Effects of graded hyperventilation on cerebral blood flow autoregulation in experimental subarachnoid hemorrhage

An impaired CBF autoregulation can be restored by hyperventilation at a Pac o(2) level of about 2.9 to 4.1 kPa (22 to 31 mm Hg). However, ie is uncerta in whether the restoring effect can take place at lesser degrees of hypocap nia. In the current study, CBF autoregulation was studied at four Paco(2) l evels: 5.33 kPa (40 mm Hg, normoventilation), 4.67 kPa (35 mm Hg, slight hy perventilation), 4.90 kPa (30 mm Hg, moderate hyperventilation), and 3.33 k Pa (25 mm Hg, profound hyperventilation). At each Paco(2) level, eight rats 2 days after experimental subarachnoid hemorrhage (SAH) and eight sham-ope rated controls were studied. The CBF was measured by the intracarotid Xe-13 3 method. The CBF auturegulation was found to be intact in all controls but completely disturbed in the normoventilated SAH rats. However, by slight h yperventilation, CBF autoregulation was restored in seven of eight SAH rats with a decline in CBF of 10%. The CBF autoregulation was found intact in a ll of the moderately or profoundly hyperventilated SAH rats, whereas the de cline 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.