Omitted decompression stop or rapid ascent

Omitted decompression stop or rapid ascent

– what are the options?

by Dr Lynn Taylor (PhD) and Dr Simon Mitchell
The experience related by Neil Vincent in ‘Watch out for the Noahs’ (Dive New Zealand October/November 2000 issue) brings home the fact that even well planned dives can go wrong. Decompression time can be shortened, or even omitted, due to unexpected circumstances. Two divers were encircled and ‘charged at’ by sharks, whilst on their decompression stop. They decided to surface early when the risk of omitted decompression loomed as the better alternative to the probability of being attacked (putting this into perspective, shark attacks on divers are extremely rare!).

After any omitted decompression there is an increased risk of decompression illness (DCI).’What do I do in such a situation?’ is a commonly asked question. In this article we will discuss four practical options, and the relative advantages and disadvantages of each.

Before we begin, there are two very important points which we wish to emphasise. Firstly, the term ‘decompression’ is not just confined to technical diving where planned deco-stops are conducted during the ascent. Even in recreational ‘no decompression’ dives, the slow ascent is a form of decompression. Therefore, whilst the above example is a clear omission of planned decompression, a rapid ascent from a dive that has not entered decompression requirements can also be considered omitted decompression, albeit of a less serious type. Secondly, there is no standard textbook advice for these situations, and probably no right or wrong response. However, there is one piece of undisputed medical advice that applies no matter which of the subsequent strategies is adopted: a diver who develops symptoms of DCI should not re-enter the water, should breathe 100% oxygen, and the Diver Emergency Service should be contacted as soon as possible.

So what are some options for omitted or shortened decompression time?

Option 1 – Do nothing, but monitor the diver for symptoms of DCI.

Clearly, if decompression is omitted the risk of DCI is greater than if it had been completed. Moreover, it should be borne in mind that the worse the omission, the more likely the development of DCI. However, it is very important to appreciate that just because decompression has been omitted, it is not inevitable that the diver will suffer DCI. For example in Neil Vincent’s experience, one diver developed symptoms and required treatment in a recompression chamber while the other didn’t. Thus, one plausible option is for the diver to rest and monitor themselves carefully for the development of any symptoms indicative of DCI.

Strenuous activity after a dive has been shown to precipitate DCI, so the diver should not do any hard physical work or exercise such as pulling up a heavy anchor or lifting heavy equipment. In this, and all other options, the diver should not dive again for at least 24 hours. If the diver develops symptoms, proactive intervention becomes imperative. The Divers Emergency Services should be contacted (via the free DES or DAN phone lines, see end of this article) and the usual first aid procedures for DCI should be initiated without delay. This means lying the diver down and administering 100% oxygen whilst monitoring their airway, breathing and circulation (pulse).

The advantages of the ‘do nothing’ approach are that it is simple, requires no resources, time or effort, and the chances are that many divers in this situation will not develop symptoms of DCI (depending on the degree of omitted decompression). The disadvantages are that nothing is actively being done to decrease the risk of DCI. It follows that of all the options discussed here, this is the most likely to result in DCI.

Option 2 – Re-enter the water with a fresh supply of air and complete the omitted decompression procedure.

One thing is certain, re-entering the water breathing air is NOT a credible option if symptoms of DCI are present, and this manoeuvre should NEVER be used as a treatment for DCI. The reasons for this are two-fold. Re-entering the water after symptoms have developed has frequently proved ineffectual and just delays the correct treatment and management. Symptoms may resolve initially under pressure but rarely is the relief long-term. Also there is a real risk that symptoms can worsen and the diver can rapidly deteriorate underwater, putting them in considerable potential danger.

However, various diving authorities have advocated this option if the diver has NO SYMPTOMS OF DCI, there is sufficient air supply to complete the omitted decompression procedure, and the procedure can be initiated quickly. Most published protocols involve going deeper than the first omitted stop, and conducting a series of stops over a longer period than the original omitted decompression. For example, the US Navy recommends that where a three metre decompression stop is omitted, the diver should initially descend to 12 metres. He or she should remain there for a quarter of the omitted three metre stop time, then nine metres for a third of the three metre stop time, six metres for half of the three metre stop time, and finally three metres for one and a half times the scheduled three metre time.

There would be little harm in adapting your own approach with even longer spent at the shallower stops. However, whatever you do, it needs to be initiated quickly; preferably within five minutes. If the delay between surfacing and re-entering the water is greater than 10 to 15 minutes there is probably very little to be gained since bubbles start to form within five minutes of surfacing. If this option is taken, the diver must always be escorted during the omitted decompression procedure. Two buddies from the same dive would be OK completing it together since the chance of both developing DCI simultaneously is small. However, if a large amount of decompression time was omitted, the diver(s) should be escorted by another who was not involved in the omitted decompression dive.

The advantages of this procedure are that if it is instituted early (within five to 10 minutes) it can be effective and, it is an active intervention that is likely to reduce the risk of DCI. However, the disadvantages are that it requires both knowledge of the correct protocols, and the time and resources to set up a proper decompression line. There is also the potential for hypothermia and the development of symptoms of DCI whilst the diver is underwater.

Option 3 – Breathe 100% oxygen at the surface.

Erring on the side of caution, one might lie the diver down and administer 100% oxygen upon exiting the water, even if no symptoms indicative of DCI had developed.

The advantages of this are that it is relatively easy to do and it is almost certainly effective if done for at least 30 to 60 minutes. The disadvantages are that using the oxygen for this diver depletes the supply, which could be needed for another if an emergency were to arise later. There are also technical limitations in the equipment making it difficult to deliver 100% oxygen to both divers in a buddy pair, unless two sets of oxygen equipment are available.

Option 4 – Breathe 100% oxygen under water.

The only circumstances in which we would consider relaxing the edict against re-entering the water with symptoms of DCI is where the incident involves a group of properly trained technical divers who are appropriately equipped for in-water oxygen recompression. For example, extreme technical divers in isolated locations who have a decompression stage, full-face masks, adequate thermal protection, and a system that can deliver 100% oxygen underwater may choose in-water recompression for early management of severe symptoms of DCI. It is NOT considered a valid option for recreational divers who have omitted decompression because they are unlikely to have the knowledge or equipment to do it ‘safely’. Quite simply, the potential risk is too great compared with the potential benefit. The major risk is oxygen toxicity manifest as a seizure. Unless the diver is properly equipped, this would almost certainly result in drowning.

Which option would we recommend?

As we said in the beginning, there is no textbook answer and probably no definitive right or wrong response. What would we do? To some degree our choice would depend upon the degree of omitted decompression. If it were minor, we would lean towards no active intervention and monitoring the diver for symptoms of DCI. If there was a significant omission of decompression, we would favour the more active interventions such as breathing 100% oxygen at the surface, or possibly immediate in-water recompression with air provided the diver was asymptomatic (i.e. NOT showing any signs of DCI).

Whatever option is taken the diver should rest and not dive for at least 24 hours. If symptoms develop contact should be made with medical personnel via the DAN / DES free phone numbers and evacuation begun immediately whilst the diver is breathing 100% oxygen.

NB: The views presented in this article are those of the authors and may not necessarily be representative of the advice given by all dive agencies.


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