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ANESTHETIC PHARMACOLOGY

Anesthetic Properties of Carbon Dioxide in the Rat

Robert J. Brosnan, DVM, PhD^*, Edmond I. Eger, II, MD, Michael J. Laster, DVM, and James M. Sonner, MD

From the *Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis; and Department of Anesthesia and Perioperative Care, University of California, San Francisco, California.

Address correspondence and reprint requests to Robert J. Brosnan, DVM, PhD, Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616. Address e-mail to rjbrosnan{at}ucdavis.edu.

BACKGROUND: Carbon dioxide decreases halothane minimum alveolar concentrations (MAC) in dogs when Paco₂ exceeds 95 mm Hg. We sought to confirm these findings for several potent inhaled anesthetics in rats.

METHODS: Groups of eight rats were anesthetized with halothane, isoflurane, or desflurane. MAC was determined for each anesthetic alone, and then with increasing concentrations of inspired CO₂. A fourth group was given CO₂ alone to determine the MAC of CO₂.

RESULTS: Increasing inspired CO₂ concentrations produced a linear dose-dependent decrease in MAC of each potent inhaled anesthetic. With elimination of CO₂, the MAC of isoflurane and desflurane returned to the original MAC. As determined by extrapolating these data to 0% of the inhaled anesthetic, the MAC of CO₂ was approximately 50% of 1 atm. Given alone, CO₂ proved lethal.

CONCLUSIONS: Unlike dogs, no threshold for the CO₂-MAC response arose with halothane, isoflurane, or desflurane in rats. The ED₅₀ for CO₂ is also approximately 50% greater in rats than reported in dogs.

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