Muscle capillary blood flow kinetics estimated from pulmonary O2 uptake and near-infrared spectroscopy

The near-infrared spectroscopy (NIRS) signal ( deoxyhemoglobin concentration; [HHb]) reflects the dynamic balance between muscle capillary blood flow ((Q) over dot(cap)) and muscle O-2 uptake ((V) over dot O-2m) in the microcirculation. The purposes of the present study were to estimate the time course of (Q) over dot(cap) from the kinetics of the primary component of pulmonary O-2 uptake ((V) over dot O-2p) and [ HHb] throughout exercise, and compare the (Q) over dot(cap) kinetics with the (V) over dot O-2p kinetics. Nine subjects performed moderate- ( M; below lactate threshold) and heavy-intensity ( H, above lactate threshold) constant-work-rate tests. (V) over dot O-2p (l/min) was measured breath by breath, and [ HHb] (mu M) was measured by NIRS during the tests. The time course of (Q) over dot(cap) was estimated from the rearrangement of the Fick equation [(Q) over dot(cap) = (V) over dot O-2m/(a- v) O-2, where (a-v) O-2 is arteriovenous O-2 difference] using (V) over dot O-2p ( primary component) and [ HHb] as proxies of (V) over dot O-2m and ( a- v) O-2, respectively. The kinetics of [ HHb] [ time constant (tau) + time delay [HHb]; M = 17.8 +/- 2.3 s and H = 13.7 +/- 1.4 s] were significantly ( P < 0.001) faster than the kinetics of (V) over dot O-2 [tau of primary component (tau P); M = 25.5 +/- 8.8 s and H = 25.6 +/- 7.2 s] and (Q) over dot(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 (Q) over dot(cap) and tau(P)-(V) over dot O-2 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 (Q) over dot(cap) kinetics in humans during exercise. The resulting overall (Q) over dot(cap) kinetics appeared to be tightly coupled to the temporal profile of (V) over dot O-2m.