Mixed venous oxygen saturation (SvO2) can provide useful information regarding a patient’s clinical condition. As such, there continues to be an interest in this value as a clinical predictor of outcomes. SvO2 is measured at the level of the pulmonary artery. It reflects the oxygen saturation of the blood returning from the body to the heart. To obtain a true measurement, a blood sample is drawn from the distal tip of a pulmonary artery catheter. This allows the blood to be a true mixture of superior and inferior vena cava as well as coronary sinus blood (venous return from all parts of the body). More sophisticated monitoring exists whereby mixed venous oxygenation is displayed continuously via specialized pulmonary artery catheters. This technology allows for early changes in clinical status to be detected, though it has not been proven to be superior to periodic measurement via a standard pulmonary artery catheter.
Normal SvO2 is 70%, with a range of 60%–80%. The absolute number is an indicator of the percentage of reduced hemoglobin left after the body’s organs and tissues have extracted oxygen.
The Fick equation is vital to understanding mixed venous oxygen saturation. It states that:
SvO2 = SaO2 − VO2/(CO × 1.34 × Hb)
In this equation, SvO2 is the mixed venous oxygen saturation, SaO2 the arterial oxygen saturation, VO2 the oxygen consumption, CO the cardiac output, and Hb the hemoglobin. This equation shows that SvO2 decreases as oxygen utilization increases. If tissues extract or utilize more oxygen, less is returned to the heart, thus a lower SvO2. If SvO2 is low due to increased tissue utilization of oxygen, one must increase oxygen delivery to meet the body’s needs. Oxygen delivery is dependent on CO and oxygen content of blood. The oxygen content of blood is largely determined by the hemoglobin level and oxygen saturation of arterial blood. To improve oxygen delivery, one must either improve CO, correct anemia, or improve oxygen saturation. Alternatively, a high SvO2 may indicate problems as well. High SvO2 can signify a decrease in tissue oxygen delivery (inadequate CO) or a decrease in tissue oxygen extraction (adequate CO) (Figure 168-1).
Common physiologic, pathologic, and therapeutic factors that influence venous oxygen saturation during the perioperative period. (Reproduced with permission from Shepherd S, Pearse, Rupert M. Role of central and mixed venous oxygen saturation measurement in perioperative care, Anesthesiology. 2009;111(3):649-656.)
SvO2 is a value obtained from the entire body’s venous return, and hence can be misleading. This number does not indicate the status of specific organ perfusion. Intracardiac shunting, liver failure, severe sepsis, or focal ischemia ...