Low-dose halothane produces airway dilatation but does not alter parenchymal mechanics in the normal canine lung

The purpose of this study was to examine whether halothane reduces the contractile tone of the normal lung and to distinguish the effects of halothane on airways from those on lung tissue. We also tested whether a mathematical model was capable of quantitatively describing the mechanical changes in the lung produced by halothane. We measured lung impedence (ZL(omega) a complex function of real (lung resistance) and imaginary (reactance) parts) at low frequencies in dogs using a forced volume oscillation technique before and during 1 MAC halothane anaesthesia. Halothane produced small changes in ZL(omega). The lung resistance tended to decrease during halothane anaesthesia whereas the lung reactance did not show change. Using an alveolar capsule technique to separate the airways from the lung tissue components, these lung mechanical changes were induced mainly by alterations in lung tissue and not in the airways. Our mathematical model featured a single airway leading to an alveolar region surrounded by a viscoelastic lung tissue. In the model analysis, estimates of airway resistance and inertance decreased by the administration of halothane. In contrast, estimates of lung tissue elastance and resistance did not change during halothane anaesthesia. These modeling results were consistent with those obtained by direct alveolar pressure measurements. Our results suggest that a low concentration of halothane dilates the airways but does not alter the parenchymal mechanics in the normal lung, and that the model provides a quantitative tool to assess lung mechanics precisely, if respiratory signals are measured only at the true airway opening.