where [Hb] is the hemoglobin concentration, AJSO2 is the arteriojugular venous oxygen saturation difference, PaO2 is the arterial O2 partial pressure (in mmHg), and PjO2 is the jugular venous O2 partial pressure (in mmHg). If the arterial saturation, [Hb], and the position of the Hb dissociation curve remain constant, the jugular bulb saturation ( SjO2) is proportional to the ratio of global cerebral blood flow to cerebral metabolism.
Normal values for cerebral blood flow, CMRO2, and SjO2 are 40 to 50 ml/100 g/min, 3 to 3.5 ml/100 g/min, and 54 to 75 per cent respectively. If arterial oxygen saturation (SaO2) is estimated using pulse oximetry, the global cerebral oxygen extraction ratio (OER) can be derived:
Continuous monitoring of arterial and jugular venous oxygen saturation enables the adequacy of global cerebral perfusion to be assessed. With a normal SaO2 of 97 per cent and a hemoglobin concentration greater than 12 g/dl, an SjO2 of less than 50 per cent indicates global relative hypoperfusion. Once SjO2 falls below 40 per cent (AJDO2 > 9.0 ml per cent, OER > 57 per cent), global cerebral ischemia is likely and will be associated with the production of lactic acid.
The lactate oxygen index (LOI) can also be used to identify cerebral ischemia:
LOI is normally less than 0.03. In the event of significant regional or global cerebral ischemia the lactate oxygen index will become elevated above 0.08. However, regional cerebral ischemia may be associated with increased lactate production in the presence of a normal or even elevated SjO2. Reduced SjO2 indicates global hypoperfusion and requires urgent correction, but normal or increased SjO2 should not be interpreted as indicating adequacy of cerebral perfusion to all areas of the brain. Proper management of patients at risk of cerebral ischemia requires monitoring of both arteriojugular lactate levels and oxygen delivery.
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