Allison J. Harper

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 to those of femoral artery blood flow and estimated muscle oxygen uptake . 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 was measured breath by breath, 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 was estimated by rearranging the Fick equation (i.e. ), (arterio – venous O2 difference) using the primary component of to represent and [HHb] as a surrogate for (a−v)O2. The overall kinetics of (mean response time, MRT, 13.7 ± 7.0 s), (τ, 27.8 ± 9.0 s) and (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 kinetics may not be a reasonable approximation of blood flow kinetics in the microcirculation , 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.

Kinetics of estimated human muscle capillary blood flow during recovery from exercise.

The kinetic characteristics of muscle capillary blood flow during recovery from exercise are controversial (e.g. one versus two phases). Furthermore, it is not clear how the overall kinetics are temporally associated with muscle oxygen uptake kinetics. To address these issues, we examined the kinetics of estimated from the rearrangement of the Fick equation using the kinetics of pulmonary ( , primary component) and deoxy‐haemoglobin concentration ([HHb]) as indices of andC  (a − v)O 2(arterio‐venous oxygen difference) kinetics, respectively. (l min−1) was measured breath by breath and [HHb] (μm) was measured by near infrared spectroscopy during moderate (M; below lactate threshold, LT) and heavy exercise (H, above LT) in nine subjects. The kinetics of were biphasic, with an initial fast phase (τI; M = 9.3 ± 4.9 s and H = 6.0 ± 3.8 s) followed by a slower phase 2 (τP; M = 29.9 ± 8.6 s and H = 47.7 ± 26.0 s). For moderate exercise, the overall kinetics of (mean response time [MRT], 36.1 ± 8.6 s) were significantly slower than the kinetics of (τP; 27.8 ± 5.3 s) and [HHb] (MRT for [HHb]; 16.2 ± 6.3 s). However, for heavy exercise, there was no significant difference between MRT‐[HHb] (34.7 ± 10.4 s) and τP for (32.3 ± 6.7 s), while MRT for (48.7 ± 21.8 s) was significantly slower than MRT for [HHb] and τP for . In conclusion, during recovery from exercise the estimated kinetics were biphasic, showing an early rapid decrease in blood flow. In addition, the overall kinetics of were slower than the estimated kinetics.

Matching of blood flow to metabolic rate during recovery from moderate exercise in humans

It is unclear whether measurement of limb or conduit artery blood flow during recovery from exercise provides an accurate representation of flow to the muscle capillaries where gas exchange occurs. To investigate this, we: (a) examined the kinetic responses of femoral artery blood flow ( ), estimated muscle capillary blood flow ( ) and estimated muscle oxygen uptake ( ) following cessation of exercise; and (b) compared these responses to verify the adequacy of O2 delivery during recovery. Pulmonary ( ) was measured breath by breath, was measured using Doppler ultrasonography, and deoxy‐haemoglobin/myoglobin (deoxy‐[Hb/Mb]) was estimated by near‐infrared spectroscopy over the rectus femoris in nine healthy subjects during a series of transitions from moderate knee‐extension exercise to rest. The time course of was estimated by rearranging the Fick equation [i.e. ], using the primary component of to represent and deoxy‐[Hb/Mb] as a surrogate for arteriovenous O2 difference. There were no significant differences among the overall kinetics of (τ, 31.4 ± 8.2 s), [mean response time (MRT), 34.5 ± 20.4 s] and (MRT, 31.7 ± 14.7 s). The kinetics were also significantly correlated (P < 0.05) with those of both and . Both and appear to be coupled with during recovery from moderate knee‐extension exercise, such that extraction falls (thus cellular energetic state is not further compromised) throughout recovery.