The calculation of oxygen content

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, 6^th Ed, Churchill Livingstone, 2004.

Andrew Wilkey, M.D. is a Cardiothoracic Anesthesia Fellow at the University of Pennsylvania Health System

Comments

[Literature Review]

Blogs are so informative where we get lots of information on any topic. Nice job keep it up!!

[research term papers]

Thanks a lot for a bunch of good tips. I look forward to reading more on the topic in the future. Keep up the good work! This blog is going to be great resource. Love reading it.

[Dissertation Help]

Nice, accurate and to the point. Not everyone can provide information with proper flow. Good post. I am going to save the URL and will definitely visit again. Keep it up.

[literature review help]

Your blog is really helps for my search and i really like it.. Thanks a lot..:)