Arterial oxygen content tells you how much oxygen is actually packed into each decilitre of blood. Saturation and PaO2 describe oxygen’s pressure and binding, but content is what determines, together with cardiac output, how much oxygen reaches the tissues. It is a cornerstone calculation in critical care and anaesthesia.
How it works
Oxygen travels two ways in blood: bound to haemoglobin and dissolved in plasma. The content formula adds both:
CaO2 = (Hb x 1.34 x SaO2) + (0.0031 x PaO2)Haemoglobin is in g/dL, saturation is a fraction (not a percentage), and PaO2 is in mmHg. The constant 1.34 is the oxygen-carrying capacity of one gram of haemoglobin in mL — Hufner’s constant — and 0.0031 is the plasma solubility coefficient for oxygen per mmHg.
Worked example: normal versus anaemic patient
Normal patient: Hb 14 g/dL, SaO2 98%, PaO2 95 mmHg
- Bound: 14 × 1.34 × 0.98 = 18.4 mL/dL
- Dissolved: 0.0031 × 95 = 0.29 mL/dL
- CaO2 ≈ 18.7 mL/dL
Anaemic patient: Hb 7 g/dL, SaO2 98%, PaO2 95 mmHg
- Bound: 7 × 1.34 × 0.98 = 9.2 mL/dL
- Dissolved: 0.0031 × 95 = 0.29 mL/dL
- CaO2 ≈ 9.5 mL/dL
The anaemic patient carries roughly half the oxygen per decilitre of blood. The dissolved fraction (0.29 mL/dL) is essentially unchanged because PaO2 is the same — this is why supplemental oxygen does little to rescue a patient whose problem is anaemia rather than low saturation. Treating the cause (transfusion vs oxygen therapy) depends entirely on knowing which component of the content equation is deficient.
From CaO2 to oxygen delivery
Oxygen delivery (DO2) is the rate at which oxygen arrives at the tissues, and it equals cardiac output scaled with content:
DO2 (mL/min) = cardiac output (L/min) x CaO2 x 10A normal cardiac output of about 5 L/min with CaO2 of 20 mL/dL gives DO2 ≈ 1000 mL/min. Resting oxygen consumption is roughly 250 mL/min, leaving a significant reserve. When CaO2 falls (anaemia) or cardiac output falls, DO2 drops. The body can compensate by extracting more oxygen from each mL of blood — raising oxygen extraction ratio — but there is a limit, and when DO2 falls below consumption, tissue hypoxia results.
Clinical interpretation notes
- A high PaO2 barely moves CaO2. Increasing PaO2 from 100 to 400 mmHg (hyperoxia) only adds about 0.9 mL/dL via the dissolved term — far less than raising haemoglobin by 1 g/dL.
- Saturation is the sensitive variable when Hb is normal. A drop from SpO2 98% to 88% on a Hb of 14 g/dL reduces bound oxygen by about 1.9 mL/dL.
- Which constant to use? Values of 1.34, 1.36, and 1.39 are all cited; the differences arise from in-vivo versus theoretical measurements. This tool uses 1.34. The clinical impact of the choice is small.
This is an educational tool. All clinical decisions must be made in the context of the full clinical picture by a qualified practitioner.