Decompression sickness is caused by the development of nitrogen bubbles in the blood and tissues as a result of a reduction of atmospheric pressure which happens too quickly for the body to dispose of the excessive nitrogen. The most common symptom of decompression sickness is ‘the Bends’, manifested by pain in and around the large joints of the body; other common symptoms include chest pains, difficulty breathing, skin irritation, and cramps. Severe cases can result in paralysis or death. Excessive rates of atmospheric pressure reduction typically can occur due to:
- Rapid or explosive decompression of an aircraft.
- Rapid ascent while SCUBA diving from depth to the surface.
- Exposure to typical aircraft cabin altitudes (5,000 to 8,000ft) too soon after SCUBA diving.
Decompression sickness normally only occurs following long exposures (more than half an hour) to altitudes above 25,000 ft. As the response to cases of aircraft decompression is immediate descent, it is rare for decompression sickness to occur in aviation
Physiology of Decompression Sickness
Under normal, stable conditions, all gases within the body cavities, tissues or fluids, are in equilibrium with the external environment. A fall in external pressure (rise in altitude) will give rise to pressure gradients between these gases and the external environment, and this gradient will slowly decrease with time until a new balance is reached. If the depressurisation is large and rapid the expanding body gases may cause physical damage to organs and tissues, or give rise to severe pain, such as in the ear.
Under normal and stable conditions the partial pressures of the most common gasses (Oxygen, CO2 and Nitrogen) remain in equilibrium. When the external pressure drops, both Oxygen and CO2 are absorbed, diffused or utilised by the body. Whereas Nitrogen ‘emerges’ from solution (tissues and fluids, including blood) and forms bubbles of gas, which take a long time to disperse from the body. It is these bubbles of gas (similar to those in a fizzy drink) which migrate to the joints (in the case of the Bends) and other areas of the body and cause pain.
Prevention and Cure
To prevent decompression sickness it is required that crew members (recommended for passengers) cease SCUBA diving at a definite time period before a planned flight. Times vary depending on the depth of dive, time of dive and number of dives; crew members should consult their own company rules and national regulations. Typically a single dive not below 10m will require a 24 hour break for crew members, and multiple dives, or single dives below 10m may require a break of 48 hours.
Rapid descent, following an aircraft decompression, to an altitude below 18,000ft, should prevent decompression sickness. In effect this is similar to placing a diver into a barometric chamber and increasing the external pressure; it reduces the internal-to-external pressure gradient and returns gaseous Nitrogen back into solution.
- ^ Aircraft Depressurisation: Cabin Crew Information Bulletin – Australian Transport Safety Bureau.
- ^ ICAO Doc 8984 Manual of Civil Aviation Medicine.
- ^ [Professional Association of Dive Instructors (PADI) Guidance on Flying after Recreational (no decompression) Dives.]