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Dynamic cerebral autoregulation during exhaustive exercise in humans

¹Department of Integrative Physiology, University of North Texas Health Science Center, Fort Worth, Texas; and ²Department of Anesthesia, Copenhagen Muscle Research Center, Rigshospitalet, University of Copenhagen, Denmark

Submitted 10 September 2004 ; accepted in final form 15 October 2004

We investigated whether dynamic cerebral autoregulation is affected by exhaustive exercise using transfer-function gain and phase shift between oscillations in mean arterial pressure (MAP) and middle cerebral artery (MCA) mean blood flow velocity (V_mean). Seven subjects were instrumented with a brachial artery catheter for measurement of MAP and determination of arterial PCO₂ (Pa_CO2) while jugular venous oxygen saturation (Sv_O2) was determined to assess changes in whole brain blood flow. After a 10-min resting period, the subjects performed dynamic leg-cycle ergometry at 168 ± 5 W (mean ± SE) that was continued to exhaustion with a group average time of 26.8 ± 5.8 min. Despite no significant change in MAP during exercise, MCA V_mean decreased from 70.2 ± 3.6 to 57.4 ± 5.4 cm/s, Sv_O2 decreased from 68 ± 1 to 58 ± 2% at exhaustion, and both correlated to Pa_CO2 (5.5 ± 0.2 to 3.9 ± 0.2 kPa; r = 0.47; P = 0.04 and r = 0.74; P < 0.001, respectively). An effect on brain metabolism was indicated by a decrease in the cerebral metabolic ratio of O₂ to [glucose + one-half lactate] from 5.6 to 3.8 (P < 0.05). At the same time, the normalized low-frequency gain between MAP and MCA V_mean was increased (P < 0.05), whereas the phase shift tended to decrease. These findings suggest that dynamic cerebral autoregulation was impaired by exhaustive exercise despite a hyperventilation-induced reduction in Pa_CO2.

cerebral autoregulation; carbon dioxide; blood flow; metabolism

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