GEM Premier 3000 blood gas analyzer was used to analyze levels of the arterial oxygen ( CaO2 ) , internal jugular venous oxygen ( CjvO2 ) , arterio-venous oxygen content difference ( Da-jvO2 ) and cerebral oxygen ( CERO2 ) uptake rates of blood samples before controlled hypotension ( T0 ) , 20 min after controlled hypotension ( T ) .