Graded hypercapnia and cerebral autoregulation during sevoflurane or propofol anesthesia

Background Hypercapnia abolishes cerebral autoregulation, but little is kno wn about the interaction between hypercapnia and autoregulation during gene ral anesthesia. With normocapnia, sevoflurane Cup to 1.5 minimum alveolar c oncentration) and propofol do not impair cerebral autoregulation, This stud y aimed to document the level of hypercapnia required to impair cerebral au toregulation during propofol or sevoflurane anesthesia. Methods: Eight healthy subjects received a remifentanil infusion and were a nesthetized with propofol (140 mug . kg(-1) . min(-1)) and sevoflurane (1.0 -1.1% end tidal) in a randomized crossover study. Ventilation was adjusted to achieve incremental increases in arterial carbon dioxide partial pressur e (Paco(2) until autoregulation was impaired. Cerebral autoregulation was t ested by increasing the mean arterial pressure (MAP) from 80 to 100 mmHg wi th phenylephrine while measuring middle cerebral artery flow velocity by tr anscranial Doppler. The autoregulation index, which has a value ranging fro m 0 to 1, representing absent to perfect autoregulation, was calculated, an d an autoregulation index of 0.4 or less represented significantly impaired autoregulation. Results: The threshold Pace, to significantly impair cerebral autoregulatio n ranged from 50 to 66 mmHg. The threshold averaged 56 +/- 4 mmHg (mean +/- SD) during sevoflurane anesthesia and 61 +/- 4 mmHg during propofol anesth esia(P = 0.03), Carbon dioxide reactivity measured at a MAP of 100 mmHg was 30% greater than that at a MAP of 80 mmHg, Conclusions: Even mild hypercapnia can significantly impair cerebral autore gulation during general anesthesia. There is a significant difference betwe en propofol anesthesia and sevoflurane anesthesia with respect to the effec t of hypercapnia on cerebral autoregulation. This difference occurs at clin ically relevant levels of Pace,. When inducing hypercapnia, carbon dioxide reactivity is significantly affected by the MAP.