Influence of phase I duration on phase II (V) over dotO2 kinetics parameter estimates in older and young adults

Murias JM, Spencer MD, Kowalchuk JM, Paterson DH. Influence of phase I duration on phase II (V) over dotO(2) kinetics parameter estimates in older and young adults. Am J Physiol Regul Integr Comp Physiol 301: R218-R224, 2011. First published April 13, 2011; doi:10.1152/ajpregu. 00060.2011.-Older adults (O) may have a longer phase I pulmonary O-2 uptake kinetics ((V) over dotO(2p)) than young adults (Y); this may affect parameter estimates of phase II (V) over dotO(2p). Therefore, we sought to: 1) experimentally estimate the duration of phase I (V) over dotO(2p) (EE phase I) in O and Y subjects during moderate-intensity exercise transitions; 2) examine the effects of selected phase I durations (i.e., different start times for modeling phase II) on parameter estimates of the phase II (V) over dotO(2p) response; and 3) thereby determine whether slower phase II kinetics in O subjects represent a physiological difference or a by-product of fitting strategy. (V) over dotO(2p) was measured breath-by-breath in 19 O (68 +/- 6 yr; mean +/- SD) and 19 Y (24 +/- 5 yr) using a volume turbine and mass spectrometer. Phase I(V) over dotO(2p) was longer in O (31 +/- 4 s) than Y (20 +/- 7 s) (P +/- 0.05). In O, phase II +/- (V) over dotO(2p) was larger (P +/- 0.05) when fitting started at 15 s (49 +/- 12 s) compared with fits starting at the individual EE phase I (43 +/- 12 s), 25 s (42 +/- 10 s), 35 s (42 +/- 12 s), and 45 s (45 +/- 15 s). In Y, +/- (V) over dotO(2p) was not affected by the time at which phase II (V) over dotO(2p) fitting started (tau(V) over dotO(2p) +/- 31 +/- 7 s, 29 +/- 9 s, 30 +/- 10 s, 32 +/- 11 s, and 30 +/- 8 s for fittings starting at 15 s, 25 s, 35 s, 45 s, and EE phase I, respectively). Fitting from EE phase I, 25 s, or 35 s resulted in the smallest CI +/- (V) over dotO(2p) in both O and Y. Thus, fitting phase II (V) over dotO(2p) from (but not constrained to) 25 s or 35 s provides consistent estimates of (V) over dotO(2p) kinetics parameters in Y and O, despite the longer phase I(V) over dotO(2p) in O.