Venous Oximetry Overview


OXYGEN: DEMAND

The amount of oxygen necessary to satisfy the metabolic requirements of all body tissues is called oxygen demand. Tissues and organs demand oxygen, but they cannot store it for future use.

 


OXYGEN: CONSUMPTION

The amount of oxygen actually used by the tissues is called oxygen consumption.  It is the difference between the oxygen delivered by the arterial system and the amount of oxygen returned to the heart by the venous system.

In healthy individuals

Oxygen Consumption = Oxygen Demand

In some disease states, cells in some tissues are unable to assimilate and/or process the needed oxygen. In these tissues
 
Oxygen Consumption < Oxygen Demand

leading to local tissue hypoxia.

 


OXYGEN: ScvOand SvO

Hemoglobin in arterial blood is highly saturated with oxygen (SaO2).  Normal levels of SaO2 delivered to tissues are 95 - 98%.

Central venous (ScvO2) and mixed venous oxygen saturation (SvO2) are measurements of the relationship between oxygen consumption and oxygen delivery in the body.   Normal values of mixed venous oxygen saturation (SvO2) are 60 - 80%.  Central venous oxygen saturation (ScvO2) values represent regional venous saturations with a normal value of ~70%.  ScvO2 usually measures slightly higher than SvO2 as it has not mixed with the venous blood from the coronary sinus.  Although the values may differ, they trend together. 


 


OXYGEN: BALANCE

Oxygen balance is necessary to sustain life. SvO2 is the "watchdog" of this balance and is often called the "fifth vital sign." When monitored, it serves as an early trouble indicator and can help clinicians adjust therapies. It is also a means of interpreting other clinical variables.

In a healthy individual, normal SvO2 is between 60 percent and 80 percent.
  • If SvO2 is low (below 60 percent) then the oxygen supply is insufficient or the oxygen demand has increased.
  • If SvO2 is high (above 80 percent) then the oxygen demand has declined or the oxygen supply has increased.



OXYGEN: BALANCING SUPPLY AND DEMAND

When the oxygen supply and demand balance is threatened, the body will compensate in the following three ways:

Increased cardiac output and/or heart rate
  • Initial response to decreasing supply or increasing demand.
Increased oxygen extraction
  • Tissues take more oxygen from arterial blood. This results in lower return of oxygen to the venous system and therefore a lower SvO2 reading.
Diverted blood flow
  • Blood flow is redirected to areas where the oxygen is needed the most.  Redirected blood flow cannot be measured clinically and therefore does not provide early warning signals.

 



GLOBAL TISSUE HYPOXIA

An indicator of serious illness, global tissue hypoxia is a key development preceding multiorgan failure and death.  In cases of circulatory abnormalities, an imbalance between systemic oxygen delivery and oxygen demand, results in global tissue hypoxia.