Undeserved hits

I am currently going through all the case notes of decompression illness at our hyperbaric unit since 2008, to make sure we learn from past experience. This will take some months but I promise to make the results available in a future article. All information will be anonymous, of course!

Some bends are described as undeserved, which means the dive profiles were not expected to result in the bends .(aka decompression illness or DCI) However there are a few points to remember. All tables and computers have a hit rate, which means that if you do enough diving, you will get bent. We do not fully understand why some divers get bent, sometimes after a dive profile they have done safely many times before. A patent foramen ovale (PFO) can be associated with undeserved DCI, but the relationship is not clear. 7.3% of divers have a PFO that is large enough to be significant, but 7.3% of divers do not get DCI. Bear in mind that almost all dives done in air below 10m result in bubbles. In the lungs, the bubbles are filtered out and blood is oxygenated before entering the arterial circulation. If bubbles are not removed, blockage of arteries occurs with symptoms of DCI. In a common form of strokes, blood clots cause blockage in arteries in the same way as bubbles. The site of blockage is usually different, as bubbles behave differently to clot, but the physical effect is the same. So given that that bubbles are so common and the effects can be so dramatic, minimising your risk is worthwhile.

“You can’t get bent, if you don’t go diving”

However, if that is not an option, then the following the advice we give all divers after DCI, will minimise your bubble load. In fact diving no deeper than 15m is even better. For those with a PFO, following this advice, including diving no deeper than 15m, is an option, rather than PFO closure.

Divers Emergency Service UK Post DCI advice:

1. No decompression diving.
2. Do a safety stop or extend it.
3. No diving deeper than 30m.
4. Use nitrox on air tables, but only if appropriately trained.
5. Do not dive to the depth or time limits on the dive tables or dive computer.
6. Remember that some dive computers only use decompression as an emergency procedure: Limits are there to stay away from, not work to.
7. Stay well hydrated while diving.
8. Dives involving the cold or heavy exercise should be even more conservative in terms of depths and times.
9. Always adhere to safe diving practises and dive within your training and experience.

While we clearly do not have all the answers, following safe diving practises is very important. However every year we will continue to see cases of DCI that are apparently undeserved. However something has to happen for DCI to occur. In my experinence of DCI, dehydration is important and comes up time and again. Especially in the more severe cases. This would make sense, as dehydrated blood is thicker and bubbles are more likely to get stuck. So please stay well hydrated and allow the bubbles to flow!

Question 1: Are there any other factors that might be involved in ‘underserved hits’?

There are modifiable factors, including depth, time and ascent rate which are clearly important and also avoiding reverse profiles, short surface intervals, cold, medical conditions, dehydration, alcohol/hangovers, exercise after the dive, valsalva, lack of sleep, poor overall fitness, obesity and tight straps and seals around limbs. A PFO is modifiable, as it can be closed. However there are also unmodifiable factors, including increasing age, presence of lung shunts, genetic disposition, and previous injury including DCI.

Question 2: Why can computers not tailor depth and time limits to the individual?

The simple answer is that this is too complicated. Computers are not calibrated to actual nitrogen absorption but use mathematical models and equations. Human physiology and every dive vary too much for this process to be tailor made. Take into account the list of factors above and the amount each contributes to the risk and the answer is impossible to achieve. Perhaps the only way to get close to an answer would be to create a deco wiki, as suggested on scubaboard, with millions of divers and dive profiles.

Question 3: So what sort of “cushion” should I leave in terms of decompression schedule?

A safety stop is a cushion. Ascending more slowly, diving more shallow and for less time are all cushions. These are all good things, as is avoiding all the factors outlined in question 1 and using nitrox on air tables (If appropriately trained). But the best single rule would be to not approach the limits set by your computer.

Question 4: So if we don’t understand what causes undeserved hits and the significance of the modifiable and unmodifiable factors, how can we have a safe dive schedule?

The bottom line is that computers use calculations, based on an ‘ideal’ person and including safety margins, to provide depth and times that fit profiles that are apparently safe. All computers and tables have a hit rate; hence if you do enough diving you will get DCI. Although the bigger the cushion, the less likely you are to suffer DCI. For example, the PADI tables are based on the US Navy tables but PADI have included a larger safety margin because a lower hit rate is more acceptable in recreational diving. So we all take a risk based approach to diving and concentrate on safe diving practices.

Question 5: In which case, how do different computer algorithms work to reduce the risk of DCI?

Alert Diver Online has answered this. (http://www.alertdiver.com/?articleNo=340)
“Questions about algorithms often overshadow all other safety issues. However, dive computer manufacturers generally do not provide information about their algorithms, their use in particular computer models or their impact on DCS risk. There are at least two reasons for this: 1) dive computers are not regulated; 2) validation of decompression safety is complicated and expensive. Thus, in most cases manufacturers do not have the data necessary to support claims of risk control or risk reduction — an important issue for divers.”