Systemic and vastus lateralis muscle blood flow and O2 extraction during ramp incremental cycle exercise

During ramp incremental cycling exercise increases in pulmonary O-2 uptake (V?o2p) are matched by a linear increase in systemic cardiac output (Q). However, it has been suggested that blood flow in the active muscle microvasculature does not display similar linearity in blood flow relative to metabolic demand. This study simultaneously examined both systemic and regional (microvascular) blood flow and O-2 extraction during incremental cycling exercise. Ten young men ((V) over dot(O2) peak = 4.2 +/- 0.5 l/min) and 10 young women ((V) over dot(O2) peak = 3.2 +/- 0.5 l/min) were recruited to perform two maximal incremental cycling tests on separate days. The acetylene open-circuit technique and mass spectrometry and volume turbine were used to measure Q (every minute) and breath-by-breath (V) over dot(O2p), respectively; systemic arterio-venous O-2 difference (a-vO2diff) was calculated as (V) over dot(O2)p/Q on a minute-by-minute basis. Changes in near-infrared spectroscopy-derived muscle deoxygenation (?[HHb]) were used (in combination with (V) over dot(O2) data) to estimate the profiles of peripheral O-2 extraction and blood flow of the active muscle microvasculature. The systemic Q-to-(V) over dot(O2p) relationship was linear (similar to 5.8 l/min increase in Q for a 1 l/min increase in (V) over dot(O2)) with a-vO2diff displaying a hyperbolic response as exercise intensity increased toward (V) over dot(O2) peak. The peripheral blood flow response profile was described by an inverted sigmoid curve, indicating nonlinear responses relative to metabolic demand. The ?[HHb] profile increased linearly with absolute (V) over dot(O2)p until high-intensity exercise, thereafter displaying a near-plateau. Results indicate that systemic blood flow and thus O-2 delivery does not reflect the profile of blood flow changes at the level of the microvasculature.