Muscle Capillary and Femoral Artery Blood Flow Kinetics following the Onset of Exercise

Abstract

The kinetics of limb blood flow following the onset of exercise have been shown to be similar to, or faster than, the kinetics of estimated muscle oxygen consumption (VO). This information has been used to support the notion that VO2m kinetics are not limited by blood flow (and thus oxygen delivery) at least for moderate exercise. However no direct comparisons have been made to determine if limb blood flow kinetics are a reasonable approximation of blood flow kinetics in the capillary bed where oxygen exchange occurs. PURPOSE: To compare the kinetics of estimated capillary blood flow (Qca) to those of femoral artery blood flow (QFA) and VO2m. METHODS: Nine healthy subjects performed a series of transitions from rest to moderate (< estimated lactate threshold, LT, 6 min bouts) knee extension exercise. Pulmonary VO2 (VO2) was measured breath-by-breath, QFA was measured using Doppler ultrasonography, and deoxy-hemoglobin ([HHb]) was estimated by near-infrared spectroscopy (NIRS) over the rectus femoris throughout the tests. VO2(second-by-second), [HHb] and QFA were time-aligned to the onset of exercise for each exercise bout and ensemble-averaged across bouts for each subject to generate a single data set for each variable, filtered in the frequency domain, then fit using nonlinear regression. The time course of Q was estimated by rearranging the Fick equation (i.e., Qca (t) p= VO2m(t)/[HHb](t)), using the primary component of pulmonary oxygen consumption (VO2) to represent VO2m and [HHb] as a surrogate for (a-v)O2. RESULTS: The overall kinetics of QFA (mean response time, MRT, 13.7 ± 7.0 s), VO2m (α, 27.8 ± 9.0 s) and Qcap (MRT, 41.4 ± 19.0 s) were significantly (P <0.05) different from each other. CONCLUSIONS: For <LT knee extension exercise, conduit artery blood flow (QFA) kinetics may not be a reasonable approximation of blood flow kinetics in the micro circulation (Qca), the site of gas exchange. This temporal dissociation suggests that blood flow may be controlled differently at the conduit artery level than in the micro circulation. The extent to which the slower kinetics of Qca reflect a possible limitation in O delivery, thereby limiting VO kinetics, is unclear from the results of this study.