Anesthesiology, November 2000, Vol. 93, No. 5, pp. 1205-1209

Reviewer: John F. Heath, M.D.

Summary: Eight healthy subjects were tested for the effect of hypercapnia on cerebral autoregulation during anesthetic maintenance with propofol or sevoflurane.  During normocapnia, neither sevoflurane (up to 1.5 MAC) nor propofol impair autoregulation.  However, hypercapnia itself is known to impair autoregulation.  The patients each had a lower limb ortho procedure during the anesthetic.  Autoregulation was measured by assessing middle cerebral artery flow velocities (Vmca) (by transcranial doppler) at different MAP's (by arterial line) then calculating the autoregulation index (ARI).  An ARI = 1.0 means no change in Vmca despite the increase in MAP (80-100mm Hg).  An ARI of 0 means autoregulation has been abolished and cerebral blood flow is pressure-passive and totally dependent on MAP.  An ARI of 0.4 or less means autoregulation has been impaired.  The average PaC02 which impaired autoregulation was 56 during sevoflurane anesthesia and 61 during propofol anesthesia.  Also there was greater carbon dioxide reactivity (30 % greater) at the higher MAP.  This is distinctly different from normocapnia which is associated with no significant change in Vmca when the MAP is raised from 80 to 100 mm Hg.  Therefore, even mild hypercapnia can impair cerebral autoregulation during general anesthesia with sevoflurane or propofol (sevo = greater impairment than propofol).  Carbon dioxide reactivity is greater at higher MAP's.