Effect of 3% CO2 inhalation on respiratory exchange ratio and cardiac output during constant work-rate exercise

The aim of this study was to examine whether the decrease in respiratory exchange ratio (RER) during constant work-rate exercise (CWE) with 3% carbon dioxide (CO2) inhalation could be caused by the combination of the decrease in CO2 output (V̇CO2) and the increase in oxygen uptake (V̇O2). In addition, we investigated the effect of 3% CO2 inhalation on cardiac output (Q̇) during CWE.

Seven males (V̇O2max: 44.1±6.4 mL/min/kg) carried out transitions from low-load cycling (baseline; 40w) to light intensity exercise (45% V̇O2 max; 89.3±12.5 W) and heavy intensity exercise (80% V̇O2max; 186.5±20.2 W) while inhaling normal air (Air) or an enriched CO2 gas (3% CO2, 21% O2, balance N2). Each exercise session was 6 min, and respiratory responses by Douglas bag technique and cardiac responses by thoracic bio-impedance method were measured during the experiment.

Ventilation for 3% CO2 was higher than for air through the experiment (P<0.05). Steady and non-steady state RER and V̇CO2 for 3% CO2 were less than for air in both light and heavy intensities (P<0.05), but V̇O2 and Q̇ did not differ between the two conditions.

3% CO2 inhalation induced the decrease in RER during CWE at light and heavy intensities, which was due to the decrease in V̇CO2. The promoted ventilation with 3% CO2 did not lead to the increase in V̇O2. Moreover, 3% CO2 inhalation did not affect Q̇ during CWE at light and heavy intensities.