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):
a) O2ER = VO2 / DO2
b)
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:
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Increased oxygen delivery:
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Increased oxygen consumption:
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Decreased oxygen consumption:
|
Abnormal circulation:
|
Abnormal circulation:
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Measurement artifact:
|
Measurement artifact:
|
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.