Oxygenation

Oxygenation Core

Oxygen delivery is limited by Hb, not lungs, most of the time.

You don’t treat numbers — you treat CaO₂ × cardiac output.

Airway

• Fix the airway first - Oxygen does NOT bypass obstruction

Breathing

• Breathing ≠ oxygenation

• Ventilation failure ≠ oxygenation failure (and vice versa)

• Oxygenation = alveoli + diffusion + Hb + flow

Circulation

• CaO₂ = Hb × SaO₂ × 1.34

• 1 gram of Hb can carry 1.34 mL of O₂

• SpO₂ & PaO₂ indicate how full the truck is (Hb = number of trucks, SpO2 = number of seats). CaO₂ tells you how much oxygen is actually being delivered.

• Content matters more than saturation. 

Hb 6 with SpO₂ 100% = still hypoxic tissue

High PaO₂ with low Hb still delivers poor oxygen.

1. Normal adult: Hb = 15 g/dL, SaO₂ = 1.0. CaO₂ = 15 × 1.0 × 1.34  ≈ 20 mL O₂ per 100 mL blood

 That’s normal oxygen carriage.

2. Severe anemia: Hb = 7, SaO₂ = 1.0. CaO₂ = 7 × 1.34 = 9.4

     Patient is hypoxic despite perfect saturation.

3. Lung disease: Hb = 15, SaO₂ = 0.80. CaO₂ = 16

    Better than an anaemic patient with Hb 7!

PaO₂ barely matters: There is a dissolved oxygen part: CaO₂ = (Hb × SaO₂ × 1.34) + (PaO₂ × 0.003)

PaO₂ 100 → dissolved O₂ = 0.3 mL

            Compared to 20 mL from Hb-bound O₂

=> Ventilators fix PaO₂

=> Blood transfusion fixes CaO₂

=> SpO₂ ≠ oxygen delivery

Anaemia causes tissue hypoxia even with SpO₂ 100%

Improving Hb improves oxygen delivery more than pushing FiO₂. This is why:

• Hb <7 crashes lactate tolerance

• Hb <8 matters in shock, sepsis, MI, TBI

Oxygen delivery (DO₂) — the equation that actually matters

DO₂ = Cardiac Output × CaO₂

DO₂ = CO × (Hb × SaO₂ × 1.34)

• Lungs don’t deliver oxygen.

• Blood flow delivers oxygen. Blood + flow fixes this.

Tissue hypoxia determinants (in order of importance) 

1. Cardiac output

2. Hemoglobin

3. Oxygen saturation

4. PaO₂ (almost irrelevant)

Priority in a shock patient should be

1. Restore flow (fluids / pressors / inotropes)

2. Correct anemia

3. Then worry about SpO₂

=> Liberal oxygen without anemia correction = false reassurance. Transfuse early if:

• ongoing shock

• rising lactate

• myocardial ischemia

• brain injury