List 5 causes of a mixed venous oxygen saturation (SvO2) recording of 86%

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College Answer

Septic shock
Left to right shunt
High FIO2
Hyperbaric oxygenation
Measurement error (poor calibration)
Reduced oxygen consumption – Hypothermia, NM blockade, hypothyroidism, general anaesthesia

Discussion

This question tests the candidate's appreciation for the relevance of an abnormally high SvO2 (or low tissue oxygen extraction ratio). It is a means of assessing the adequacy of oxygen delivery.

One can usually group these "data interpretation" answers into a series of subcategories:

Measurement error

  • bubble of gas in the sample
  • poorly calibrated ABG machine

Anatomical error

  • Left to right shunt
  • Microvascular shunt (eg. in sepsis)

Increased supply

  • Hyperbaric oxygen therapy
  • High FiO2
  • Hyperdynamic circulation, eg. phaeochromocytoma or ECMO

Reduced demand

  • Neuromuscular blockade
  • Deep sedation / anaesthesia
  • Hypothermia
  • Hypothyroidsm
  • Failure of mitochondrial oxygen use (eg. cyanide poisoning, sepsis)

A general overview of mixed and central venous saturation can be found elsewhere.

There is also an excellent article with satisfying explanations of the physiology behind oxygen delivery and extraction.

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.