DIFFUSING-CAPACITY DEPENDENT ON LUNG-VOLUME AND AGE IN NORMAL SUBJECTS

In this study we determined reference values of total diffusing capaci ty of carbon monoxide (DL(CO)) and DL(CO) per liter alveolar volume (D L(CO)/VA) at total lung capacity (TLC) and at lung volumes below TLC i n sitting position. In 55 healthy nonsmoking volunteers (20-85 yr old) , we determined reference values at TLC level in which age was the onl y parameter. In a subgroup (n = 16) these references did not change by correction for normal variability in hemoglobin concentration. In all volunteers DL(CO) decreased and DL(CO)/VA increased with decreasing V A. The increase in DL(CO)/VA was linear and less in older subjects. We derived equations to calculate reference values of DL(CO)/VA for lung volumes at and below TLC with two methods: 1) ''random coefficients l inear'' model, which calculates the reference values directly, and 2) a conversion method, which calculates DL(CO)/VA for lower VA levels fr om reference values at TLC. An advantage of the conversion method is t he suitability of DL(CO)/ VA reference values at TLC of other populati ons. A disadvantage is the greater standard deviation of these referen ce values compared with those obtained by the random coefficients line ar method. DL(CO) can be found by multiplying DL(CO)/VA with VA.