A sensitive five-breath N2 washout test of distribution of ventilation.

The amount of nitrogen eliminated in the first five breaths after start of oxygen breathing was compared with the amount that would be removed in these breaths, if alveolar mixing were uniform. The ratio of these two amounts is a measure of the degree of unevenness of ventilation; it was determined in 41 nonsmokers, 22 heavy smokers, and 6 patients with chronic obstructive pulmonary disease. The "five-breath index" (ratio x 100) in the nonsmokers ranged between 90 and 100. In 16 of the smokers, it was less than 87.5 (95% confidence limit of normals), although in only 5 of these, spirometric tests (MMEF) were considered abnormal. The index in the 6 patients ranged between 51 and 83. The test is highly reproducible, simple to perform, and might be of value in the detection of early lung disease.