Chronic obstructive pulmonary disease (COPD) is associated with impaired gas mixing and increased dead space, but little is known about the effect of improving alveolar gas sample by complete correction of dead space in an attempt to significantly improve the final result of transfer factor compared with standard guidelines of the European Respiratory Society (ERS) and American Thoracic Society (ATS). By using a rapid infrared analyzer, TLCO was measured by the single breath method in 152 COPD patients at different stages of severity (FΕV₁:57% predicted; CI 95%:24–91). Standard washout volume of 0.75 liter was insufficient to clear phases I and II in 36 patients (23.7%). In 19 subjects (12.5%), a washout volume larger than 1 liter was necessary for complete dead space clearance, although in these patients, correction visually adequate to complete clear phases I and II resulted in higher TLCO values. Only in 5 patients (3,3%) did the final result change by more than 5% from the previous value. A vital capacity higher than 3 liters, rather than the degree of airflow limitation was a better predictor for larger washout volume requirements. We conclude that in the measurement of TLCO by the breathholding method, ERS and ATS recommendations for washout volume can be safely used for clinical purposes in a wide range of patients with mild to severe obstruction.

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