SERIAL DISTRIBUTION OF AIRWAY MECHANICAL-PROPERTIES IN DOGS - EFFECTSOF HISTAMINE

We measured respiratory input impedance (Zin; 8-2,048 Hz) in five dogs (anesthetized, tracheostomized, vagotomized, and mechanically ventila ted) during 80 s of apnea after a bolus intravenous injection of salin e or histamine (5.0 mg). In the control case, three antiresonances in Zin were found in four of the dogs, whereas in the remaining dog only two were found. The magnitude and frequency of these antiresonances we re significantly altered after bronchoconstriction. To interpret Zin, a model incorporating detailed airway geometry, asymmetrical branching , and nonrigid airway walls was developed. The model fit both the sali ne and histamine Zin data well and predicted a serial distribution of bronchoconstriction consistent with known effects of histamine; i.e., the diameters of the most peripheral airways were reduced (26% of thei r control values), whereas tracheal diameters were not significantly a ffected. The model provided estimates of tracheal diameters that were well correlated (r = 0.92) with direct measurements. Control estimates of soft tissue viscosity (1.63 +/- 0.42 cmH(2)O.s) and Young's modulu s (406 +/- 125 cmH(2)O) compared closely with values in the literature . These results indicate that bronchoconstriction induced by histamine results in significant changes in Zin over this frequency range and t hat by using this data analysis approach definitive physiological para meters relative to airway geometry and wall mechanical properties can be obtained from measurements made at the airway opening.