Clamping end-tidal carbon dioxide during graded exercise with control of inspired oxygen

• Decreases in the partial pressure of end-tidal carbon dioxide (PETCO2), influence several physiological functions in the body.
• Several breathing systems have been developed to clamp PETCO2 in spontaneously breathing humans.
• However, most systems cannot accommodate high rates of ventilation or fluctuating levels of inspired oxygen (FIO2).
• In this study, we describe a novel system that can maintain PETCO2 during voluntary exercise and with progressive changes in FIO2.

Exercise- and hypoxia-induced hyperventilation decreases the partial pressure of end-tidal carbon dioxide (PETCO2), which in turn exerts many physiological effects. Several breathing circuits that control PETCO2 have been previously described, but their designs are not satisfactory for exercise studies where changes in inspired oxygen (FIO2) may be desired. This study is the first report of a breathing system that can maintain PETCO2 constant within a single session of graded submaximal exercise and graded hypoxia. Thirteen fit and healthy subjects completed two bouts of exercise consisting of three 3 min stages on a cycle ergometer with increasing exercise intensity in normoxia (Part A; 142 ± 14, 167 ± 14, 192 ± 14 W) or with decreasing FIO2 at a constant exercise intensity (Part B; 21, 18, and 14%). One bout was a control (CON) where PETCO2 was not manipulated, while during the other bout the investigator clamped PETCO2 within 2 mmHg (CO2Clamp) using sequential gas delivery (SGD). During the final 30 s of each exercise stage during CO2Clamp, PETCO2 was successfully maintained in Part A (43 ± 4, 44 ± 4, 44 ± 3 mmHg; P = 0.44) and Part B (45 ± 3, 46 ± 3, 45 ± 3 mmHg; P = 0.68) despite the increases in ventilation due to exercise. These findings demonstrate that this SGD circuit can be used to maintain isocapania in exercising humans during progressively increasing exercise intensities and changing FIO2.