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

Abstract

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 (CO<inf>2</inf>) inhalation could be caused by the combination of the decrease in CO<inf>2</inf> output (V̇CO<inf>2</inf>) and the increase in oxygen uptake (V̇O<inf>2</inf>). In addition, we investigated the effect of 3% CO<inf>2</inf> inhalation on cardiac output (Q̇) during CWE. Seven males (V̇O<inf>2max</inf>: 44.1±6.4 mL/min/kg) carried out transitions from low-load cycling (baseline; 40w) to light intensity exercise (45% V̇O<inf>2 max</inf>; 89.3±12.5 W) and heavy intensity exercise (80% V̇O<inf>2max</inf>; 186.5±20.2 W) while inhaling normal air (Air) or an enriched CO<inf>2</inf> gas (3% CO<inf>2</inf>, 21% O<inf>2</inf>, balance N<inf>2</inf>). 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% CO<inf>2</inf> was higher than for air through the experiment (P<0.05). Steady and non-steady state RER and V̇CO<inf>2</inf> for 3% CO<inf>2</inf> were less than for air in both light and heavy intensities (P<0.05), but V̇O<inf>2</inf> and Q̇ did not differ between the two conditions. 3% CO<inf>2</inf> inhalation induced the decrease in RER during CWE at light and heavy intensities, which was due to the decrease in V̇CO<inf>2</inf>. The promoted ventilation with 3% CO<inf>2</inf> did not lead to the increase in V̇O<inf>2</inf>. Moreover, 3% CO<inf>2</inf> inhalation did not affect Q̇ during CWE at light and heavy intensities.