Many divers are introduced to this complication of diving during their basic open water course. However, many still are unsure of what decompression illness is all about, even those at much higher levels in their diving career. To make things worse, many medical personnel are also unprepared to handle such cases due to its rarity or assumed rarity of presentation. This assumption is however incorrect as more and more indulge into the sport of scuba diving.
Decompression illness is a disorder caused by bubbles forming in body tissues or in the blood where they should never be.
The first source is bubble formation from nitrogen that has been dissolved in tissues during the dive. This is otherwise known as DCS (Decompression Sickness)
- During descent and diving, air is breathed at increased pressure, and therefore more of the nitrogen molecules from the air can dissolve into the blood. The nitrogen enters the blood in the lung capillary bed, and is distributed to the tissues via the arteries where, nitrogen leaves the blood and diffuses into tissues.
- The deeper the dive, the faster the nitrogen is taken up from the air we breathe, and the longer the dive, the more time it has to accumulate in the tissues.
- During the ascent, pressure falls and less nitrogen can remain dissolved in the tissues and therefore, the nitrogen diffuses out of tissues and into the venous blood of the organs, back to the lungs for elimination. Ideally, this elimination process occurs fast enough to dissipate the nitrogen molecules without bubble formation.
- However, different tissues in the body have differing rates of elimination depending on their blood supply. Those with very good blood supply tend to accumulate and dissipate nitrogen fast and are called “fast” tissues. “Slower” tissues such as tendons are often not a problem after a short dive, even if it is deep, because they don’t have enough time to accumulate significant quantities of nitrogen but they become more important during long dives, or repetitive dives, when nitrogen can build up over a long time. Then there are the intermediates or “medium” tissues such as the nerves and spinal cord, where the accumulation is fast but the dissipation is slow especially if the bottom time is long or ascent is slightly faster.
- When nitrogen is not dissipated fast enough, the pressure of dissolved nitrogen will exceed the ambient (surrounding) pressure at some point during the ascent and the molecules of dissolved nitrogen will form bubbles.
- Bubbles formed in the blood (usually the venous system) are mainly filtered as it passes the lungs and rarely enters the arterial circulation except in foetus and some with a vascular heart anomaly (patent foramen ovale).
The second potential source of bubbles in DCI is the introduction of air bubbles to the arterial circulation because of lung overexpansion. This is better known as Arterial Gas Embolism(AGE)
- This has nothing to do with dissolved nitrogen, or time and depth for that matter. Indeed, this problem can arise during ascent from depths as shallow as 1 – 2 metres. The most important rule in scuba diving is “to breathe normally at all times; never hold your breath”. This is because any air trapped in the lungs during an ascent (by holding the breath for example), will expand as pressure decreases. If there is sufficient over-expansion of the lung it may rupture some of the small airways and the associated blood vessels. Such damage is referred to as pulmonary barotrauma.
- This barotrauma causes introduction of air bubbles to the lung capillary circulation which are then circulated throughout the body as air embolus. Bubbles arriving in the circulation of the brain may cause stroke-like symptoms. These arterial bubbles are therefore considered very dangerous.
Embolus = foreign particles that float and travel freely in the blood e.g air, clots Arteries = blood vessels that carry blood from the heart to the rest of the body Veins = blood vessels that carry blood from the body back to the heart