Informative value of simple multibreath nitrogen washout measurements for clinical and research purposes

Some simple multibreath nitrogen washout indexes quantifying inspired gas distribution and ventilatory efficiency were obtained in a group of patients with mild to advanced chronic obstructive pulmonary disease (COPD) and studied in their relationships with routine pulmonary function tests. The indexes (lung clearance index (LCI), mixing ratio (MR) and data obtained by graphic analysis of the washout curve) were correlated with spirometric, pulmonary mechanics and arterial blood gas measurements, but only 8-38% of the interindividual variation in these indexes was explained by the above routine tests. An additional 5-13% of the variation was explained by the washout tidal volume (VT); this finding may reflect changes in gas distribution with VT and/or the influence of the dead space on ventilatory efficiency. Our data indicate that, in patients with COPD, nitrogen washout indexes tend to change in parallel with routine pulmonary function tests, reflecting the severity of the disease; these indexes also contain specific information (in addition to that provided by routine physiologic tests), presumably related to the distribution and efficiency of ventilation. Nitrogen washout measurements may thus represent a helpful adjunct to routine pulmonary function testing; LCI and MR appear to be particularly convenient for practical purposes because of their simplicity, and an informative content comparable with that of more complex indexes.