Learning Center/Formulas/CaO₂

oxygenation

Oxygen Content of Blood

CaO₂ = (Hgb × 1.34 × SaO₂) + (PaO₂ × 0.003)

Normal: 18–20 mL O₂/dL arterial blood; 13–15 mL O₂/dL mixed venous blood

Variables

CaO₂

Arterial oxygen content(mL O₂/dL blood)

Normal: 18–20 mL/dL

Hgb

Hemoglobin concentration(g/dL)

Normal: 12–17 g/dL

1.34

O₂ carrying capacity of Hgb(mL O₂/g Hgb)

Normal: Constant

SaO₂

Arterial oxygen saturation(decimal (0–1))

Normal: 0.95–1.0

PaO₂

Arterial PO₂(mmHg)

Normal: 80–100 mmHg

0.003

O₂ dissolved in plasma per mmHg(mL/dL/mmHg)

Normal: Constant

What It Measures

Oxygen content is the total amount of oxygen carried in blood, both bound to hemoglobin (the dominant component) and dissolved in plasma (minor component). It reveals why anemia can cause tissue hypoxia even when SpO₂ is 100% — hemoglobin is the oxygen carrier, not plasma. The dissolved component contributes < 1.5% of total O₂ at normal PaO₂.

When to Use

Calculate when assessing oxygen delivery (DO₂) in critically ill patients, when anemia may limit oxygen transport despite normal saturation, or when evaluating the benefit of blood transfusion versus supplemental oxygen. Required for true shunt calculation.

Interactive Calculator

Hemoglobin(g/dL)

SaO₂(decimal (0–1))

PaO₂(mmHg)

Worked Examples

Normal patient

→ 19.98 mL/dL

Normal oxygen content

Severe anemia, saturating

→ 9.68 mL/dL

CaO₂ reduced by 50% despite 100% saturation — anemia limits O₂ delivery

Clinical Significance

Normal CaO₂ ≈ 20 mL/dL. Anemia reduces CaO₂ dramatically — a Hgb of 7 g/dL with SaO₂ 100% gives CaO₂ = 9.4 mL/dL (vs 20 mL/dL with Hgb 15). This is why SpO₂ is NOT a measure of oxygen delivery — a fully saturated anemic patient may have severely impaired oxygen delivery.

Memory Tips

💡CaO₂ = Hgb × 1.34 × SaO₂ + (PaO₂ × 0.003) — the bound part dominates; dissolved is negligible at normal PaO₂
💡1.34 is the Hüfner constant — how many mL of O₂ can 1 g of Hgb hold
💡SpO₂ measures SATURATION — not how much hemoglobin is present. A low Hgb = low CaO₂ even at 100%

NBRC Pearls

Transfusion increases CaO₂ by increasing Hgb — more impactful than increasing FiO₂ in anemic patients
CO poisoning: SaO₂ reads normal (pulse oximetry) but CaO₂ is severely reduced (Hgb bound to CO, not O₂)
CvO₂ (mixed venous O₂ content) is used with CaO₂ to calculate C(a-v)O₂ and cardiac output

← All Formulas

Variables

CaO₂

Arterial oxygen content(mL O₂/dL blood)

Normal: 18–20 mL/dL

Hgb

Hemoglobin concentration(g/dL)

Normal: 12–17 g/dL

1.34

O₂ carrying capacity of Hgb(mL O₂/g Hgb)

Normal: Constant

SaO₂

Arterial oxygen saturation(decimal (0–1))

Normal: 0.95–1.0

PaO₂

Arterial PO₂(mmHg)

Normal: 80–100 mmHg

0.003

O₂ dissolved in plasma per mmHg(mL/dL/mmHg)

Normal: Constant

What It Measures

Memory Tips

💡CaO₂ = Hgb × 1.34 × SaO₂ + (PaO₂ × 0.003) — the bound part dominates; dissolved is negligible at normal PaO₂

💡1.34 is the Hüfner constant — how many mL of O₂ can 1 g of Hgb hold

💡SpO₂ measures SATURATION — not how much hemoglobin is present. A low Hgb = low CaO₂ even at 100%

NBRC Pearls

Transfusion increases CaO₂ by increasing Hgb — more impactful than increasing FiO₂ in anemic patients

CO poisoning: SaO₂ reads normal (pulse oximetry) but CaO₂ is severely reduced (Hgb bound to CO, not O₂)

CvO₂ (mixed venous O₂ content) is used with CaO₂ to calculate C(a-v)O₂ and cardiac output