The physiologic basis for continuous mixed venous oxygen saturation monitoring

Critically ill patients often have conditions that reduce oxygen delivery and increase oxygen demand. Routine nursing care, such as suctioning, positioning, and bathing, also increases the patient's oxygen demand. If the patient's oxygen demand exceeds the supply, dysrhythmias, hypotension, altered level of consciousness, and other adverse responses can occur. We describe use of continuous monitoring of mixed venous oxygen saturation (SvO2) as a tool to assess the patient's supply/demand balance during nursing care. The physiology of oxygen transport is reviewed, and oxygen delivery, reserve, and consumption are defined. Conditions that decrease oxygen delivery and increase oxygen demand are discussed, and the effects on SvO2 are illustrated. With continuous SvO2 monitoring, critical care nurses can see the effect of their nursing care on the patient's oxygenation and can adjust their care according to the patient's tolerance.