CENTRAL VENOUS/MIXED VENOUS OXYGEN SATURATION

What is it?

Mixed venous oxygen saturation (SvO2) is the percentage of oxygen bound to hemoglobin in blood returning to the right side of the heart.  This refects the amount of oxygen "left over" after the tissues remove what they need. It is used to help us to recognize when a patient's body is extracting more oxygen than normally. An increase in extraction is the bodies way to meet tissue oxygen needs when the amount of oxygen reaching the tissues is less than needed.

A true mixed venous sample (called SvO2) is drawn from the tip of the pulmonary artery catheter, and includes all of the venous blood returning from the head and arms (via superior vena cava), the gut and lower extremities (via the inferior vena cava) and the coronary veins (via the coronary sinus). By the time the blood reaches the pulmonary artery, all venous blood has "mixed" to reflect the average amount of oxygen remaining after all tissues in the body have removed oxygen from the hemoglobin. The mixed venous sample also captures the blood before it is re-oxygenated in the pulmonary capillary.

Because pulmonary artery catheter use has declined dramatically, ScvO2 measurements obtained from internal jugular or subclavian catheters are often used and interpreted in the same manner. An ScvO2 refers to a central venous sample. An ScvO2 measurement is a surrogate for the SvO2.  It may be used to identify changes in a patient's tissue oxygen extraction. We usually assume (possibly incorrectly at times) that a blood gas sample obtained from the internal jugular or subclavian (which reflects only head and upper extremities) will have the same meaning as an SvO2.

What does it tell us?

Mixed venous oxygen saturation (SvO2) can help to determine whether the cardiac output and oxygen delivery is high enough to meet a patient's needs.  It can be very useful if measured before and after changes are made to cardiac medications or mechanical ventilation, particularly in unstable patients.

Normal SvO2 60-80%. Normal ScvO2 (from an internal jugular or subclavian vein) is > 70%. 

ATP (energy) is needed for all cell function and survival. Tissues require oxygen in order to make ATP (energy).  If the amount of oxygen 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:

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 shown below.

 

Oxygen Delivery (DO2) = Cardiac Output (HR X Stroke Volume) X Oxygen Content (Hb X SaO2)

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 oxygen remaining in the blood as it returns to the right side of the heart (decreased SvO2).

 

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 to produce tissue ATP.  Anaerobic metabolism is inefficient, producing a large amount of metabolic waste (e.g. lactic acid) that is difficult for the body to eliminate quickly.  It also produces a relatively poor supply of ATP.  Prolonged anaerobic 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.  In otherwords, a decreased SvO2 indicates that the cardiac output is not high enough to meet tissue oxygen needs.  Thus, SvO2 can indicate whether an individual's cardiac output is high enough to meet their needs.A rise in SvO2 demonstrates a decrease in oxygen extraction, and usually indiates that the cardiac output is meeting the tissue oxygen need.  A return of the SvO2 to normal suggests patient improvement.  However, a rise in SvO2 in the presence of a rising lactate is inappropriate - the patient who has resorted to anaerobic metabolism (third compensation) should have evidence of a high cardiac output and increased extraction.  This is an ominous finding, suggesting that the tissues are unable to extract.  It can be seen in late septic shock, or in cell poisoning such as cyanide.Cardiac output is routinely measured to evaluate drug effectiveness.  Unfortunately, the cardiac output measurement only gives us a value, it does not indicate whether the measured cardiac output is meeting the patient's needs.  For some individuals, a cardiac output that falls below the normal range may be adequte, whereas for others, a normal or elevated cardiac output value may be too low.   An SvO2  in the normal range, along with a normal lactate, suggests that the cardiac output is adequate.SVO2 can be very helpful when attempting to determine whether a change in therapy is beneficial.  Measuring SvO2 before and after a change can assist in determining whether the therapy made the patient better or worse.SvO2 can also be useful when evaluating changes to ventilator therapy, especially in unstable patients.  Changes may be made to the ventilator to increase the oxygen content of the blood, which is important to the total oxygen delivery (cardiac ouptut X oxygen content).  Increased PEEP may be required to increase the oxygen content, however, increased levels of PEEP can decrease the cardiac output.  By measuring the SvO2 before and after a change in PEEP, the optimal level of PEEP can be determined.  The "best" PEEP is the level that improves the SaO2 without causing the SvO2 to fall.

Tissue oxygen need is met when the amount of oxygen being delivered to the tissues is sufficient to meet the amount of oxygen being consumed (VO2).  When the oxygen delivery falls below oxygen consumption needs, lactic acidosis develops.

There are 4 fundamental causes for a drop in SvO2:

1. The cardiac output is not high enough to meet tissue oxygen needs
2. The Hb is too low
3. The SaO2 is too low
4. Oxygen consumption has increased without an increase in oxgyen delivery