Methacholine responsiveness in infants assessed with low frequency forced oscillation and forced expiration techniques

Background-The contribution of the pulmonary tissues to the mechanical beha viour of the respiratory system is well recognised. This study was undertak en 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 in ertance (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 (FEV0.5). Lung function w as 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 FEV0.5 wa s achieved (PC15FEV0.5). The response to Mch was defined in terms of the co ncentration of Mch provoking a change in lung function parameters of more t han two standard deviation units (threshold concentration). Results-At PC15FEV0.5 a response in Raw, law, G, and tl, but not H, was det ected (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 fou nd between threshold concentrations of LFOT parameters and FEV0.5. Conclusions-Inhaled Mch alters both airway and respiratory tissue mechanics in infants.