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An impaired CBF autoregulation can be restored by hyperventilation at a PaCO 2  level of about 2.9 to 4.1 kPa (22 to 31 mm Hg). However, it is uncertain whether the restoring effect can take place at lesser degrees of hypocapnia. In the current study, CBF autoregulation was studied at four PaCO 2  levels: 5.33 kPa (40 mm Hg, normoventilation), 4.67 kPa (35 mm Hg, slight hyperventilation), 4.00 kPa (30 mm Hg, moderate hyperventilation), and 3.33 kPa (25 mm Hg, profound hyperventilation). At each PaCO 2  level, eight rats 2 days after experimental subarachnoid hemorrhage (SAH) and eight shamoperated controls were studied. The CBF was measured by the intracarotid  133 Xe method. The CBF autoregulation was found to be intact in all controls but completely disturbed in the normoventilated SAH rats. However, by slight hyperventilation, CBF autoregulation was restored in seven of eight SAH rats with a decline in CBF of 10%. The CBF autoregulation was found intact in all of the moderately or profoundly hyperventilated SAH rats, whereas the decline in CBF was 21% and 28%, respectively. In conclusion, hyperventilation to a PaCO 2  level between 4.00 and 4.67 kPa (30 to 35 mm Hg) appears to be sufficient for reestablishing an impaired autoregulation after SAH.

Effects of Graded Hyperventilation on Cerebral Blood Flow Autoregulation in Experimental Subarachnoid Hemorrhage