Define oxygen delivery  and  describe  the means  of assessing the adequacy  of oxygen delivery to the tissues in a critically ill patient.

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College Answer

Definition: Oxygen delivery is the total amount of oxygen delivered to the tissues by the cardiac
output and is calculated as the product of blood flow (QT) and arterial oxygen content (CaO2). The normal global oxygen delivery is approximately 1000 ml/min.
The adequacy of oxygen delivery can assessed by various means including:
•    Clinical examination: (ie signs of inadequate oxygen delivery) can result from hypovolemia, sepsis, myocardial dysfunction or severe hypoxemia in the face of a normal circulation. Clinical features include shock, cyanosis, Kussmaul respiration, pallor, raised or lowered JVP, bounding pulses if sepsis, gallop rhythm.
•    Monitoring:     pulse  oximetry, Hb,  PaO2,  cardiac output,  MV  oximetry,  ABGs,  serum lactate and creatinine, central venous oxygen saturation, tonometry, sublingual capnometry.
•    Absolute value: No precise data exist for what is adequate in critical illness. There was a vogue for achieving supranormal oxygen delivery in sepsis and ARDS in the early 1990s, but this approach has been shown to result in excess mortality in the critically ill.

Discussion

Oxygen delivery? Surely they must mean DO2.

One might vaguely recall 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.

And indeed, 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 ... is that adequate?

The adequacy of oxygen delivery can  be determined by a variety of ways. Instead of listing various methods of assessment in a disorganised fashion, I expect the college would have preferred to see a systematic approach.

Thus:

Methods of assessing adequacy of DO2

  • Adequacy of oxygen delivery into the organism
    • FiO2
  • 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 derived 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.