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
- 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