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

Abstract

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 (Q̇cap) and muscle O2 uptake (V̇o2m) in the microcirculation. The purposes of the present study were to estimate the time course of Q̇cap from the kinetics of the primary component of pulmonary V̇o2 (V̇o2p) and [HHb] throughout exercise, and compare the Q̇cap kinetics with the V̇o2p kinetics. Nine subjects performed moderate- (M; below lactate threshold) and heavy-intensity (H, above lactate threshold) constant-work-rate tests. V̇o2p (l/min) was measured breath by breath, and [HHb] (μM) was measured by NIRS during the tests. The time course of Q̇cap was estimated from the rearrangement of the Fick equation [Q̇cap = V̇o2m/(a-v)O2, where (a-v)O2 is arteriovenous O2 difference] using V̇o2p (primary component) and [HHb] as proxies of V̇o2m and (a-v)O2, respectively. The kinetics of [HHb] [time constant (τ) + 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 V̇o2 [τ of primary component (τP); M = 25.5 ± 8.8 s and H = 25.6 ± 7.2 s] and Q̇cap [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 Q̇cap and τP-V̇o2 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 Q̇cap kinetics in humans during exercise. The resulting overall Q̇cap kinetics appeared to be tightly coupled to the temporal profile of V̇o2m.