DIFFERENTIAL RESPONSES OF GLOBAL AIRWAY, TERMINAL AIRWAY, AND TISSUE IMPEDANCES TO HISTAMINE

The forced oscillation and alveolar capsule techniques were applied to determine the input impedance of the lungs and the airway transfer im pedances between 0.2 and 20 Hz in six open-chest dogs in the control s tate, during intravenous infusion of histamine at seven rates between 0.25 and 16 mu g . kg(-1). min(-1), and after the infusion. In each co ndition, the input impedances seen from the alveolar capsules, i.e., t erminal airway impedance (Zaw,ter), were measured by imposing 2- to 20 0-Hz oscillations from the capsules (B. L. K. Davey and J. H. T. Bates . Respir. Physiol. 91: 165-182, 1993). Airway resistance (Raw) and ine rtance and tissue damping and elastance were derived from the lung imp edance data. For all dogs, histamine progressively increased Raw and t he real part of airway transfer impedance (airway transfer resistance) , reaching, at 16 mu g . kg(-1) min(-1), 241 +/- 109 (SD) and 370 +/- 186%, respectively, of the control values, but caused greater, althoug h locally highly variable, increases (769 +/- 716% of the control valu e) in the real part of Zaw,ter extrapolated to zero frequency (RO). Wi th increasing doses of histamine, the changes in RO always preceded th ose in Raw and airway transfer resistance implying that bronchoconstri ction developed first in the lung periphery. It is therefore concluded that the measurement of Zaw,ter offers a sensitive method for the det ection of early nonuniform responses to bronchoconstrictor stimuli tha t are not yet reflected by the values of the overall Raw. In one-half of the cases, significant increases in tissue damping and elastance oc curred before any change in RO; this suggests that the mechanisms of a irway and parenchymal constrictions may be unrelated.