Assessment of the adequacy of oxygen delivery

By Alex Yartsev - 09/08/2015
Last updated 27/08/2015 - 02:48
 Topic: Equipment and Procedures

Oxygen delivery is described by this equation:

DO2 = Qt x CaO2

Where Qt is the cardiac output in L per minute, and CaO2 is the oxygen content of whole blood;and where the oxygen content of whole blood is the (tiny) fraction of dissolved O2, and the product of Hb (g/L) x 1.39 (in ml, the oxygen-carrying capacity of hemoglobin), multiplied by the saturation of hemoglobin.

With a cardiac output of about 5L/min and a oxygen carrying capacity of 1.39ml O2 per 1g Hb, at a Hb of 150, and at a saturation of 100%, one can calculate that the DO2 is around 1042.5ml of O2 per minute.

But, how do you know this is adequate?

Methods of assessing adequacy of DO2

  • Adequacy of oxygen delivery into the organism
    • FiO2: Is the patient breathing a hypoxic mixture?
    •  
  • Adequacy of oxygen transport into the bloodstream
    • A-a gradient
    • SaO2, SpO2
    • Hb concentration
    • Proportion of ineffective haemoglobin (eg. methaemoglobin, carboxyhaemoglobin)
  • Adequacy of macrocirculation
    • Mean arterial pressure
    • Cardiac output indices, including advanced haemodynamic data eg. CI dreived from PAC or PiCCO
  • Adequacy of microcirculation
    • Physical examination, particularly
      • Capillary refill
      • Mottling
      • Temperature of the extremities
  • Adequacy of oxygen utilisation at the cellular level
    • mixed or central venous oxygen saturation (thus allowing the calculation of the oxygen extraction ratio)
    • arteriovenous CO2 gradient
    • arterial lactate

References

This old article is the result of a meeting where several luminaries put their heads together about what the best method is for assessing tissue oxygenation:
Haglund, U., and R. G. Fiddian-Green. "Assessment of adequate tissue oxygenation in shock and critical illness: oxygen transport in sepsis, Bermuda, April 1+ 2, 1989." Intensive care medicine 15.7 (1989): 475-477.

Gutierrez, Juan A., and Andreas A. Theodorou. "Oxygen Delivery and Oxygen Consumption in Pediatric Critical Care." Pediatric Critical Care Study Guide. Springer London, 2012. 19-38.

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