Tidal airway closure during bronchoconstriction in asthma: usefulness of lung volume measurements

The presence and extent of tidal airway closure is not routinely assessed in asthma. The objective of this study was to provide a simple functional tool able to detect tidal airway closure during bronchoconstriction in asthma.

In 20 subjects with mild persistent asthma, we sequentially performed the measurement of functional residual capacity (FRC) by body plethysmography (pleth) and multibreath helium dilutional technique (He) and then computed residual volume (RV) and total lung capacity (TLC) at baseline, at the end of methacholine (MCh) challenge and after bronchodilator (albuterol).

Despite substantial bronchoconstriction (fall in FEV(1) = 35 ± 7%), TLC,pleth did not change following MCh challenge, but FRC,pleth because of dynamic pulmonary hyperinflation (+0.68 ± 0.54 L) and RV,pleth because of air trapping (+0.65 ± 0.37 L), invariably increased (on average by 22% and 46%, respectively). In contrast, FRC,He (and RV,He and TLC,He) could either increase, as seen in 13 subjects (Group I), or decrease, as seen in 7 subjects (Group II). Hence, the difference between FRC,pleth and FRC,He (Diff. FRC,pleth - FRC,He) was much greater in Group II (1.03 ± 0.41 L) than in Group I (0.22 ± 0.20 L) (p < .01). No functional differences were found between the two groups, including baseline PD(20)FEV(1) and absolute and percent change in forced vital capacity (FVC) at the end of the MCh challenge.

Comparison between FRC,pleth and FRC,He is useful to identify asthmatics prone to tidal airway closure during MCh-induced bronchoconstriction and Diff. FRC,pleth - FRC,He can be used to measure the overall unventilated lung volume upstream of the airways closed at end-expiratory lung volume (EELV).