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Efficacy of Anticholinergic and $bT-Adrenergic Agonist Treatment of Maximal Cholinergic Bronchospasm in Tracheally Intubated Rabbits

Departments of Anesthesiology, Medicine (Pulmonary and Critical Care), and Physiology and Biophysics, University of Washington School of Medicine, Seattle, Washington, and the Department of Anesthesiology, Chang Gung Memorial Hospital and Chang Gung Medical School, Linkou Medical Center, Linkou, Taiwan, Republic of China

Abstract

Cholinergically induced bronchoconstriction is thought to be a major cause of bronchospasm during anesthesia. We used tracheally intubated rabbits (4-mm endotracheal tube) stimulated with methacholine to assess the efficacy of β-adrenergic agonist and anticholinergic treatment in reversing the increases in respiratory system resistance. Four groups were compared: (a) inhaled metaproterenol, 20 puffs via metered dose inhaler (0.65 mg/puff); (b) inhaled ipratropium bromide, 20 puffs from a metered dose inhaler (18 µg/puff); (c) 2 mg of intravenous atropine; and (d) no treatment after methacholine challenge as a control group. Methacholine increased respiratory system resistance from 0.041 ± 0.001 (mean ± SEM) to 0.098 ± 0.006 cm H₂O mL^-1 s^-1 (P < 0.001). Whereas β-adrenergic agonist treatment was ineffective in ameliorating bronchoconstriction, inhaled ipratropium bromide and atropine were highly effective, causing an 86%-88% reversal in the methacholine-induced increase in respiratory system resistance. Both these agents were also effective in improving dynamic compliance. We conclude that inhaled ipratropium bromide is effective in treating cholinergic bronchospasm even when administered via a small endotracheal tube and that the β-adrenergic agonist metaproterenol is ineffective in rabbits in the face of maximal cholinergic stimulation.