Arterial Oxygen Content Calculator (CaO2 and DO2)

What is the arterial oxygen content (CaO2) formula?

The arterial oxygen content formula is CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂), with hemoglobin in g/dL, SaO₂ as a fraction and PaO₂ in mmHg, giving mL/dL. The first term is oxygen bound to hemoglobin, the second oxygen dissolved in plasma. Oxygen delivery is DO₂ = CaO₂ × cardiac output × 10, in mL/min.

Why the Hüfner factor here is 1.34, not 1.39

The 1.34 is Hüfner’s factor, the mL of oxygen that one gram of fully saturated hemoglobin binds, and textbooks print values from 1.34 to 1.39 for it. 1.39 is the theoretical ceiling: each hemoglobin tetramer, about 64,500 g/mol, binds four oxygen molecules. Whole blood measures lower. Gregory (1974) found 1.306 mL/g in adult blood after correcting for carboxyhemoglobin and methemoglobin, and attributed most of the gap to the cyanmethemoglobin standard, which he found read hemoglobin about 6 percent high.

This calculator uses 1.34, the figure the bedside form of the equation is usually written with and the constant the oxygen dissociation curve simulator uses, so the two pages give the same content for the same blood. If your course uses 1.39, read the result named “Content with 1.39 instead of 1.34” under the answer. At a hemoglobin of 15 g/dL and a saturation of 98 percent the two differ by about 0.7 mL/dL.

Hemoglobin carries almost all of the oxygen

At a hemoglobin of 15 g/dL, a saturation of 98 percent and a PaO₂ of 100 mmHg, the bound term is 19.7 mL/dL and the dissolved term 0.3 mL/dL, so plasma carries about 1.5 percent of the total. Halve the hemoglobin to 7.5 g/dL and the content falls to about 10.1 mL/dL. The saturation, the PaO₂ and the pulse oximeter reading stay where they were, because saturation is a share of whatever hemoglobin is present, so anemia lowers oxygen content without moving either oxygen number on a blood gas.

Supplemental oxygen adds little once hemoglobin is saturated. Raising PaO₂ from 100 to 600 mmHg multiplies the dissolved term by six, from 0.3 to 1.8 mL/dL, and adds almost nothing to the bound term, so it raises the content of anemic blood by not much more than 1.5 mL/dL. Dissolved oxygen only becomes large at hyperbaric pressures: a PaO₂ of 2,000 mmHg dissolves 6 mL/dL.

SaO2, SpO2 and carbon monoxide

The saturation in the equation multiplies total hemoglobin, so it has to be oxyhemoglobin as a share of total hemoglobin. A standard pulse oximeter reads carboxyhemoglobin as oxyhemoglobin, so in carbon monoxide poisoning SpO₂ can look normal while the content is low. Pulse oximeters also miss low arterial saturations more often in Black patients than in White patients (Sjoding and colleagues, New England Journal of Medicine, 2020), so an SpO₂ can overstate the content without any warning on the screen.

With carbon monoxide or methemoglobin present, use the oxyhemoglobin fraction a co-oximeter reports, often shown as FO₂Hb. A functional saturation leaves those dyshemoglobins out of its denominator while the hemoglobin in the equation keeps them in, so pairing the two overstates the content.

From oxygen content to oxygen delivery

Content is a concentration and delivery is a flow. Multiplying by cardiac output in L/min gives oxygen per minute, and the factor of 10 converts the content from per dL to per L so that the units agree. A content of 20 mL/dL at a cardiac output of 5 L/min delivers 1,000 mL/min.

Because delivery is a product, each factor scales it in proportion: a fall of a third in hemoglobin, in saturation or in cardiac output cuts delivery by about a third, and a rise in one can offset a fall in another. For a figure that compares people of different sizes, use the cardiac index in place of cardiac output, or divide by a surface area from the body surface area calculator, which gives the indexed delivery, DO₂I, in mL/min/m².

The same equation applied to mixed venous blood, with the venous saturation and PvO₂, gives the venous content. The arterial minus venous difference, times cardiac output and the same factor of 10, is the oxygen the tissues took up, which is the Fick principle.

Common mistakes

• Saturation as a fraction in a percent field. Typing 0.98 instead of 98 makes the bound term a hundred times too small. The calculator warns when a saturation is 1 or below.
• Hemoglobin in g/L. UK, Canadian and Australian reports give g/L, and 135 read as g/dL would make the content ten times too high. Divide by 10 first; the field refuses anything above 30 g/dL.
• The 0.003 with PaO₂ in kPa. The coefficient is per mmHg. Per kPa it is about 0.0225, and using 0.003 with a kPa value makes the dissolved term 7.5 times too small. This calculator converts kPa to mmHg before using it.
• Dropping the 10 in DO₂. The answer comes out ten times too low, 100 mL/min where it should be 1,000.
• Comparing answers built on different factors. A 1.39 answer and a 1.34 answer for the same blood can both be correct. Check which factor a worked answer used before deciding which of you made the error.

Converting units first? Use the pressure conversion table.