In the presence of a right-to-left shunt of the heart, such as a patent foramen ovale, venous bubbles may enter the arterial system, resulting in an arterial gas embolism. A similar effect, known as ebullism, may occur during explosive decompression, when water vapour forms bubbles in body fluids due to a dramatic reduction in environmental pressure. Inert gases
The main inert gas in air is nitrogen, but nitrogen is not the only gas that can cause DCS. Breathing gas mixtures such as trimix and heliox include helium, which can also cause decompression sickness. Helium both enters and leaves the body faster than nitrogen, so different decompression schedules are required, but, since helium does not cause narcosis, it is preferred over nitrogen in gas mixtures for deep diving. There is some debate as to the decompression requirements for helium during short-duration dives. Most divers do longer decompressions; however, some groups like the WKPP have been experimenting with the use of shorter decompression times by including deep stops. The balance of evidence as of 2020 does not indicate that deep stops increase decompression efficiency. Any inert gas that is breathed under pressure can form bubbles when the ambient pressure decreases. Very deep dives have been made using hydrogen-oxygen mixtures (hydrox), but controlled decompression is still required to avoid DCS. Isobaric counterdiffusion
DCS can also be caused at a constant ambient pressure when switching between gas mixtures containing different proportions of inert gas. This is known as isobaric counterdiffusion, and presents a problem for very deep dives.
These cases should be assessed by a specialist in diving medicine, which can generally be done remotely by telephone or internet.For joint pain, the likely tissues affected depend on the symptoms, and the urgency of hyperbaric treatment will depend largely on the tissues involved. Sharp, localised pain that is affected by movement suggests tendon or muscle injury, both of which will usually fully resolve with oxygen and anti-inflammatory medication. Sharp, localised pain that is not affected by movement suggests local inflammation, which will also usually fully resolve with oxygen and anti-inflammatory medication. Deep, non-localised pain affected by movement suggests joint capsule tension, which is likely to fully resolve with oxygen and anti-inflammatory medication, though recompression will help it to resolve faster. Deep, non-localised pain not affected by movement suggests bone medulla involvement, with ischaemia due to blood vessel blockage and swelling inside the bone, which is mechanistically associated with osteonecrosis, and therefore it has been strongly recommended that these symptoms are treated with hyperbaric oxygen. Epidemiology
The incidence of decompression sickness is rare, estimated at 2.8 to 4 cases per 10,000 dives, with the risk 2.6 times greater for males than females. DCS affects approximately 1,000 U.S. scuba divers per year. In 1999, the Divers Alert Network (DAN) created "Project Dive Exploration" to collect data on dive profiles and incidents.