A simple method to clamp end-tidal carbon dioxide during rest and exercise

Carbon dioxide regulates ventilation and cerebral blood flow during exercise. There are significant limitations in breathing systems designed to control end-tidal gas concentrations when used during high-intensity exercise. We designed a simple, inexpensive breathing system which controls end-tidal carbon dioxide (PET CO2) during exercise from rest to peak work capacity (W(max)). The system is operated by an investigator who, in response to breath-by-breath PET CO2, titrates flow of a 10 % CO(2), 21 % O(2) mixture into an open-ended 5-L inspiratory reservoir. To demonstrate system efficacy, nine fit male subjects performed two maximal, incremental exercise tests (25 W min(-1) ramp) on a cycle ergometer: a poikilocapnic control trial in which PET CO2 varied with work intensity, and an experimental trial, in which we planned to clamp PET CO2 at 50 mmHg. With our breathing system, we maintained PET CO2 at 51 ± 2 mmHg throughout exercise (rest, 50 ± 2; W(max), 52 ± 5 mmHg; mean ± SD) despite large changes in ventilation (range 27-65 at rest, 134-185 L min(-1) BTPS at W (max)) and carbon dioxide production (range 0.3-0.7 at rest, 4.5-5.5 L min(-1) at W (max)). This simple, inexpensive system achieves PET CO2 control at rest and throughout exercise.