RESUMO
Although techniques for the short-term control of end-tidal gases exist, the lack of a satisfactory technique for longer-term control of the end-tidal gases has limited protracted physiological experiments of this nature. We have constructed a chamber in which subjects can be comfortable for many hours while having their end-tidal gas composition monitored and controlled. The system for controlling the end-tidal gas composition is based on a principle described by Swanson and Bellville (J. Appl. Physiol. 39: 377-385, 1975) in which end-tidal PO2 (PETO2) and PCO2 (PETCO2) are monitored and deviations of the actual PETO2 and PETCO2 (PETCO2) are monitored and deviations of the actual PETO2 and PETCO2 from the desired values are corrected by a feedback mechanism that adjusts the inspired gas composition accordingly. End-tidal and inspired gas tensions are measured via a nasal catheter connected to a mass spectrometer. A computer averages the end-tidal and inspired gas tensions and, at 5-min intervals, adjusts the gas composition inside the chamber. During 8 h of isocapnic hypoxia, the system held the 5-min average value for PETO2 within 2 Torr of the desired value (55 Torr) and the value for PETCO2 within 0.35 Torr of the desired value (the resting value for each subject) in four subjects.