Background: The measurement of single-breath diffusing capacity (DlcoSB) assumes that diffusing capacity per liter of alveolar volume (Dlco/VA) determined in a 750-mL gas sample represents the diffusing capacity (Dlco) of the entire lung. Fast-responding gas analyzers provide the opportunity to verify this assumption because of the possibility to measure CO and CH4 fractions continuously throughout the entire expiration. Continuous gas sampling provides more information per measurement, but this information cannot be expressed in the traditional parameters. Our goals were to find new parameters to express the extra information of the continuous gas sampling, and to compare these new parameters with the traditional parameters.
Methods: We compared a new method to determine Dlco with the traditional method in 62 healthy volunteers and 26 COPD patients. Traditionally, DlcoSB is determined by multiplying Dlco/VA with alveolar volume, both calculated from gas concentrations in a 750-mL gas sample. The new method calculates total-breath Dlco (DlcoTB) by integration of Dlco/VA against exhaled volume.
Results: In healthy volunteers, Dlco/VA shows a slight upward slope during exhalation, while in COPD patients Dlco/VA shows a horizontal line. Total-breath total lung capacity (TLC) is larger than single-breath TLC both in healthy volunteers and in COPD patients, leading to a DlcoTB that is significantly larger than DlcoSB in both groups (p < 0.001).
Conclusion: The assumption that a 750-mL gas sample represents the entire lung seems to be correct for Dlco/VA but not for the CH4 fraction in case of ventilation inhomogeneity.