Oxygen Content vs Delivery: EDAIC Worked Examples

Oxygen content is the amount of oxygen in a volume of blood. Oxygen delivery is the amount carried to the systemic circulation each minute. A saturation value alone supplies neither answer: you also need haemoglobin concentration, and delivery additionally requires cardiac output.

This is educational exam practice, not a patient-management protocol. The examples below are original teaching calculations using deliberately simplified assumptions.

Keep four quantities separate

PaO₂ is a pressure, usually expressed in mmHg or kPa. SaO₂ describes haemoglobin oxygen saturation. CaO₂ is arterial oxygen content, often expressed as mL O₂/dL blood. DO₂ is oxygen delivery, commonly mL O₂/min. Writing the unit beside the symbol prevents the mistake of treating these as interchangeable measurements. OpenAnesthesia explains their physiological relationships.

For the exercises here, use:

CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂)

Hb is in g/dL, SaO₂ is a fraction between zero and one, and PaO₂ is in mmHg. The result is mL O₂/dL. The first term represents haemoglobin-bound oxygen; the second represents dissolved oxygen. The chosen binding coefficient is 1.34 mL O₂/g Hb, a conventional empirical value described in the NCBI oxygen-transport chapter. Different references use slightly different coefficients: retain the one specified in a question.

Worked example: a saturation that conceals the difference

Compare two fictional samples. Both have SaO₂ 98% and PaO₂ 100 mmHg. Sample A has Hb 14 g/dL; sample B has Hb 8 g/dL. Assume no important dyshemoglobinaemia and use the same coefficient throughout.

For A:

CaO₂ = (1.34 × 14 × 0.98) + (0.003 × 100)

= 18.3848 + 0.3 = 18.68 mL O₂/dL.

For B:

CaO₂ = (1.34 × 8 × 0.98) + 0.3

= 10.5056 + 0.3 = 10.81 mL O₂/dL.

The identical saturation does not produce identical content. Sample B carries about 58% as much oxygen per decilitre as A in this model. Notice the wording: this comparison describes the samples, not their clinical severity or an intervention threshold.

Before touching a calculator, predict the direction. Lower Hb with the other inputs unchanged must lower the bound term. If your calculation produces higher content, inspect the saturation conversion or multiplication.

Add flow to calculate delivery

When cardiac output is recorded in L/min and content in mL/dL, the conversion is:

DO₂ = cardiac output × CaO₂ × 10

The ten appears because one litre contains ten decilitres. At a hypothetical cardiac output of 5 L/min, A gives 5 × 18.68 × 10 = 934 mL O₂/min, whereas B gives approximately 541 mL O₂/min.

Now keep sample A's content fixed but reduce the model cardiac output to 3 L/min. Delivery becomes 3 × 18.68 × 10 = 560 mL O₂/min. Content has not changed; delivery has. This isolates the flow term rather than inventing a new oxygenation problem.

Global delivery is not the same as tissue oxygen consumption or adequate oxygenation of every organ. Distribution of flow and tissue extraction also matter; the BMJ review separates these concepts. A numerical DO₂ result therefore does not, by itself, establish that an individual patient is adequately perfused.

Three common calculation traps

Putting 98 into the equation. In the stated formula, 98% becomes 0.98. Using 98 multiplies the bound term by one hundred. If the stem gives Hb in g/L, divide by ten before using this version of the equation.

Substituting kPa for mmHg without changing the coefficient. The dissolved-oxygen coefficient above is paired with mmHg. Either convert the pressure or use a correctly specified alternative coefficient; mixing conventions silently changes the answer.

Calling PaO₂ “the amount of oxygen”. In sample A, the dissolved term is only 0.3 mL/dL. If PaO₂ rises from 100 to 200 mmHg while the exercise holds saturation fixed, that term rises to 0.6 mL/dL. The calculated content increases by 0.3 mL/dL, not twofold.

Original self-test

Judge each statement independently under the assumptions used above.

1. At unchanged Hb, SaO₂ and PaO₂, doubling cardiac output doubles calculated DO₂. True: flow multiplies unchanged content.
2. A normal SaO₂ establishes a normal CaO₂. False: haemoglobin concentration is still missing.
3. The ×10 conversion is required when content is already expressed in mL O₂/L. False: litres already match; adding it creates a tenfold error.
4. At fixed cardiac output, two equal CaO₂ values give equal calculated global DO₂. True: the formula has the same inputs, although this does not prove equal regional oxygenation.
5. Increasing PaO₂ from 100 to 200 mmHg doubles total content in sample A. False: only the small dissolved term doubles in the stated model.

For revision, draw a four-column card headed pressure, saturation, content and delivery. Put the symbol, unit and missing input beneath each. Recalculate one example with only a single input changed; explain the direction before the arithmetic. Use the physiology topic guide to place this exercise in your wider Paper A plan, then practise the distinction in the EDAIC question bank.

Sources

• OpenAnesthesia: Oxygen Physiology.
• NCBI Bookshelf: Oxygen Transport.
• BMJ: Oxygen transport—1. Basic principles.

Prepared with AI assistance for examination revision; this article is not a patient-care protocol or a substitute for current primary guidance. Sources checked 7 October 2026.