Methacholine-induced lung function changes measured with infant body plethysmography^†

Several techniques have been applied to measure airway responsiveness (AR) in infants, but there are limited data on lung function changes measured by body plethysmography during induced bronchoconstriction. The aim of this study was to compare changes in maximum forced expiratory flow measured at functional residual capacity (V'_maxFRC) by rapid thoracoabdominal compression (RTC) technique with plethysmographic measurements of specific airway conductance (sG_aw), and to investigate whether changes in functional residual capacity (FRC) occur during methacholine-induced bronchoconstriction in infants. We examined 94 infants with recurrent airway symptoms using methacholine airway challenge test including RTC and plethysmographic measurements. A significant association between changes in V'_maxFRC and sG_aw (r = 0.30; P = 0.004) was observed, but after adjustments with baseline variability the changes in V'_maxFRC were greater and showed a closer association with changes in oxygen saturation. At the point of maximal airway obstruction, there was a poor agreement between V'_maxFRC and sG_aw to indicate a significant methacholine-induced bronchoconstriction. Airway challenge was also associated with a significant increase in FRC (P < 0.001), with decreasing V'_maxFRC. We conclude that in infants undergoing airway challenge with methacholine, plethysmographic measurements of sG_aw correlate with the changes in V_maxFRC, but the agreement is poor and the methods cannot be used interchangeably. V_maxFRC is also more sensitive to detect airway obstruction than sG_aw. However, methacholine-induced bronchoconstriction was associated with significant increases in FRC, which may affect the validity of V_maxFRC measurements during the test. Pediatr Pulmonol. 2011; 46:362–368. © 2010 Wiley-Liss, Inc.