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NEUROSURGICAL ANESTHESIA

Does Tissue Oxygen-Tension Reliably Reflect Cerebral Oxygen Delivery and Consumption?

Neurovascular Laboratory, Department of Neurosurgery, University of Freiburg, Freiburg, Germany

Address correspondence and reprint requests to Dr. med. Kai-Michael Scheufler, Abt. Allgemeine Neurochirurgie, Universitätsklinikum Freiburg, Breisacher Str. 64, D-79106 Freiburg, Germany. Address e-mail to scheufle{at}nz11.ukl.uni-freiburg.de

We investigated the value of brain oxygen partial pressure (P_brO₂) with respect to predicting cerebral energetic failure in a rabbit model of global cerebral ischemia and hypoxia. Local cortical blood flow (l_coCBF), P_brO₂, extracellular lactate, pyruvate, and glutamate concentrations, as well as microvascular hemoglobin saturation (S_mvO₂), cytochrome oxidase redox level (Cyt a+a₃ oxidation), and brain electrical activity, were assessed during variable degrees of cerebral ischemia and hypoxia, induced by cisternal infusion of artificial cerebrospinal fluid or an admixture of nitrous oxide to inspiratory gas in 10 animals each. Arteriovenous difference in oxygen content, cerebral metabolic rate for oxygen, and oxygen extraction were derived from multimodal data. P_brO₂, S_mvO₂, and Cyt a+a₃ oxidation were closely related to cerebral blood flow and indices of oxidative metabolism. P_brO₂ 8 mm Hg corresponded to l_coCBF 15 mL · 100 g^-1 · min^-1, S_mvO₂ 9%, Cyt a+a₃ oxidation 20%, and progressive loss of brain electrical activity. Adequate tissue oxygenation was reflected by cerebral metabolic rate for oxygen 2.8 mL · 100 g^-1 · min^-1, arteriovenous difference in oxygen content 12.5 mL O₂ · 100 mL^-1, and oxygen extraction 60%. Meaningful interpretation of low P_brO₂, especially with respect to definition of energetic thresholds, requires complementary information from simultaneous assessment of l_coCBF and tissue oxygen extraction.

IMPLICATIONS. The relationship between brain oxygen partial pressure and several variables of energy metabolism was investigated during variable degrees of cerebral ischemia and hypoxia in a rabbit model. Correct interpretation of individual brain oxygen partial pressure values, especially with respect to definition of energetic thresholds, requires complementary information from assessment of cerebral blood flow and tissue oxygen extraction.

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