Anesthetic properties of carbon dioxide in the rat.
BACKGROUND
Carbon dioxide decreases halothane minimum alveolar concentrations (MAC) in dogs when Paco(2) 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 CO(2). A fourth group was given CO(2) alone to determine the MAC of CO(2).


RESULTS
Increasing inspired CO(2) concentrations produced a linear dose-dependent decrease in MAC of each potent inhaled anesthetic. With elimination of CO(2), 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 CO(2) was approximately 50% of 1 atm. Given alone, CO(2) proved lethal.


CONCLUSIONS
Unlike dogs, no threshold for the CO(2)-MAC response arose with halothane, isoflurane, or desflurane in rats. The ED(50) for CO(2) is also approximately 50% greater in rats than reported in dogs.
Brosnan
Robert J
RJ
Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA. rjbrosnan@ucdavis.edu
Eger
Edmond I
EI
Laster
Michael J
MJ
Sonner
James M
JM
D018685:Anesthetics, Inhalation / Q000008:administration & dosage*; D000818:Animals; D002245:Carbon Dioxide / Q000008:administration & dosage*; D008297:Male; D011650:Pulmonary Alveoli / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013045:Species Specificity
D018685:Anesthetics, Inhalation; D002245:Carbon Dioxide