A study of cardiorespiratory dynamics with step and ramp exercise tests in normoxia and hypoxia

Open Access
Publisher Website
Google Scholar
Abstract
The ability of the cardiorespiratory system to adapt to the demands of increased exercise intensity was studied under the effects of hypoxia with two different submaximal cycling exercise protocols. A step transition in work rate from 25 W to 105 W and a ramp increase in work rate from a baseline of 25 W with a slope of 40 W·min−1 were used. Eight healthy male subjects each completed maximal exercise tests to exhaustion, as well as six repetitions of the step and the ramp, during normoxia and during hypoxia (FIO2 = 14%, balance N2). o2 was measured breath by breath and cardiac output () was estimated beat by beat by impedance cardiography. Kinetic analyses of the o2 and data were performed to obtain the gain and the total lag time. With hypoxia, the gain of the o2 response was no different from normoxia, at 10.6(SEM 0.1) ml·min−1·W−1, but total lag time increased from 23.0(2.2) s to 32.5(2.6) s (p2 response from 9.4(0.2) ml·min−1·W−1 in normoxia to 9.0(0.1) ml·min−1·W−1 in hypoxia, while total lag time was no different between normoxia and hypoxia, at 19.7(3.3) v 21.4(2.9) s. Both the gain and total lag time of the step tests were significantly different from the ramp test. Kinetics of during the step transition did not differ between normoxia and hypoxia, at 26.5(2.3) v 31.0(4.9) s, respectively (NS). Slower kinetics of o2 during the step increase in work rate appear to be caused by a reduction in arterial O2 content. The apparent lack of difference in the kinetics of o2 with hypoxia in the ramp exercise test is not a result of improved O2 transport. Rather, the reduction in the slope of the o2 versus time relationship shows a progressive inability to meet the metabolic demands by O2 transport. The ramp test protocol is not suited for the study of dynamic characteristics of the cardiorespiratory system.