Contribution of residual volume to the increased alveolar plateau with bronchoconstriction.

We studied 10 normal volunteers with single-breath nitrogen tests before and after methacholine-induced bronchoconstriction. On the first day we challenged each subject with increasing methacholine concentrations to obtain a 20% drop in the partial forced expired volume in 1 s/partial forced vital capacity ratio. This resulted in an increased residual volume (RV) from 1,536 +/- 138 to 2,371 +/- 216 ml (mean +/- SEM, p less than 0.01). On the following day we did single-breath nitrogen tests from RV, RV + delta V (delta V is the individual's RV increase after bronchoprovocation) and after a similar methacholine rechallenge. Baseline slope of phase III was 0.88 +/- 0.11% N2/1 (mean +/- SEM), 1.36 +/- 0.22% N2/1 with the RV + delta V test (p less than 0.01), and 2.84 +/- 0.68% N2/1 with bronchoconstriction (p less than 0.05). The results show that the changes in RV per se accounted for 24% of the increase in the slope of phase III seen with bronchoprovocation.