Mechanism of Action of Inhalational Anesthesia on Airways

Open Access
Publisher Website
Google Scholar
Abstract
To separate reflex from direct actions of anesthetics on airways, the effects of halothane and isoflurane (1.5 MAC [minimum anesthetic concentration]) on Ascaris antigen-induced (a mixed reflex and direct stimulus) and methacholine-induced (a direct acting stimulus) airway constriction [asthma models] in Basenji-Greyhound dogs. Prior to aerosol challenge, pulmonary resistance (RL) and dynamic compliance (Cdyn) were not different during control (thiopental), halothane and isoflurane anesthesia. RL was 1.93 .+-. 0.15, 1.81 .+-. 0.23 and 2.1 .+-. 0.12 cm H2O .cntdot. l-1 .cntdot. s during thiopental, halothane and isoflurane, respectively. Cdyn was 116 .+-. 8, 106 .+-. 16 and 110 .+-. 9 ml/cm H2O during thiopental, halothane and isoflurane anesthesia, respectively. In control studies (thiopental), Ascaris antigen increased RL by 9.4 .+-. 2.44-fold and decreased Cdyn to 0.29 .+-. 0.02 times the prechallenge value. Both halothane and isoflurane anesthesia significantly attenuated the increase in RL provoked by Ascaris antigen challenge and halothane significantly attenuated the decrease in Cdyn. During halothane and isoflurane anesthesia, Ascaris antigen increased RL. by 3.8 .+-. 0.96- and 3.5 .+-. 0.57-fold, respectively, and decreased Cdyn to 0.48 .+-. 0.09 and 0.38 .+-. 0.07 times the prechallenge value, respectively. In control studies (thiopental anesthesia), methacholine produced dose-related increases in RL and decreases in Cdyn. Both halothane and isoflurane attenuated the increase in RL. and the decrease in Cdyn provoked by methacholine with halothane being more effective than isoflurane with regards to Cdyn. 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.