Decompression Sickness

How it happens

Air is about 79 percent nitrogen. Under pressure, more nitrogen dissolves into your blood and tissues, much as more gas dissolves into a soda bottle when it is sealed under pressure. The deeper and longer you dive, the more nitrogen your body absorbs. This is Henry's law: the amount of gas that dissolves in a liquid is proportional to the pressure of that gas.

As you ascend and the pressure drops, the nitrogen comes back out and is carried to the lungs and breathed away. If you come up slowly and within limits, this happens quietly. If the pressure drops too fast, or you have absorbed too much nitrogen, it can form bubbles in tissues and blood vessels, like the fizz when you open a soda. Those bubbles cause decompression sickness.

Symptoms

Symptoms usually begin within a few hours of surfacing, and most appear within 24 hours. They range from mild to life-threatening. Doctors often group them in two broad types.

• Milder forms (often called Type I): joint or limb pain, often in the shoulders or elbows; unusual fatigue; itching; a blotchy skin rash.
• Serious forms (often called Type II): numbness, tingling or weakness; dizziness or loss of balance; difficulty breathing or chest pain; confusion, headache or vision changes; problems with bladder or bowel control; collapse.

Any of these after a dive should be treated as possible decompression sickness until a doctor says otherwise. Divers often deny symptoms or blame them on a hard day. Don't.

First aid

1. Lay the diver flat and keep them still and comfortable.
2. Give the highest concentration of oxygen available, ideally 100 percent through a demand valve or non-rebreather mask, from someone trained to do it.
3. Call emergency services and a dive emergency hotline such as DAN, which can advise on the nearest recompression chamber.
4. Give water to drink if the diver is fully conscious and able to swallow.
5. Note the dive profile, times and symptoms for the medical team.

Don't take the diver back into the water to "recompress" them. In-water recompression without specialist equipment and training can make things much worse.

Treatment

The standard treatment is recompression in a hyperbaric chamber. Raising the pressure shrinks the bubbles, and breathing high-concentration oxygen helps the body clear the nitrogen. Treatment works best when it starts early, though it can still help when delayed.

Risk factors

Exceeding your no-decompression limits is the biggest risk, but people can get decompression sickness on dives within limits. Factors that are thought to raise the risk include fast ascents, missed safety stops, several dives a day over many days, deep dives, hard exercise during or after a dive, cold, dehydration, and flying after diving too soon. A heart condition called patent foramen ovale (a small opening between the heart's chambers) is also linked to some forms.

Prevention

• Plan every dive with a dive computer or tables and stay well inside the limits.
• Ascend slowly, no faster than your training and computer allow, and always make a safety stop.
• Make the deepest dive of the day first.
• Stay warm and hydrated, and avoid hard exercise right after diving.
• Wait before flying or driving to altitude.
• Consider enriched air nitrox with air limits on your computer for an extra margin.

Frequently asked questions