Measurement of total lung capacity (TLC) by the reference, plethysmography (PL), should be similar to methane dilution (MD) during a single breath diffusion capacity (DLCO) measurement in subjects with normal lungs. To test this hypothesis, 50 nonsmoking subjects with no history of any medical problems were enrolled in a comparison study.
Plethysmography and single breath DLCO measurements were obtained with a Collins system. An acceptable effort for DLCO included breath-holding for 9–11 seconds at 90% or more of vital capacity. TLC was determined as functional residual capacity + inspiratory capacity. Lung volumes were expressed BTPS. Deming regression and the concordance correlation coefficient (CCC) were used to determine agreement since both methods measure with a certain degree of uncertainty. All values are mean + S.D. unless otherwise stated. Not significant is NS.
There were 28 female and 22 male subjects with an age range of 22 to 55 (32.5 + 8.4 years old). The average PL and MD TLC values were 4.66 + 0.97 (95% C.I., 4.38–4.94) and 4.55 + 0.91 (95% C.I., 4.42–4.68) liters, respectively (NS, paired t test). The Pearson correlation coefficient was 0.88 (p<0.01; 95% C.I., 0.79–0.93). The CCC was 0.87 (95% C.I., 0.78–0.92). Deming regression revealed a slope of 0.93 (p=0.17, null b=1; 95% C.I., 0.84–1.03) and a y intercept of 0.20 (p=0.39, null a=0; 95% C.I., −0.27-+0.70).
The MD estimate of TLC obtained during single breath DLCO measurements is a reasonable estimate of TLC in subjects with normal lungs when plethysmography is not available.CLINICAL IMPLICATION: A TLC in the normal range obtained by methane dilution suggests lungs relatively free of disease.
G.R. Pesola, None.