Question 13.2

Last updated 24/06/2018 - 04:46
 Topic: Equipment and Procedures
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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
 

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College Answer

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.

Or, 

O2ER = VO2/DO2 = (CaO2-CvO2)/CaO2


or, as Walley (2010) abbreviates, 

O2ER = (SaO2-SvO2)/SaO2

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.

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