What it is
Arterial oxygen content (CaO₂) is the total amount of oxygen carried in each decilitre of arterial blood. Almost all of it is bound to haemoglobin; a tiny fraction is dissolved in plasma. CaO₂ is the bridge between the numbers clinicians watch (Hb, SaO₂) and what actually reaches tissue — oxygen delivery — and it explains why a saturation of 98% is cold comfort in a profoundly anaemic patient.
The method
CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂)
- Hb in g/dL; SaO₂ as a fraction (entered as a percentage and converted internally).
- 1.34 mL O₂/g is haemoglobin’s oxygen-carrying capacity (Hüfner’s constant).
- 0.003 mL O₂/dL/mmHg is the solubility coefficient for the small dissolved term.
Normal CaO₂ is roughly 16–20 mL O₂/dL. Oxygen delivery follows directly: DO₂ = CaO₂ × cardiac output × 10.
When to use it
Use CaO₂ when oxygenation looks adequate by pulse oximetry but tissue oxygen delivery is in question: anaemia, haemorrhage, shock, sickle crisis, or any decision about whether transfusion will buy more than a small bump in FiO₂. It reframes resuscitation around delivery rather than saturation alone, and underpins DO₂/VO₂ reasoning in critical care.
Worked example
Hb 8 g/dL, SaO₂ 98%, PaO₂ 95 mmHg:
- Bound = 1.34 × 8 × 0.98 = 10.5 mL/dL
- Dissolved = 0.003 × 95 = 0.29 mL/dL
- CaO₂ = 10.5 + 0.29 ≈ 10.8 mL O₂/dL
Now hold SaO₂ at 98% but restore Hb to 14 g/dL: bound = 1.34 × 14 × 0.98 = 18.4 mL/dL, CaO₂ ≈ 18.7 mL/dL — nearly double. The lesson is stark: correcting the anaemia roughly doubled oxygen content while the saturation never moved.
Pitfalls and caveats
- Hb is the dominant term. A fall from 14 to 7 g/dL halves content; a fall in SaO₂ from 98% to 90% trims it only ~8%. Chasing the saturation while ignoring the haemoglobin misses the bigger lever.
- The dissolved term is negligible at normal pressures — it only becomes relevant under hyperbaric oxygen.
- Content is not delivery. A high CaO₂ with poor cardiac output still starves tissue; always pair it with the DO₂ logic.
- SaO₂ from a co-oximeter is more reliable than peripheral SpO₂ here, especially with dyshaemoglobinaemias (carboxy-/methaemoglobin) where SpO₂ misleads.
Run it: Arterial Oxygen Content (CaO₂)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.