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LETTER TO THE EDITOR

Kinetics of muscle oxygen use, oxygen content, and blood flow during exercise

Ferreira, Leonardo F., Dana K. Townsend, Barbara J. Lutjemeier, and Thomas J. Barstow. Muscle capillary blood flow kinetics estimated from pulmonary O₂ uptake and near-infrared spectroscopy. J Appl Physiol 98: 1820–;1828, 2005. First published January 7, 2005; doi:10.1152/japplphysiol.00907.2004.— The near-infrared spectroscopy (NIRS) signal (deoxy-hemoglobin concentration; [HHb]) reflects the dynamic balance between muscle capillary blood flow (_cap) and muscle O₂ uptake (O_2m) in the microcirculation. The purposes of the present study were to estimate the time course of _cap from the kinetics of the primary component of pulmonary O₂ (O_2p) and [HHb] throughout exercise, and compare the _cap kinetics with the O_2p kinetics. Nine subjects performed moderate- (M; below lactate threshold) and heavy-intensity (H, above lactate threshold) constant-work-rate tests. O_2p (l/min) was measured breath by breath, and [HHb] (µM) was measured by NIRS during the tests. The time course of _cap was estimated from the rearrangement of the Fick equation [_cap = O_2m/(a-v)O₂, where (a-v)O₂ is arteriovenous O₂ difference] using O_2p (primary component) and [HHb] as proxies of O_2m and (a-v)O₂, respectively. The kinetics of [HHb] [time constant () + time delay [HHb]; M = 17.8 ± 2.3 s and Ç = 13.7 ± 1.4 s] were significantly (P < 0.001) faster than the kinetics of O₂ [ of primary component (_P); M = 25.5 ± 8.8 s and H = 25.6 ± 7.2 s] and _cap [mean response time (MRT); M = 25.4 ± 9.1 s and H = 25.7 ± 7.7 s]. However, there was no significant difference between MRT of _cap and _P-O₂ for both intensities (P = 0.99), and these parameters were significantly correlated (M and H; r = 0.99; P < 0.001). In conclusion, we have proposed a new method to noninvasively approximate _cap kinetics in humans during exercise. The resulting overall _cap kinetics appeared to be tightly coupled to the temporal profile of O_2m.

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Kansas State University
Manhattan, Kansas
e-mail: tbarsto{at}ksu.edu

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