The influence of a cryogenic brain injury on the cerebrovascular response to isoflurane in the rabbit

To determine if an acute neurologic injury alters the cerebrovascular response to isoflurane, rabbits were anesthetized with morphine/N₂O, mechanically ventilated, surgically instrumented, and assigned to one of three groups. Group I animals (n = 8) served as controls and received no injury. In Groups 2 (n = 9) and 3 (n = 8), a 30-s cryogenic injury was produced in the left parietal region using liquid N₂ poured into a funnel affixed to the surface of the skull. Regional CBF was measured using microspheres. In Groups 2 and 3, flow was determined before and then 30 and 90 min after injury or at equivalent times in Group 1. After the 90-min data were collected, 1% 1≈0.5 minimal alveolar concentration (MAC)] and then 2% (≈1.0 MAC) isoflurane was administered to uninjured rabbits in Groups 1 and to lesioned rabbits in Group 3. A mean arterial pressure of ≥80 mm Hg was maintained during isoflurane administration by an infusion of angiotensin II. In uninjured rabbits (Group 1), 2% isoflurane produced bilaterally symmetrical increases in hemispheric CBF, from 76 ± 21 (mean ± SD) to 150 ± 48 ml 100 g^-1 min^-1. CBF in the hindbrain increased from 91 ± 25 to 248 ± 102 ml 100 g^-1 min^-1. By contrast, in the lesioned rabbits of Group 3, 2% isoflurane resulted in CBF in the lesioned hemisphere changing from 56 to only 77 ml 100 g^-1 min^-1 (NS), while in the contralateral hemisphere, CBF rose from 68 to 97 ml 100 g^-1 min^-1 (also NS). These results indicate that a cryogenic injury attenuates the normal CBF response to a volatile anesthetic, both in the damaged hemisphere as well as in apparently uninjured regions distant from the injury focus. In a separate group of animals, a similar cryogenic injury abolished the CBF response to changing P_aco₂ in the injured hemisphere, but not in either the contralateral hemisphere or the cerebellum. It is possible that the CBF effects of isoflurane may be mediated via some intermediary neurogenic and/or biochemical process.