Regulation of middle cerebral artery blood velocity during dynamic exercise in humans: influence of aging

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 arterial carbon dioxide (Pa-CO2) were assessed at rest and during steady-state cycling at 30% and 50% heart rate reserve (HRR) in 9 young (24 +/- 3 yr; mean +/- SD) and 10 older middle-aged (57 +/- 7 yr) subjects. Transfer function analysis between changes in MAP and mean MCAV (MCAV(mean)) in the low-frequency (LF) range were used to assess dynamic CA. No age-group differences were found in Pa-CO2 at rest or during cycling. Exercise-induced increases in MAP were greater in older subjects, while changes in MCAV(mean) were similar between groups. The cerebral vascular conductance index (MCAV(mean)/MAP) was not different at rest (young 0.66 +/- 0.04 cm.s(-1).mmHg(-1) vs. older 0.67 +/- 0.03 cm.s(-1).mmHg(-1); mean +/- SE) or during 30% HRR cycling between groups but was reduced in older subjects during 50% HRR cycling (young 0.67 +/- 0.03 cm.s(-1).mmHg(-1) vs. older 0.56 +/- 0.02 cm.s(-1).mmHg(-1); P < 0.05). LF transfer function gain and phase between MAP and MCAV(mean) was not different between groups at rest (LF gain: young 0.95 +/- 0.05 cm.s(-1).mmHg(-1) vs. older 0.88 +/- 0.06 cm.s(-1).mmHg(-1); P > 0.05) or during exercise (LF gain: young 0.80 +/- 0.05 cm.s(-1).mmHg(-1) vs. older 0.72 +/- 0.07 cm.s(-1).mmHg(-1) at 50% HRR; P > 0.05). We conclude that despite greater increases in MAP, the regulation of MCAV(mean) is well maintained during dynamic exercise in healthy older middle-aged subjects.