Kety and Schmidt's application of the Fick Principle in the 1940s has served as the technical basis for the human cerebral metabolic studies still in use today. In this technique, arterial and jugular venous blood samples are collected, analysed for oxygen, glucose and lactate, and an arterial-jugular venous difference (AVD) is calculated. The cerebral metabolic rate is then calculated as the product of the AVD and the simultaneous measurement of cerebral blood flow. To further understand the relationship between the cerebral metabolic components, an oxygen-carbohydrate index (OCI), calculated by CMR_O_2 (CMR_glucose + 1/2CMR_lactate), or a metabolic ratio (OGI), calculated by CMR_O_2/CMR_glucose, is used to determine how much fuel substrate is oxidized, where a value of 6 represents the complete oxidation of glucose.
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