Clinical Aspects of Cerebral Hypoxia
Cerebral hypoxia is the most common cause of disability and death in neurological and neurosurgical patients. It can be divided into hypoxic hypoxia, a reduction in the O2 content of the circulating blood, and ischemic hypoxia, a reduction in the volume of circulating blood per unit time. The normal value for the partial pressure of O2 (PaO2) in circulating blood is approximately 90 torr. A value below the normal range is defined as systemic hypoxemia; but the intact brain can function well at a PaO2 far below the normal range and does not suffer irreversible damage until the PaO2 is markedly depressed. Furthermore, in borderline cerebral hypoxia metabolism is deranged, but only in some cells, and the metabolic derangement may not be detectable clinically; vital neurons continue to produce and utilize a normal amount of energy, and the patient is neurologically normal. It is desirable, therefore, to define four levels or categories of systemic hypoxemia: (1) below the normal range of PaO2; (2) the range of PaO2 which results in an alteration of metabolism that is not detectable by neurological examination; (3) the range of PaO2 that depresses neurological function; (4) PaO2 values that result in irreversible hypoxic brain damage. The same categories are applicable to brain ischemia; mean cerebral blood flow (CBF) ranges from normal to values that result in permanent ischemic brain damage.
KeywordsCerebral Blood Flow Cerebral Autoregulation Brain Metabolism Systemic Arterial Pressure Ischemic Hypoxia
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