V/Q ratio and gas exchange

V/Q Ratio Basics - The Balancing Act

• Ventilation (V): The volume of gas reaching the alveoli.
• Perfusion (Q): The volume of blood passing through the alveolar capillaries.
• Efficient gas exchange requires a delicate balance between V and Q.
• The ideal overall V/Q ratio is approximately 0.8.

• Mismatch Types:
  • High V/Q (Dead Space): Good ventilation, poor perfusion. Alveoli are ventilated but not perfused, so no gas exchange occurs. Think of a pulmonary embolism.
  • Low V/Q (Shunt): Poor ventilation, good perfusion. Blood flows past unventilated alveoli, failing to get oxygenated.

⭐ In the upright lung, both ventilation and perfusion are highest at the base, but perfusion increases more dramatically than ventilation. This results in a higher V/Q ratio at the apex and a lower V/Q ratio at the base.

V/Q Mismatch - When Ratios Go Rogue

• The ventilation/perfusion (V/Q) ratio compares alveolar ventilation (V) to pulmonary blood flow (Q).
• Ideal V/Q ≈ 0.8: optimizes gas exchange, matching airflow to blood flow.
• Gravity creates a physiological V/Q gradient:
  • Apex: ↑ V/Q ratio (>3.0). Gravity pulls blood down, so perfusion (Q) is lower. Creates physiologic dead space.
  • Base: ↓ V/Q ratio (<0.6). Gravity enhances blood flow, so perfusion (Q) is higher. Creates physiologic shunt.

• V/Q = 0 (Shunt): Airway obstruction (e.g., mucus plug, foreign body).

  • No ventilation, but perfusion continues.
  • Blood passes through but isn't oxygenated. $P_{A}O_2$ is low, resembling venous blood.

• V/Q = ∞ (Dead Space): Blood flow obstruction (e.g., pulmonary embolism).

  • Ventilation occurs, but no perfusion.
  • Air reaches alveoli but doesn't contact blood. $P_{A}O_2$ is high, resembling inspired air.

⭐ High-Yield: A shunt (V/Q = 0) is not corrected by 100% O₂ because the unventilated alveoli cannot exchange gas, regardless of inspired oxygen concentration. In contrast, diffusion limitations and most V/Q mismatches will correct.

Regional Lung V/Q - Apex vs. Base

In an upright lung, gravity pulls both air and blood down, but its effect on blood flow (perfusion, Q) is far greater than on ventilation (V). Both V & Q are highest at the base.

• Base: Resembles a physiologic shunt. Alveoli are well-perfused but relatively under-ventilated.

⭐ Overall gas exchange in the lung is dominated by the base due to its significantly higher blood flow, making its gas pressures more representative of arterial blood gas values.

High‑Yield Points - ⚡ Biggest Takeaways

• The ideal V/Q ratio is ~0.8, as perfusion (Q) is normally greater than ventilation (V).
• The lung apex has the highest V/Q ratio (>3.0), acting as physiologic dead space.
• The lung base has the lowest V/Q ratio (<0.6), acting as a physiologic shunt.
• High V/Q areas have ↑PAO2 and ↓PACO2; Low V/Q areas have ↓PAO2 and ↑PACO2.
• Hypoxic pulmonary vasoconstriction is the primary mechanism to correct V/Q mismatch.
• V/Q mismatch is the most frequent cause of hypoxemia.