Evaluation of single-breath helium dilution total lung capacity in obstructive lung disease.

We examined the effect of obstructive lung disease on the single-breath helium dilution method of measuring total lung capacity (TLC). In test results of 79 patients, we found this dilution method underestimated the TLC determined by a radiographic method by a mean of 2.3% in normal subjects, 10.4% in patients with mild, 21.8% in those with moderate, and 38.0% in those with severe obstruction. The ratio of forced expiratory volume in one second to forced vital capacity was used to group the patients; the differences in the degree of underestimation of TLC between groups were significant (p less than 0.05). In addition, we found a strong correlation between the degree of underestimation of TLC by the helium method and the severity of obstruction when patients were considered as a continuous population. Using a regression equation based on this correlation, we derived a method for correcting the helium dilution TLC. This allows use of this test in patients whose obstructive disease would otherwise render it inaccurate.