The Bends: Understanding Decompression Sickness And How to Avoid It

Photo credit: Max Gotts via Unsplash

Decompression sickness is a risk every diver is aware of from their first lesson. We’ve all heard the cautionary tales, but what is “the bends” and, most importantly, how do we avoid it?

Decompression Sickness Explained

Decompression sickness (DCS) is a serious condition caused by a rapid decrease in the pressure surrounding the body, whether it is air or water. When diving with compressed air, we breathe in extra oxygen and nitrogen. While oxygen is used by the body and converted to carbon dioxide in the lungs, nitrogen is absorbed into the tissues at high pressures.

Under normal circumstances, as we return to a lower pressure area (the surface), the gradual decrease in surrounding pressure allows nitrogen to be released back into the bloodstream and cleared from the body through the lungs. This is known as “off-gassing”. DCS occurs when pressure changes too quickly, preventing the normal off-gassing process. Nitrogen remains in the tissues and begins to form bubbles, which can interfere with blood flow and oxygen supply to organs and other tissues.

Signs and Symptoms of Decompression Sickness

The most common signs and symptoms of DCS are joint and muscle pain, extreme fatigue, nausea, mild numbness and tingling, and rash. When vital organs such as the brain or lungs are affected, more serious complications can arise, including confusion, loss of consciousness, seizures, breathing difficulties, and paralysis. Arterial gas embolism (AGE) is a severe form of decompression illness where gas bubbles travel in the bloodstream to distant organs. In rare cases, AGE and severe DCS can be fatal.

Signs and symptoms of DCS typically start within 15 minutes of ascending, but can take up to 12 hours to develop. Delayed onset can occur in some cases, particularly if the diver takes a flight within 24 hours of diving. Any signs or symptoms that develop within 24 hours of a dive should be treated with suspicion and reviewed by a medical professional who is knowledgeable about diving.

Treatment of Decompression Sickness

Where signs and symptoms do develop, the first line of treatment is high-flow oxygen. It should be administered immediately and should continue until the diver reaches a centre with a hyperbaric chamber. Hyperbaric oxygen therapy is the primary treatment for DCS. It involves placing the diver in a pressurised chamber (2.5 times the pressure of normal air), which is pumped full of pure oxygen. Hyperbaric therapy increases oxygenation of tissues and organs and reduces the number of nitrogen bubbles in the bloodstream. Therapy typically lasts a few hours and, in more severe cases, may need to be repeated.

All cases of DCS should be reviewed by a diving medical specialist to determine whether hyperbaric therapy is required. Even if symptoms improve at atmospheric pressure or with high-flow oxygen, they may recur after a period of time, and long-term complications may arise if hyperbaric therapy is not administered. It’s essential that the diver is reviewed by a specialist and kept under observation for 24 hours regardless of the treatment given. It’s a good rule of thumb to know who to contact for advice and guidance wherever you are planning to dive.

Risk Factors and Prevention

So what are the risk factors for decompression sickness, and how do we avoid it? Obviously, the most common cause is ascending too quickly. Deeper dives, longer dive times, and breath-holding all increase the risk. Other factors include diving in cold temperatures, dehydration, recent alcohol intake, doing multiple dives in a short period of time, obesity, flying after diving, and certain heart or lung conditions (including asthma, obstructive lung disease, or heart defects).

Most of these risk factors can be avoided with careful planning. Good pre-dive practices include staying hydrated, being well-rested, and avoiding alcohol. Ascending slowly, breathing normally, and performing safety stops are essential to reduce the risk of decompression sickness. Dive tables and dive computers are important in planning safety stops, maximum bottom time, and surface intervals between repetitive dives. Slow, gradual ascent and normal breathing are essential. Divers should avoid flying within 24 hours of a dive, particularly if they have exceeded their no-decompression limit. Overall fitness is also important, and divers should get medical advice if they are concerned about any medical conditions, particularly lung or heart problems.

Dive Tables

First introduced in the early 20^th century, dive tables have long been one of the principal ways divers plan the depth and duration of their dive and how to counteract the associated risks. There are many variations of dive tables available. Most major dive organisations, such as PADI and the US Navy, have their own version, but the principles remain the same. Calculations should be conservative—divers must round up their planned depth and time to allow a safe margin, rather than run the risk of finding themselves in trouble.

Dive tables serve two main functions: calculating the no-decompression limit for a particular depth and, where repetitive dives are planned, informing the required surface interval between dives. Based on the depth and duration of the dive, the No-Decompression Limit table outlines when safety stops are needed and the no-decompression limit for that depth. Exceeding the no-decompression limit is a major risk factor for decompression sickness. If the limit is exceeded, the diver must carry out mandatory safety stops (8 minutes if the limit is exceeded by less than 5 minutes and 15 minutes if exceeded by more than 5 minutes). They must also increase their surface interval if planning multiple dives, specifically 6 hours if exceeding the limit by less than 5 minutes and 24 hours if more than 5 minutes.

For repetitive dives, providing the no-decompression limit is not exceeded, divers can use the Repetitive Dive table to calculate the correct surface interval and safe bottom time for each dive. As all divers know, your first dive should be your deepest, and all subsequent dives should be shorter and shallower.

Dive Computers

As useful as dive tables are, and as important as it is for all divers to understand them, they have their limitations. The potential for human error is high, given the manual calculations that are required. Another disadvantage is that tables are based on “square profiles”, where the assumption is that divers will go to the deepest depth and stay there for the total length of time. The inability to factor in multi-level dives means that divers can lose out on valuable bottom time.

These days, tables have been largely replaced by dive computers. Computers allow real-time tracking of depth, bottom time, the gas mixture used, heart and respiration rate, and the number of dives undertaken. Most offer safety stop reminders, ascent rate monitoring, and alerts about no-decompression limits. Based on real-time, dynamic figures, dive computers allow for longer bottom time and shorter surface intervals, while automating the safety process and reducing the risk of human error.

That said, the ultimate responsibility for safe diving lies with divers themselves. Dive computers are not perfect, and there is always a risk of mechanical failure. Divers need to be able to read and understand dive tables and apply safe limits, particularly in the case of repetitive dives.

Decompression Sickness: A Serious But Avoidable Risk

Decompression sickness is rare, occurring in approximately 0.03% of recreational dives, but it can have serious consequences. All divers should be aware of the risk factors, the signs and symptoms, and how to prevent DCS. Careful planning of dive depth and duration, good pre-dive practices including rest and hydration, and ensuring a gradual, safe ascent are essential. Dive tables and computers play an important role, particularly when it comes to repetitive dives. Always err on the side of caution, and act quickly if you or any of your dive group experience unusual symptoms after a dive.