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1 Department of Integrative Physiology, University of North Texas Health Science Center, Fort Worth, Texas, USA
2 Department of Anesthesia, The Copenhagen Muscle Research Center, Rigshospitalet, Copenhagen, Copenhagen, Denmark
* To whom correspondence should be addressed. E-mail: Sogoh{at}hsc.unt.edu.
We investigated whether dynamic CA is affected by exhaustive exercise using transfer function gain and phase shift between oscillations in mean arterial pressure (MAP) and middle cerebral artery mean blood flow velocity (MCA Vmean). Seven subjects were instrumented with a brachial artery catheter for measures of MAP and determination of PaCO2, while jugular venous oxygen saturation (SvO2) 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) continued to exhaustion with a group average time of 26.8 ± 5.8 min. Despite no significant change in MAP during exercise, MCA Vmean decreased from 70.2 ± 3.6 to 57.4 ± 5.4 cm/s and SvO2 from 68 ± 1% to 58 ± 2% at exhaustion and both correlated to PaCO2 (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/(glucose + 1/2 lactate) from 5.6 to 3.8 (P<0.05). At the same time, the normalized low frequency gain between MAP and MCA Vmean was increased (P<0.05), while the phase shift tended to decrease. These findings suggest that dynamic CA was impaired by exhaustive exercise despite a hyperventilation-induced reduction in PaCO2.
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