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NEUROSURGICAL ANESTHESIA

The Effect of Hypocapnia on the Autoregulation of Cerebral Blood Flow During Administration of Isoflurane

Timothy J. McCulloch, MBBS, FANZCA^*, Tillman W. Boesel, MBBS, FANZCA^*, and Arthur M. Lam, MD, FRCPC

*Department of Anaesthetics, Royal Prince Alfred Hospital and University of Sydney, Sydney, Australia; and Department of Anesthesiology, University of Washington School of Medicine, Seattle, Washington

Address correspondence and reprint requests to Dr. Timothy J. McCulloch, Department of Anaesthetics, Royal Prince Alfred Hospital, Missenden Rd., Camperdown, NSW 2050, Australia. Address e-mail to tmccull{at}usyd.edu.au.

Isoflurane impairs autoregulation of cerebral blood flow in a dose-related manner. Previous investigations in several other conditions have demonstrated that impaired autoregulation can be restored by hyperventilation. We hypothesized that hypocapnia may restore cerebral autoregulation impaired by isoflurane anesthesia. We administered isoflurane in 100% oxygen to 12 healthy patients aged 21–59 yr scheduled for elective nonneurological surgery. Isoflurane end-tidal concentration was individualized at 0.1% to 0.2% less than that required to induce short periods of isoelectric electroencephalogram. This resulted in an end-tidal isoflurane concentration of 1.6% ± 0.2% (mean ± sd) corresponding to an age-adjusted minimum alveolar anesthetic concentration multiple of 1.4. Mean arterial blood pressure was reduced to <80 mm Hg, by infusion of remifentanil if required. Cerebral autoregulation was assessed by infusing phenylephrine to increase mean arterial blood pressure to 100 mm Hg while monitoring middle cerebral artery blood flow velocity with transcranial Doppler ultrasonography. The change in flow velocity was used to calculate the autoregulation index (ARI). The ARI ranges between 0 and 1 and an ARI 0.4 indicates significantly impaired autoregulation. Autoregulation was tested twice in randomized order: once during normocapnia (Paco₂ 38–43 mm Hg) and once during hypocapnia (Paco₂ 27–34 mm Hg). The median (interquartile range) ARI was 0.29 (0.23–0.64) during normocapnia and 0.77 (0.70–0.78) during hypocapnia (P < 0.005). Of the 12 subjects, autoregulation was significantly impaired in 8 subjects during normocapnia and none during hypocapnia (P = 0.001). Hypocapnia restored cerebral autoregulation in normal subjects during isoflurane-induced impairment of autoregulation.

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