What Is Decompression Sickness? A scuba diver during a wreck penetration dive.
A scuba diver during a wreck penetration dive. Photo by Marek Okon on Unsplash.

What is decompression sickness or DCS? In short, decompression sickness is getting sick when changing from a high-pressure environment to a lower pressure environment. Humans can safely do this and scuba divers do it on a regular basis. There are some safety rules that they adhere to in order to not get sick. Namely monitoring the amount of absorbed gas as well as the rate at which they decompress.

So, what are these safety rules, can you still get sick following the rules? For decompression sickness explained in more detail, keep scrolling.

Decompression Illness

For the scope of this article, we are going to simplify this medical condition. In scuba diving, the more widely used term is Decompression Illness. This term encompasses both Decompression Sickness as well as Lung Over Expansion Injuries. This makes it easy to understand and removes the need for an exact diagnosis among first responders.

More inclusively the term Dysbarism, describes decompression sickness, arterial gas embolism, and barotrauma. This word is formed from the Greek prefix ‘dys’ which denotes difficulty, pain, or trouble, and ‘bar’ which is a metric pressure measurement.

If you are interested in a detailed study of diving gas embolism, this is a great book to read.

Decompression Sickness

When we change ambient pressure environments, our body reacts by trying to equalise to the surrounding gas pressures. For example, when we go to high altitudes, our body will over time acclimatize to the lesser ambient pressure.

This is a natural process and does not cause any sickness provided our body can equalise the pressures gradually.

Scuba divers absorb gas which is stored in their body tissues in solution. This gas is sometimes referred to as Silent Bubbles, gas bubbles so small that they are undetectable by ultrasound and do not harm the body.

If we however decrease surrounding pressure too rapidly or there are simply too many of these silent bubbles, we risk decompression sickness. Essentially one or many of these silent bubbles expand into a physical bubble that can cause injury to our body.

Medical Oxygen used in DCI treatment
A common oxygen setup used in the diving industry. Photo by Samuel Ramos on Unsplash

In order to understand this concept in more detail, we need to understand partial pressures & pressure gradients.

Partial Pressures

A partial pressure of a gas is the how much a given gas represents in the atmospheric pressure.

At 0 meters in altitude, so the beach, we are surrounded by 1 bar of air. Air consists of 21% oxygen and 79% nitrogen. Therefore their respective partial pressures are 0.21 and 0.79.

Now both of these gases are fully saturated in our bodies. Think blood, bone, skin, flesh, muscle, fat, hair, absolutely everything!

You can express this like so 0.21 PPO2 and 0.79 PPN2. (PPO2 stands for partial pressure of oxygen & PPN2 nitrogen respectively).

If you remember from above that at sea level we are in 1 bar of air, so 100%. If we enter a higher pressure environment, say 3 bar, the partial pressures change accordingly. You guessed it, we have three times the pressure 3 bar = 300%) so we multiply each gas’ partial pressure by 3. This leaves us with 0.63 PPO2 and 2.37 PPN2.

Over time, the different partial pressures of the environment and our body will reach equilibrium. This is due to a physics principle known as Henry’s Law.

Pressure Gradients

So, what’s the problem then? Scuba divers go to higher pressures than that regularly and return unscathed. This is due to our body’s ability to equalise gas pressures naturally.

The two main factors that determine whether we will get sick or not, are the amount of gas dissolved and the rate at which the pressure changes. This goes both ways, entering a higher pressure environment or a lower pressure environment than we are currently in.

The greater the difference between different partial gas pressures, the faster they equalise. This is why diving deeper (higher hydrostatic pressure) means we absorb nitrogen faster initially. This results in shorter no decompression limits.

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Pressure gradients are also responsible for ascent rates. When divers ascend, they are supersaturated (higher partial gas pressure in the body than the surrounding environment) and they need to ensure not to exceed the safe pressure difference to allow the body to off-gas naturally.

A practical example

A scuba diver going for his first dive of the day. At this point, the diver has the same partial gas pressures saturated in their body as everyone else on the planet.

Now the diver descends and goes on their dive to 30 meters depth.

He is now in 4 bar of hydrostatic pressure. The physics principle Boyle’s Law defines this. Very briefly, we add 1 bar of pressure for every 10 meters of depth.

Remembering that our body tries to equalise this pressure, it does exactly that during the dive. The longer the diver stays underwater, the more additional oxygen and nitrogen he absorbs.

Now, if the diver follows no decompression rules and ascends at a safe rate, they are very unlikely to get decompression sickness. These rules are based on what the human body can equalise safely.

Breaking these rules, by either absorbing too much gas and not completing decompressing stops or ascending too fast can cause decompression sickness.

Essentially the diver is forcing their body to decompress quicker than it can do safely and the gas forms physical bubbles. These bubbles can escape and end up under the skin (skin bends), they can form in your arteries (arterial gas embolism). They can even disrupt your nervous system (CNS DCS).

Lung Over Expansion Injuries

Another injury a diver can get is an over expansion injury. In this case, the diver has either held their breath while ascending or had a considerable blockage in their lung.

The former is far more common. Because gas expands during the ascent, the excess air has to go somewhere. If the diver continues to breathe normally, they equalise their lungs naturally.

However, if they were to hold their breath and ascend, that air could potentially rupture the lung somewhere, causing severe damage to the lungs.

This sounds quite graphic and scary, however, it is also very easily avoided by simply breathing continuously and never holding your breath while scuba diving.

DCS in Freediving

For quite some time it was believed that only scuba divers are vulnerable to DCI due to the compressed gas consumed at depth. However with freediving growing in popularity it has been observed in freedivers also. Mostly in competitive freediving where extreme depths are attempted and divers remain deep for several minutes.

During that time the pressure gradient is extreme and the body may absorb some gas stored in the diver’s lungs. Because freedivers ascend rapidly, some of that absorbed gas can in fact form physical bubbles.

This is why scuba divers should never duck dive or freedive after scuba diving. The residual nitrogen can still cause decompression sickness if rapid pressure decreases are occurring several hours after the dive. This is the same reason divers need to wait a minimum of 12 hours after a single scuba dive before they fly.

Decompression Sickness Symptoms

Decompression sickness physiology is quite complex. This sickness is generally classified into two types.

Type 1 decompression sickness, considered less severe, concerning skin symptoms and lymphatic swelling.

Type 2 DCS, considered more severe, including neurological, inner ear and cardiopulmonary symptoms.

Here is a list of common DCS signs & symptoms:

  • Joint pain or muscle pain
  • Numbness, tingling and weakness
  • Extreme fatigue
  • Nausea, vomiting
  • Dizziness, vertigo
  • Skin rash or skin itch
  • Impaired mental function
  • Difficulty breathing
  • Lack of coordination
  • Reduced levels of consciousness
  • Lymphatic swelling
  • Bladder or bowel dysfunction
  • Compromised cardiovascular function

This is not an exclusive list. Decompression sickness symptoms can be idiosyncratic. This means the patient’s primary complaints draw attention away from potentially less obvious but more severe symptoms.

The most prominent symptoms observed are immobility and pain caused by bubbles in a victim’s joints. This is why DCS is also known as the bends. Read more on the history of this term in why is decompression sickness called the bends?

It is important to note that DCS symptoms generally occur after a dive, not during. Either several minutes upon surfacing or up to several hours after surfacing.

All signs and symptoms of DCS need to be taken seriously. Immediate and precautionary treatment can dramatically affect treatment and improve recovery time and success.

LOE Symptoms

Lung Over Expansion Injury (LOE) signs & symptoms:

The main difference is that LOE symptoms usually occur immediately upon surfacing and are quite obvious.

  • Difficulty breathing
  • Severe coughing
  • Purple appearance
  • Pink foam from mouth and nose
  • Loss of consciousness

Preventing Decompression Sickness

The most common decompression incidents for regular people occurs when scuba diving. This doesn’t mean you shouldn’t scuba dive!

A large part of scuba diving training is learning safety protocols and understanding these basic physics concepts in order to avoid decompression illness. In this case, prevention is simply adhering to these rules.

There is no room in diving for showing off and breaking limits to later brag about. These types of divers are seeking to get decompression illness.

The most basic rules besides that are to keep very well hydrated, lead an active lifestyle and eat a balanced diet.

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Technical divers that venture far deeper and stay for far longer, learn even more procedures to return safely. The severity of DCI is higher in technical diving. This is why their training is hyper-focused on prevention. Some of the additional procedures involve conducting decompression stops and using different gas mixtures.

Treatment for Decompression Sickness

Decompression sickness is treatable. Treatment involves the use of a hyperbaric chamber. Also known as Decompression Chambers, they enclose a patient and put them under pressure.

The reason this treatment works is Boyle’s law. Increasing a victim’s surrounding pressure to be equal or higher than the dissolved gas forces bubbles back into solution. The goal here is to shrink the bubbles’ volume enough to force them to become small enough for the body to naturally expel the excess gas. Gradually releasing the pressure ensures the body has enough time to gradually release the excess gas.

Decompression Chamber
A common decompression chamber used to treat divers in. Photo by SSS Recompression Chamber Network

During treatment, patients are administered high concentrations of oxygen. This increases the pressure gradient and aids the body in releasing residual nitrogen more efficiently.

Some chambers are just big enough for a grown human to fit inside of, others are large enough to accompany several patients.

For the keen readers among you, you might be thinking if pressure is all I need to treat DCS, why not just go on another dive? There are several reasons why this is not a good idea. Let’s say someone complains about DCS symptoms post-dive and we throw them back in the water.

It would be very hard, if near impossible to monitor signs and symptoms underwater. We would not be able to communicate with the patient. Extended periods of time in the water cools the body and can cause hypothermia. Most likely they would be breathing compressed air, not increasing the pressure gradient.

While in-water recompression works in theory, however, it is only attempted in extreme and rare cases. Usually with very experienced technical or Navy divers when no other options are available.

Staying Safe

Now that we know exactly what decompression sickness is, we understand the importance of avoiding it. As a certified diver it is quite easy to stay safe. What causes decompression sickness in most cases is diver error. Therefore we can avoid decompression sickness by diving conservatively and following the rules and safe diving practices.

This is a big part as to why you need a certification to go scuba diving.

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