Oxygen Extraction Ratio Calculator
Oxygen Extraction Ratio Calculator
The fraction of delivered oxygen the tissues take up, from arterial and venous oxygen content. A rising extraction ratio means delivery is failing to meet demand — the physiological question behind lactate and ScvO₂ monitoring.
Oxygen extraction ratio
6 inputs → O₂ERHb 13 g/dL, SaO₂ 97%, PaO₂ 95 mmHg, SvO₂ 70%, PvO₂ 40 mmHg
Formula
CaO₂ = 1.34 × Hb × SaO₂/100 + 0.003 × PaO₂
CvO₂ = 1.34 × Hb × SvO₂/100 + 0.003 × PvO₂
Surrogate: O₂ER ≈ (SaO₂ − SvO₂) ÷ SaO₂
- 1.34
- Hüfner's constant, the millilitres of oxygen one gram of haemoglobin binds when fully saturated. The same figure this site uses on the arterial oxygen content calculator; published values range from 1.34 to 1.39 and the choice shifts content by under 4%
- 0.003
- the solubility of oxygen in plasma, mL per dL per mmHg. At a normal arterial pO₂ this contributes about 0.3 mL/dL — under 2% of arterial content — which is why the saturation surrogate works at ordinary pressures
- why the surrogate works
- haemoglobin appears in both the numerator and the denominator and cancels almost exactly, leaving (SaO₂ − SvO₂) ÷ SaO₂. On this page's default inputs the content form gives 0.28 and the surrogate 0.28. They diverge in severe anaemia and in hyperbaric or high-FiO₂ conditions, where the dissolved term stops being negligible
- O₂ER = VO₂ ÷ DO₂
- the same quantity from the other direction: consumption divided by delivery. That equivalence is why the ratio is a statement about the balance between the two and not about either alone
- normal
- 0.2 to 0.3 at rest, corresponding to a venous saturation of roughly 70 to 80%. About a quarter of delivered oxygen is used
- the critical ratio
- Ronco and colleagues measured it directly in critically ill patients and found 0.61 ± 0.05 in septic and 0.59 ± 0.16 in non-septic patients, with no significant difference, at a critical oxygen delivery of 3.8 to 4.5 mL/min/kg. Above roughly 0.6, consumption becomes delivery-dependent. 0.5 is a clinical alarm level, not the physiological threshold
- SvO₂ or ScvO₂
- mixed venous from a pulmonary artery catheter, or central venous from a superior vena cava line. ScvO₂ usually reads a few per cent higher in critical illness, the offset is not constant, and no conversion is applied here
Worked example
Hb 13 g/dL, SaO₂ 97%, PaO₂ 95 mmHg, SvO₂ 70%, PvO₂ 40 mmHg
CaO₂ = (1.34 × 13 × 0.97) + (0.003 × 95) = 16.897 + 0.285 = 17.18 mL O₂/dL
CvO₂ = (1.34 × 13 × 0.70) + (0.003 × 40) = 12.194 + 0.120 = 12.31 mL O₂/dL
Extracted: 17.18 − 12.31 = 4.87 mL O₂/dL
4.87 ÷ 17.18 = 0.28 — normal, and consistent with the SvO₂ of 70%
By the surrogate: (97 − 70) ÷ 97 = 27 ÷ 97 = 0.28. The two agree to two decimal places, which is the whole case for using the surrogate at the bedside
Enter the same tensions as 12.7 kPa and 5.3 kPa with the unit set to kPa and the answer is unchanged, because the dissolved term is small either way
Drop the SvO₂ to 50% with everything else unchanged and the ratio rises to 0.49 — approaching the alarm level, and not far from the measured critical ratio of about 0.6
Halve the haemoglobin to 6.5 g/dL with the same saturations and the ratio barely moves, to 0.29. The ratio is nearly blind to anaemia; oxygen DELIVERY has halved. That is why the two calculations belong together
Reading the extraction ratio
| O₂ER | Approximate SvO₂ | What it usually means |
|---|---|---|
| Below 0.20 | Above 80% | Supranormal delivery, or impaired extraction — sepsis with microcirculatory shunting, cyanide, severe hypothermia. A low ratio with a high lactate is the dangerous version |
| 0.20 – 0.30 | 70 – 80% | Normal resting balance between delivery and consumption |
| 0.30 – 0.50 | 50 – 70% | Compensating for reduced delivery or increased demand. Look at haemoglobin, saturation, cardiac output — and at fever, shivering, agitation and work of breathing |
| 0.50 – 0.60 | 40 – 50% | Marked compensation, approaching the measured critical ratio. Send a lactate and act on delivery |
| Above 0.60 | Below 40% | At or beyond the critical extraction ratio measured by Ronco and colleagues. Consumption becomes delivery-dependent; anaerobic metabolism and rising lactate follow |
The three levers on delivery, and the one on demand
| Lever | How it moves delivery | Typical intervention |
|---|---|---|
| Haemoglobin | Delivery is directly proportional to it. Halving haemoglobin halves delivery | Transfusion, on a threshold and a patient rather than a number |
| Arterial saturation | Directly proportional through the 1.34 × Hb × SaO₂ term | Oxygen, positive pressure, treating the lung |
| Cardiac output | Directly proportional. DO₂ = CaO₂ × cardiac output × 10 | Fluid, inotropes, rate and rhythm control |
| Arterial pO₂ above saturation | Almost nothing. Dissolved oxygen is about 0.3 mL/dL of 17 | Raising FiO₂ in an already saturated patient adds very little content |
| Consumption (VO₂) | The other half of the ratio and the one most often ignored | Antipyretics, stopping shivering, analgesia, sedation, supporting work of breathing |
Why the saturation surrogate is usually safe
| Situation | Content form | Surrogate (SaO₂ − SvO₂) ÷ SaO₂ | Agree? |
|---|---|---|---|
| Hb 13, SaO₂ 97, SvO₂ 70, PaO₂ 95, PvO₂ 40 | 0.28 | 0.28 | Yes |
| Hb 6.5 (anaemia), same saturations and tensions | 0.29 | 0.28 | Yes — within 0.01 |
| Hb 13, SaO₂ 100, SvO₂ 70, PaO₂ 500, PvO₂ 40 (high FiO₂) | 0.35 | 0.30 | No — the dissolved oxygen in the arterial sample is now 1.5 mL/dL |
| Hb 4, SaO₂ 100, SvO₂ 70, PaO₂ 500, PvO₂ 40 (profound anaemia on high FiO₂) | 0.44 | 0.30 | No — dissolved oxygen is a large share of a small content |
The question behind lactate and ScvO₂ monitoring
Oxygen delivery and oxygen consumption are two different quantities, and almost every measurement at the bedside reports one of them while the clinical question is about the relationship between the two. A haemoglobin of 7 g/dL is dangerous in a septic patient with a fever and unremarkable in a stable one with chronic anaemia; a cardiac output of 4 L/min is generous for a sleeping adult and inadequate for one who is shivering. The extraction ratio collapses both into a single number: it is consumption divided by delivery, or equivalently the fraction of the oxygen arriving in arterial blood that the tissues remove before the blood returns. At rest that fraction is about a quarter, which is why a normal mixed venous saturation is around 70 to 75%.
Extraction is the body's first defence against falling delivery, and it is a good one. Halve a patient's cardiac output and the tissues simply take a larger share of what arrives; consumption is maintained and nothing goes anaerobic. That compensation is visible as a falling venous saturation and a rising extraction ratio long before the lactate moves, which is precisely what makes the ratio useful — it reports a problem while it is still being compensated. But the reserve is finite. Ronco and colleagues measured the point at which it runs out, in critically ill patients at the withdrawal of life support, and found a critical extraction ratio of about 0.6 — 0.61 in septic patients and 0.59 in non-septic ones, with no significant difference between them. Beyond it, consumption starts to follow delivery, cells respire anaerobically and lactate rises.
That measured figure is worth holding against the number most often quoted. A ratio above 0.5 is a sensible clinical alarm level and this page bands it there, but 0.5 is not the physiological threshold and should not be described as one. The threshold is around 0.6, it is the same in sepsis as outside it — which is itself a useful finding, since sepsis is often said to impair extraction — and the corresponding critical oxygen delivery in that study was 3.8 to 4.5 mL/min/kg. A patient at 0.55 has less reserve than a comfortable phrase like 'raised' suggests.
Two cautions finish the picture. The ratio is a global measure and can be entirely normal while one organ bed is ischaemic, so it does not exclude regional hypoperfusion and never explains a rising lactate on its own. And a low ratio is not automatically reassuring: in distributive shock, microcirculatory shunting and mitochondrial dysfunction leave oxygen in the venous blood because the cells cannot use it, so a high venous saturation with a high lactate is one of the most ominous combinations in critical care. Read the ratio with the lactate, with the oxygen delivery, and with the patient in front of you — it supports a clinician's judgement rather than replacing it.
Frequently asked questions
What is a normal oxygen extraction ratio?
About 0.25, with a usual resting range of 0.20 to 0.30. That corresponds to a venous oxygen saturation of roughly 70 to 80%: the tissues take about a quarter of the oxygen delivered and the rest returns to the right heart.
At what extraction ratio does oxygen consumption become delivery-dependent?
At about 0.6. Ronco and colleagues measured it directly in critically ill patients and found a critical extraction ratio of 0.61 in septic and 0.59 in non-septic patients, with no significant difference, at a critical oxygen delivery of 3.8 to 4.5 mL/min/kg. The figure of 0.5 often quoted is a reasonable clinical alarm level rather than the physiological threshold.
Can I use (SaO₂ − SvO₂) ÷ SaO₂ instead of the full content calculation?
Usually yes. Haemoglobin appears in both the numerator and denominator of the content form and cancels almost exactly, so the saturation surrogate matches the full calculation to two decimal places in ordinary conditions. It diverges where dissolved oxygen stops being negligible — profound anaemia combined with a very high inspired oxygen, and hyperbaric conditions.
What does a low extraction ratio mean?
Either that delivery is generous, or that the tissues cannot use the oxygen arriving. The second is the dangerous one: distributive shock with microcirculatory shunting, mitochondrial dysfunction in sepsis, cyanide poisoning and severe hypothermia all leave oxygen in the venous blood. A low extraction ratio with a rising lactate identifies that group and is an emergency.
Are SvO₂ and ScvO₂ interchangeable?
No. Mixed venous saturation comes from a pulmonary artery catheter and central venous saturation from a superior vena cava line. ScvO₂ typically reads a few per cent higher in critical illness, the difference is not constant, and no reliable single conversion exists — so no offset is applied here. Record which sample the value came from.
Should I enter the pO₂ in kPa or mmHg?
Either — set the selector to match your analyser, and it applies to both the arterial and the venous value. The 0.003 solubility coefficient is per mmHg, so a kPa entry is multiplied by 7.50062 first. In practice the dissolved term changes the answer very little: a normal arterial pO₂ contributes about 0.3 mL/dL of a total near 17.
Related calculators
References
- Ronco JJ, Fenwick JC, Tweeddale MG, et al. Identification of the critical oxygen delivery for anaerobic metabolism in critically ill septic and nonseptic humans. JAMA. 1993;270(14):1724–30.
- Vincent JL, De Backer D. Circulatory shock. N Engl J Med. 2013;369(18):1726–34.
- Rivers E, Nguyen B, Havstad S, et al. Early goal-directed therapy in the treatment of severe sepsis and septic shock. N Engl J Med. 2001;345(19):1368–77.
- Walley KR. Use of central venous oxygen saturation to guide therapy. Am J Respir Crit Care Med. 2011;184(5):514–20.
- Leach RM, Treacher DF. The pulmonary physician in critical care 2: oxygen delivery and consumption in the critically ill. Thorax. 2002;57(2):170–7.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.