Oxyhaemoglobin dissociation · Severinghaus 1979

Severinghaus O₂ Calculator

Convert between PO₂ and SO₂, correct for temperature, pH and PCO₂, then estimate pulmonary shunt on VV ECMO without a mixed venous sample. The page opens with example values.

Pressure

PO₂ ↔ SO₂

Dash 2016

PO₂ kPa

Hb g/L

Curve model

Correct for temperature, pH (plasma), PCO₂

Temp °C

PCO₂ kPa

2,3-DPG and transfusion

Patient's own RBC 2,3-DPG mmol/L

Transfused RBCs % of circulating

Donor RBC 2,3-DPG mmol/L

Donor P50 if measured kPa

Normal RBC 2,3-DPG is about 4.65 mmol/L. Stored red cells lose most of it within 2–3 weeks and rebuild it over days after transfusion. Donor and patient cells are modelled as two populations with their own curves, mixed by proportion.

SO₂–

P50 at these conditions–

Hill n at P50–

Slope at this PO₂–

O₂ content–

Standardised (virtual) PO₂–PO₂ giving the same SO₂ at 37 °C, pH 7.40, PCO₂ 5.3 kPa, normal 2,3-DPG

Patient, from the values enteredNormal: 37 °C, pH 7.40, PCO₂ 5.3 kPa, 2,3-DPG 4.65 mmol/L

Red dot: operating point. The PO₂ you entered and the saturation it gives on the patient's curve. The hollow dot at the same PO₂ is where the normal curve would put it, and the dashed line between them is the deviation.

Black dot: P50. The PO₂ at which haemoglobin is half saturated, on the patient's curve. The hollow dot beside it is the normal P50. Right of normal means lower affinity (O₂ released more easily); left means higher affinity.

Neither is the inflection point. The curve is steepest just below P50, at . Above about 8 kPa (60 mmHg, ~90%) it flattens into the plateau.

Shunt on VV ECMO

Berggren · no PAC

Pulmonary-artery O₂ content is built from circuit gases: the oxygenator return mixes with native venous blood in proportion to effective ECMO flow. Hb is shared with the panel above.

Use the dissociation curve from the panel above

Patient at FiO₂ test

FiO₂ 0–1

PaO₂ kPa

PaCO₂ kPa

SaO₂ measured %

Barometric kPa

Circuit and flow

Cardiac output L/min

ECMO blood flow L/min

Post-oxy PO₂ kPa

Pre-oxy SO₂ %

Recirculation %

Optional check against a PAC

Measured SvO₂ (PA) %

Qs/Qt estimated–

PA content (CvO₂)–

End-capillary CcO₂–

Arterial CaO₂–

Native venous content–pre-oxy blood, recirculation removed

Effective BF / CO–BF × (1 − recirculation) ÷ CO

PaO₂ for each shunt with current ECMO venous contentSame patient, venous blood without ECMO

Reading the curve. Each line is the PaO₂ the Berggren equation predicts across shunt 0–1, holding Hb and venous content fixed. The bend sits where arterial blood leaves the flat top of the dissociation curve. Above it, extra O₂ is only dissolved, so PaO₂ falls steeply with small rises in shunt. Below it, PaO₂ barely moves while shunt climbs. ECMO lifts venous content and shifts the whole curve right, which is why PaO₂ alone separates severe shunt values poorly.