Simple contrivance “clamps” end-tidal P CO 2 and P O 2 despite rapid changes in ventilation

The device described in this study uses functionally variable dead space to keep effective alveolar ventilation constant. It is capable of maintaining end-tidal$P{\text{CO}}_2$ and$P{O}_2$ within ±1 Torr of the set value in the face of increases in breathing above the baseline level. The set level of end-tidal $P{\text{CO}}_2$ or$P{O}_2$ can be independently varied by altering the concentration in fresh gas flow. The device comprises a tee at the mouthpiece, with one inlet providing a limited supply of fresh gas flow and the other providing reinspired alveolar gas when ventilation exceeds fresh gas flow. Because the device does not depend on measurement and correction of end-tidal or arterial gas levels, the response of the device is essentially instantaneous, avoiding the instability of negative feedback systems having significant delay. This contrivance provides a simple means of holding arterial blood gases constant in the face of spontaneous changes in breathing (above a minimum alveolar ventilation), which is useful in respiratory experiments, as well as in functional brain imaging where blood gas changes can confound interpretation by influencing cerebral blood flow.