Dive medicine

Gas laws

• Boyle’s Law
  • P1V1 = P2V2
  • Pressure and volume of an ideal gas are inversely related
  • At 10m below surface, lung volume halves until take a breath in
• Dalton’s Law
  • Total pressure exerted by a mixture of gases is the sum of the partial pressures of each gas
  • Partial pressure of nitrogen = 0.79 x ambient pressure, so rises with depth
  • If ambient pressure rises, the partial pressure of each gas must rise
• Henry’s Law
  • Quantity of a gas in solution in a liquid is proportional to the partial pressure of the gas
  • Explains uptake of inert gas (nitrogen) into tissues at depth
• 1 atm = 760mmHg = 760 torr = 14.7 psi = 1.013 bar = 101.325 kPa
• 1 atm = 10m = 33ft
  • Slightly higher for freshwater i.e.34ft to reach 1 atm

Barotrauma of descent

• Types
  • Barotitis (ear squeeze)
  • External ear squeeze
  • Sinus barotrauma
  • Inner ear barotrauma
  • Face, tooth and dry-suit squeeze
• Pathophysiology
  • Volume of gas in all air-containing spaces reduces
  • Tympanic membrane is bent inwards, causing fullness/pain in ear
  • If unsuccessful at equalising, prolonged pain, tympanic membrane injury and barotitis (ear squeeze) will occur
• Barotitis (ear squeeze)
  • Pain or fullness without otoscopic changes through to haemorrhage within TM or haemorrhage into middle ear with haemotympanum
  • Ultimately TM may rupture resulting in relief of pain but influx of water into middle ear leading to calorically-induced vertigo and potential panic, drowning or other injury
  • Treatment
    • Analgesics and decongestants
    • Antibiotics if TM ruptured, especially if diving in contaminated water
    • Refrain from diving until perforation healed
    • Referral to ENT if larger perforation or when healing does not occur
    • If no perforation, refrain from diving until able to equalise normally

• External ear squeeze
  • If external canal occluded by wax or plug, TM bends outwards as cannot equalise between external canal and TM
• Sinus barotrauma
  • If ostia occluded, air cannot enter causing pain, mucosal oedema and submucosal oedema and stripping of sinus mucosa from bone, haemorrhage and rarely paraesthesia of the infraorbital nerve distribution
  • Similar traumatic neuropathy can occur to the facial nerve with middle ear barotrauma
  • Treatment – Analgesics, decongestants and possibly antibiotics

• Inner ear barotrauma
  • Can cause significant long-term damage
  • If diver attempts forceful Valsalva, pressure differential between CSF, transmitted through vestibular and cochlear structures and the middle ear air space can cause rupture of the oval or round windows, tearing of the vestibular membrane or combination
  • If diver is then able to equalise, a rapid increase in middle ear pressure occurs, which can cause similar injury in reverse
  • Present with unilateral roaring tinnitus, sensorineural hearing loss and profound vertigo
  • Can differentiate from inner ear decompression illness, CAGE or alternobaric vertigo by history of difficulty equalising and occurring on descent
  • Immediate complications are potential panic, leading to drowning or rapid ascent
  • Treatment
    • Urgent ENT
    • Immediate exploration vs. bed rest sitting upright, medications for vertigo and avoidance of raising ICP (e.g. avoiding nose blowing or straining)
    • If undergo hyperbaric treatment for decompression illness, need urgent tympanostomy to prevent same pressure differentials that led to injury in the first place

• Face squeeze
  • Air not added to mask on descent, resulting in facial bruising, conjunctival injection or haemorrhage, changes in vision and retrobulbar haemorrhage 
• Tooth squeeze
  • Air spaces inside tooth collapse
• Dry-suit squeeze
  • Folds in suit cause painful red streaks on skin

Barotrauma of ascent

• Alternobaric vertigo
  • Air flows through ostia of sinuses with expansion and Eustachian tube
  • If air is trapped in one middle ear cavity but not the other, can get unequal vestibular impulses to brain leading to vertigo
  • Usually transient

• Pulmonary barotrauma
  • If closed glottis on ascent, can get parenchymal lung injury
  • Can occur even in a swimming pool
  • Pneumomediastinum – Symptomatic treatment and may be subtle on CXR
  • Can get subcutaneous emphysema
  • Pneumothorax
  • Can occur in those with congenital cysts, obstructive pulmonary disease or other causes of air trapping without a closed glottis
  • Air embolism into pulmonary venous circulation can lead to arterial embolism, the most dangerous of which is cerebral arterial gas embolism (CAGE)
    • Any neurological symptom/sign in the setting of barotrauma of ascent is CAGE until proven otherwise

Decompression sickness

• Due to obstructive and inflammatory effects of inert gas bubbles in tissues and vascular system
• Bubbles are necessary but not sufficient to cause decompression illness
• Bubbles can form in tissues or in the circulation (usually low-pressure venous system)
• Air-blood and air-endothelial interfaces initiate inflammatory and thrombotic processes together with production of microparticles of lipid bilayer membraneous vesicles extruded from endothelial cells that together cause impaired flow
• Related to depth and duration of dive and rare if dive <10m (unlike CAGE)

• Clinical features
  • Type I (pain only)
    • Joints, extremities and skin (cutis marmorata)
    • Lymphatic obstruction can cause lymphoedema
  • Type II (serious)
    • Involves CNS (mainly spinal cord in compressed air divers and brain in high altitude decompressions)
    • Vestibular symptoms (staggers) and cardiopulmonary symptoms (chokes)
  • Type III (Above + CAGE)
• Symptoms usually occur within minutes to hours of resurfacing
  • But can take days rarely
  • May improve with recurrent dives or worsen precipitously with flight

• Pain
  • Deep pain, unrelieved but not worsened by movement
  • Thought to be due to distension by gas bubbles in ligaments or fascia, intramedullary bone bubbles or activation of stretch receptors in tendons
  • Knees and shoulders mostly
  • Usually one joint only
  • Sport divers are more prone to spinal cord effects vs. seasoned commercial divers who tend to get joint involvement
  • Poorly localised and difficult to describe back and abdominal pain may herald more serious spinal cord involvement

• Pulmonary
  • Cough, haemoptysis, dyspnoea and substernal chest pain
  • Can cause cardiovascular collapse
• Neurological
  • Truncal constriction or girdle-like pain
  • Wooly feeling in feet, developing into ascending paralysis (transverse myelitis)
  • Rapid in onset affecting cervical and thoracic spine typically
  • Does not necessarily cause a clear spinal level or syndrome e.g. anterior/posterior
  • Autonomic involvement is uncommon
  • Seems to occur due to initial bubbling in venous plexus that first impedes and then obstructs venous outflow from the cord, preventing further bubbles from egressing and subsequent in situ bubble formation within the cord (autochthonous bubbles)

• Vestibular
  • Usually after deep, long dives
  • Vertigo, hearing loss, tinnitus and disequilibrium
  • Differentiated from inner ear barotrauma as occurs after out of water and generally immediately after a forced Valsalva
  • Headache, nausea, unusual fatigue can also occur
• Patent foramen ovale
  • Seems to have an increased prevalence in patients with inner ear and cutaneous decompression sickness
  • Reasonable to screen divers with recurrent unexplained decompression sickness for a PFO
  • Unclear if closure reduces incidence of subsequent decompression sickness

• Arterial gas embolism
  • Most often due to pulmonary trauma with air entering pulmonary veins and left side circulation
  • Can also occur during CVL placement if right-to-left pulmonary or cardiac shunts exist
  • Distribution depends on blood flow rather than gravity
  • Can occur from any depth including <10m
  • Clinical features
    • CNS – Stroke syndromes
    • Immediate apnoea and cardiac arrest due to air in entirety of large arteries and veins of the central vascular bed
    • Effects from pulmonary trauma-derived arterial gas embolism tend to occur on ascent or immediately upon resurfacing
    • If doesn’t die immediately, often get LOC, seizure, blindness, disorientation and hemiplegia
    • Symptoms may improve spontaneously if bubbles move into venous circulation after a spike in blood pressure
    • Parietal signs and symptoms are easily overlooked
    • Get cascade of inflammatory changes
  • Lab tests
    • Creatine phosphokinase levels correlate with embolism severity
    • May get raised troponin but most likely does not represent occlusive coronary artery disease
• Treatment of decompression sickness and arterial gas embolism
  • 100% O2, IV fluids and rapid recompression
  • Supine positioning recommended
    • Used to recommend Trendelenberg or left lateral decubitus to trap air in left ventricle, however, air usually distributed already and positioning leads to raised ICP, reduced cerebral perfusion and interferes with first aid
  • Hyperbaric chamber
    • Reduces size of bubbles and high partial pressure of oxygen in solution increases inert nitrogen washout
    • Mass action dictates a gas will move down pressure gradients so nitrogen will move out of bubbles and oxygen will move in and then be absorbed and utilised by cells
    • Decreases tissue oedema, increases oxygen delivery to ischaemic tissues and reduces neutrophil adhesion to endothelium and neutrophil activation
  • IV lignocaine for CAGE 1mg/kg
    • Appears to decrease neuropsychiatric symptoms when given during cardiopulmonary bypass procedures

Special scenarios

• Asthma
  • Slightly higher relative risk of barotrauma (2x general population) but absolute risk is still low
  • Should be assessed by dive medicine specialist, exercise pulmonary function testing performed and if have normal testing on treatment and understand risks can be cleared for diving
• Immersion pulmonary oedema
  • Unknown cause and is not treatable by recompression
  • Some divers will experience recurrent episodes but others never again
• Nitrogen narcosis
  • When air breathed at depth of 30m or greater
  • Loss of fine motor skills and higher order brain function
  • For each 10m beyond 30m = one martini (martini rule)
  • At depths greater than 100m, unconsciousness may occur due to anaesthetic effect of nitrogen
  • Helium often used instead of nitrogen to prevent this at dives >70m
• Oxygen toxicity
  • Cerebral oxygen toxicity can occur in divers using oxygen-enriched air
  • Twitching, nausea, paraesthesia, dizziness and seizuresn
  • Seen in <1/1000 of hyperbaric chamber patients

Last Updated on November 23, 2021 by Andrew Crofton