CHAMBER FOR CONTROLLING END-TIDAL GAS TENSIONS OVER SUSTAINED PERIODS IN HUMANS

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 (PET(O2)) and PCO2 (PET(CO2)) are monitored and deviations of the actual PET(O2) and PET(CO2) 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 PET(O2) within 2 Torr of the desired value (55 Torr) and the value for PET(CO2) within 0.35 Torr of the desired value (the resting value for each subject) in four subjects.