A Simple Method to Clamp Carbon Dioxide During Exercise

Abstract

PURPOSE: We previously described a rebreathing circuit to clamp end-tidal carbon dioxide (PETCO2) during maximal exercise, but encountered increased inspiratory resistance, and decreased inspired oxygen concentrations which limited the circuit's utility. To overcome these limitations, we designed a new circuit to clamp PETCO2 levels without relying on rebreathing. We conducted a series of investigations to test its efficacy. METHODS: The circuit was made of an open-ended 24 inch length of 4 inch diameter plastic tubing acting as an inspiratory reservoir, connected via 0.5 m of flexible tubing to a standard 2 way T- valve, open to the room on the expiratory side. To achieve our experimental goals, we titrated a small amount of compressed 21% oxygen - 15% CO2 mix into the ambient air present in the reservoir, in response to a real-time PETCO2 monitor. Six competitive male cyclists (180.0 ± 6.5 cm, 80.0 ± 11.3 kg, 34 ± 10 years), participating in a larger study and previously familiarized with the incremental exercise tests in a hypobaric chamber, performed two maximal exercise tests at each of two altitudes (Pb = 630 mmHg, Pb = 425 mmHg) on two separate days. At each altitude, subjects performed a poikilocapnic test and a test with PETCO2 clamped at 50 mmHg, in a randomly assigned and counterbalanced order. Metabolic gases, ventilation, and pulse oximetry were monitored continuously across trials. RESULTS: PETCO2 was effectively clamped at maximal work rates at both altitudes (low altitude: 51 ± 3 vs. 33 ± 2 mmHg: mean ± SD, P < 0.01; high altitude: 53 ± 5 vs. 26 ± 3 mmHg, P< 0.01) despite very high ventilatory rates (low altitude: 169.9 ± 7.8 L/min (clamp) vs 174 ± 6.2 L/min (control), P=0.35; high altitude: 172.4 ± 10.4 L/min (clamp) vs. 164.9 ± 17.6 L/min (control), P=0.50). Oxygen saturation did not vary between trials at either altitude at maximal exercise (low altitude: 89.4 ± 5.1 (clamped) vs. 89.7 ± 5.0% (control), P= 0.82; high altitude: 73.7 ± 7.9% (clamp) vs. 71.5 ± 4.6% (control) P= 0.28). CONCLUSIONS: Our circuit can accurately and precisely clamp end-tidal gas concentrations during exercise, making it a valuable tool for future studies requiring precise control of PETCO2.