Aetiology

Decompression Illness:

• caused by intravascular or extravascular bubbles that are formed as a result of reduction in environmental pressure (decompression)

Multiple factors are involved in the pathogenesis of the Decompression Illness

• divers with bronchial asthma, atrial septal defect, patent foramen ovale, or obesity are more prone to develop
• depth of the dive below the sea surface, the temperature of the water, and the speed of ascent are considered as the main contributory factors for development of Decompression Illness. When divers ascend at a speed of 9-10 meters/min, they have minimal risk of developing Decompression Illness. If the ascent is faster (>19 meters/min), the risk of Decompression Illness is significantly higher (2)

Decompression Illness starts with the formation and increase in size of extravascular and intravascular bubbles when the sum of the dissolved gas tensions (oxygen, carbon dioxide, nitrogen, helium) and water vapour exceeds the local absolute pressure

• in diving and during compressed-air tunnel and caisson work, this state of supersaturation is made possible by the increase in tissue inert gas partial pressure that occurs when the gas (usually nitrogen, but occasionally helium) is respired at high pressure.
• supersaturation arises during decompression if the rate of ambient pressure reduction exceeds the rate of inert gas washout from tissue.

Most divers breathe compressed air, which is roughly 78% nitrogen

• dives deeper than that are typically made using a mixture of helium and oxygen since helium has almost no narcotic properties
  • many technical divers use a combination of helium, nitrogen, and oxygen ("trimix") at shallower depths to help offset the effects of nitrogen narcosis (and the considerable cost of using only helium as a diluent)
  • note that nitrogen is not chemically inert, it is often referred to by divers as an "inert" gas

Changes in dissolved inert gas in the body

• at atmospheric pressure
  • the dissolved inert gas in the body is in equilibrium with that of the atmosphere
    
    
• as the diver descends
  • as the pressure of the diver's breathing gas increases with increasing depth
    • partial pressure of inert gas in the breathing mix rises
    • creates a positive pressure gradient between the inert gas in the lungs and the gas dissolved in the blood and body tissues
    • inert gas molecules in the lungs then pass through the alveolar-capillary interface and become dissolved in the body as a function of partial pressure and time
      • therefore the farther a diver descends and the longer he or she stays at depth, the more inert gas becomes dissolved in the blood and body tissues
        
        
• as a diver ascends toward the surface
  • the inert gas pressure in the lungs decreases and the pressure gradient between the lungs and the body equilibrates and then reverses
  • when the partial pressure of dissolved inert gas in the body is higher than the partial pressure of inert gas in the lungs, the tissues become supersaturated
    • if the tissues are supersaturated then
      • gas molecules in the body then pass through the alveolar/capillary membrane into the lungs and are exhaled
      • this is a simplistic description of the process known as decompression
      • body tissues will tolerate some supersaturation; however, "silent," or asymptomatic; bubbles may form in the venous blood even after normal, uneventful decompression
      • nitrogen bubbles that form in the circulation during the phase of de-decompression are normally filtered out by the pulmonary capillaries
        • in the presence of anatomic defects such as an atrial septal defect or a patent foramen ovale, the risk of decompression illness increases significantly
        • the bubbles pass through the right heart and become lodged in the arterial side of the pulmonary capillaries, where they are gradually reduced in size and eliminated by the process described above
        • if the pressure gradient becomes too great then these venous bubbles may become large and/or numerous enough to obstruct the flow of blood through the pulmonary vasculature which can result in rapid onset hypoxemia, hypercarbia, and death

Reference:

1. Phatak UA et al. Decompression syndrome (Caisson disease) in an Indian diver.Ann Indian Acad Neurol. 2010 Jul-Sep; 13(3): 202-203.
2. Vann RD et al. Decompression illness.Lancet 2010; 377: 153-64
3. Hexdall EJ, Cooper JS. Chokes.StatPearls [Internet]