Lesson 5: Ventilation/Perfusion Mismatch Summary

Why do dead space and shunt rarely exist in absolute form in the lung?

• Most lung units exist on a continuum of ventilation and perfusion rather than pure states.

• Therefore, most clinical pathology produces varying degrees of V/Q mismatch rather than true dead space or true shunt.

What is the normal V/Q ratio and why is it not exactly 1?

• Normal V/Q ≈ 0.8, meaning ventilation is about 80% of perfusion.

• This occurs because pulmonary blood flow slightly exceeds alveolar ventilation under normal physiology.

What physiologic conditions correspond to the extremes of the V/Q spectrum?

Condition	V/Q
Dead space	∞
Normal lung	~0.8
Ideal matching	1
Shunt	0

• Dead space = ventilation without perfusion, while shunt = perfusion without ventilation.

Why does pulmonary embolism produce dead space physiology?

• A pulmonary embolism blocks pulmonary blood flow to ventilated alveoli, producing ventilation without perfusion.

• This results in V/Q → ∞ (alveolar dead space).

Why does atelectasis produce shunt physiology?

• Atelectasis causes alveoli to collapse, eliminating ventilation while perfusion persists.

• Blood leaving these units remains poorly oxygenated, creating a right-to-left shunt (V/Q = 0).

Why is atelectasis the most common cause of hypoxemia in the PACU?

• General anesthesia decreases FRC, reducing radial traction that normally keeps alveoli open.

• This leads to atelectasis → right-to-left shunt → V/Q mismatch → hypoxemia.

Why do patients with V/Q mismatch usually have more difficulty with oxygenation than CO₂ elimination?

• CO₂ diffuses ~20 times faster than oxygen, allowing compensation through increased ventilation.

• Oxygenation cannot compensate as easily, so hypoxemia occurs earlier than hypercapnia.

Why does CO₂ retention indicate severe V/Q mismatch?

• Mild V/Q mismatch can be compensated by increased ventilation in well-ventilated lung regions.

• CO₂ retention suggests compensatory mechanisms have failed, indicating severe mismatch.

In an upright patient, how does V/Q ratio change from lung base to apex?

• Base: V < Q → low V/Q

• Apex: V > Q → high V/Q

This gradient occurs because perfusion decreases more dramatically than ventilation toward the apex.

Why is pulmonary blood flow often used interchangeably with “Q” in V/Q discussions?

• Perfusion in the lung reflects pulmonary blood flow, which is largely determined by cardiac output.

• Therefore Q may represent pulmonary blood flow or cardiac output.

How does V/Q mismatch affect oxygen and carbon dioxide transport in underventilated alveoli?

• Blood leaving these units retains CO₂ and fails to fully oxygenate.

• This contributes to systemic hypoxemia and hypercapnia.

How does V/Q mismatch affect gas exchange in overventilated alveoli?

• Overventilated units eliminate large amounts of CO₂, but cannot proportionally increase oxygen content.

• Once hemoglobin reaches ~100 mmHg PaO₂, it becomes nearly fully saturated, limiting additional oxygen uptake.

Why can well-ventilated alveoli compensate for CO₂ retention but not hypoxemia?

• The oxyhemoglobin dissociation curve plateaus at high PaO₂, limiting additional oxygen loading.

• CO₂ transport lacks this plateau effect, allowing hyperventilated alveoli to remove excess CO₂.

Why is the PaO₂–PAO₂ gradient typically large in V/Q mismatch?

• Oxygen from well-ventilated alveoli cannot fully compensate for poorly ventilated regions.

• Therefore arterial oxygen remains lower than alveolar oxygen.

Why does the PaCO₂–ETCO₂ gradient often remain small during mild V/Q mismatch?

• Overventilated alveoli eliminate CO₂ efficiently, compensating for poorly ventilated units.

• Thus CO₂ gradients remain relatively small unless mismatch becomes severe.

What physiologic mechanisms compensate for V/Q mismatch?

Two key compensatory mechanisms:

1. Bronchoconstriction

• Reduces ventilation to poorly perfused alveoli (dead space regions).

2. Hypoxic pulmonary vasoconstriction (HPV)

• Reduces perfusion to poorly ventilated alveoli (shunt regions).

Why is hypoxic pulmonary vasoconstriction critical during anesthesia and one-lung ventilation?

• HPV diverts blood away from poorly ventilated alveoli toward ventilated lung units.

• This reduces shunt and improves oxygenation during one-lung ventilation.

Which anesthetic drugs can impair hypoxic pulmonary vasoconstriction?

• Volatile anesthetics can partially inhibit HPV.

• High concentrations may therefore increase shunt and worsen hypoxemia during thoracic surgery.

Why does increasing FiO₂ improve V/Q mismatch but not severe shunt?

• In V/Q mismatch, some ventilation remains, so increased oxygen can improve alveolar oxygen content.

• In shunt, blood bypasses ventilated alveoli entirely, so oxygen therapy has limited effect.