hypercapnia, and hypoxemia, the p ercentage 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 f or 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 i nduced by hyperventilation. Because the change in CBV reflects the pot ential change in ICP in response to treatment, therapeutic hyperventil ation may impair CBF to a greater degree than it reduces ICP. From the se data, it can be concluded that (1) CBV measurements are feasible us ing 99m-Tc-labeled RBCs, (2) CBV changes may not be proportional to CB F changes, and (3) measurement of CBV may be useful in assessing the e ffects of procedures in the early management of head-injured patients.