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REGIONAL ANESTHESIA

Isoflurane Dosage for Equivalent Intraoperative Electroencephalographic Suppression in Patients With and Without Epidural Blockade

Andrew P. Morley, FRCA^*, James Derrick, FANZCA^*, Paul T. Seed, MSc CStat, Perpetua E. Tan, MPhil^*, David C. Chung, MD FRCA, FRCPC^*, and Timothy G. Short, MD FANZCA

*Department of Anaesthesia and Intensive Care, Faculty of Medicine, Chinese University of Hong Kong, Prince of Wales Hospital, Sha Tin, New Territories, Hong Kong Special Administrative Region; Maternal and Fetal Research Unit, Department of Obstetrics and Gynaecology, Guy’s, King’s and St. Thomas’ School of Medicine, King’s College, London, United Kingdom; and Department of Anaesthesia, Auckland Hospital, Grafton, Auckland, New Zealand

Address correspondence and reprint requests to Andrew Morley, Department of Anaesthesia, St. Thomas’ Hospital, Lambeth Palace Rd., London SE1 7EH, United Kingdom. Address e-mail to ukmorty{at}aol.com

We conducted a prospective, randomized, controlled trial to establish the effect of epidural blockade on isoflurane requirements for equivalent intraoperative electroencephalographic (EEG) suppression. Fifty patients undergoing abdominal hysterectomy received combined epidural and general anesthesia or general anesthesia alone with isoflurane and alfentanil. Isoflurane was administered by computer-controlled closed-loop feedback to maintain an EEG 95% spectral edge frequency of 17.5 Hz, a target chosen on the basis of a pilot study. In epidural patients, end-tidal isoflurane concentration (FE'_ISO) was 0.19% smaller (95% confidence interval [CI], -0.32% to -0.06%; P < 0.01), mean arterial blood pressure was 17 mm Hg lower (95% CI, -24 to -9 mm Hg; P < 0.0001), and body temperature was 0.4°C lower (95% CI, -0.7 to 0°C; P < 0.05) than in controls. EEG bispectral index (BIS) was 4 points higher (95% CI, 1 to 7; P < 0.05). EEG median frequency and heart rate were similar in both groups. Epidural patients were 76% more likely (95% CI, 58% to 94%; P < 0.001) to require metaraminol for hypotension and were 28% more likely (95% CI, 3% to 53%; P < 0.05) to require glycopyrrolate for bradycardia. After surgery, the time to eye opening in epidural patients was 2.3 min shorter (95% CI, -4.2 to -0.5 min; P < 0.05). Time to eye opening correlated better with FE'_ISO in the last 30 s of anesthesia (FE'_ISO = 0.07 x time to eye opening + 0.31; r² = 0.59; P < 0.0001) than with BIS from the same period (BIS = 64 - 1.25 x time to eye opening; r² = 0.22; P < 0.001) (P < 0.0001). To maintain similar intraoperative spectral edge frequency, patients receiving combined epidural and general anesthesia require 21% less isoflurane than those receiving general anesthesia alone. This smaller isoflurane dose is associated with faster emergence from anesthesia.

IMPLICATIONS: The dose of general anesthetic required to maintain similar intraoperative suppression of brain electrical activity is 21% less in patients with nerve blockade than in those without. This dose reduction results in faster waking times in patients with nerve blockade, which may reflect lighter intraoperative anesthesia. The dose of general anesthetic required to maintain similar intraoperative suppression of brain electrical activity is 21% less in patients with nerve blockade than in those without. This dose reduction results in faster waking times in patients with nerve blockade, which may reflect lighter intraoperative anesthesia.

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