Comparison of four methods for calculating the total lung capacity measured by body plethysmography.

Static lung volumes were measured plethysmographically one hour apart in healthy subjects (n = 14) and in patients with chronic pulmonary disorders of various etiologies (n = 25). The total lung capacity (TLC) obtained from paired measurements of functional residual capacity (FRC) and inspiratory capacity (IC) was calculated according to the four following methods: 1) average FRC plus the largest IC, 2) average FRC plus the average IC, 3) largest sum of FRC and corresponding IC, and 4) average of individual FRC and IC sums. The data, analysed for average values and for reproducibility in the group as a whole and in the healthy subjects and patients separately indicate that: a) For the group as a whole the largest average TLC values were found with method 3 followed by methods 4 and 1. The differences were statistically significant for all comparisons but one (method 1 vs method 4). A similar pattern was found for the healthy subjects and patients separately. b) For the group as a whole, the one hour reproducibility tended to be worse from method 1 through method 4 but the intermethod differences were not statistically significant. For the healthy subjects, the reproducibility tended to be better for methods 3 and 4 and for the patients this was the case for methods 1 and 2. The average reproducibiltiy of methods 1 and 2 was similar for both the healthy and patient groups and these methods seemed more suitable for TLC calculations. Because it is more widely employed, method 1 is recommended.