Human femoral artery and estimated muscle capillary blood flow kinetics following the onset of exercise

The purpose of this study was to compare the kinetics of estimated capillary blood flow ((Q)over dot(cap)) to those of femoral artery blood flow ((Q)over dot(FA)) and estimated muscle oxygen uptake ((V)over dot(O2m)). Nine healthy subjects performed a series of transitions from rest to moderate (below estimated lactate threshold, 6 min bouts) knee extension exercise. Pulmonary oxygen uptake ((V)over dot(O2)) was measured breath by breath, (Q)over dot(FA) was measured continuously using Doppler ultrasound, and deoxyhaemoglobin ([HHb]) was estimated by near-infrared spectroscopy over the rectus femoris throughout the tests. The time course of (Q)over dot(cap) was estimated by rearranging the Fick equation (i.e. (Q)over dot(cap) = (V)over dot(O2m)/(a-v)O-2), (arterio-venous O-2 difference) using the primary component of (V)over dot(O2) to represent (V)over dot(O2m) and [HHb] as a surrogate for (a-v) O-2. The overall kinetics of (Q)over dot(FA) (mean response time, MRT, 13.7 +/- 7.0 s), (V)over dot(O2m) (tau, 27.8 +/- 9.0 s) and (Q)over dot(cap) (MRT, 41.4 +/- 19.0 s) were significantly (P < 0.05) different from each other. We conclude that for moderate intensity knee extension exercise, conduit artery blood flow ((Q)over dot(FA)) kinetics may not be a reasonable approximation of blood flow kinetics in the microcirculation ((Q)over dot(cap)), the site of gas exchange. This temporal dissociation suggests that blood flow may be controlled differently at the conduit artery level than in the microcirculation.