Decompression Theory

Gas Laws & Bubble Formation - Pressure's Pesky Puffs

• Boyle's Law: $P_1V_1 = P_2V_2$. Gas volume expands on ascent (↓pressure); risk of barotrauma (lungs, ears, sinuses).
• Dalton's Law: $P_{total} = \sum P_{partial}$. Governs partial pressures of inspired gases (O2, N2); relevant for gas toxicities.
• Henry's Law: Gas dissolved in liquid $\propto$ its partial pressure ($C = kP_{gas}$). N2 dissolves in tissues at depth.
• Bubble Formation:
  • Ascent (↓pressure) → N2 supersaturation → N2 gas bubbles form from dissolved state.
  • Bubbles in tissues/blood cause Decompression Sickness (DCS).
  • Critical for staged decompression.

⭐ Nitrogen narcosis ("Martini's Law"): Euphoric/impaired effects similar to alcohol, typically noticeable below 30m depth (approx. 1 effect unit per 10m beyond 20m).

DCS Pathophysiology & Types - The Bends Breakdown

• Rapid ↓ ambient pressure (ascent) → dissolved inert gases ($N_2$) exceed saturation → form bubbles in tissues/bloodstream.
• Bubble Consequences:
  • Mechanical: Tissue distortion; vascular obstruction (gas embolism).
  • Biochemical: Endothelial damage; inflammation, platelet, coagulation activation.

DCS Prevention & Risk Factors - Dive Smart, Surface Safe

Prevention Pathway:

• Key Risk Factors (↑ DCS):
  • Dive Profile: Fast ascent, missed stops, deep/long dives, repetitive dives.
  • Physiological: Dehydration, obesity, fatigue, PFO, poor fitness, older age.
  • Environmental: Cold water.
  • Post-dive: Flying too soon (violating no-fly times), strenuous exercise.

⭐ DAN Recommends: Wait 12 hrs after single no-stop dives; 18 hrs after multiple dives/days; 24 hrs after dives requiring decompression stops before flying to altitude.

DCS Management & Decompression Models - Taming Tiny Terrors

• DCS Management (First Aid & Definitive)

  • Immediate: 100% Oxygen, oral/IV fluids, supine position.
  • Definitive: Recompression therapy (Hyperbaric Oxygen Therapy - HBOT) is crucial.
    • US Navy Treatment Table 6 for Type II DCS.
  • Adjuncts: NSAIDs for pain.

• Decompression Models (Predicting N₂ Load)

  • Haldanian Model:
    • Assumes inert gas dissolves in various body tissues ("compartments") at different rates.
    • Each has a specific gas uptake/elimination half-time ($t_{1/2}$).
    • Defines M-values (Maximum allowable tissue tension).
  • Bühlmann Model (e.g., ZH-L16C):
    • Popular algorithm in dive computers (e.g., ZH-L16), refining Haldane's concepts.
    • ZH-L16 indicates 16 tissue compartments.
  • Bubble Models (e.g., VPM, RGBM):
    • Consider bubble mechanics (formation, growth, elimination) directly.
    • Aim to limit bubble size/quantity below critical thresholds.

⭐ Exam Favourite: The most critical immediate step in managing suspected Decompression Sickness (DCS) is the administration of 100% normobaric oxygen.

High‑Yield Points - ⚡ Biggest Takeaways

• Decompression Sickness (DCS): Results from nitrogen bubbles forming in tissues/blood due to rapid ascent.
• Henry's Law: States that ↑ambient pressure leads to ↑dissolved gas (primarily N₂) in tissues.
• DCS Types: Type I involves musculoskeletal pain ("the bends"), skin manifestations; Type II is severe, affecting neurological, cardiorespiratory, or vestibular systems.
• Prevention: Crucially involves slow ascent, adherence to decompression stops, and using dive tables/computers.
• Treatment: Immediate 100% Oxygen administration, followed by definitive hyperbaric recompression therapy.
• Arterial Gas Embolism (AGE): A critical emergency often due to pulmonary barotrauma during ascent; requires urgent recompression.