Hypocapnic Bronchoconstriction and Inhalation Anesthetics

The effects of halothane, enflurane, and methoxyflurane on hypocapnic bronchoconstriction (increased airway resistance and decreased compliance of the lung) were studied in vivo in the isolated left lower lobe of the canine lung. Hypocapnic bronchoconstriction, induced by altering the concentration of CO2 in gas ventilating the lobe, was repeated in the presence and absence of various concentrations of anesthetic gases (halothane: 0.5, 1.0, and 3.0 per cent; enflurane: 1.0, 3.0, and 5.0 per cent; methoxyflurane: 0.25, 0.50, and 1.0 per cent). In the higher concentrations, all three drugs blocked the bronchoconstrictor effect produced when the inspired CO2 was decreased from 5 to 0 per cent. In lower concentrations, halothane was the most effective blocking drug. Propranolol did not affect the ability of the three anesthetics to block hypocapnic bronchoconstriction, nor did the beta-receptor blocking drug sotalol affect the blocking effects of halothane. The ability of these anesthetics to block hypocapnic bronchoconstriction probably is mediated not through an adrenergic mechanism but by one that is nonspecific. (Key words: Lung, bronchoconstriction; Carbon dioxide, hypocarbia; Anesthetics, volatile, halothane; Anesthetics, volatile, enflurane; Anesthetics, volatile, methoxyflurane.)