Arterial Oxygen Content (CaO₂) Calculator
Arterial Oxygen Content (CaO₂) Calculator
Calculate CaO₂ from haemoglobin, saturation and pO₂ — and see why anaemia costs far more oxygen content than a modest desaturation.
Arterial Oxygen Content (CaO₂)
3 inputs → contentHb 13 g/dL, SaO₂ 97%, PaO₂ 95 mmHg
Formula
- 1.34
- Hüfner's constant — mL of O₂ bound per gram of fully saturated haemoglobin (1.36 and 1.39 are also quoted in the literature)
- Hb
- haemoglobin, g/dL
- SaO₂
- arterial oxygen saturation, %
- 0.003 × PaO₂
- oxygen dissolved in plasma — trivial at atmospheric pressure, about 0.3 mL/dL against roughly 17 mL/dL carried by haemoglobin
Worked example
Hb 13 g/dL, SaO₂ 97%, PaO₂ 95 mmHg
1.34 × 13 × (97 ÷ 100) = 16.90 mL O₂/dL bound to haemoglobin
0.003 × 95 = 0.29 mL O₂/dL dissolved in plasma
16.90 + 0.29 = 17.18 mL O₂/dL
Which term dominates
| Change | Effect on CaO₂ |
|---|---|
| Hb 13 → 6.5 g/dL (halved) | CaO₂ falls by about 50% |
| SaO₂ 99% → 90% | CaO₂ falls by under 10% |
| PaO₂ 95 → 500 mmHg (100% O₂) | CaO₂ rises by only about 1.2 mL/dL — hardly at all |
Why haemoglobin matters more than saturation
Arterial oxygen content has two components with very different weight. The haemoglobin-bound term, 1.34 × Hb × SaO₂, carries the overwhelming majority of the oxygen — around 17 mL/dL in a healthy adult. The dissolved term, 0.003 × PaO₂, is trivially small at atmospheric pressure, contributing roughly 0.3 mL/dL against that 17. Even breathing 100% oxygen and driving PaO₂ to 500 mmHg adds only about 1.2 mL/dL of dissolved oxygen — a small fraction of total content, and clinically almost irrelevant once haemoglobin is saturated.
This asymmetry has a direct teaching point that is easy to lose sight of at the bedside: halving haemoglobin halves oxygen content, but dropping saturation from 99% to 90% — a fall that looks alarming on a monitor — reduces content by under 10%. A patient with a haemoglobin of 6.5 g/dL and a saturation of 99% is carrying roughly the same oxygen content as a patient with normal haemoglobin and a saturation in the high 80s. Chasing saturation on a monitor while ignoring a falling haemoglobin is a common and consequential error, particularly in the critically ill and in gastrointestinal bleeding.
1.34 is Hüfner’s constant, the oxygen-carrying capacity of fully saturated haemoglobin; some laboratories and texts use 1.36 or 1.39, reflecting different assay methods, and the choice makes only a small difference to the result.
Frequently asked questions
What is the formula for arterial oxygen content?
CaO₂ = (1.34 × Hb × SaO₂/100) + (0.003 × PaO₂), where Hb is in g/dL, SaO₂ in %, and PaO₂ in mmHg. The result is in mL O₂ per dL of blood.
Why does haemoglobin matter more than saturation?
Because the haemoglobin-bound term dominates the equation — about 17 mL/dL against roughly 0.3 mL/dL dissolved in plasma. Halving haemoglobin halves content; dropping saturation from 99% to 90% reduces content by under 10%.
Does giving 100% oxygen meaningfully raise oxygen content?
Only slightly, once saturation is already near 100%. Driving PaO₂ from 95 to 500 mmHg adds about 1.2 mL/dL of dissolved oxygen — a small addition to a total already carried mostly by haemoglobin.
What is Hüfner's constant?
The amount of oxygen, in mL, that binds to one gram of fully saturated haemoglobin — conventionally 1.34, though 1.36 and 1.39 also appear depending on assay method.
Related calculators
References
- Leach RM, Treacher DF. The pulmonary physician in critical care: oxygen delivery and consumption. Thorax. 2002;57(2):170–7.
- Marino PL. The ICU Book, 4th edition — oxygen transport.