Autoregulation Remains Intact during Stable Xenon Inhalation in the Baboon
For the measurement of cerebral blood flow (CBF) using the stable xenon/CT method to be recognized as a valuable adjunct to clinical management of ischemic brain syndromes and to both clinical and animal research, it must provide assessment of perfusion reserve. Does blood flow remain under control by metabolic need (autoregulation) or has it become pressure dependent, that is, on the elbow of the autoregulation curve? Usually, resting gray matter flow below 40 to 50 mL/(100 cm3-min) indicates that autoregulation is exceeded or impaired, and levels of 20 to 25 mL/(100 cm3-min) imply exhaustion of reserve with imminent danger of ischemic injury. Since xenon, in concentrations of 30% to 35% as used for clinical CBF determination, is known to alter blood flow with reports of 15% to 20% flow increase toward the end of the 5 min period of Xe breathing, it is important to be certain that autoregulation is not disturbed by the methodology. It has been shown that agents which are known to alter cerebrovascular resistance (CVR) and CBF do not necessarily affect the ability of the brain to regulate flow in the face of varying cerebral perfusion pressure (CPP). Examples are hypocapnea, a potent cerebral vasoconstrictor that does profoundly alter CBF in dogs but does not substantially affect the ability to autoregulate CBF , and Mannitol, which improves O2 transport by lowering viscosity but does not raise CBF due to a compensatory elevation in CVR in cats and humans [2,3]. By contrast, the calcium entry blocker, nimodipine, has been shown to impair CBF autoregulation in rats .
KeywordsCerebral Blood Flow Cerebral Perfusion Pressure Calcium Entry Blocker Cerebral Blood Flow Measurement Alter Blood Flow
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