Isoflurane Increases Brain Oxygen Reactivity in Dogs

We tested the possibility that large-dose isoflurane will produce a loss of brain tissue oxygen regulation in dogs. A total of 12 dogs were anesthetized with isoflurane, a craniotomy was performed, and a probe was inserted to measure brain tissue oxygen pressure (PtO(2)), carbon dioxide, and pH. Baseline measures were made during 1.5% end-tidal isoflurane with 30% oxygen ventilation, followed by 95% oxygen ventilation. Six dogs (Group 1) were treated with 3% isoflurane and 30% oxygen, followed by a second oxygen challenge with 95% O(2). Six dogs (Group 2) received propofol to produce a similar suppression of the electroencephalogram as in Group 1, followed by 95% oxygen ventilation. Brain tissue oxygen reactivity was calculated by the increase in PtO(2) divided by the increase in arterial PO(2). During 1.5% isoflurane and propofol anesthesia, PtO(2) increased from 42 to 62 mm Hg with oxygen ventilation, and brain tissue oxygen reactivity was 0.14% per mm Hg(-1). Brain tissue oxygen reactivity did not change during propofol anesthesia. With 3% isoflurane, PtO(2) increased from 52 to 113 mm Hg and brain tissue oxygen reactivity was 0.36% per mm Hg(-1) (P: < 0.05). These results suggest that the cerebrovasodilator and vasoplegic effects of large-dose isoflurane attenuate brain oxygen regulation.