Effects of thiopental on airway calibre in dogs: direct visualization method using a superfine fibreoptic...

Induction of anaesthesia with thiopental sometimes causes bronchospasm. Although the mechanism by which thiopental induces bronchospasm may involve cholinergic stimulation, direct spastic effect and histamine release, the spastic effects of thiopental have not been comprehensively defined. In this study, we have assessed the effect of thiopental on in vivo airway smooth muscle tone using direct visualization method with a superfine fibreoptic bronchoscope as previously reported. Twenty-one mongrel dogs were anaesthetized with pentobarbital (30 mg kg-1) and paralysed with pancuronium (200 micrograms kg-1 h-1). The trachea was intubated with a tube that had a second lumen for insertion of the bronchoscope (od: 2.2 mm) to continuously measure bronchial cross-sectional area. The tip of the bronchoscope was placed between the second and third bronchial bifurcation of the right lung. The dogs were allocated to three groups of seven: group T, A+T, H+T. In group T, thiopental 0 (saline), 0.1, 1.0 and 10 mg kg-1 was given i.v. In group A+T, saline i.v., 5 min later atropine 0.1 mg kg-1 i.v., and 5 min later thiopental 10 mg kg-1 was administered. In group H+T, bronchoconstriction was produced with histamine 10 micrograms kg-1 i.v. followed by infusion at 500 micrograms kg-1 h-1. Thirty minutes later, thiopental 0, 1.0 and 10 mg kg-1 were given. Arterial blood sampling was performed for measurement of plasma catecholamines and histamine. In group T, thiopental significantly reduced bronchial cross-sectional area (maximally by 28.7 (5.6% at 0.5 min after thiopental 10 mg kg-1), which returned to the baseline in 3 min, while any changes in plasma concentrations of catecholamines and histamine were not observed except norepinephrine level at 1 min following thiopental 10 mg kg-1 i.v. Atropine pretreatment completely prevented thiopental-induced bronchospasm in group A+T. In group H+T, thiopental 10 mg kg-1 transiently but significantly decreases bronchial cross-sectional area. Therefore, the present study indicates that the mechanism of thiopental bronchospasm may result from cholinergic nerve stimulation.