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

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). VO2 was measured breath by breath and cardiac output (Q) was estimated beat by beat by impedance cardiography. Kinetic analyses of the VO2 and Q data were performed to obtain the gain and the total lag time. With hypoxia, the gain of the VO2 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 (p less than 0.05). In contrast, with ramp exercise there was a significant reduction in the gain or slope of the VO2 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 Q during the step transition did not differ between normoxia and hypoxia, at 26.5(2.3) v 31.0(4.9) s, respectively (NS).(ABSTRACT TRUNCATED AT 250 WORDS)