# Question 13.2

Created on Wed, 05/13/2015 - 00:24
Last updated on Fri, 04/28/2017 - 18:26
Pass rate: 44%
Highest mark: 8.3

## Other SAQs on this topic

How do you calculate the oxygen extraction ratio (O2ER)?

In a patient with septic shock, how would you interpret the following values for the oxygen extraction ratio (O2ER):

• O2ER = 0.5
• O2ER = 0.2

a)  O2ER = VO2 / DO2

b)

• The normal value is around 0.2 – 0.3 and if the value is higher this suggests that the tissues are extracting excessive amounts because oxygen delivery is inadequate due to inadequate cardiac output from either inadequate contractility or inadequate preload and may respond to inotropes and/or fluid resuscitation.
• A low normal OER in this patient suggests failure of the microcirculation with inadequate oxygen uptake due to shunting and microvascular occlusion and resultant tissue ischaemia. This would be confirmed by rising lactate levels.

## Discussion

The Oxygen Extraction Ratio is very simply the proportion difference between the oxygen entering your patient and the oxygen exiting your patient.

A professional-sounding equation is what is called for in this scenario, and that equation is O2ER = VO2 / DO2

One can (and I have) fall tumbling into the rabbit-hole of metabolic physiology when faced with a question like this. One must remember that it is asking "How do you calculate the OER", not "How do you get the objective data which allows you to calculate OER" or "critically evaluate the use of central venous oxygen saturation in the ICU"

An OER of 0.5 suggests that about 50% of arterial oxygen is gone by the time the blood returns to the heart. This corresponds to an ScVO2 of about 50%, and suggests that something is woring with the circulation, i.e. it may be too sluggish. Similarly, an OER of 20% suggests something is wrong with the circulation (it might be too fast). Unfortunately, there is nothing specific about the OER; it only describes the matching of supply and demand, but it is powerless to identify the cause of a mismatch.

For an example, here is a table listing the causes of an abnormal oxygen extraction ratio:

 An abnormally HIGH O2ER An abnormally LOW O2ER Inadequate oxygen delivery: Hypoxia Anaemia Blood flow insufficiency: shock states of all sorts Increased oxygen delivery: Hyperbaric oxygen Polycythaemia Hyperdynamic circulation Artificial circulation, eg. ECMO High cardiac output state, eg.  sepsis, cirrhosis, anxiety, Increased oxygen consumption: Increased muscle activity: Exercise, including respiratory effort Shivering Seizures States of inflammation, eg. sepsis Increased metabolic rate: Hyperthermia Hyperthyroidism Catecholamine excess Response to massive injury or burns Decreased oxygen consumption: Decreased muscle activity: Sedation Paralysis Atrophy Mechanical ventilation Decreased metabolic rate: Hypothermia Hypothyroidism Starvation Failure of oxygen utilisation Mitochondrial dysfunction in sepsis Cyanide toxicity (among others) Abnormal circulation: Right-to-left shunt (cyanotic defect) Arteriovenous malformations Portosystemic shunts (in liver disease) Abnormal circulation: Left-to-right shunt (non-cyanotic defect) Microcirculatory shunt (eg. in sepsis) Tourniquet (large fraction of the circulation excluded by occlusion, eg. aortic crossclamp) Measurement artifact: Post-collection error in the VBG (prolonged sample-to-machine transit time) Measurement artifact: Central venous rather than mixed venous samples (SvO2 is frequently higher) Inadequate mixing of blood (PA catheter in the wrong position)

## References

Walley, Keith R. "Use of central venous oxygen saturation to guide therapy."American journal of respiratory and critical care medicine 184.5 (2011): 514-520.

McLellan, S. A., and T. S. Walsh. "Oxygen delivery and haemoglobin." Continuing Education in Anaesthesia, Critical Care & Pain 4.4 (2004): 123-126.

Leach, R. M., and D. F. Treacher. "The pulmonary physician in critical care• 2: Oxygen delivery and consumption in the critically ill." Thorax 57.2 (2002): 170-177.

Ronco, Juan J., et al. "Identification of the critical oxygen delivery for anaerobic metabolism in critically ill septic and nonseptic humans." JAMA: the journal of the American Medical Association 270.14 (1993): 1724-1730.

Orlov, David, et al. "The clinical utility of an index of global oxygenation for guiding red blood cell transfusion in cardiac surgery." Transfusion 49.4 (2009): 682-688.

Bakker, Jan, et al. "Blood lactate levels are superior to oxygen-derived variables in predicting outcome in human septic shock." CHEST Journal 99.4 (1991): 956-962.