Questions: Ventilation-Perfusion Matching and Gas Exchange Efficiency

Question 1

A patient with severe pneumonia has fluid-filled alveoli in the right lower lobe with intact blood flow — blood traverses the lobe without any gas exchange. You increase their inspired oxygen to 100% FiO₂. What is the expected effect on arterial oxygenation?

• A. Complete correction of hypoxemia, because 100% oxygen fully saturates all available hemoglobin
• B. Moderate improvement, because the higher FiO₂ boosts diffusion across the fluid-filled alveoli
• C. Minimal improvement, because the shunted blood never contacts the extra oxygen regardless of FiO₂
• D. Paradoxical worsening, because high FiO₂ inhibits hypoxic pulmonary vasoconstriction throughout the lung

Answer: (C)

This is a true shunt (V/Q = 0): blood passes through completely unventilated alveoli and returns to the left heart deoxygenated. Since this blood never contacts alveolar gas, raising FiO₂ cannot correct the problem — the extra oxygen is simply not available to the shunted fraction. Hemoglobin in the ventilated regions is already near 100% saturated, so supplemental oxygen adds little additional oxygen content to compensate for the fixed shunt fraction.

Question 2

A pulmonary embolism completely obstructs blood flow to the right upper lobe while ventilation continues normally. What is the V/Q ratio of the affected lobe, and what clinical term describes this?

• A. V/Q = 0, called a shunt — no ventilation reaching the perfused alveoli
• B. V/Q approaches infinity, called dead space — ventilation with no perfusion
• C. V/Q = 1.0, called optimal matching — embolism has no effect on V/Q ratio
• D. V/Q < 0.6, called a low V/Q zone — blood flow exceeds ventilation

Answer: (B)

Dead space is ventilation without perfusion — air arrives at the alveoli but no blood is present to pick up oxygen. A pulmonary embolism eliminates perfusion while ventilation continues, making V/Q → ∞ in that region. A shunt is the opposite: perfusion without ventilation (V/Q = 0). This distinction matters clinically: dead space responds to supplemental oxygen, while true shunt does not.

Question 3

Hypoxic pulmonary vasoconstriction (HPV) is a local response that diverts blood away from poorly ventilated alveoli — the opposite of how systemic blood vessels respond to low oxygen in peripheral tissues.

• T. True
• F. False

Answer: True

In systemic tissues, hypoxia signals that local metabolic demand exceeds oxygen delivery, so arterioles dilate to increase blood flow. In the lung, the logic is reversed: a hypoxic alveolus has poor ventilation, so directing blood there would waste perfusion on a non-functional gas exchange unit. Pulmonary arterioles therefore constrict in response to low alveolar PO₂, redirecting blood to better-ventilated regions. This local optimization improves overall V/Q matching.

Question 4

Supplemental oxygen can correct hypoxemia from any form of V/Q mismatch, including true shunt, if the FiO₂ is increased high enough.

• T. True
• F. False

Answer: False

True shunt (V/Q = 0) does not respond to supplemental oxygen regardless of FiO₂, because shunted blood bypasses ventilated alveoli entirely and never encounters the extra oxygen. In contrast, hypoxemia from dead space or low V/Q mismatch does respond to supplemental oxygen, because increasing alveolar PO₂ in functional regions can compensate for the impaired ones. The response to 100% O₂ is therefore a key clinical test for distinguishing shunt from other causes of hypoxemia.

Question 5

Why does true shunt (V/Q = 0) not respond to supplemental oxygen, even at 100% FiO₂? Explain the mechanism.

Model answer: In true shunt, a fixed fraction of blood flows through completely unventilated lung units — it never contacts alveolar gas, regardless of how high the inspired oxygen concentration is raised. The oxygenated blood exiting normal lung units is already near 100% hemoglobin saturation, so increasing FiO₂ adds only a trivial amount of dissolved oxygen (not enough to compensate for the deoxygenated shunt fraction being added to the output). The resulting mixture remains hypoxemic because the shunted fraction is a fixed oxygen debt that supplemental therapy cannot reach.

This contrasts with V/Q mismatch from dead space, where functional lung units can be 'rescued' by raising alveolar PO₂. The shunt fraction is immune to FiO₂ manipulation because the blood physically bypasses the gas exchange surface.