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Decompression

Decompression. Decompression - Introduction. Nitrogen Depth & Time Safe Ascent Various Theories Varying Permeability Model (VPM) Reduced Gradient Bubble Model (RGBM) Gradient Factors. General Theory. Inert Gas Partial Pressure Supersaturation Slow & Fast Tissues M-Values

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Decompression

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  1. Decompression

  2. Decompression - Introduction • Nitrogen • Depth & Time • Safe Ascent • Various Theories • Varying Permeability Model (VPM) • Reduced Gradient Bubble Model (RGBM) • Gradient Factors

  3. General Theory • Inert Gas • Partial Pressure • Supersaturation • Slow & Fast Tissues • M-Values • Tissue Compartments • Leading Tissues • Avoid Exceeding M-Value

  4. General Theory • Supersaturation • Gradient • PP of dissolved inert gas in a tissue (High) • Ambient pressure of gas in alveoli (Low) • Steep Gradient • Faster Decompression • Level of supersaturation nearer to M-Value • Greater risk of DCS • Lower Gradient • Slower Decompression • Less risk of DCS • Balance

  5. General Theory • Haldanian Models Empirical • Bubbles cause DCS • Silent Bubbles – Doppler • Newer theories look at bubble formation • Algorithms • Deep stops – Attempt to limit bubble formation • Dissolved gas and free gas • Bubble Models • Varying Permeability Model (VPM) • Reduced Gradient Bubble Model (RGBM)

  6. General Theory • Ideally inert gas remains in solution • Bubbles • Damage tissue • Clog up pulmonary capillary bed • Bubble Nuclei

  7. Physics of the Bubble Pgas – Partial Pressure of Gas in the Bubble Ptension – Surface Tension Pamb – Ambient Pressure Ptissue – Tension from surrounding Tissue PGas PTension Pamb PTissue

  8. Physics of the Bubble • Reducing Forces > Expanding Forces – Bubble Shrinks • Expanding Forces > Reducing Forces – Bubble Expands • Equilibrium • Gases diffuse into and out of Bubble • Partial Pressure in bubble = Tissue Tension outside bubble • At Depth • pp inert gas in bubble is high • pp of dissolved gas in tissue is low • Reduces bubble size • Deep stops beneficial

  9. Oxygen Window • Inherent Unsaturation • Oxygen metabolised • CO2 highly soluble • Gas tension lower than in arterial blood • Aids removal of gas from bubbles • Zero Supersaturation • Nitrogen Supersaturation no greater than Inherent Unsaturation • Ambient Pressure not exceeded

  10. Gas Elimination • Low partial pressure of inert gas in alveoli • Achieved at Shallower Depths • M – Values create ceiling • Low FN2 in breathing mix • Rich Mix on Ascent • Decompression Schedules • Deep Stops reduce bubble formation • Shallow stops remove dissolved inert gas • Balance

  11. Bubble Models • Varying Permeability Model • Gas Nuclei • Surfactant Skin • Skins permeable at low ambient pressure • Gas diffusion reduced at high ambient pressure • Bubble growth and contraction varies with pressure • Critical total volume of bubbles

  12. Varying Permeability Model • Bubble Growth • Tissue tension vs. Bubble pressure Pb = Pamb + S/r Pb – Bubble Pressure, Pamb – Ambient pressure S – Constant based on bubble skin, r – Bubble radius • Larger bubbles limit permitted tissue tension • Need to know bubble size

  13. VPM – Bubble Size • Ambient pressure • Dissolved gas tissue tension • Fast tissues – More dissolved gas- Larger bubbles • Speed of descent, tissue type, breathing mixture

  14. VPM – Bubble Size • Fast Descent – Lower tissue tension • Increased crushing pressure – smaller bubble • Bubble crushing & growth calculated for differing tissue tensions & bubble sizes • Higher ambient pressure – smaller bubbles – Less decompression? • Increased amount of dissolved gas increases decompression obligation • Volume of bubbles on ascent limited to theoretical critical value

  15. Bubble Models • Reduced Gradient Bubble Model (RGBM) • Oxygen window included in algorithm • Assumes a distribution of bubble nuclei • Calculations based on limiting total volume • Assumes bubble skins permeable at all pressures • Contrary to VPM • Considers bubbles in slow compartments • Eliminated over several days • Repetitive, Multi-Day diving • Conservative no-stop times – Doppler measurements

  16. Bubble Models Gradient Factors M-Value Line GF 20% Inert Gas Pressure in Tissue Compartment Ambient Pressure Line Gradient Factor Line GF 80% Ambient Pressure

  17. Bubble Models – Gradient Factors • M-Value Line • Theoretical limit for avoiding DCS • 100% permitted supersaturation • Tissue Tension = Ambient Pressure • 0% supersaturation • Limit level of supersaturation – Gradient Factor eg. Level of supersaturation of 80% - GF 80 • Different conservatism at different depths • GF Low (Eg. 20%) • GF High (Eg. 80%) • GF 20/80

  18. Bubble Models – Gradient Factors • GF Low • Generates Deep Stops • Reduce microbubbles • Fast Tissues offgassing • Slow Tissues ongassing • Longer deco stops at shallower depths • Gas supply • Gradient Factors • Which to use? • Personal Decision

  19. Summary • Nitrogen • Depth & Time • Safe Ascent • Various Theories • Varying Permeability Model (VPM) • Reduced Gradient Bubble Model (RGBM) • Gradient Factors

  20. Questions?

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