B. Shunt

1. Technique: inhale pure O₂ and calculate AaΔO₂

2. Rationale: 100% P_I-O2 reduces the contribution to AaΔO₂ of D_O2 ↓ and Ventilation/Perfusion non-uniformity, but does not improve desaturation due to shunt

3. Approximate estimate: when inhaling pure O2

%SHUNT @ 1% x ( AaΔO2 / 20 mmHg )

4. Example

Patient breathing pure O2 gives the following values systemic arterial blood gas values:

P_a-O2 = 270 mmHg
P_a-CO2 = 43 mmHg

Calculations:

P_I-O2 = 760 - 47 = 713 mmHg (inhaled gas, saturated)
P_A-O2 = P_I-O2 - P_A-CO2 (alveolar air equation when breathing pure O₂ )

but P_A-CO2 ≈ P_a-CO2, so

P_A-O2 = 713 - 43 = 670 mmHg
AaDO₂ = 670 - 270 = 400 mmHg Þ 20% shunt

5.    Simplified method (more approximate but acceptable)

When inhaling pure oxygen, sample systemic arterial blood and use the following guidelines:

Pa-O2	Percent Shunt
570 mmHg	5% shunt (high normal)
470 mmHg	10% shunt
370 mmHg	15% shunt
270 mmHg	20% shunt