Nonhomogeneity of lung response to inhaled histamine assessed with alveolar capsules

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Abstract
To assess the homogeneity of airway responses to inhaled histamine, regional alveolar pressure excursions (PA) arising from small-amplitude oscillations applied at the airway opening (Pao) were examined. In 5 anesthetized and vagotomized dogs the sternum was split and the anterior right lung field exposed. PA was sampled using 4 capsules affixed to the right apical and middle lobes while lung impedance (ZL) and airway impedances (Zaw) were measured during conventional tidal breathing and during forced oscillations (2-60 Hz at 10 cmH2O distending pressure). During tidal breathing after exposure to aerosol histamine, regional PA could be separated into 3 groups by plotting Lissajous figures of PA vs. Pao: PA in phase with Pao (no looping), PA lagging Pao (moderate looping) and PA decreasing while Pao was increasing and vice versa (paradoxical looping), suggesting unresponsive, responsive, and closed pathways, respectively, between the airway opening and specific alveolar zones. During high-frequency oscillation the corresponding PA spectra were markedly different from control spectra and revealed resonant amplification, overdamped resonance and marked attenuation, respectively. With induced bronchospasm resonant amplification of PA was damped on average. The more obstructed and closed pathways were protected from resonant amplification, and the more open (nonlooping) pathways were subjected to resonant amplification greater than in the control state. In spite of this markedly nonhomogeneous behavior, frequency dependence of ZL was consistent with the model by Mead which ignores nonhomogeneity of peripheral compartments. The response of airways to inhaled histamine is nonhomogeneous, but frequency dependence of ZL > 2 Hz is not sufficient to characterize this nonhomogeneity.

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