Use of acetylene breathing to determine cardiac output in young and older adults

Purpose: The aims of this investigation were: 1) to establish the day-to-day reproducibility of open-circuit acetylene breathing for measuring exercise cardiac output (Q(C)) in young and older adults; and 2) to compare estimates of Q(C) from open-circuit acetylene breathing with estimates of Q(C) from previously established closed-circuit acetylene rebreathing. Methods: Twenty men (10 young: 28 +/- 1 yr; 10 older: 61 +/- 1 yr (mean +/- SE)) performed cycle ergometry exercise on 3 separate days. Q(C) was estimated using open-circuit acetylene breathing on 2 d, and closed-circuit acetylene rebreathing on I d. Results: Open-circuit acetylene breathing was highly reproducible (young: standard error of measurement (SEM) = 1.52 L(.)min(-1), limits of agreement (LOA) = 0.2 +/-4.2 L(.)min(-1), coefficient of variation 6% < CV < 8%, day 2 = (0.9 X day 1) + 2.4, r = 0.90, P < 0.001, r(2) = 0.82; older: SEM = 0.94 L-min(-1), LOA = 0.1 +/- 2.8 L(.)min(-1), 4% < CV < 10%, day 2 = (1.0 X day 1) + 0, r = 0.91, P < 0.001, r(2) = 0.82). Estimates of Q(C) from open-circuit acetylene breathing demonstrated good agreement with closed-circuit acetylene rebreathing (young: SEM = 1.52 L(.)min(-1), LOA = 0.9 +/- 4.4 L-min(-1), 5% < CV < 10%, open-circuit = (1.0 X closed-circuit) + 1.5, r = 0.89, P < 0.001, r(2) = 0.79; older: SEM = 1.13 L(.)min(-1), LOA = 0.1 +/- 3.2 L(.)min(-1), 5% < CV < 9%, open-circuit = (0.9 X closed-circuit) + 1.6, r = 0.88, P < 0.001, r(2) = 0.78). Conclusion: These results demonstrate that open-circuit acetylene breathing provides reproducible measurements of Q(C) during exercise that demonstrate good agreement with values obtained from the acetylene rebreathing procedure in young and older healthy men.