Ventilation-perfusion mismatch

Pathophysiology/Etiology

• An imbalance between the amount of air reaching the alveoli (Ventilation) and the amount of blood perfusing them (Q).
• The ideal V/Q ratio is ~0.8 to 1.0. Any significant deviation impairs gas exchange, leading to hypoxemia.
• An elevated A-a gradient is a hallmark of V/Q mismatch.

Two Extremes of V/Q Mismatch

• 1. Low V/Q (Shunt): Perfusion without Ventilation (V/Q → 0)

  • Mechanism: Blood flows past alveoli that are not ventilated (e.g., collapsed or filled with fluid), so it cannot be oxygenated. This is called a physiologic shunt.
  • Causes: Airway obstruction (e.g., foreign body, mucus plug), pneumonia, pulmonary edema, atelectasis (lung collapse), ARDS.
  • Key Feature: Hypoxemia that does not correct with 100% O2 because the supplemental O2 cannot reach the perfused capillaries.
    • In a healthy, ventilated alveolus, hemoglobin passing by is already ~97-98% saturated with oxygen even on room air (21% O2).
    • Only part of the blood redirect to healthy lungs, thus 100% O2 can't entirely fix.

• 2. High V/Q (Dead Space): Ventilation without Perfusion (V/Q → ∞)

  • Mechanism: Alveoli are ventilated, but there is no blood flow to them for gas exchange to occur. This is called alveolar dead space.
  • Causes: Pulmonary embolism (PE) is the classic cause. Others include emphysema (capillary destruction), and conditions with low cardiac output or pulmonary hypertension.
  • Key Feature: Primarily impairs CO2 elimination, leading to "wasted ventilation." Hypoxemia occurs and typically corrects with 100% O2.
    • Because all obstructed blood redirect to healthy arteries.

Clinical Presentation

• Hypoxemia is the primary consequence.
• Symptoms depend on the underlying cause but can include dyspnea, tachypnea, dizziness, and cyanosis.

Diagnosis

• Arterial Blood Gas (ABG): Shows hypoxemia (↓ PaO2).
• A-a Gradient: Increased A-a gradient is a key finding in V/Q mismatch, distinguishing it from hypoxemia due to hypoventilation or low inspired O2 (which have a normal A-a gradient).
• V/Q Scan: A nuclear medicine scan that directly measures and compares the distribution of ventilation and perfusion in the lungs. Classically used to diagnose PE.
• Imaging (CXR, CT): Helps identify the underlying cause (e.g., pneumonia, edema, PE).

Management/Treatment

• Treat the underlying cause: This is the most crucial step (e.g., antibiotics for pneumonia, anticoagulation for PE, bronchodilators for asthma).
• Supplemental O2: Effective for high V/Q dead space but poorly effective for low V/Q shunts.
• Mechanical Ventilation: May be required for severe respiratory distress, often with PEEP to recruit collapsed alveoli in cases of shunting (e.g., ARDS).

Key Differentiating Factors (High-Yield)

• Hypoxemia Correction with 100% O2:
  • Corrects: Dead Space (e.g., PE).
  • Does NOT correct: Shunt (e.g., pneumonia, pulmonary edema).
• A-a Gradient:
  • Increased: V/Q Mismatch, Shunt, Diffusion limitation.
  • Normal: Hypoventilation, High Altitude.