[Non-invasive measurement of stroke volume in steady-state and unsteady-state workload with CO2 rebreathing]

During steady-state exercise the noninvasive measurement of cardiac output using CO2-rebreathing has been found to be reliable and reproducible. In contrast, reliability of cardiac output measurement during unsteady state exercise is unclear. The ability to determine cardiac output (CO) noninvasively during steady state and unsteady state exercise was assessed in nine healthy students aged 25.7 +/- 7.4 years. Two cycle ergometer exercise tests were performed, one maximal unsteady state test with 25 watts increment of workload per minute, and also one steady state test at 25, 50, and 75 percent of max. VO2. CO was measured using the equilibrium CO2-rebreathing technique during unloaded cycling in both tests, at 75 and 150 watts in the unsteady state test and at all workloads during steady state exercise. Mean max. VO2 was 31.4 +/- 5.9 ml/kg/min and mean VO2 at the anaerobic threshold 24.5 +/- 7.2 ml/kg/min, respectively. During unsteady state exercise the CO2/workload slope was linear (r = 0.973), as with steady state exercise (r = 0.976). There was no difference concerning the slopes of both curves, but the elevation of VO2 with unsteady state exercise was lower, compared to steady state (p < 0.005). The relationships of CO/VO2 during unsteady and steady state exercise were best expressed by linear equations: CO = 7.49 x VO2 + 2.35 (r = 0.866) and CO = 8.24 x VO2 + 1.4 (r = 0.852), respectively. Similar to VO2/workload, both regressions did not have different slopes, but did have different elevations (p < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)