Age-related prolongation of phase I of V̇o₂ on-kinetics in healthy humans

Abstract

Data are lacking regarding age-related modifications of phase I (PhI) of pulmonary Vo(2) on-kinetics during moderate-intensity exercise. We studied three groups (aged 20-30, 40-50, and 60-70 years) of 10 normal subjects, who underwent one incremental and four below-gas exchange threshold constant-power cardiopulmonary exercise tests. Data from constant-power tests were time-aligned and averaged, and the PhI-phase II transition (PhI-IItr) determined when a sharp decrease from baseline of respiratory exchange ratio occurred. The Vo(2) phase II time constant (tau) was obtained by an exponential fitting starting 1) from PhI-IItr ("experimental" fitting strategy) and 2) after 20 s from exercise onset ("fixed-duration" fitting strategy). Assuming estimated arterial-venous O(2) concentration difference not to change with respect to resting value, cardiac output (CO) values at rest and PhI-IItr were obtained according to Fick's principle. Average pulmonary flow acceleration (AFA) during PhI was calculated as the ratio between CO increase during PhI and PhI duration. PhI duration was related to age (r = 0.74, P < 0.0001), increasing from 21 +/- 3 s to 27 +/- 3 s to 32 +/- 4 s in the 20-30, 40-50, and 60-70 age groups, respectively, and to AFA (r = -0.60, P < 0.001), but not to CO increase during PhI. With respect to the experimental fitting strategy, the fixed-duration strategy overestimated Vo(2) phase II tau the more the higher the subject's age, with a lower goodness of fit in the 60-70 group (SE 0.035 vs. 0.056, P < 0.01). In conclusion, PhI duration is related to age in healthy male humans and is linked to CO acceleration-rather than to increase-during PhI. A significant overestimation of phase II tau thus may occur in healthy elderly subjects and patients with a pathologically induced longer PhI duration when fitting data where the PhI-PhIItr was not experimentally determined but assumed to be a set value (i.e., 20 s).

Influence of phase I duration on phase II V̇o2 kinetics parameter estimates in older and young adults

“…Recently published data by Mezzani et al (28) found that in older adults (with an experimentally determined phase I of 31 s) using a fixed value of 20 s as the starting point for the fitting of phase II yielded a longer V O 2p of 47 s versus a fit from the experimentally estimated phase I with a V O 2p of 34 s. The present data with a fit from 15 s in O agree that the inclusion of phase I data in the fit can lengthen the phase II (by ϳ6 s). Our results differ from those of Mezzani et al (28) as to the magnitude of the effect of inclusion of the phase I data in the phase II fit. The discrepancy between these studies resides in the strategy used for fitting phase II V O 2p .…”

“…The discrepancy between these studies resides in the strategy used for fitting phase II V O 2p . In the paper of Mezzani et al (28), although the exponential fit function included a TD it is clear that it was not allowed to vary freely; that is, the TD was actually constrained to the fixed duration assigned to the phase I V O 2p (20 s) phase II exponential fit through the 20-s phase I point and resulted in a poor fit of the on transient between 20 and 60 s (see Fig. 1 in Mezzani et al for which the estimated phase II is 57 s versus the fit from 38 s in this subject giving a of 32 s).…”

“…Frequently, the phase I V O 2p duration has simply been assigned to last a fixed period of time (ϳ20 s) such that phase II would consistently start at the same point for every subject (7,9,25,26,34). Although a blood transit time from exercising muscles to pulmonary capillaries of ϳ20 s may be an acceptable physiological assumption, it has recently been shown that the cardiodynamic phase is longer in older adults, presumably the result of age-related changes in cardiovascular function (28). According to the modeling study of Barstow et al (2), there is an intrinsic relationship between phase I and phase II V O 2p such that for a given rate of adjustment of muscle O 2 utilization (V O 2m ) a slower rate of adjustment of blood flow (i.e., larger Q by ϳ10 s) at exercise onset would result in a lengthened phase I V O 2p (by ϳ2 s) and, as a consequence, a smaller estimated…”

“…As such, the slower phase II V O 2 kinetics in older (O) compared with young (Y) adults consistently reported in the literature (1,7,9,11,12,14,20) has been challenged as it may (in part) be the result of the fitting strategy instead of the actual physiological response as proposed elsewhere (28). Indeed, the implication from the Mezzani et al (28) study is that the difference in the phase II V O 2p kinetics between the older and young individuals consistently reported in these previous studies is explained by the method used to fit the data. However, it remains unclear whether, and to what extent, a longer phase I V O 2p would affect the fitting of the primary component V O 2p .…”

Similar level of impairment in exercise performance and oxygen uptake kinetics in middle-aged men and women with type 2 diabetes

“…The conditional expressions (U 1 and U2) limit the fitting of a particular phase to the period at and beyond the time delay associated with that phase. Fitting the cardiodynamic phase allowed us to more accurately establish the onset of the second phase (25), although this phase cannot be always described by an exponential term (18) and so, only its amplitude and duration (TD 2 Ϫ TD1) are presented ( Table 2).…”