Key messages

• An absolute reduction in Do2 could occur as a result of a reduction in hemoglobin, PaO2, and hence SaO2, or from a reduction in cardiac output.

• If Do2 falls but oxygen demand continues and Vo2 is maintained, there will be an increase in the oxygen extraction ratio (OER) and a fall in SVo2 if the ability of the tissues to extract oxygen is intact.

• Despite the high values of Do2 found in severe sepsis, there is evidence of an inability of peripheral tissues to utilize oxygen, and hence an inappropriately low Vo2.

• It would seem reasonable to try, wherever possible, to achieve an OER as close to normal as possible. In some situations, such as septic shock, acute respiratory distress syndrome, and trauma, the aim should be a value of about 20 per cent, provided that underlying cardiac disease has been excluded.

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