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INTRODUCTION

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Mixed venous oxygen saturation (SvO2) is a measurement of whole body oxygenation. Mixed venous blood in measured in the pulmonary artery to sample deoxygenated blood entering the pulmonary artery before passing through the lungs. The pulmonary artery receives a mixture of blood from the superior vena cava, inferior vena cava, and coronary sinus. It serves as a sample of whole body oxygen utilization. Pulmonary artery blood can be sampled periodically by withdrawing blood through a pulmonary artery catheter or continuously with an oximetric Swan–Ganz catheter.

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The normal mixed venous oxygen saturation is about 70%–75%. This value reflects the fact that the body normally extracts only 25%–30% of oxygen carried in the blood.

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Mixed venous oxygen saturation can be explained using the modified Fick equation:

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where SaO2 is the arterial Hgb saturation (%),VO2 is the oxygen consumption (mL/min), Q is the cardiac output (L/min), and Hgb is hemoglobin (g/dL).

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The majority of oxygen in blood in bound to hemoglobin. A very small amount in dissolved in the blood as indicated by the arterial oxygen content equation

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The Fick equation can be rewritten to express the relationship between oxygen consumption, oxygen content, and cardiac output. The difference between CaO2 and CvO2 is the amount of oxygen utilized by the tissues:

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where CaO2 is the arterial oxygen content (mL/dL) and CvO2 is the mixed venous oxygen content (mL/dL).

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When oxygen delivery is reduced, oxygen consumption remains constant by increasing oxygen extraction as well as cardiac output. This mechanism is protective until tissues extract about 50%–60% of oxygen from the blood. Once oxygen extraction reaches this maximum, oxygen consumption is supply-dependent and lactic acidosis due to cellular hypoxia develops. SvO2 can be used as an indirect indicator of cardiac output in the presence of constant SaO2, VO2, and Hgb.

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DECREASED MIXED VENOUS OXYGEN SATURATION

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A decrease in SvO2 signifies insufficient oxygen delivery or increased oxygen consumption (Table 4-1). This problem occurs in low cardiac output, anemia, hypoxemia, or hypermetabolic states. The body compensates to maintain aerobic respiration by increasing oxygen extraction from hemoglobin. The body can maximally extract 50%–60% of oxygen carried in the blood, decreasing SvO2 to 40%–50%. Once the tissues reach maximal oxygen extraction, further reduction in oxygen delivery results in anaerobic metabolism, acidosis, and multiorgan failure.

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Table Graphic Jump Location
TABLE 4-1Cause of Decreased SvO2
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INCREASED MIXED VENOUS OXYGEN SATURATION

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