Dynamic response of cerebral perfusion during low-intensity cycling exercise: A study using transcranial Doppler ultrasound
We investigated the changes in cerebral perfusion during constant low-intensity exercise and the effect of a late increase in workload. Eleven healthy males participated in this study. Four subjects performed 15 min of constant low-intensity exercise on a cycle ergometer, while seven subjects performed 20 min of exercise consisting of 17 min of low-intensity exercise, which was incremented to moderate-intensity for the final 3 min. As an index for cerebral perfusion, the time-averaged mean velocity of flow in the middle cerebral artery (MCAVmean) was measured using transcranial Doppler ultrasound. During the 15-min of low-intensity exercise (78 ± 3 Watts), pulmonary oxygen consumption (VO2) increased to 19.1 ± 2.5 ml/min/kg within 3-6 min, while end-tidal pressure of carbon dioxide (PETCO2) increased to 42.0 ± 3.2 mmHg, and MCAVmean increased to a peak 19.2 ± 9.1 % above the pre-exercise resting level then gradually decreased back toward the resting level. During the 20-min exercise at low and then moderate intensity (77 ± 4 and 111 ± 7 Watts for low and moderate intensity, respectively), VO2 increased from 19.1 ± 2.8 to 24.2 ± 3.6 ml/min/kg after the late increment in exercise intensity, while PETCO2 remained unchanged (p = 0.48, Tukey’s post hoc test), and MCAVmean tended to increase but did not change significantly. Kinetic analysis of VO2 and MCAVmean at low and moderate exercise intensities using a monoexponential model revealed that the time constant (τ) for VO2 was significantly related to the τ for MCAVmean at low (n=9) (R2 =0.52, P =0.03) and moderate intensity (n=6) (R2 = 0.69, P =0.04). These findings imply that cardiac output exerts an indirect effect to alter cerebral perfusion and that cerebral autoregulation likely operates to stabilize cerebral blood flow during prolonged exercise at a constant workload.
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