METHACHOLINE RESPONSIVENESS USING THE RAISED VOLUME FORCED EXPIRATIONTECHNIQUE IN INFANTS

Infant lung function can be assessed with the tidal volume ''squeeze'' technique or, over an extended volume range, with the newer raised vo lume forced expiration technique (RVFET). We assessed methacholine res ponsiveness in 11 infants, measuring both maximal expiratory flow at f unctional residual capacity (Vmax,FRC) with the tidal volume technique , and forced expiratory volume/time (FEVt) with RVFET. We used a stand ard methodology for the former. FEVt was measured by inflating the inf ant's lungs to 20 cm H2O and forcing expiration using a jacket setup t o transmit a pressure of 20 cm H2O to the airway. Lung function was me asured at baseline and after methacholine inhalations, increasing from 0.1 g/L to 10 g/L in half log dosage increments (DI). The provocative concentrations (PC) of methacholine leading to a 40% fall in Vmax,FRC and a 15 or 20% fall in FEVt were calculated. The mean provocative co ncentration of methacholine required to produce a 40% fall in Vmax,FRC was less than that required to produce a 20% fall in FEV0.5 by 0.39 D I (95% CII -0.60 to 1.38) and less than that required to produce a 20% fall in FEV0.75 by 0.42 DI (95%, CI, -0.54 to 1.39). Similarly, the p rovocative concentration of methacholine required to produce a 40% fal l in Vmax,FRC was less than that required to produce a 15% fall in FEV 0.5 by 0.14 DI (95% CI, -0.99 to 1.28) or a 15% fall in FEV0.75 by 0.1 3 DI (95% CI, -0.80 to 1.08), but the differences were small and not s ignificant. Despite these differences the agreement between the two me thods was good, and bronchoconstriction was not attenuated by the forc ed inspiration delivered by the raised volume maneuver. We conclude th at the raised volume forced expiration technique is able to detect met hacholine-induced bronchoconstriction.