CaO₂ Calculator
Calculate arterial oxygen content using hemoglobin, SaO₂, and PaO₂. This calculator helps show why a patient can have normal PaO₂ and SpO₂ but still have low oxygen content when hemoglobin is low.
Calculator
Calculate Arterial Oxygen Content
Formula: CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂). Enter SaO₂ as a percent or decimal.
CaO₂
—
mL O₂/dL blood
Enter Hb, SaO₂, and PaO₂ to calculate oxygen content.
Formula
CaO₂ Formula
CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂)
Example: Hb 15, SaO₂ 0.98, PaO₂ 95
CaO₂ = (1.34 × 15 × 0.98) + (0.003 × 95)
CaO₂ ≈ 20.0 mL/dL
Example: Hb 15, SaO₂ 0.98, PaO₂ 95
CaO₂ = (1.34 × 15 × 0.98) + (0.003 × 95)
CaO₂ ≈ 20.0 mL/dL
RT Memory Trick
Most Oxygen Rides on Hemoglobin
The first part of the formula is usually much larger than the dissolved oxygen part.
(1.34 × Hb × SaO₂) = oxygen bound to hemoglobin
(0.003 × PaO₂) = dissolved oxygen
(0.003 × PaO₂) = dissolved oxygen
Remember:
PaO₂ tells you dissolved oxygen pressure. CaO₂ tells you how much oxygen is actually contained in arterial blood.
PaO₂ tells you dissolved oxygen pressure. CaO₂ tells you how much oxygen is actually contained in arterial blood.
Interpretation
How to Interpret CaO₂
| CaO₂ Pattern | Meaning | Common Clinical Connection |
|---|---|---|
| About 18–22 mL/dL | Typical adult range | Often seen with normal Hb and normal saturation. |
| Low CaO₂ with low PaO₂ | Hypoxemia is reducing oxygen loading | Pneumonia, ARDS, V/Q mismatch, shunt. |
| Low CaO₂ with normal PaO₂ | Oxygen-carrying capacity may be low | Anemia or abnormal hemoglobin problem. |
| Low CaO₂ with poor perfusion | Content may be low and delivery may also be impaired | Shock, low cardiac output, severe anemia. |
Clinical Pearl
Low Hemoglobin Can Cause Hypoxia Without Hypoxemia
Hypoxemia means low PaO₂. Hypoxia means inadequate tissue oxygen. A patient with severe anemia may have normal PaO₂ and normal saturation, but low CaO₂ because there is not enough hemoglobin to carry oxygen.
Normal PaO₂
Dissolved oxygen pressure may look fine.
Dissolved oxygen pressure may look fine.
Low Hb
Oxygen-carrying capacity is reduced.
Oxygen-carrying capacity is reduced.
Low CaO₂
Total arterial oxygen content is low.
Total arterial oxygen content is low.
Clinical connection
Always connect ABGs with hemoglobin, perfusion, and patient signs.
Always connect ABGs with hemoglobin, perfusion, and patient signs.
Common Student Mistakes
Avoid These Errors
Using SaO₂ as 98 instead of 0.98
The calculator converts percentages, but the formula uses a decimal.
The calculator converts percentages, but the formula uses a decimal.
Thinking PaO₂ equals oxygen content
PaO₂ is dissolved oxygen pressure, not total oxygen content.
PaO₂ is dissolved oxygen pressure, not total oxygen content.
Ignoring hemoglobin
Hb is the major oxygen carrier and often the biggest driver of CaO₂.
Hb is the major oxygen carrier and often the biggest driver of CaO₂.
Forgetting oxygen delivery
CaO₂ is only part of delivery; cardiac output and perfusion also matter.
CaO₂ is only part of delivery; cardiac output and perfusion also matter.
Continue Learning
Connect Oxygen Content to Oxygen Delivery
CaO₂ explains oxygen content. Oxygen delivery also depends on cardiac output, perfusion, and tissue demand.