BACKGROUND: The purposes of this study were to assess the resting volume of the modified bronchial cuff of left-sided double-lumen bronchial tubes (DLT) and to determine the maximum range of the mainstem bronchial diameter indicated for DLT. METHODS: Left-sided DLTs (Broncho-Cath(R)) of 35-41 Fr (n = 5 each) were used for the study. The cuff was inflated with air in 0.5-ml increments to a volume of 5 ml and the corresponding cuff pressure was recorded. The smallest cuff volume, beyond which a 0.5 ml increase resulted in more than 10 mm Hg increase in cuff pressure, was considered to be the resting volume of that cuff. The resting volume was also calculated by differentiation on the fitted curve. The minimum required bronchial diameter was considered to be the reported OD of the bronchial tube and the maximum diameter was the measured OD of the bronchial cuff at a cuff pressure of 30 mmHg, which was measured with a precision caliper (0.1 mm intervals) at the midcuff level. RESULTS: The resting volume of the bronchial cuff, measured both traditionally and using the curve fitting analysis, were comparable. A DLT of any size > or = 35 Fr can be used for a bronchus with a diameter of 10.7-20.6 mm. CONCLUSIONS: There were extensive overlaps in the range of bronchial diameters indicated for each DLT size. Contrary to a common belief, the upper limits of the bronchial diameters indicated for all the DLTs > or = 35 Fr seemed to be the same regardless of the DLT size.