Dr. Wilkey discusses the calculation of oxygen content
Oxygen is poorly soluble in water; therefore without an adjunctive means of transport, it cannot be transported in blood in quantities sufficient to sustain life. That adjunct comes in the form of hemoglobin. Oxygen can bind to hemoglobin at any of four active sites on each molecule of hemoglobin. When measured per gram of hemoglobin, its capacity to carry oxygen is 1.39 ml. As will be seen below, the term for O2 per gram Hgb is 1.34 rather than 1.39. (Some sources may report values ranging from 1.31 to 1.36). This is due to binding of non-O2 species to hemoglobin, thereby changing its conformation to one that will not bind oxygen. The ~0.05 ml/gm is comprised of compounds such as methemoglobin and carboxyhemolobin.
The equation for the concentration of oxygen in arterial blood is as follows:
CaO2 = (SaO2 x Hgb x 1.34) + (0.003 x PaO2)
The term for SaO2 is expressed as a fraction of 1.0 rather than a percentage (i.e. 0.98 instead of 98%). Hemoglobin is entered in grams and PaO2 is in mmHg.
The term 0.003, derived from oxygen solubility coefficients at different temperatures, assumes a normal body temperature of 37ºC and is expressed as mL O2/dl/mmHg. (The solubility of oxygen in blood is 0.03 ml O2/l/mmHg. This is divided by ten to bring the units into agreement with others in the equation).
The above equation can then be expressed with units included as:
CaO2 = (SaO2 x Hgb
gm/dl x 1.34 ml/ gm) + (0.003 O2 ml/dl/ mmHg x PaO2 mmHg) = ml/dl. (Terms that cancel are lined out).
In a hypothetical patient with SaO2 1.0 (100%), Hgb of 15 gm/dl and PaO2 of 100 mmHg, the oxygen content of arterial blood is:
CaO2 = (1.0 x 15 gm/dl x 1.34 ml/gm) + (0.003 ml/dl/mmHg x 100 mmHg) = 20.4 mL/dl.
Note that the contribution of dissolved oxygen to the total is only 0.3 mL/dl or less than 1.5% of the total—not nearly enough to sustain life. An exception to this is with the application of hyperbaric oxygen therapy. 100% O2 at 3 atmospheres would lead to a PaO2 as high as 2200mmHg.
Keep in mind that while dissolved O2 usually contributes little to the total arterial content of oxygen, it can make the difference between two hypothetical patients or patient scenarios. Recalculating this equation with a SaO2 of 75% will approximate mixed venous oxygen content. This equation is one of the most fundamental in anesthesia and must be committed to memory—being able to do the calculation quickly and without the use of a calculator is useful as well.
Also of note is the strong contribution of Hgb to oxygen content. Though other factors such as tissue perfusion and oxygen uptake and extraction can play significant roles in end organ function or dysfunction, often the addition of more hemoglobin is the only readily modifiable way to increase arterial oxygen capacity and its subsequent delivery to tissues.
1) Stoelting RK, Pharmacology & Physiology in Anesthetic Practice, Lippincott Williams & Wilkins; 3Rev Ed, 1999.
2) Morgan GE, Mikhail MS, Clinical Anesthesiology, McGraw-Hill Medical; 4 Ed, 2005.
3) Miller RD, Miller’s Anesthesia, 6th Ed, Churchill Livingstone, 2004.
Andrew Wilkey, M.D. is a Cardiothoracic Anesthesia Fellow at the University of Pennsylvania Health System