Lesson 5: Ventilation/Perfusion Mismatch Summary

Lesson 5: Ventilation/Perfusion Mismatch Overview

  • Dead space and shunt exist on a continuum; lung units have varying degrees of ventilation and perfusion.

  • Normal V/Q ratio is 0.8; V/Q mismatch occurs when this ratio is disrupted.

Definitions

  • Dead Space: Ventilation without perfusion (V/Q = ∞).

  • Shunt: Perfusion without ventilation (V/Q = 0).

  • Atelectasis: Primary cause of hypoxemia in PACU, leading to right-to-left shunt.

Effects of V/Q Mismatch

  • It is harder for patients with V/Q mismatch to oxygenate than to eliminate CO2.

  • CO2 retention indicates significant V/Q mismatch.

V/Q Ratio Context

  • V/Q ratios:

    • V/Q = ∞ (dead space)

    • V/Q > 1 (overventilated)

    • V/Q = 1 (ideal match)

    • V/Q < 1 (underventilated)

    • V/Q = 0 (shunt)

  • Patient positioning affects V/Q relationship; apex: V > Q, base: V < Q.

Clinical Implications

  • Hypoxemia in PACU due to smaller FRC post-anesthesia leads to atelectasis.

  • Treatment includes humidified O2 and airway reopening maneuvers (mobility, deep breathing).

Consequences in Alveoli

  • Underventilated Alveoli: Blood retains CO2 and lacks oxygen uptake.

  • Overventilated Alveoli: Blood eliminates excess CO2 but cannot absorb enough O2 due to saturation limits.

Whole Lung Response to V/Q Mismatch

  • CO2 compensation occurs from overventilated areas; however, oxygen uptake remains inadequate due to the mismatch.

  • The body attempts to balance ventilation and perfusion:

    • Bronchioles constrict in dead space (zone 1).

    • Hypoxic pulmonary vasoconstriction occurs in shunt areas (zone 3).