Cerebral blood flow and metabolism during exercise

During exercise regional cerebral blood flow (rCBF), as blood velocity in major cerebral arteries and also brood flow in the internal carotid artery increase, suggesting an increase in blood flow to a large part of the brain. Such an increase in CBF is independent of the concomitant increase in blood pressure but is modified by the alteration in arterial carbon dioxide tension (PaCO2). Also, the increase in middle cerebral artery mean blood velocity (MCA V-mean) reported with exercise appears to depend on the ability to increase cardiac output (CO), as demonstrated in response to beta-1 blockade and in patients with cardiac insufficiency or atrial fibrillation. Near-infrared spectroscopy (NIRS) determined cerebral oxygenation supports the alterations in MCA V-mean during exercise. Equally, the observation that the cerebrovascular CO2-reactivity appears to be smaller in the standing than in the sitting and especially in the supine position could relate to the progressively smaller CO. In contrast, during exercise "global" cerebral blood flow (gCBF), as determined by the Kety-Schmidt technique is regarded as bring constant. One limitation of the Kety-Schmidt method for measuring CBF is that blood flow in the two internal jugular veins depends on the origin of drainage and it has not been defined which internal jugular venous flow is evaluated. Such a consideration is equally relevant for an evaluation of cerebral metabolism during exercise. While the regional cerebral uptake of oxygen (O-2) increases during exercise, the global value is regarded as being constant. Yet, during high intensity exercise lactate is taken up by the brain and its O-2 uptake also increases. Furthermore, in the initial minutes of recovery immediately following exercise, brain glucose and O-2 uptake are elevated and lactate uptake remains high. A maintained substrate uptake by the brain after exercise suggests a role for brain glycogen in cerebral activation, but the fate of brain substrate uptake has not yet been determined. (C) 2000 Elsevier Science Ltd. All rights reserved.