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Learning Outcomes?
- • Describe the physics of evolved gases in decompression, and the pathophysiology of DCI
- • Describe the clinical manifestations of DCI and their diagnostic features
- • State the pre-disposing and precipitating factors for DCI
- • Describe methods of preventing DCI from hypobaric exposures
- • Describe treatment of suspected DCI in-flight and post- flight
- • Locate the reference documents regarding hypobaric exposures in the ADF
- • Recognise, manage and recommend prevention strategies for decompression illness from hypobaric exposures
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Definitions Dysbarism?
- • any disease induced by change in pressure, includes both trapped AND evolved gases
- • decompression sickness, arterial gas embolus and barotrauma
- • Decompression Sickness (DCS)
- • evolved gas disease
- • Arterial Gas Embolus
- • pulmonary barotrauma with embolisation (usually cerebral)
- • Decompression Illness (DCI)
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Decompression Illness?
- • Decompression illness is a risk with any exposure to reduced ambient pressure and can occur in:
- • compressed gas divers
- • estimated to occur <0.05%
- • aviators
- • astronauts
- • pressurised chamber workers
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Incidence of Sub-atmospheric DCI?
- • Diving (sub-aquatic) vs. aviation (sub-atmospheric)
- • Large diving study (DAN) with incidence of ~0.05%
- • DCI in aerospace operations:
- • most cases arise from hypobaric chamber training
- • cases do happen operationally:
- • rapid decompressions (rare)
- • HAPO
- • pressurisation ground checks
- • specific risk factors in crew
- • space
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PATHOPHYSIOLOGY
Decompression Illness
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Inert Gas (N2) Uptake
- • Determined by:
- • Tissue perfusion
- • Tissue solubility
- • Blood solubility
- • Rate of diffusion
- • Temperature
- • Tissue pH & PCO2
- • At depth, inert gas diffuses into tissues down concentration gradient
- • On ascent, the elimination of inert gas mirrors the process of uptake
- Dalton’s Law
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The Physics of DCI - Henry’s Law?
- At a constant temperature, the amount of a given gas dissolved in a given type & volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid
- “Fizzyology” - “Coke Bottle Law”
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With a reduction in ambient pressure, a pressure differential exists between the partial pressures of N2 in the tissues &
- environment (until equalisation)?
- jQuery1101021855516431892696_1488026318906
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If a state of “supersaturation”?
develops, bubbles can form out of solution
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The Physics of Bubbles?
- • Formation of bubbles depends on:
- • Henry’s law
- • Haldane’s critical supersaturation theory
- • Pre-existing conditions:
- • Bubble micronuclei
- • Hydrophobic interfaces
- • Vascular turbulence
- • Tribonucleation/ cavitation
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Pathophysiology of DCI?
- Bubbles – evolved or embolic
- Primary effects of bubble formation:
- • fragment
- • compress
- • obstruct
- Secondary effects of bubble formation?
- • Stimulation of inflammatory processes
- • Complement cascades
- • Kinin
- • Coagulation (and platelets)
- • Damage endothelial cells
- • Immune system eg. leucocytes
- • Often delayed
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Venous Gas Emboli?
- • Arise de novo
- • low pressure
- • flow in venous plexi may favour formation
- • promoted by presence of micronuclei
- • Effects:
- • haematological consequences
- • venous infarction
- • embolic via right heart to the lungs
- • Increase in MPAP
- Incidence of VGE
- • Poor correlation to clinical DCI
- • Measured by precordial Doppler US
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Arterial bubbles?
- • Unlikely to form de novo
- • High pressure
- • Rarely supersaturate
- • Arise via:
- • VGE (paradoxical gas embolism)
- • R to L shunts (eg. PFO)
- • Passage through bubble filter
- • Pulmonary barotrauma
- • Effects:
- • Damage endothelium
- • Strip surfactant
- • Increased permeability
- • Haemoconcentration
- • Activate inflammatory cascades
- Role of PFO
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• Incidence - ~30% of general population?
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Theoretical risks:?
- •R to L shunting
- •Increased R atrial pressure (eg. lung bubble load)
- •CAGE
- •Anti-G straining
- •PPB
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Manifestations of DCI?
- • Musculoskeletal
- • Cutaneous
- • Gastrointestinal
- • Cardiopulmonary
- • Neurological
- • Peripheral nerves
- • CNS
- • Cerebrum
- • Cerebellum
- • Spinal cord
- • Audio-vestibular
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DCI in Aerospace Operations?
- • First symptom is almost always joint awareness or discomfort
- • May be masked by operational distractions
- • Staying at altitude leads to evolution/ more severe symptoms
- • Many resolve on recompression to sea level
- • differs from sub-aquatic DCI
- • “Landing is treatment”
- • ALL require presentation to Health Services
- • Joint pain - 60 %
- • usually knee, shoulders, elbows
- • Neurological - 34%
- • motor, sensory, visual, cerebellar
- • Both - 23%
- • Respiratory - 8%
- • Collapse is rare in aviation
- • 1 in 140 cases, 136,000 exposures
- • 40-54% have symptoms persisting on return to sea level
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Musculoskeletal DCI?
- • “The Bends”
- • Peri-articular joint pain is the most common symptom
- • minor niggle to severe pain
- • Onset usually early, but can be delayed (24-48 hrs)
- • Better with pressure, worse with use
- • Migratory polyarthralgia/myalgia
- • Examination is usually unremarkable
- • Exact cause of bubble-induced pain is uncertain:
- • ?peri-osteum, medullary cavity, ligaments or tendons
- • ?due to mass effects &/or inflammatory mediators
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Cutaneous DCI?
- • “The Creeps”
- • Subcutaneous bubble formation
- • Initiates inflammatory reaction, generally causing symptoms such as pruritis or hyperaesthesia
- • May also have erythematous macular rash/marbling
- • Severe form is “cutis marmorata”
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Respiratory DCI?
- • “The Chokes”
- • Lungs act as “bubble filters”, but can be overwhelmed
- • This can lead to arterial bubbles
- • Increase in pulmonary venous pressure
- • Can lead to shunting across PFO
- • Pain, chest tightness, cough
- • Rare - ?other causes with similar presentation
- • Beware associated pulmonary barotrauma
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Neurological DCI?
- • Neurological DCI includes:
- • Brain
- • Cerebrum
- • Cerebellum - “The Staggers”
- • Spinal cord
- • Peripheral nerves
- • Inner ear
- • Embolic vs autocthonus
- • de novo bubble formation in the brain is unlikely due to the large perfusion such that supersaturation is unlikely to develop
- • most cases of CNS DCI are consistent with a vascular event (CAGE)
- • bubbles can cause secondary CNS effects due to BBB permeability changes and initiation of inflammatory reactions
- • Symptoms are varied in both presentation and severity
- • eg. mild short-lived paraesthesia to permanent paralysis
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Spinal cord DCI?
- • Multiple possible aetiologies:
- • vascular
- • arterial emboli
- • venous infarction
- • compression - tissue bubbles
- • Poor prognosis generally
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Other Symptoms of DCI?
- • Constitutional DCI
- • Non-specific symptoms such as malaise, lethargy, headache, myalgias
- • likely secondary to inflammatory activation
- • Lymphatic DCI
- • Lymphoedema, unusual swelling
- • Inner ear DCI
- • Need to distinguish from inner ear barotrauma
- • Unique inert gas kinetics of labyrinth – very rare in aviation
- • Other effects:
- • Haemoconcentration, pro-coagulant
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Cerebral Arterial Gas Embolism (CAGE)?
- • Rare in aviators, incidence unknown
- • Typically due to breath holding with diving
- • increased risk with pulmonary disease that can predispose to gas trapping or shearing forces
- • can occur at trans-pulmonary pressures of >60-80mmHg (eg. ascent to surface from ~1m depth!)
- • Usual presentation is sudden collapse immediately or very soon after decompression
- • Gas embolism can also affect other vascular beds
- • eg. coronary, splenic, hepatic, renal
- • Don’t forget PBT (risk of pneumothorax)
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RISK FACTORS Decompression Illness Predisposing Factors?
- • Most important risk factors are:
- • “Dose”
- • altitude/pressure
- • duration of exposure
- • Rate of decompression
- • ?pre-oxygenation
- • Risk factors for DCI classified as:
- • Individual
- • Sortie
- • Post-sortie period
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Predisposing Factors DCI?
- • Individual risk factors:
- • Anatomy, eg. PFO, other shunts
- • Physiology:
- • Likely - age and fitness
- • Unproven - BMI (body fat %), gender), diet (fat), hydration
- • Individual susceptibility - increased risk if previous DCI
- • Sortie risk factors:
- • Recent exposure to hyperbaric environment
- • eg. SCUBA, aircraft pressurisation testing, HEED training
- • Sortie profile – repetitive, high altitude
- • Exercise
- • Cold
- • Post-sortie risk factors:
- • Level of exercise post-sortie
- • Rewarming
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Flying after diving ?
- • DI(AF) OPS 6-13 (DI(G) OPS 22-1)
- • No decompression diving
- • CA<8,000ft – TMUFF 12hrs for aircrew; 9 hours others
- • CA>8,000ft – TMUFF 48hrs for all
- • Decompression diving
- • CA < 8,000ft – TMUFF 24 hrs
- • CA > 8,000ft – TMUFF 48hrs
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• Flying after HEEDs/HABD ?
• 9hrs for Navy, ?for Army
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• Flying after chamber runs ?
- • Traditional
- • CADO (nil restriction)
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Individual susceptibility?
• Reports of unprovoked DCS at:
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Decompression Illness Prevention?
- • Operational sortie (“dose”) limits:
- • Altitude limits
- • Minimise time of exposure
- • Denitrogenation (pre-oxygenation)
- • Keep warm
- • Pressure suits
- • Mission planning
- • DI(G) OPS 22-2
- • Only fly when healthy
- • Do not fly after compressed air diving
- • Restrictions depend on gas mix and dive profile
- • DI(G) OPS 22-1
- • Pre-oxygenation regulations
- • DI(AF) OPS 3-5
- • AVMED SI(OPS) 1-1
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Pre-oxygenation?
- • 100% oxygen (good seal required)
- • Longer = better (diminishing returns)
- • Enhanced by exercise
- • 30 mins eliminates ~30%
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Decompression Illness Treatment Aircrew Immediate Actions?
- • Descend ASAP
- • Use 100% oxygen
- • Keep warm
- • Minimise activity
- • Declare emergency
- • Seek AVMO review
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Decompression Illness Treatment Medical Management?
- • Patient positioning
- • 100% Oxygen
- • IV Fluids
- • Pharmacological adjuncts
- • Recompression
- • Transport
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DCI & Collapse?
- • Manage as with all unconscious patients
- • DRABCDE:
- • AIRWAY (& c-spine)
- • BREATHING (100%O2)
- • CIRCULATION
- • DISABILITY
- • ENVIRONMENT
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Medical Report on an Aircraft Incident or Accident?
- • Notifications:
- • AVMED on call SAVMO (24/7) - 24/7 - MUST be notified ASAP for hypobaric DCI in aircrew
- • SUMU (HMAS PENGUIN 24/7) - should be contacted as soon as practicable to seek advice and to notify if an ADF member needs recompression
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DCI History & Examination?
- • Signs and symptoms
- • Medical history
- • symptoms
- • evolution
- • Response to landing/Rx
- • Risk factors
- • Sortie history
- • Sortie profile (inert gas load)
- • breathing gas
- • ?pre-oxygenation
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DCI History & Examination>?
- • Systems review
- • Other injuries
- • Neurological examination
- • including MMSE
- • if stable
- • thorough
- • including mini mental state
- • sharpened Rhomberg’s
- • Do not stand up if CAGE
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DCI Investigations?
- • DCI is a clinical diagnosis
- • Ix indicated to exclude other:
- • Bloods
- • FBC (increased Hb & Hct)
- • Biochem
- • BSL
- • CPK (elevated in CAGE)
- • Radiology
- • CXR
- • CT
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DCI Positioning?
- • Supine
- • Do not sit/stand up to examine or transfer if risk of CAGE
- • Head down not recommended
- • May contribute to cerebral oedema
- • Airway challenges
- • Increases venous return – more bubbles centrally
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DCI Oxygen Therapy?
- • 100% oxygen is mandatory
- • Don’t forget risk of oxygen toxicity
- • Must be a good seal
- • Aviators mask ideal, often impractical
- • Enhances denitrogenation
- • HD130 - For “pain only” DCI which resolves
- • 2 hrs normobaric 100% oxygen and observe
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DCI Pharmacological Adjuncts?
- • Analgesics
- • Paracetamol +/- opioids PRN
- • Avoid aspirin
- • NSAIDs ?
- • Lignocaine infusion
- • promising initial studies of benefit in cerebral protection in neurological DCI/CAGE
- • more recent evidence ?
- • monitoring requirements are challenging
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DCI Recompression?
- • HD130 - symptoms that clear on descent with normal neurological exam
- • 100% oxygen for 2hrs
- • aggressive hydration
- • observe 24hrs and re-evaluate
- • recompression may not be required
- • Always discuss with AVMED +/- SUMU
- • unfit to fly 72 hours
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DCI Recompression?
- • Symptoms that persist at ground level or recur recompression
- • 100% oxygen
- • IV fluids
- • hyperbaric therapy
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In all cases the nearest hyperbaric facility must be consulted?
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Recompression Aims:?
- • Reduce the size of bubbles (pressure effect)
- • Denitrogenate bubbles (oxygen effect)
- • increase diffusion gradient out of bubbles
- • Relieve ischaemia and hypoxia
- • help restore normal tissue function in affected areas
- • Some anti-inflammatory effects
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Quiz question 12
- Patent foramen ovale is thought to increase risk of clinical DCI in aviation activities which:
- a. Decrease right atrial pressure
- b. Increase left atrial pressure
- c. Increase pulmonary artery pressures
- d. Increase right atrial pressure
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Quiz question 13
- An example of a “fast tissue” in which nitrogen uptake and washout are rapid is:
- a. Bone
- b. Muscle
- c. Fat
- d. Tendon
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