Comparison of low concentrations of halothane and isoflurane as bronchodilators

Although high concentrations of all currently used inhalational anesthetics are thought to be good bronchodilators, studies using traditional measures of airway tone fail to show differences in airway responsiveness during halothane, enflurane, and isoflurane use. Using a more sensitive technique, the authors compared the ability of halothane and isoflurane to dilate histamine-constricted airways at equivalent MAC concentrations. Responses of histamine-constricted individual airways to increasing doses of halothane and isoflurane were directly measured using high-resolution computed tomography (HRCT). Fifteen studies were performed in five dogs. All dogs were initially anesthetized with thiopental 15 mg/kg followed by a 10-mg.kg-1 x h-1 maintenance dose. Following tracheal intubation, the lungs were mechanically ventilated (15 ml/kg, 15 bpm). The airways were constricted with intravenous histamine 200 micrograms/min. On alternate days, the dogs subsequently received increasing concentrations of either halothane or isoflurane (0.6, 1.1, and 1.7 MAC). On a separate day, the dogs received atropine 0.2 mg/kg after the histamine infusion and the study was repeated. Histamine decreased airway area 34 +/- 2.5% (mean +/- SEM). All preconstricted airways showed a significant dose-dependent dilation to halothane and isoflurane at concentrations of 0.6, 1.1, and 1.7 MAC. Halothane significantly dilated airways to a greater extent than isoflurane at 0.6 and 1.1 MAC (P < 0.001). This effect was most pronounced in airways less than 3 mm in diameter. At 1.7 MAC, there was no significant difference between the two agents (P = 0.42). Atropine (0.2 mg/kg) reversed the airway constriction elicited by intravenous histamine. The histamine-preconstricted airways area increased 370 +/- 34% (P < 0.0001) after atropine. Halothane and isoflurane dilate histamine-constricted airways in a dose-dependent manner. However, at low concentrations, halothane was a more effective bronchodilator than isoflurane at equivalent MAC doses.