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The cerebral circulation is protected from systemic blood pressure surges by a complex branching system and two resistance elements: the first of these lies in the large cerebral arteries, and the second in vessels of diameter <100 μm. Endothelial cells in cerebral capillaries contain few pinocytic vesicles and are sealed with tight junctions, without any anatomical gap. Several endogenous substances including catecholamines and vascular growth enhancing factor can dynamically modulate blood-brain barrier (BBB) permeability. Classical cerebral autoregulation assessment does not consider the latency of autoregulatory mechanisms, focusing instead on the maintenance of cerebral blood flow (CBF) at different steady state levels of cerebral perfusion pressure (CPP). Methods of measuring CBF may be regional or global, and may be applicable either to humans or primarily to experimental animals. Severe head injury is accompanied by both direct and indirect effects on CBF and metabolism, which show both temporal and spatial variations.
A number of technologies aimed at detecting oxygen supply/demand imbalance have been developed of which jugular bulb oximetry is the most mature. More recently, near-infrared spectroscopy and direct brain tissue oxygen measurement have become clinically available. The brain extracts oxygen from arterial blood at a rate to supply its global metabolic requirements leaving an oxygen-poor venous effluent. Jugular venous oxygenation can be measured intermittently by serial blood sampling or continuously by fibre-optic oximetry. Jugular bulb oximetry can be used to detect disorders of both cerebral autoregulation and carbon dioxide reactivity. Cerebral oxygenation has been studied by jugular bulb cannulation during aneurysm clipping surgery. Jugular venous saturation has been studied as a potential prognostic marker in comatose patients in which spontaneous circulation has been restored after cardiac arrest. Experience with brain tissue oxygenation microsensors is increasing and clearly these provide a very direct measurement of tissue metabolism.