Comparison of respiratory inductance plethysmography with thoracoabdominal compression in bronchial challenges in infants and young children

Respiratory inductance plethysmography measuring thoracoabdominal asynchrony (TAA) has been claimed to be a useful tool for measuring changes in airway resistance in infants. In this study we evaluated the response to methacholine by thoracoabdominal compression and respiratory inductance plethysmography. Seventeen infants (mean age, 13.1 ± 4.7 mo) with recurrent episodes of cough or wheeze underwent bronchial challenge with inhaled methacholine. Lung function was evaluated by measuring maximal expiratory flow at resting lung volume (V̇maxFRC), and the degree of TAA was measured by phase angle (θ). Methacholine was inhaled for 1 min during tidal breathing using increasing doubling concentrations until a fall of at least 40% in V̇maxFRC was achieved (final concentration). All infants responded to the final concentration of methacholine by a significant fall in V̇maxFRC (from 31 ± 10 to 12 ± 5 ml/s/kg, p < 0.001). All but one infant responded to methacholine at the final concentration with a significant increase in phase angle (median θ increased from 11.7 to 31.7 degrees, p < 0.001). In two other infants there was an early response in θ compared with the response in V̇maxFRC. Phase angle increase after methacholine was expressed as Z-scores (the difference between postmethacholine θ and postbuffer θ divided by the standard deviation of postbuffer θ). An increase of at least 2.0 Z-scores in θ was observed at the same concentration of methacholine when V̇maxFRC fell by at least 40% in 15 of the 17 infants (88%). We conclude that respiratory inductance plethysmography is a sensitive method to measure bronchial reactivity to methacholine in most of the infants studied (14 of 17, 82%). A concentration of methacholine causing an increase in θ of at least 2.0 standard deviations above baseline is equivalent to the concentration causing a 40% fall in V̇maxFRC.