Background: The contribution of the pulmonary tissues to the mechanical behaviour of the respiratory system is well recognised. This study was undertaken to detect airway and lung tissue responses to inhaled methacholine (Mch) using the low frequency forced oscillation technique (LFOT).
Methods: The respiratory system impedance (Zrs, 0.5-20 Hz) was determined in 17 asymptomatic infants. A model containing airway resistance (Raw) and inertance (Iaw) and a constant phase tissue damping (G) and elastance (H) was fitted to Zrs data. Tissue hysteresivity (eta) was calculated as eta=G/H. The raised volume rapid thoracic compression technique (RVRTC) was used to generate forced expiratory volume in 0.5 seconds (FEV(0.5)). Lung function was determined at baseline and following inhaled Mch in doubling doses (0.25-16 mg/ml) until the maximal dose was reached or a fall of 15% in FEV(0.5) was achieved (PC(15)FEV(0.5)). The response to Mch was defined in terms of the concentration of Mch provoking a change in lung function parameters of more than two standard deviation units (threshold concentration).
Results: At PC(15)FEV(0.5) a response in Raw, Iaw, G, and eta, but not H, was detected (mean (SE) 61.28 (12.22)%, 95.43 (34.31)%, 46.28 (22.36)%, 44.26 (25.83)%, and -6.48 (4.94)%, respectively). No significant differences were found between threshold concentrations of LFOT parameters and FEV(0.5).
Conclusions: Inhaled Mch alters both airway and respiratory tissue mechanics in infants.