HFOV Mechanisms of Gas Transport

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1. Direct bulk flow

2. Taylor dispersion

3. Pendeluft

4. Asymmetric velocity profiles

5. Cardiogenic mixing

6. Molecular diffusion

HFOV Mechanisms of Gas Transport

Krishnan CHEST 2000

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Convection (Bulk Flow) Ventilation

• Even with small tidal volumes, direct alveolar ventilation occurs to short path length units that branch off of the primary airways.

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Augmented (Taylor) dispersion

• Complicated interplay between convective forces and molecular diffusion

• Addition of convective flow to a diffusive process significantly increased dispersion of molecules

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Penduluft

• At high frequencies, distribution becomes strongly influenced by time constant inequalities. Gas from fast units (short time constants) will empty into the slow (long time constants) units.

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Asymmetrical Velocity Profile

Inspiration

Net Effect

Expiration

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Cardiogenic Mixing

• The heart beat adds to the the peripheral gas mixing.

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Molecular Diffusion

• Is felt to be one of the major mechanisms for gas exchange in the alveolar regions.

• It is responsible for the gas exchange across the AC membrane and also contributes to the transport of O2 and CO2 in the gas phase near the membrane.

• This may be due to the increased turbulence of molecules.