Effects of volatile anaesthetic agents on enhanced airway tone in sensitized guinea pigs

Background. Although volatile anaesthetics afford protection against bronchospasm, their potential to reverse a sustained constriction of hyperreactive airways has not been characterized. Accordingly, we investigated the ability of halothane, isoflurane, sevoflurane and desflurane to reverse lung constriction induced by prolonged stimulation of the muscarinic receptors in guinea pigs sensitized to ovalbumin. Methods. Pulmonary input impedance (ZL) was measured using forced oscillations in five groups of ovalbumin-sensitized, mechanically ventilated guinea pigs. ZL was measured under baseline conditions, during steady-state bronchoconstriction induced by an i.v. infusion of methacholine (MCh), and after administration of one of the volatile agents at 1 MAC after the induction of a steady-state bronchoconstriction. Airway resistance (Raw), and parenchymal tissue resistive and elastic coefficients were extracted from ZL by model fitting. Results. All four volatile agents exhibited an initial relaxation of the MCh-induced airway constriction followed by gradual increases in Raw. The bronchodilatory effect of isoflurane was the most potent (-28.9 (SE 5.5)% at 2 min, P<0.05) and lasted longest (7 min); sevoflurane and halothane had shorter and more moderate effects (-21.1 (3.9)%, P<0.05, and -6.1 (1.7)%, P<0.05, respectively, at 1 min). Desflurane caused highly variable changes in Raw, with a tendency to enhance airway tone. Conclusions. Volatile agents can reverse sustained MCh-induced airway constriction only transiently in sensitized guinea pigs. Isoflurane proved most beneficial in temporally improving lung function in the presence of a severe constriction of allergic inflamed airways. Desflurane displayed potential to induce further airway constriction.