No-decompression limits: fantasy or reality?

By Dr Simon Mitchell

One of the first diving medicine stories that Des Gorman ever told me concerned an Australian diver who tolerated four days of mild paralysis down one side of his body before seeking help, because ‘I knew I wasn’t bent because I was within the limits of the tables.’ In response to the incredulous observation ‘But you’re paralysed down one side of your body!’ he replied ‘Yeah, I was a bit worried about that.’

Now, the fact that he was Australian has nothing to do with it. Some of my best friends are Aussies (Hi Jenny J, Wighty, Mike, Ben and Dave), and at least some of them are intelligent. No, there are plenty of Kiwis who operate under the same misapprehension: that if you stick to your dive table no-decompression limits, you cannot suffer decompression illness (DCI). 50% of the patients we treat at the unit for DCI will tell you that this is wrong.

Dive tables: you know, those things we all learn to use during dive courses. They tell you how long you can spend at a particular depth and make a normal ascent directly to the surface without decompression stops. These safe dive duration estimates are called no-decompression limits, and the tables allow you to calculate them for single dives, repetitive dives and multilevel dives. Of course, increasing numbers of divers are using dive computers instead of tables. These devices calculate no-decompression limits automatically, and constantly adjust them with each change in depth.

Let me state right here and now that I’m not about to launch into a vitriolic tirade about inadequacies in dive tables or computers. On the contrary, one or the other is mandatory for all divers, and I repeat my often-stated belief that all divers should have downloadable dive computers in this modern age. No, it is the way we interpret the data provided by our table or computer that is sometimes inappropriate. Tables are technical looking, colourful and cleverly formatted, and therefore very believable. The computer user is even less likely to question the data his or her computer generates because, well hell, it’s a computer!! But blind obedience to anything is rarely a good thing, and the concept of ‘safe’ no-decompression limits provided by either tables or computers deserves scrutiny. Specifically, it is critical that the diver understands that compliance with no-decompression limits does not eliminate the risk of DCI.

Those who design tables and computers face a difficult task. We know that the risk of DCI after ascent from any depth increases with the time spent there. Importantly, there is no critical time at which the risk of DCI suddenly ascends from very low to very high. This means the designer is forced to draw a line in the sand somewhere along the continuum of time (and risk), where the risk of decompression illness is ‘acceptable.’ That time then becomes your no-decompression limit.

Designers have different ways of determining no-decompression limits, often based on mathematical modelling. This has generated a plethora of mumbo jumbo technical terms, such as multiple tissue models, controlling tissues, ‘m’ values, outgassing half lives, and the list goes on and on. Claims of superiority for one table over another on the basis of more fancy maths are basically bollocks. What really counts is whether the table (or computer) works. In other words, do the limits provided result in an actual incidence of DCI that is acceptable? I will return to how we answer this question a little later.

But understand this if you understand nothing else: a dive to the no-decompression limit provided by a dive table or computer does carry a risk of DCI. Even a dive within that limit will carry a level of risk. If Dr Gorman’s Australian patient had understood this concept, he might not have sat around in a hemiparetic state watching TV for four days before seeking help. Some no-decompression limit dive profiles have been tested enough times in standardised conditions so that the risk associated with them is statistically established. These profiles help illustrate some of the above points. Take a dive to 18 metres (60 feet). The US Navy table specifies a no-decompression limit of 60 minutes. We know from repeated testing that if 100 divers completed this profile (without safety stops), you could expect 1.4 cases of DCI. In other words, the risk of this profile is 1.4%. The PADI RDP and the Doppler modified SSI table specify a no-decompression limit of 50 minutes. Testing has shown the risk of this profile to be 1.1%; less than 1.4%, but not perhaps by as much as you might expect. Of course, we all know that you should not dive ‘right up to’ a no-decompression limit, so let’s say we dive a conservative 18 metres for 40 minutes profile. The risk of this dive is still 0.9%. So we can still expect one in 100 divers to get sick diving this profile, which is well within the no-decompression limit. I think the point is clearly made.

Of course, I am not trying to suggest that tables and computers are no good because DCI can still arise when diving within their limits. I am merely trying to debunk the misconception of absolute safety if dives are within limits, and by association, the misconception that everyone who suffers DCI is a fool and has done something wrong. Male divers in particular somehow consider the diagnosis of DCI to be an emasculating event. I haven’t had anyone come into the unit with a blanket over their head yet, but many look as though they are being led into court to face a murder charge, such is the shame of it all. It is important to understand that DCI victims are often just unlucky, and not to fly into an investigative frenzy looking for the cock-up they made.

Education is the key here. I remember learning to dive in the less-PC early ’70s. We were inculcated into the creed of blame: if you got DCI then you were a bloody idiot because you had obviously gone outside the tables. If you stuck to the tables then you were safe. I think instructors these days are more enlightened. I acknowledge the tact required in introducing students to the concept that they cannot necessarily rely on the no-decompression limits provided by their tables. But it has to be done.

Earlier, I raised the issue of how we establish that the no-decompression limits provided by a dive table or computer result in an actual incidence of DCI that is acceptable. The risk data I have cited for 18 metre dive profiles comes from multiple field testing of the actual profiles in carefully controlled dives. Many repetitions of each profile are required to estimate risk with any degree of accuracy. That is why almost none of the commonly used tables or computers (the latter with their infinite number of multi-level profiles) have ever been tested to this exacting standard. This is not necessarily a criticism, since the difficulty of such an undertaking cannot be overstated. However, the important message is that not only is there a risk of DCI associated with computer and table no-decompression limits, but also, in most cases, we don’t know what that risk is. I reiterate that this is not a reason to lose faith in tables or computers. Just don’t be surprised when you hear of divers getting DCI while working within the limits provided by such devices. My thoughts on the best way to avoid DCI: work within your table or computer no-decompression limits as they are probably a good indicator of low risk; slow controlled ascents; and safety stops. Ponder on this: the US Navy no-decompression limit for a 30 metre dive is 25 minutes. The risk of DCI in diving 30 metres for 25 minutes is 1.8%. If you go to 30 metres for 30 minutes, the USN table tells you to complete a decompression stop at three metres for three minutes. The risk of DCI for this dive is around 1%!! (Almost half the risk of the shorter dive without the stop.) This was a ‘compulsory’ decompression stop, but that is irrelevant. Safety stops really work.

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