MORPHINE-ISOFLURANE INTERACTION IN DOGS, SWINE AND RHESUS-MONKEYS

In monkeys, dogs and swine (six each) we tested the reduction of the i soflurane MAC (minimal alveolar concentration) produced by 2 mg.kg-1 m orphine intravenously (i.v.) and the concurrent effect on Pco2 with sp ontaneous ventilation. MAC fell to a minimum of 55% of control at 53 m in in monkeys, 50% at 38 min in dogs and 13% at 33 min in swine. PaCO2 rose at constant MAC with morphine to 55-60 mmHg, but did not fall ov er the next several hours despite the decline of plasma morphine conce ntration, and the resulting needed rise in isoflurane concentration to keep the anaesthesia depth at 1 MAC. After isoflurane concentration h ad returned to pre-morphine control levels, naloxone immediately reduc ed Paco2 to or below control level. Morphine pharmacokinetics in the t hree species studied conformed to a two-compartment model.