NONINVASIVE MEASUREMENT OF CARDIAC-OUTPUT IN STEADY-STATE AND UNSTEADY-STATE EXERCISE USING CO2-REBREATHING

During steady-state exercise the noninvasive measurement of cardiac ou tput using CO2-rebreathing has been found to be reliable and reproduci ble. In contrast, reliability of cardiac output measurement during uns teady state exercise is unclear. The ability to determine cardiac outp ut (CO) noninvasively during steady state and unsteady state exercise was assessed in nine healthy students aged 25.7 +/- 7.4 years. Two cyc le ergometer exercise tests were performed, one maximal unsteady state test with 25 watts increment of workload per minute, and also one ste ady state test at 25, 50, and 75 percent of max. VO2. CO was measured using the equilibrium CO2-rebreathing technique during unloaded cyclin g 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 +/- 72 ml /kg/min, respectively. During unsteady state exercise the CO2/workload slope was linear (r = 0.973), as with steady state exercise (r = 0.97 6). 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 unstea dy 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 = 0.85 2), respectively. Similar to VO2/workload, both regressions did not ha ve different slopes, but did have different elevations (p < 0.0001). W e conclude that measurement of cardiac output can be obtained during s teady state and unsteady state exercise in healthy subjects using the equilibrium CO2-rebreathing technique. During unsteady state exercise similar values at comparable VO2 to those obtained in steady state exe rcise can be measured.