DCI and Dive Computers –
what’s the story?
By Lynn Taylor
In the June/July issue of Dive New Zealand (issue 52), an interview with Professor Des Gorman reported on his recent observations on divers coming to the recompression chamber in Devonport (the Slark Unit). He commented that recently divers have been presenting with more serious manifestations of decompression illness (DCI). Professor Gorman linked the seriousness of the cases to the increased reliance divers now have on using dive computers during repetitive diving, in combination with divers’ disregard to the common rules of safe diving practices.
In the same issue there was a letter to the editor from Rex Gilbert, NZ Underwater’s accident recorder, pointing out that it is not the dive computers themselves that injure divers, but their misuse.
Essentially the comments made by these two highly respected authorities are reinforcing the same message. A dive computer is a tool which must be used appropriately; it does not replace the basic rules of dive planning and common sense.
The interview with Professor Gorman seems to have created a ripple throughout the dive industry in relation to concerns over the safety of using dive computers. Indeed, in this current issue, the article entitled ‘Culture Clash’ by Lawrence Goldberg and David Merritt emphasises the need for recreational and technical divers to be more cautious in their dive planning and take some lessons from commercial divers. Their article brings out some important safety considerations regarding the changes that commercial divers have made in their diving practices, and the effect that these changes have had on the now-enviable safety record of commercial divers. These authors make reference to the 1997 data on DCI from the US, which is collated and reported by DAN (Divers Alert Network), regarding computer use in victims of DCI.
The purpose of my article is to take a close look at data from DAN US and the Slark Hyperbaric Unit, and see how this information can be interpreted to improve the safety of computer-assisted diving. For this article I will restrict the observations to a ‘snapshot in time’ and compare the most recently published DAN US data (1997 statistics) with NZ data from the same year. In part 1, I will look at the actual data, and in part 2, I will take a look at the bigger picture of the role of dive computers. I am grateful to the staff of the Devonport chamber for allowing me access to their 1997 database (unpublished data). The views expressed here are my personal views and may not necessarily represent those of DAN or the Slark Unit.
â What does the data say?
Number of cases in 1997: DAN (US) – 452 cases, Devonport (NZ) 45 cases.
Whilst the number of divers treated for DCI in New Zealand appears disproportionately high in relation to the population size, it should be remembered that the number of divers per capita in NZ is higher than in the US, and also that diving in New Zealand tends to be an all-year-round activity. The Devonport figures also include divers brought here for treatment from the Pacific Islands.
Method of dive planning/monitoring
|DAN (US) 452||60% (n=269)||37% (n=169)||3% (n=14)|
|Devonport (NZ) 45||29% (n=13)||51% (n=23)||
6% (n=7) nil
17% (n=2) unknown
At first glance, the US data would suggest that diving using a computer might carry a higher degree of risk of DCI than diving with tables. However, if, for example, 60% of all diving in the US were done with a computer, then it would be no surprise that 60% of the divers with DCI used computers. Also, association does not necessarily mean correlation. The type of divers who engage in more ‘risky’ diving practices may also be the same group who decide to purchase technical equipment such as dive computers. The argument can go around in circles. What is more worrying is the high percentage of divers in New Zealand that use no method of planning a dive.
Severity of symptoms
|DAN (US)||DCI-A 29.0%||DCI-A 17.2%||DCI-A 28.6%|
|DCI-B 65.8%||DCI-B 69.2%||DCI-B 57.1%|
|AGE 5.2%||AGE 13.6%||AGE 14.3%|
|Devonport (NZ)||DCI 100 %||DCI 100 %||DCI 100 %|
|AGE 0 %||AGE 0 %||AGE 0 %|
DCI-A: milder – skin, muscle, joint symptoms only.
DCI-B: more severe – neurological/cardio-respiratory symptoms.
AGE: Air Gas Embolism.
The 1997 US data indicates that the majority of divers presenting would be considered to have more ‘severe’ (Type B) DCS with the percentages being similar for computer and table users. This classification is indicative but is somewhat oversimplified. The Devonport database makes no attempt to restrict classification to these two categories, because the overlap is considered too great to make a definitive distinction. It is therefore not possible to directly compare the two countries.
What is noteworthy is that the percentage of divers diagnosed with the life-threatening Air Gas Embolism is significantly less in the divers using computers. One conclusion suggested by these data might be that the ‘rate of ascent’ indicators on dive computers could be better at slowing a diver’s ascent than judgement methods used by table divers.
Because a higher percentage of divers experiencing DCI have engaged in repetitive diving, it seems logical to conclude that repetitive diving carries greater risk. This may be true, and there is evidence to suggest that repetitive diving does carry an increased risk of DCI. However, most people probably do more than one dive in a day. Therefore, if, for example, 85% of diving in general was repetitive diving, then the proportions in the table above would be no greater than expected.
The Devonport data revealed that a higher proportion of people who dive with a computer engage in repetitive diving compared to those who dive using tables. Professor Gorman commented that some divers he has seen recently are doing four and five deep dives in one day. Common sense alone would say that anyone who challenges the basic safe diving practices in such an extreme way is exposing himself or herself to a significantly increased risk of DCI.
The data most certainly does put paid to the myth that ‘you can’t get bent on a single one tank dive’. Of the 45 cases of DCI seen at Devonport, 24 were as a result of a single dive.
Depth of deepest dive > 24m (80ft)
The data suggests that divers who use dive computers are more likely to dive deeper than those who use tables. This could be interpreted in two ways: either computers encourage deeper diving, or those divers who engage in deeper diving decide to purchase a computer.
The DAN US 1997 database revealed that computer users were more experienced divers, and had been diving more often and for a greater number of years than table users. Of the 13 computer divers presenting in New Zealand, nine (69%) had an experience level of 100 dives or more, up to 6000 dives, compared to 50% of divers using tables.
The likely explanation for the difference is that people tend to leave the financial outlay for a dive computer until they have been diving for a while. Could it also mean that divers become less conservative in their diving the more they dive? Or is it just a reflection of the fact that every dive carries a risk of DCI? For example, if each dive carried a 1% risk of DCI, then statistically you might expect that DCI would be experienced at some time within a 100-dive period.
Part 2 â
The bigger picture
When analysing data like these we are trying to make interpretations based on an overview of the information available. To keep things in perspective, we should also remind ourselves of the respective merits of diving using a table or a computer.
What safety advantages can tables offer?
â¢ Dive tables assume that the entire dive is spent at the maximum depth, therefore giving less permitted bottom time and therefore more conservative dive profiles.
â¢ Tables are not subject to equipment malfunction (only diver misinterpretation).
â¢ Tables such as PADI’s and SSI’s include reminders about general safe diving practices.
What safety advantages can computers offer?
â¢ The visual/audio ascent rate indicator assists divers to ascend at a slower rate.
â¢ A dive computer is always taken underwater with you. (How often do table divers actually take their tables underwater with them?)
â¢ Computers allow additional time to be spent in shallower water at the end of a dive (rather than ascending directly to the surface) encouraging further elimination of nitrogen.
â¢ If you do deviate from your planned dive, a computer will automatically recalculate for you (and take you through any required decompression stops).
â¢ Avoids human errors made in calculation.
â¢ Accurate records of dive profile are kept (particularly valuable to medical personnel).
â¢ Visual displays are generally more easily read than analogue gauges, making it easier to keep track of depth and time, and accurately monitor surface interval.
So are computers safe?
The answer to this question is, in my view, ‘Yes, computers are as safe as tables, if used appropriately.’ Diving with a dive computer can be likened to driving a car in a limited speed zone. There are defined boundaries, and there are sensible practices which further restrict those boundaries based on common sense and interpretation of the environment at the time. There may be a speed limit of 100km/hr imposed on the open road by law, yet if there was thick fog and the roads were wet, we would be unlikely to drive at the maximum permitted speed. Why not, when the speed sign says you can? Because common sense kicks in, assesses the situation and, based on knowledge and attitude, applies modification to behaviour to make the journey safer.
In the same way, a dive computer sets limits of maximum time and depth. It should be understood that these limits are determined from mathematical algorithms programmed into the computer which are based on theoretical calculations of nitrogen absorption during the dive, and theoretical calculations of nitrogen elimination during the surface interval. They cannot calculate the actual nitrogen load of an individual and do not take into account predisposing environmental factors. In the 1997 DAN data, 93.7% of the divers experiencing DCI were diving within the limits of the computer or table that they were using. This clearly demonstrates that nitrogen buildup is only part of the picture in the risk of DCI. Fast ascent rates, square profile dives, saw-tooth dive profiles and progressively deeper dives during a day all increase the risk of DCI considerably. Also, we know that cold water, exertion in currents and dehydration are just some of the factors that increase the susceptibility to DCI.
Whilst some computers do take into account water temperature or air consumption, I am not aware of any computer on the market that takes into account the order of dive depths. Consider a dive to 30m in the morning, a dive to 20m in the afternoon, and a night dive to 10m. A computer or table would allow these dives and the day would be likely to carry a low degree of DCI risk. Now, take exactly the same dive profiles but reverse the dive sequence. Start the day with a 10m dive and end with a 30m night dive. Would you do that? I sincerely hope that no one out there would even consider such a plan! Why not? The day would have the same amount of total nitrogen absorption and so a dive computer could, theoretically, allow such dives. The key is that common sense and the knowledge and understanding of basic nitrogen bubble formation (learned in initial dive training courses) would prevail.
Whilst DCI can occur even in divers who do dive conservatively and well within table/computer limits, there are a few basic rules which help minimise the risk:
â¢ Familiarise yourself with acceptable table profiles and basic rules of safe diving practice.
â¢ Read your dive computer manual. Ensure you understand every display mode.
â¢ Stay well within your computer limits and be especially conservative in currents or cold water.
â¢ Go to the maximum depth early in the dive, and progressively and slowly work shallower. Avoid rectangular profiles.
â¢ Ascend slowly. Recommended 9m (30ft) per minute or slower.
â¢ Always do a three-minute safety stop (at approx five metres) at the end of every dive, regardless of dive depth and time. (There is substantial evidence to show a safety stop considerably reduces the risk of DCI).
â¢ Avoid deep repetitive dives.
â¢ Limit repetitive diving to no more than three dives per day. Have a ‘day off’ after three or four days if diving on consecutive days.
â¢ Always make dives progressively shallower during a 24-hour period.
What can you do if DCI is suspected?
Early recognition of even mild symptoms of DCI and providing the appropriate management is imperative to give the best possible outcome. Divers with DCI often make a complete recovery and go on to continue diving – if they receive the appropriate treatment. Administering 100% oxygen to a diver with suspected DCI is the single most important component of early management of DCI that can be given at the dive site. Oxygen should be initiated as soon as symptoms occur and continued for as long as the supply lasts (or until advised otherwise by medical personnel). The Diver’s Emergency Service should be called, and you will be put through to medical personnel who are trained in diving medicine for further instruction. If in doubt, phone â they are there to help. The DES toll free number within New Zealand is 0-9-445 5484. In Australia call toll free 1800-088 200.
Oxygen administration requires proper training. For details of the nearest DAN Oxygen Provider Course, the next DAN Oxygen Instructor Course, or DAN First Aid Instructor Course, please phone Basil Murphy, DAN NZ, Slark Unit 0-9-445 5036. In Australia contact DAN Australia on 0-3-563 1151.
The message to take home:
Dive computers are a tool and, like any tool, we must learn how to use them correctly. I believe dive computers are as safe as tables if used appropriately, and that means as a complementary adjunct to safe diving practices. Computers have the advantage over tables of giving us an d picture of our current dive status. They assist in indicating nitrogen buildup during a dive, as well as monitoring air consumption, depth and time, and helping control ascent rate. They should be complementary to a diver’s knowledge and understanding. I believe all divers should use tables to plan their dives until the basic profiles and accepted rules of diving are committed to memory. For those of you who are wondering, yes, I do use a dive computer to assist my diving. If there is just one message you take from reading this article, it is that we must take responsibility for our own dive planning and not rely completely on a piece of equipment which was never intended to replace common sense.
Lynn Taylor (PhD) is a PADI IDC Staff Instructor and a DAN O2 instructor. Her interests in the medical and safety aspects of diving have stemmed from her science and research background. Over future issues Lynn will continue to write articles on topics relevant to improving diver safety and highlight some of the important work of DAN.