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MedCram Lecture: Hypoxemia - VQ Mismatch

Jul 15, 2024

MedCram Lecture: Hypoxemia - VQ Mismatch

Introduction

  • Topic: Mechanisms causing hypoxemia
  • Focus: VQ Mismatch
  • Definitions: V/Q Ratio
    • V (Ventilation): Air moving in and out of the lung
    • Q (Perfusion): Blood circulating to lung areas
    • Simultaneous occurrence is crucial for lung function

Background on Ventilation and Perfusion

  • Heart Chambers: Right and left
    • Right side pumps blood to lungs (low pressure: 20-30 mm Hg)
    • Left side pumps blood to body (high pressure: 80-120 mm Hg)
  • Lungs: Blood distribution influenced by gravity
    • More perfusion in lower lung areas
    • More ventilation in lower areas but less variable

VQ Ratio Explained

  • High VQ Ratio: More ventilation, less perfusion (e.g., trachea)
    • Causes high oxygen levels
    • Example: Dead space
  • Low VQ Ratio: More perfusion, less ventilation
    • Causes low oxygenation
    • Example: Shunt
  • VQ Mismatch: Difference in VQ ratios across lung units
    • Regions of both high and low VQ ratios
    • Leads to hypoxia

High VQ vs. Low VQ

  • High VQ Area:
    • Lots of ventilation, low perfusion
    • High oxygen levels (e.g., 98% O2 saturation)
  • Low VQ Area:
    • Less ventilation, high perfusion
    • Lower oxygen levels (e.g., 80% O2 saturation)

Effects of VQ Mismatch on Blood Oxygen

  • Blood from both high and low VQ areas mixes
  • New blood oxygenation level skews toward lower perfusion area due to volume
    • Example result: ~85% O2 saturation
  • 100% oxygen administration can correct VQ mismatch
    • Improves overall oxygenation (unlike shunt)

Common Causes of VQ Mismatch

  • Pulmonary embolism: Blocks perfusion to lung segment
    • Leads to high VQ at blocked segment, low VQ elsewhere
  • Other causes:
    • Pneumonia
    • COPD
    • Pulmonary hypertension
    • Asthma

Key Points

  1. Response to 100% Oxygen: Improves oxygenation
  2. Increased A-a Gradient: Present in VQ mismatch
  3. Common Causes: COPD, Pneumonia, Pulmonary Embolism, Pulmonary Hypertension, Asthma
  4. Most common cause of hypoxemia

Notes Summary: VQ mismatch is a primary cause of hypoxemia, distinguished by areas of varying ventilation and perfusion within the lung, often correctable by 100% oxygen. Common conditions leading to VQ mismatch include pulmonary embolism, COPD, and pneumonia.

Full transcript