Mechanism of action of inhalational anesthesia on airways

Abstract

To separate reflex from direct actions of anesthetics on airways, the authors studied the effects of halothane and isoflurane (1.5 MAC) on Ascaris antigen-induced (a mixed reflex and direct stimulus) and methacholine-induced (a direct acting stimulus) airway constriction in Basenji-Greyhound dogs. Prior to aerosol challenge, pulmonary resistance [R(L)] and dynamic compliance [C(dyn)] were not different during control (thiopental), halothane, and isoflurane anesthesia. R(L) was 1.93 ± 0.15 (mean ± SE), 1.81 ± 0.23 and 2.1 ± 0.12 cm H2O.l-1.s during thiopental, halothane, and isoflurane, respectively. C(dyn) was 116 ± 8, 106 ± 16 and 110 ± 9 ml/cmH2O during thiopental, halothane, and isoflurane anesthesia, respectively. In control studies (thiopental), Ascaris antigen increased R(L) by 9.4 ± 2.44 fold and decreased C(dyn) to 0.29 ± .02 times the prechallenge value. Both halothane and isoflurane anesthesia significantly attenuated the increase in R(L) provoked by Ascaris antigen challenge and halothane significantly attenuated the decrease in C(dyn). During halothane and isoflurane anesthesia, Ascaris antigen increased R(L) by 3.8 ± 0.96 and 3.5 ± 0.57 fold, respectively, and decreased C(dyn) to 0.48 ± 0.09 and 0.38 ± .07 times the prechallenge value, respectively. In control studies (thiopental anesthesia), methacholine produced dose-related increases in R(L) and decreases in C(dyn). Both halothane and isoflurane attenuated the increase in R(L) and the decrease in C(dyn) provoked by methacholine with halothane being more effective than isoflurane with regards to C(dyn). The mechanism of action of halothane and isoflurane on airways is similar and complex, involving depression of airway reflexes as well as direct effects on airway smooth muscle.