Reproducibility of Open Circuit Acetylene-Derived Cardiac Output at Moderate, Heavy, and Severe Exercise Intensities

The objective of this test-retest study was to determine the reproducibility of open-circuit acetylene uptake-derived measures of [Formula: see text] during repeated measures of cycling exercise within the moderate, heavy, and severe intensity domains. Seventeen healthy participants (mean±SD, 53±15 [range: 22-75] years; 9 females) completed a maximal incremental cycling exercise protocol (Visit 1) and two step-incremental protocols (Visits 2 and 3) over three days. The step-incremental protocol involved 10-min of cycling within the moderate-intensity domain (MOD; 85% of gas exchange threshold (GET)), then 10-min within the heavy-intensity domain (HVY; 70% between GET and respiratory compensation point (RCP)) and up to 10 min within the severe-intensity domain (SVR; 115%RCP) or until task disengagement. During MOD and HVY, open-circuit inspiration of 0.7%C 2 H 2 , 7%He, and 21%O 2 in nitrogen was applied over 10-12 breaths at 2.5, 5.0, and 7.5 min. During the SVR stage, open-circuit breathing occurred at 2.0 min and during the final min of exercise as indicated by the participant. The integrated uptake versus end-tidal change in acetylene approach was used to calculate [Formula: see text]. A two-way (trial x day) repeated measures analysis of variance and intraclass correlation coefficient (ICC) assessed within- and between-day reproducibility at each intensity. Average power output for MOD, HVY, and SVR were 98±54, 138±72, and 174±90 W, eliciting steady-state [Formula: see text]O 2 of 1.58±0.72 and 2.05±0.89 and peak [Formula: see text]O 2 of 2.31±0.97 L·min -1 . For all intensities, there were no interactions or main effects. For MOD on Day 1, [Formula: see text] were 12.0±4.6, 12.1±5.5, and 11.5±4.5 L·min -1 , at 2.5, 5.0 and 7.5 min, respectively (ICC=0.98) and on Day 2 were 11.8±4.2, 11.2±3.7, and 11.4±3.8 L·min -1 (ICC=0.990). For HVY on Day 1, [Formula: see text] were 14.1±5.4, 13.8±5.0, and 13.2±4.2 L·min -1 (ICC=0.995) and on Day 2 were 14.0±4.5, 13.6±5.0, and 13.0±3.8 L·min -1 (ICC=0.99). For SVR on Day 1, [Formula: see text] were 15.3±4.8 and 15.1±5.0 L·min -1 at 2.0 and end-exercise, respectively, and on Day 2 these values were 15.0±5.0 and 15.4±6.1 L·min -1 . Open-circuit acetylene derived measures of [Formula: see text] exhibited excellent test-retest reproducibility both within-intensities and between-days and this was consistent within all three exercise-intensity domains. Daniel A. Keir was supported by a NSERC Discovery Grant (RGPIN-2021-03980) and by the Canadian Foundation for Innovation John Evans Leadership Fund (Project: 42544). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.