Cerebral Blood Flow and Anaesthesia : a Review

Adequate cerebral blood flow to each part of the brain is essential for its normal functioning as cerebral tissues are very intolerant of hypoxia. The cessation of cerebral blood flow/ circulation for a few minutes can produce permanent damage to the brain. In addition, the CBF determines the cerebral blood volume (CBV) which itself is the net result of cerebral arterial supply and venous drainage. The CBV being a major (60-65%) and variable component of intracranial contents, plays an important role in determining its total volume. The cranium being a closed hard case (except in young children with open fontanelles or suture lines) with limited compliance, the abnormal changes in CBV and intracranial contents manifest in the form of a proportionate change in intracranial pressure (ICP). An idealized pressure/ volume (ICP / CBV) relationship can be described to represent the intracranial compliance curve (Fig.1). Initially (Phase A), when the intracranial volume increases, the intracerebral pressure does not increase suggesting the presence of compensatory mechanisms. However, when the elastance of the system is limited (Phase B), a further increase in volume translates into a rise in pressure. Finally, when the compensatory mechanisms have been exhaustated, any slight change in volume leads to an exponential rise in pressure (Phase C)1. However, if the cranium is open, an increase in CBV will cause a brain bulge (tight brain) and a decrease will result in brain relaxation. CEREBRAL BLOOD FLOW AND ANAESTHESIA : A REVIEW