ATP (energy) is needed for 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 - Cardiac Output is Increased
The cardiac output is increased in an effort to increase the oxygen delivery to the tissues as shown below.

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

If this is not sufficient to meet tissue energy needs; 

Second - Tissues Extract an Increased Amount of Oxygen. 
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; 
Third - Anaerobic Metabolism Increases.
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 SvO₂?
The normal SvO2 ranges between 60-80%.   If SvO2decreases, 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, SvO2can 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, in the presence of a normal or improving lactate, suggests patient improvement.  However, a rise in SvO2in the presence of a rising lactate is inappropriate - the patient who has resorted to anaerobic metaolism (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.

SvO2 can help to determine whether the Cardiac Output is adequate.
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 suggests that the cardiac output measured 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 causes for a drop in SvO2:

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