RELATIONSHIP OF MIDDLE CEREBRAL-ARTERY BLOOD-FLOW VELOCITY TO INTENSITY DURING DYNAMIC EXERCISE IN NORMAL SUBJECTS

Cerebral blood flow has been reported to increase during dynamic exerc ise, but whether this occurs in proportion to the intensity remains un settled. We measured middle cerebral artery blood flow velocity (nu(m) ) by transcranial Doppler ultrasound in 14 healthy young adults, at re st and during dynamic exercise performed on a cycle ergometer at a int ensity progressively increasing, by 50 W every 4 min until exhaustion. Arterial blood pressure, heart rate, end-tidal, partial pressure of c arbon dioxide (P(ET)CO2), oxygen uptake (VO2) and carbon dioxide outpu t were determined at exercise intensity. Mean nu(m) increased from 53 (SEM 2) cm.s-1 at rest to a maximum of 75 (SEM 4) cm.s-1 at 57% of the maximal attained VO2 (VO2max), and thereafter progressively decreased to 59 (SEM 4) cm.s-1 at VO2max. The respiratory exchange ratio (R) wa s 0.97 (SEM 0.01) at 57% of VO2max and 1.10 (SEM 0.01) at VO2max. The P(ET)CO2 increased from 5.9 (SEM 0.2) kPa at rest to 7.4 (SEM 0.2) kPa at 57% of VO2max, and thereafter decreased to 5.9 (SEM 0.2) kPa at VO 2max. Mean arterial pressure increased from 98 (SEM 1) mmHg (13.1 kPa) at rest to 116 (SEM 1) mmHg (15.5 kPa) at 90% of VO2max, and decrease d slightly to 108 (SEM 1) mmHg (14.4 kPa) at VO2max. In all the subjec ts, the maximal value of nu(m) was recorded at the highest attained ex ercise intensity below the anaerobic threshold (defined by R greater t han 1). We concluded that cerebral blood flow as evaluated by middle c erebral artery flow velocity increased during dynamic exercise as a fu nction of exercise intensity below the anaerobic threshold. At higher intensities, cerebral blood flow decreased, without however a complete return to baseline values, and it is suggested that this may have bee n at least in part explained by concomitant changes in arterial PCO2.