Effect of increasing bronchodilatation on the single breath nitrogen test.

To determine if and how changes of the slope of phase III of the N2 test reflect drug-induced bronchodilatation, we studied patients with moderate to severe reversible airflow limitation who inhaled increasing doses of salbutamol. The bronchodilating response on each dose level was monitored both by the N2 test and dynamic spirometry. The study shows that the slope of phase III is related to drug-induced bronchodilatation in a dose-dependent way. Furthermore, the decrease of the slope of phase III mirrors the concomitant increase of FEV1. These results are consistent with bronchomotor tone as a determinant of the slope of phase III. Alternatively, the decrease of the slope of phase III may be independent of bronchomotor tonus per se and be explained by a decrease in residual volume as reflected by the observed dose-dependent increase in vital capacity.