Understanding Lung Gas Distribution Tests

Aug 13, 2024

Exhaled Gas Concentrations and Lung Gas Distribution

Key Concepts

  • Uniformity of Inspired Gas Distribution
    • Analyzing exhaled gas concentrations helps understand how uniformly gas is distributed throughout the lungs.
    • Variations in gas concentration during expiration indicate non-uniform distribution.

Single Breath Nitrogen Washout Test

  • Procedure:

    1. Patient seated, takes two breaths of air and exhales to residual volume.
    2. Administer 100% oxygen after turning valve on mouthpiece.
    3. Patient inhales oxygen to total lung capacity and exhales slowly to residual volume.
    4. Measure nitrogen concentration and exhaled volume continuously.

  • Equipment:

    • Nitrogen meter for concentration measurement.
    • Pneumo tachograph for volume measurement.

Exhaled Nitrogen Concentration vs. Exhaled Volume

  • Phases of the Curve:
    • Phase 1: Initial exhalation, devoid of nitrogen, represents gas from proximal airways.
    • Phase 2: Mix of central airway and alveolar gas.
    • Phase 3: Mix of gases from lung apices, mid-lung zones, and bases.
    • Phase 4: Predominantly gas from apices as airway bases close at lower lung volumes.

Gas Distribution During Inspiration

  • More inspired gas goes to lung bases due to:

    • Higher alveolar density.
    • Less distension in bases allowing for greater expansion.

  • Oxygen primarily dilutes nitrogen at lung bases, causing concentration differences during exhalation.

Small Airways and Closing Volume

  • Elastic Recoil and Airway Closure:

    • Decreased recoil and narrow airways close small airways at lung bases.
    • More gas from apices exhaled as phase four progresses.

  • Closing Volume:

    • Volume from onset of phase four to end of expiration.
    • Normal: ~10% of vital capacity.
    • Increased in small airways disease, aging, and smoking (>50 years can reach 25% of vital capacity).

Additional Resources

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