Halothane decreases both tissue and airway resistances in excised canine lungs

Studies of the anesthetic effects on the airway often use pulmonary resistance (R(L)) as an index of airway caliber. To determine the effects of the volatile anesthetic, halothane, on tissue and airway components of R(L), we measured both components in excised canine lungs before and during halothane administration. Tissue resistance (Rti), airway resistance (Raw), and dynamic lung compliance (C(L,dyn)) were determined at constant tidal volume and at ventilatory frequencies ranging from 5 to 45 min^-1 by an alveolar capsule technique. Halothane decreased R(L) at each breathing frequency by causing significant decreases in both Raw and Rti but did not change the relative contribution of Rti to R(L) at any frequency. Halothane increased C(L,dyn) at each breathing frequency, although there was little change in the static pressure-volume relationship. The administration of isoproterenol caused similar changes. We conclude that halothane changes both airway and tissue components of R(L); it may act by relaxing the contractile elements in the lung. Both components must be considered when the effects of volatile anesthetics on R(L) are interpreted.