On issues of confidence in determining the time constant for oxygen uptake kinetics

Abstract

Background: τV̇O2 at the onset of constant work rate (CWR) exercise is a variable of aerobic fitness that shortens with physical training and lengthens with cardiopulmonary disease. Determination of τV̇O2 with sufficiently high confidence has typically required multiple exercise transitions limiting its clinical application. Objectives: To design a protocol to determine τV̇O2 reliably but simply. Methods: On each of three days, five healthy men performed two CWR tests on a cycle ergometer below the metabolic threshold (V̇O 2θ) for blood lactate accumulation as determined by gas exchange measurements followed by an incremental work rate (IWR) test. τV̇O2 was determined (a) from the on-transit (on-τV̇O2) and off-transit (off-τV̇O2) of six CWR tests both individually and superimposed, using non-linear regression with a monoexponential model, and (b) by geometric analysis of the IWR tests (ramp-τV̇O2). Results: Group means (SD) were: V̇O 2MAX 3.84 (0.44) litres/min, V̇O2θ 1.88 (0.23) litres/min, steady state exercise V̇O2 1.67 (0.07) litres/min, on-τV̇O2 38.0 (5.3) seconds, off-τV̇O2 39.0 (4.3) seconds, and ramp-τV̇O2 60.8 (15.4) seconds. On-τV̇O2 correlated with off-τV̇O2 (r = 0.87), V̇O2MAX (r = -0.73), and V̇O2θ (r = 0.89). The pooled mean τV̇O2 from six superimposed tests agreed with the arithmetic grand mean of the six tests. Conclusions: The average of on-τV̇O2 and off-τV̇O2 fell within the 95% confidence interval of the pooled mean by the second test. Ramp-τV̇O2 was longer and less reproducible. These findings support the use of both on- and off-transit data for the determination of τV̇O2, an approach that reduces the number of transitions necessary for accurate determination of τV̇O2, potentially enhancing its clinical application.