Submitted on February 1, 2008
Accepted on May 7, 2008
Regulation of Middle Cerebral Artery Blood Velocity during Dynamic Exercise in Humans: Influence of Aging
James P Fisher1, Shigehiko Ogoh2, Colin N. Young1, Peter B Raven2, and Paul J Fadel3*
1 Medical Pharmacology and Physiology, University of Missouri, Columbia, Missouri, United States; Veterans Affairs Medical Center, Harry S. Truman Memorial Veterans Hospital, Columbia, Missouri, United States
2 Integrative Physiology, University of North Texas Health Science Center, Fort Worth, Texas, United States
3 Medical Pharmacology and Physiology, University of Missouri, Columbia, Missouri, United States; Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri, United States; Veterans Affairs Medical Center, Harry S. Truman Memorial Veterans Hospital, Columbia, Missouri, United States
* To whom correspondence should be addressed. E-mail: fadelp{at}health.missouri.edu.
Although cerebral autoregulation (CA) appears well-maintained during mild to moderate intensity dynamic exercise in young subjects, it is presently unclear how aging influences the regulation of cerebral blood flow during physical activity. Therefore, to address this question, middle cerebral artery blood velocity (MCAV), mean arterial pressure (MAP), and the partial pressure of end-tidal carbon dioxide (PaCO2) were assessed at rest and during steady-state cycling at 30% and 50% heart rate reserve (HRR) in nine young (24±3 yrs; mean±SD) and ten older middle-aged (57±7 yrs) subjects. Transfer function analysis between changes in MAP and mean MCAV (MCAVmean) in the low-frequency (LF) range were used to assess dynamic CA. No age-group differences were found in PaCO2 at rest or during cycling. Exercise-induced increases in MAP were greater in older subjects, while changes in MCAVmean were similar between groups. The cerebral vascular conductance index (MCAVmean/MAP) was not different at rest (0.66±0.04 young vs. 0.67±0.03 older cm/s/mmHg; mean±SE) or during 30%-HRR cycling between groups, but was reduced in older subjects during 50%-HRR cycling (0.67±0.03 young vs. 0.56±0.02 older cm/s/mmHg; P<0.05). LF transfer function gain and phase between MAP and MCAVmean was not different between groups at rest (LF gain: 0.95±0.05 young vs. 0.88±0.06 older cm/s/mmHg; P>0.05) or during exercise (LF gain: 0.80±0.05 young vs. 0.72±0.07 older cm/s/mmHg at 50%-HRR; P>0.05). We conclude that despite greater increase in MAP the regulation of MCAVmean is well-maintained during dynamic exercise in healthy older middle-aged subjects.
