The PAO2 - PaO2 difference when breathing 100% O2 equals 240 mm Hg; the estimated transpulmonary shunt is?

The PAO2 - PaO2 difference when breathing 100% O2 equals 240 mm Hg; the estimated transpulmonary shunt is?

• A. 5%
• B. 12%
• C. 17%
• D. 20%

Answer: (B)

When a patient is on 100% oxygen, the difference between alveolar O2 tension (PAO2) and arterial O2 tension (PaO2) reflects venous admixture from a transpulmonary shunt. The larger the A-a gradient on 100% O2, the more blood is bypassing fully oxygenated alveoli. Compute PAO2 on 100% O2 at sea level: PAO2 ≈ 713 mmHg − (PaCO2/R). With a normal PaCO2 ~40 and respiratory quotient R ~0.8, PAO2 is about 663 mmHg. If the A-a gradient is 240 mmHg, PaO2 ≈ PAO2 − gradient ≈ 663 − 240 ≈ 423 mmHg. A practical bedside rule estimates transpulmonary shunt as roughly the A-a gradient on 100% O2 divided by 20. So 240 ÷ 20 = 12%. Therefore the estimated transpulmonary shunt is about 12%. This is a rough estimate used in the OR to gauge shunt severity; the exact value would require a full shunt calculation using O2 content and the shunt equation.

When a patient is on 100% oxygen, the difference between alveolar O2 tension (PAO2) and arterial O2 tension (PaO2) reflects venous admixture from a transpulmonary shunt. The larger the A-a gradient on 100% O2, the more blood is bypassing fully oxygenated alveoli.

Compute PAO2 on 100% O2 at sea level: PAO2 ≈ 713 mmHg − (PaCO2/R). With a normal PaCO2 ~40 and respiratory quotient R ~0.8, PAO2 is about 663 mmHg. If the A-a gradient is 240 mmHg, PaO2 ≈ PAO2 − gradient ≈ 663 − 240 ≈ 423 mmHg.

A practical bedside rule estimates transpulmonary shunt as roughly the A-a gradient on 100% O2 divided by 20. So 240 ÷ 20 = 12%. Therefore the estimated transpulmonary shunt is about 12%.

This is a rough estimate used in the OR to gauge shunt severity; the exact value would require a full shunt calculation using O2 content and the shunt equation.