Suction applied to a chest-tube drainage system produces subambient pressure within the chest tube and may increase or decrease the flow of gas passing from the airway through a bronchopleural fistula. Such gas may have participated in gas exchange and contain more CO2 and less O2 than inspired gas. Calculated or measured variables dependent upon exhaled CO2 and O2 will be erroneous unless the fistula gas values are considered. A device easily assembled from standard measuring instruments was used to titrate suction, thereby facilitating fistula closure and allowing more accurate physiologic assessment of gas content.