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SvO2 (mixed venous oxygen saturation) orScvO central venous oxygen saturation)

What is it?

Mixed venous oxygen saturation (SvO ) is the percentage of oxygen bound to hemoglobin in blood returning to the right side of theheart. This refects the amount of oxygen "left over" after the tissues remove what they need. It is used to help us to recognize when apatient's body is extracting more oxygen than normally. An increase in extraction is the bodies way to meet tissue oxygen needs whenthe amount of oxygen reaching the tissues is less than needed.

A true mixed venous sample (called SvO ) is drawn from the tip of the pulmonary artery catheter, and includes all of the venousblood returning from the head and arms (via superior vena cava), the gut and lower extremities (via the inferior vena cava) and thecoronary veins (via the coronary sinus). By the time the blood reaches the pulmonary artery, all venous blood has "mixed" to reflectthe average amount of oxygen remaining after all tissues in the body have removed oxygen from the hemoglobin. The mixed venoussample also captures the blood before it is re-oxygenated in the pulmonary capillary.

Because pulmonary artery catheter use has declined dramatically, ScvO measurements obtained from internal jugular or subclaviancatheters are often used and interpreted in the same manner. An ScvO refers to a central venous sample. An ScvO measurement is asurrogate for the SvO . It may be used to identify changes in a patient's tissue oxygen extraction. We usually assume (possiblyincorrectly at times) that a blood gas sample obtained from the internal jugular or subclavian (which reflects only head and upperextremities) will have the same meaning as an SvO .

What does it tell us?

Mixed venous oxygen saturation (SvO ) can help to determine whether the cardiac output and oxygen delivery is high enough to meeta patient's needs. It can be very useful if measured before and after changes are made to cardiac medications or mechanicalventilation, particularly in unstable patients.

What are the normal values?Normal SvO 60-80%. Normal ScvO (from an internal jugular or subclavian vein) is > 70%.How do I use this information clinically?

ATP (energy) is needed for all cell function and survival. Tissues require oxygen in order to make ATP (energy). If the amount ofoxygen being received by the tissues falls below the amount of oxygen required (because of an increased need, or decreased supply),the body attempts to compensate as follows:

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First Compensation: Cardiac Output increases

The cardiac output is increased in an effort to increase the amount of oxygen being delivered to the tissues as shownbelow.

Oxygen Delivery is the amount of oxygen being sent to the tissues, and is determined by the following:

Oxygen Delivery (DO ) =

Cardiac Output (HR X Stroke Volume)X

Oxygen Content (Hb X SaO )

If this is not sufficient to meet tissue energy needs, we move to our second compensation.Second Compensation: Tissue oxygen extraction increases.

Tissues begin to remove or extract a higher percentage of oxygen from the arterial blood. This results in a reduced amount of oxygenremaining in the blood as it returns to the right side of the heart (decreased SvO ).

If this is not sufficient to meet tissue energy needs, we move to our second compensation.

Third Compensation: Anaerobic Metabolism iIncreases

If the tissues fail to receive an adequate supply of oxygen, anaerobic metabolism becomes the only mechanism toproduce tissue ATP. Anaerobic metabolism is inefficient, producing a large amount of metabolic waste (e.g. lacticacid) that is difficult for the body to eliminate quickly. It also produces a relatively poor supply of ATP. Prolongedanaerobic metabolism leads to energy depletion and metabolic acidosis.

Why measure it?If SvO2 decreases, it indicates that the tissues are extracting a higher percentage of oxygen from the blood than normal. Inotherwords, a decreased SvO2 indicates that the cardiac output is not high enough to meet tissue oxygen needs. Thus, SvO2 canindicate whether an individual's cardiac output is high enough to meet their needs.A rise in SvO2 demonstrates a decrease in oxygenextraction, and usually indiates that the cardiac output is meeting the tissue oxygen need. A return of the SvO2 to normal suggestspatient improvement. However, a rise in SvO2 in the presence of a rising lactate is inappropriate - the patient who has resorted toanaerobic metabolism (third compensation) should have evidence of a high cardiac output and increased extraction. This is anominous finding, suggesting that the tissues are unable to extract. It can be seen in late septic shock, or in cell poisoning such ascyanide.Cardiac output is routinely measured to evaluate drug effectiveness. Unfortunately, the cardiac output measurement onlygives us a value, it does not indicate whether the measured cardiac output is meeting the patient's needs. For some individuals, acardiac output that falls below the normal range may be adequte, whereas for others, a normal or elevated cardiac output value may betoo low. An SvO2 in the normal range, along with a normal lactate, suggests that the cardiac output is adequate.SVO2 can be veryhelpful when attempting to determine whether a change in therapy is beneficial. Measuring SvO2 before and after a change can assistin determining whether the therapy made the patient better or worse.SvO2 can also be useful when evaluating changes to ventilatortherapy, especially in unstable patients. Changes may be made to the ventilator to increase the oxygen content of the blood, which isimportant to the total oxygen delivery (cardiac ouptut X oxygen content). Increased PEEP may be required to increase the oxygencontent, however, increased levels of PEEP can decrease the cardiac output. By measuring the SvO2 before and after a change inPEEP, the optimal level of PEEP can be determined. The "best" PEEP is the level that improves the SaO2 without causing the SvO2to fall.Tissue oxygen need is met when the amount of oxygen being delivered to the tissues is sufficient to meet the amount of oxygen beingconsumed (VO2). When the oxygen delivery falls below oxygen consumption needs, lactic acidosis develops.

VO2 (Oxygen Consumption) = Cardiac Output X Hb X (SaO2 - SvO2)

There are 4 fundamental causes for a drop in SvO2:1. The cardiac output is not high enough to meet tissue oxygen needs

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2. The Hb is too low3. The SaO2 is too low

4. The oxygen consumption has increased without an increase in oxgyen delivery

Brenda MorganClinical Educator, CCTCSeptember, 1999Revised: July 24, 2012

Last Updated July 24, 2012 | © 2007, LHSC, London Ontario Canada


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