Kinetics of VO2 with impulse and step exercise in humans

Abstract

The constancy of the time course (i.e., dynamic linearity) of the O2 uptake (VO2) response to exercise was examined by testing the law of superposition on data from impulse and step work rate forcings. Two impulses (10 s at a 235-W increase above a 25-W base line, I-235; and 5 s at a 475-W increase above a 25-W base line, I-475), four steps (ST) (25-65 W, ST1; 65-105 W, ST2; 25-105 W, ST3; and 25-145 W, ST4), and the corresponding off-transient responses were performed six to eight times by each of five subjects. The integrated area (G) of the VO2 response for I-235 was similar to that of ST1 and ST2 (P greater than 0.05); the I-475 G was significantly greater (P less than 0.05). The time constant of VO2 during the step function on-transient response for the second exponential component was significantly faster for ST1 and significantly slower for I-235 and I-475 than for ST2, ST3, and ST4 (P less than 0.05). However, I-235 and I-475 time constants for VO2 were not different from the ST off-transient values. Attempts to superimpose the integral of the impulse on the ST data showed that the early rapid increase in VO2 in the ST was underpredicted by the impulse and that the impulse response lagged behind the ST at all points before steady state. It can be concluded that VO2 kinetics failed the test of superposition and are therefore described by a nonlinear dynamic system.

Clinical Stress Testing in the Pediatric Age Group

“…64 -66 Consequently, because (1) most important exercise responses (eg, cardiac output, oxygen uptake, minute ventilation, heart rate) during exercise in children have response times Ͻ1 minute; (2) work rate increment is typically Ͻ30 W/min in pediatric subjects; and (3) desired submaximal measurements (eg, cardiac output) take Ͻ15 seconds to be completed, valid submaximal physiological responses to ramp changes in work rate can be obtained if appropriate and careful data analysis is performed. [67][68][69][70] …”

Oxygen Kinetics in Response to Impulse Work.

“…An impulse work load of 300 W was used for the following reasons. First, the maximum work rate of the ergometer used in this study was 300 W. Although this work rate can be set for a duration of 10 sec, as it was in a former study (Hughson et al, 1988), it takes a few seconds to attain 300 W. Thus, in order to obtain a clear effect of 300 W, we set the period of impulse work at 20 sec. The experiment was performed once a day and repeated three times on separate days.…”

“…oxygen uptake (VO2) responding to impulse work (Bakker et al, 1980;Hughson et al, 1988;Swanson, 1990;Whipp and Ward, 1991), and it has been demonstrated . that VO2 is composed of two components (phase I and phase II).…”

“…that VO2 is composed of two components (phase I and phase II). With regard to these components, Hughson et al (1988) have suggested that phase I reflects an immediate increase in pulmonary blood flow and phase II reflects . tissue VO2.…”

“…In this study, by comparing the change in deoxygenated hemoglobin (deoxy Hb) in the working muscle using NIRS during . impulse work with the change in VO2, we reexamined the hypothesis proposed by Hughson et al (1988).…”