Block 6 · DCS: The reason we need decompression theory

Introduction

All decompression theory revolves around one big question: how can we control the risk of getting DCS after a dive as well as possible?
Whenever we talk about decompression models, gradient factors, or ascent profiles, we’re essentially trying to minimise that risk as far as we can. Unfortunately, even with careful planning and conservative choices, there will still be dives that end in DCS.
In the block on Human Factors and the stigma surrounding DCS, we take a closer look at why it matters to treat DCS simply as what it is: a sports injury. This block is mainly about recognising symptoms — and acting correctly.
Before we start, test your knowledge first: can you tell the difference between facts and divers’ folklore?

What is DCS?

Decompression sickness (DCS) describes a range of symptoms that can occur when dissolved inert gas — usually nitrogen — cannot be eliminated from the body quickly enough during ascent at the end of a dive. As we descend, nitrogen dissolves into tissues in proportion to ambient pressure. During ascent, that gas has to leave the body again. If this happens too fast, or not efficiently enough, bubbles can form. These bubbles interfere with normal function, irritate tissues and blood vessels, and — depending on where they are and how large they get — can cause anything from mild complaints to life-threatening conditions.

What matters is this: DCS is rare, but not “impossible”. It can happen even after dives that stay completely within the limits of tables or computer guidance. So the point of this block is not to create fear, but to give a clear, sober picture: DCS is an uncommon but serious sports injury — and you should be able to recognise it and respond appropriately.

How does DCS happen?

To understand DCS, it helps to look at how gases move through the body. During descent, ambient pressure increases and more nitrogen dissolves into blood and tissues. How quickly this happens depends on factors such as perfusion and tissue type: well-perfused organs saturate faster, fatty tissue a bit more slowly, and poorly perfused structures like bone very slowly.

During ascent, ambient pressure drops again. The tissue becomes “supersaturated” — and the risk of bubble formation increases. Under normal conditions, nitrogen is transported in a controlled way via the bloodstream to the lungs and exhaled. If ascent is too fast, or supersaturation is too high, the gas can no longer remain fully dissolved and bubbles can form in blood or within supersaturated tissues. These bubbles can grow, merge, and cause problems in tissues or in the circulation.

Even though bubbles play a central role in DCS, the exact interaction between bubbles and other factors is not fully understood. Different people can develop very different bubble loads after the same dive; the same person can react differently on different days. The one clear pattern is this: more bubbles mean a higher statistical risk of DCS.

Symptoms of DCS

Decompression sickness can look very different from one case to another. Some divers have only mild, non-specific symptoms; others develop life-threatening deficits within minutes. That makes early interpretation difficult. This is why dive medicine follows a simple rule: better to be too cautious once than to miss a serious sign.

Why symptoms vary so much

DCS is caused by gas bubbles that form in the body after a dive. Depending on where those bubbles accumulate, you’ll see different symptoms: pain around joints, a characteristic skin pattern, or neurological deficits such as weakness or altered consciousness. Symptoms often appear within the first hour after surfacing, but they can also develop much later — cases up to 24 hours are documented.

Typical key symptoms

• Neurological: dizziness, unsteadiness, an insecure gait, sensory changes (tingling, numbness), weakness in arms or legs, confusion, visual disturbance, headache, seizures.
  These symptoms are always serious — even if they seem mild.
• Skin: itching, redness, or mottled (“marbled”) skin that can be painful and swollen. These signs often look harmless, but can indicate deeper problems.
• Musculoskeletal: joint pain (“the bends”) is the classic presentation — but not the most dangerous one. It may come in waves, migrate, or persist even at rest.
• Circulation and lungs: chest pain, shortness of breath, cough, palpitations, or dizziness from circulatory compromise. These can be life-threatening — and can be confused with asthma or a heart attack.
• General: fatigue, unusual tiredness, nausea, or vomiting. These symptoms are non-specific, but deserve extra attention after a dive.

Things that can look similar

Not everything that happens after a dive is automatically DCS. Some complaints have other causes:

• Sunburn: red patches exactly where the sun has hit.
• Gastrointestinal issues: diarrhoea or nausea may be infectious or diet-related.
• Pre-existing problems: chronic joint pain or old injuries are not suddenly caused by a dive.
• Fatigue: can simply be normal exhaustion after exertion.

Still, the rule remains: get it checked. Especially neurological signs, or unusual combinations of symptoms, should be taken seriously.

First aid for suspected DCS

We’ve already seen that DCS symptoms can be quite varied. If someone feels unwell after a dive, you should always keep DCS in mind — without losing sight of other medical problems.
What makes first aid for a diving accident different from general first aid is two things:
Oxygen: if there is even the slightest suspicion that this could be a diving-related incident, give oxygen.
Diving medical hotline: emergency services rarely deal with diving accidents. Involve a diving medical hotline through the diver’s insurance organisation. Even if the person isn’t insured there, you’ll get expert advice. If insurance is expected to cover costs, they also need to be informed as soon as realistically possible — life-threatening emergencies, of course, come first.

What to do in a diving accident or suspected DCS

Not everything that happens after a dive is immediately an emergency. A structured, stepped response helps — and it’s also the logic behind our traffic-light quiz:

• Red: life-threatening or severe neurological signs, acute shortness of breath, chest pain.
  → Give O₂ and call 112 immediately. A hotline can be involved in parallel.
• Yellow: suspicious but not immediately life-threatening symptoms (e.g. mottled skin, tingling, dizziness).
  → Give O₂, monitor closely, and call a hotline. The hotline can coordinate next steps if needed.
• Green: probably not dive-related (e.g. sunburn, simple fatigue).
  → Observe. Hotline only if unsure.

Rule of thumb

“Any unusual symptom after a dive is suspicious — better one extra dose of oxygen and one extra phone call than one too few.”