Constant positive airway pressure (CPAP) to the operative lung during one-lung ventilation (1-LV) with a double-lumen tube increases PaO2; there have been no reports of application of CPAP to the operative lung during 1-LV with the Univent bronchial blocker (BB) tube. This study determined the method of administration and the effect on PaO2 of 10 cm H2O of CPAP to the operative lung during 1-LV (1-LV + 10 CPAP) produced by the Univent BB system. We designed our CPAP system for the Univent BB using an in vitro lung model so that low O2 flow rates (2-4 L/min) yielded clinically relevant levels of CPAP (5-20 cm H2O) over a wide range of lung compliance. The CPAP system simply consisted of placing a resistance to a variable oxygen flow distal to the operative lung. Seven consenting patients who required thoracotomy and 1-LV were anesthetized and their tracheas were intubated with the Univent BB tube; the BB was inserted into the appropriate mainstem bronchus until the proximal surface of the BB cuff was just distal to the tracheal carina. PaO2 was measured in the seven patients during 12 sequences of two-lung ventilation (2-LV), one-lung ventilation (1-LV), and 1-LV with 10 cm H2O CPAP (1-LV + 10 CPAP). 1-LV + 10 CPAP was always instituted on the deflation phase of a previous single tidal inhalation. We found in our patients with a lung compliance of 32 +/- 4 mL/cm H2O that 2.4 +/- 0.2 L/min of oxygen flow produced 1-LV + 10 CPAP.(ABSTRACT TRUNCATED AT 250 WORDS)