A new trend in diving practice

A New Trend in Diving Practice

Interview with Professor Des Gormon, Head of Occupational Medicine, University of Auckland

by Dave Moran

I understand that in recent times at the Navy chamber divers have been presenting in quite serious conditions.

I think the best way to describe it is that it is a return to the sort of disease we were seeing in the late ’70s and early ’80s. There is no doubt that the type of DCI we are seeing has reverted from the late ’80s and early ’90s, when there was almost a disappearance of the critically ill diver. It was rare. You had maybe one or two a year in Auckland at the most, and then they weren’t particularly ill.

Now there’s a range of presentations of serious disease. Take pulmonary decompression illness for one. That’s the one that usually kills divers. I’m also talking about strokes and significant spinal cord lesions where people are left with the inability to control their bladder, bowels, can’t have erections, can’t walk, lose power down one side, can’t speak. I’m not talking about numbness and tingling or a bit of weakness here and there. I’m talking about quite significant disease.

Are these cases due to deep diving?

No, it’s due to repetitive diving. We currently have a very large research project in South Australia with tuna farm divers. We’ve been monitoring their diving very closely in terms of printing out their real depth time and getting exact printouts of the diving that they’re doing. We’ve been giving them daily health questionnaires so we know exactly what they do and what their health outcomes are. We’ve used this information to produce a model of decompression illness which is the best I’ve seen in the world. The person that I’m doing this research with is David Doolette, who has a PhD in neurophysiology. In my opinion he’s the world’s leading decompression schedule modeller. I think he’s five years ahead of anybody in the States, and I’m putting together a programme now with the Australian government to keep him in Australia because universities in the States are trying to poach him.

The outcome of this work is that the tuna farmers can perform very long, very deep dives (max depth 20-30m) and still have a risk of disease which doesn’t exceed 0.5%. But the extraordinary thing is that the minute they do a second dive, the risk suddenly jumps to 1.1 to 1.3%. Do a third dive and it jumps again.

The work has been illuminating in that we all knew that repetitive diving was the single most important risk factor for decompression illness, but we didn’t appreciate just how dramatic an influence it was. Really, the evidence and the science goes back to the 1960s. In about 1960 Hemplemann looked at the risk of decompression illness at 60 feet (18m) for 60 minutes and he found that it was just under 1%. He then did 60 feet for 30 minutes and a 90-minute surface interval, then 60 feet for 30 minutes. The most conservative treatment of that dive would be to assume that the second dive was equivalent to 60 feet for 60 minutes. And yet the risk of disease is about 1.4%. The risk goes up by a third, yet as divers the most conservative treatment schedule that I am aware of is to treat the two dives as one.

The conventional approach is to assume you’re off-gassing during the surface interval. The reality is that bubbles form in the majority of decompressions. Once the bubbles form, the elimination of inert gas from the body slows profoundly. So the concept of off-gassing during surface intervals is flawed. People assume that the gas comes out very rapidly – exponentially. It doesn’t, it comes out slowly (linear), which means that all the models predicting off-gassing wildly overestimate the amount of gas coming out.

Repetitive diving was the major factor in DCI in the late ’70s to mid ’80s. But the extent of the practice decreased. We had probably a decade of very mild disease. The numbers were probably about the same, but the disease was mild, people with flu-like illness, aches and pains, and maybe some numbness and tingling or a bit of heaviness in a limb. But these were people who generally did very well. Afterwards a lot of them became depressed, and had trouble at work, but in fact it’s difficult to distinguish there between the effects of decompression illness and the effects of any illness. Once you have any form of illness you tend to have a period of depression, particularly if you are hospitalised.

During that ten-year period, were there any indications of why that had happened?

The beginning of that period was the time when we had seen a dramatic improvement in diver education. The only factor that I could ever identify, which is a sensible explanation of ten years of reasonably mild decompression illness, was a general pattern of diving where most people were trained, and most people were diving reasonably conservatively. It didn’t matter what tables they were using, the diving pattern was conservative. In other words people were multi-level diving, but assuming square profiles with their decompression tables. That is where you assume you spent the entire dive at the maximum depth.

There’s no doubt that in ten years we saw very few, if any, serious illness. It’s not a fluke, because ten years is too long a period for a fluke. There’s also no doubt that the pattern began to change again two or three years ago. It was only a suspicion then, but over the last 12 months it’s gone well beyond suspicion. We are now seeing disease profiles which, in my opinion, are exactly the same as I was seeing in 1980, in that every second, third or fourth diver was turning up with dramatic disease, i.e. not being able to walk, stand, use a limb, have no control of limbs, losing control of bladder or bowel functions etc. This is what we’re seeing again now.

What have they had in common? Some of them had quite innocent exposures and have just got very sick. There have been some who have been lunatics, quite frankly, and got sick. The majority have been doing extensive repetitive diving, which would not have been possible under the traditional approach to decompression practice, but is unquestionably possible with the use of computers. There is no doubt that there is a range of diving practice which is possible with a computer and impossible with tables.

So these patients have in common multiple repetitive diving practices. Some of them were doing four or five dives a day, whereas probably the old tables would have said stop diving after the second dive, and always after the third. We’ve had people who were diving to more than 30 metres four times a day.

The pattern of disease has changed and we’ve been looking for an explanation. The only one we can find is that the pattern of diving has changed. Generally we’ve explained the huge increase in decompression illness over the last 20 years on the basis that there are simply more people in the water. The incidence probably hasn’t changed; that is, the number of cases per 1000 hours of exposure. All that’s happened is the denominator has gone up so the numerator goes up. You could argue that in the last 18 months we’ve seen more critically ill divers because the numbers have gone up again, but I don’t think that’s true. I know that in the last two or three years there’s been virtually no change at all in the number of active divers.

If what we were seeing was purely a matter of more divers in the water, what we should have seen was a huge increase in the number of total cases, but the proportion of seriously ill shouldn’t have changed. What we’ve seen is exactly the same number of cases, but the proportion of seriously ill has dramatically increased. I don’t believe it’s remotely explainable on the basis of more people in the water, without changing the risk of more people getting injured. Several times a year now we’re getting people leaving the Navy Hospital and going into a spinal unit. That’s what we used to see in the ’80s.

I’ve had experienced divers present with serious decompression illness. When I’ve discussed their dive profile on their computer, they’ve been fully aware that that’s a dive profile they would never have attempted in the past, but they’ve been happy to do so now because the computer has been in the green zone. In other words, you have an experienced diver, who has learned what is reasonable practice, engaging in utterly unreasonable practice because a machine tells him it’s OK.

The other concern that I have is that we have a whole population of young divers growing up who have no feel for decompression tables. They’ll go to Vanuatu and do four dives at 40 metres in one day, whereas an experienced diver wouldn’t consider doing it.

Because the majority of divers these days are using computers, what safeguards should be taken?

I know PADI had a programme for computer assisted diving, and I think that’s what they’ve got to go back to. People need to use the tables to plan their dive for maximum depth and time, and then use the computer as a secondary device. It’s something that can monitor air supply and their ascent rate. It acts as a backstop. If they’re going to dive to 30 metres, at the end of the day they should decide how long that dive should last, not the computer. In other words, they should establish a profile. The answer is to simply go back to using conservative decompression table practice to impose boundaries on the diving for the day, and then within that you use the computer as a secondary device.

There’s no doubt that using a computer, because of the real depth time format, is taking diving practice to an extraordinarily liberal extreme. If you increase the time at a particular depth the risk of decompression illness climbs profoundly. It increases slowly at first, and then for a very short increase in time it increases dramatically.

Are the manufacturers of dive computers aware of what’s happening out there?

Well, they certainly ensure that the documentation they produce for their computers gives them legal protection from being sued. The pages and pages of theory that come with computers are largely nonsense. The computer designers are trying to satisfy the marketplace, which is trying to maximise dive time, so their fear is that if they produce a computer which offers very conservative practice, it may not sell purely because it’s so restrictive. This is where the advertising hype, in my opinion, has been unethical. If I say to you, ‘I want you to buy this product because it will increase your dive time,’ I must concede in the same breath that it will increase your risk, because you can’t have one without the other.

Most young divers or new divers buy a computer because the general opinion out there is that computers are conservative; also because most divers don’t do a square profile when they’re diving, and the computer averages everything out and therefore ‘the computer is safer than a square profile’, which is flawed reasoning when you think about it. If you assume your whole dive is a square profile dive, the total time of the dive must be more conservative than taking an average depth for the dive to establish your maximum time limit. It demonstrates to me that there’s a whole generation of divers who know absolutely nothing about decompression.

From what I can see, people do their course and are taught the tables, but after the course they’re sold a computer. So they never really get to practice planning their dives using tables.

I agree entirely. That familiarity I was referring to before regarding what’s on and what’s not on comes from years of using the tables. You build up a repository, almost an innate knowledge.

Is this a worldwide phenomenon?

It’s hard to tell, because very few units see remotely the number of patients that the Naval Hospital sees in Auckland. Probably the only units in the world that are as busy as this one are the units in Townsville, Australia; Hawaii; and Catalina Island. The vast majority of units do not see a lot of divers. They may see a dozen a year, so it takes a long time before trends become known. The number of divers per capita in New Zealand is much higher than anywhere else in the world, and New Zealand essentially has diving 12 months of the year. Also, New Zealanders tend to be adventurous in the water. It’s easy for us to recognise patterns because of the sheer volume of patients we see per week across the year. Trends become obvious pretty quickly, whereas a unit that treats one patient a month can take five years to recognise what we would see in six months.

I would have thought that places like Melbourne would see more divers with decompression illness.

They would have less than half the number of treatments that we have in Auckland. There are less divers in Melbourne than there are in Auckland, and also the diving sites are far less attractive around that city than they are around Auckland. Aucklanders have access to the Coromandel and its offshore islands, through to the Northland coast with the Mokohinaus, Poor Knights, and Cavalli Islands. So there are far more accessible world-class diving spots.

So what is the message for divers?

The strong message that divers should take on board is the familiar phrase of ‘plan your dive and dive your plan,’ with the added provision of ‘plan using your dive tables and use your computer as a supplementary reference.’

Thanks for your time, Des. I am sure I speak for the whole dive industry and the Naval Decompression Unit in that we all want to see empty beds at the Naval Hospital.

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