Nekton

Exercise Fizziology and Diving

I have borrowed the title of this article (the "fizziology" part of it at least) from my colleague Dr David Doolette in Adelaide. We have both been closely following a series of papers coming out of Alf Brubakk's lab in Norway. These papers describe a series of studies that have the potential to revolutionise diving safety, and it all revolves around the relationship between physical exercise and diving. Potentially, (notice I use that word again) this is by far the most important new discovery in the field of diving medicine for years.

A recap on decompression illness
Before we go any further, let's have a quick review of the most basic mechanism by which decompression illness may arise. Remember that when we dive we are breathing air at the same pressure as the water that surrounds us. The deeper we go, the higher that pressure is. Since air is composed mainly of nitrogen and oxygen, this means we are breathing these gases at a higher pressure. It is the nitrogen we are concerned with. At higher pressures it is more soluble (that is, the nitrogen molecules will more readily move from the gas of the lungs into the liquid of the blood) and we therefore absorb more of it from the lungs into the blood. Similarly, more of these nitrogen molecules can leave the blood and dissolve into the liquid of the various body tissues as the blood passes through them. Over the course of our bottom time we accumulate nitrogen in the tissues. It makes sense that the deeper we go and the longer we stay there, the more time the nitrogen has to build up in these tissues.

When we begin to ascend, these processes are reversed. As the pressure around us falls, the solubility of nitrogen falls and those molecules of nitrogen we absorbed during the dive "want" to form a gas again. What we want to happen is for the blood to pick up these molecules from the tissues, and carry them back to the lungs where they can diffuse out of the blood and straight into the gaseous form. Unfortunately, nitrogen is not able to escape from all tissues as quickly as we would like, and there will often come a point in our ascent where the "pressure" of dissolved nitrogen in some tissues will exceed the surrounding water pressure. This situation is termed "supersaturation" and it tends to provoke the nitrogen molecules into becoming a gas (by forming a bubble) then and there. These bubbles can form in a variety of body tissues (some are more vulnerable than others) and they also form in the blood that is carrying nitrogen away from the tissues back toward to the lungs. That is why we can frequently detect bubbles in the veins using Doppler ultrasound following diving.

When we begin to ascend, these processes are reversed. As the pressure around us falls, the solubility of nitrogen falls and those molecules of nitrogen we absorbed during the dive "want" to form a gas again. What we want to happen is for the blood to pick up these molecules from the tissues, and carry them back to the lungs where they can diffuse out of the blood and straight into the gaseous form. Unfortunately, nitrogen is not able to escape from all tissues as quickly as we would like, and there will often come a point in our ascent where the "pressure" of dissolved nitrogen in some tissues will exceed the surrounding water pressure. This situation is termed "supersaturation" and it tends to provoke the nitrogen molecules into becoming a gas (by forming a bubble) then and there. These bubbles can form in a variety of body tissues

(some are more vulnerable than others) and they also form in the blood that is carrying nitrogen away from the tissues back toward to the lungs. That is why we can frequently detect bubbles in the veins using Doppler ultrasound following diving.

If there are sufficient bubbles of sufficient size, then we may develop symptoms of decompression illness. Interestingly, bubbles form during and after ascent from most "normal" dives, which obviously includes a huge number of dives that don't result in decompression illness. All the attention we give to following the rules imposed by our dive tables and computers is an exercise in limiting bubble formation to a safe level rather than preventing it entirely. This is relevant because if one realises this, it is not hard to imagine how problems might be triggered by even relatively subtle changes in variables (other than time, depth and ascent rate) that have a bearing on bubble formation or behaviour.

Exercise and diving
One such variable is exercise. Up until this point in the history of diving medicine we have established that heavy exercise during or after a dive is bad. Most divers have heard this. Indeed, we used to be taught extra safety factors that one incorporated into dive table calculations if one was expecting a hard working dive. More recently, we have seen "air-integrated" dive computers that monitor air consumption during the dive and if it is high, they assume you are working hard and adjust the decompression algorithm for that dive to be more conservative. Clever stuff. Our view on exercise during diving has not changed; it is still well established as a risk factor for decompression illness. This is because exercise speeds up the whole process of nitrogen accumulation in many tissues. During exercise, the blood is circulating faster and carrying dissolved nitrogen to many tissues faster. Typically, after a hard working dive the ascent is much more relaxed and blood flow falls. So at the very time we want plenty of blood flow to carry nitrogen from the tissues to the lungs, there is actually less flow than there was when the tissues were loading with nitrogen during the bottom time. I assume you get the picture. It's like filling a bath with a big hose, then trying to empty it in the same time with a small one. This is not a good situation, and it is why training agencies advice divers to stay relaxed and avoid hard work underwater.

I'm sure some of you are thinking that this problem could be solved by continuing to exercise hard during the ascent, so that high blood flows to the tissue are maintained. There is some superficial logic to this idea, and it is not totally invalid as you will see. However, it is proven that heavy exercise during decompression can actually provoke bubble formation. This probably occurs by a number of mechanisms, some of which we don't fully understand. The generation of momentary areas of negative pressure in moving tissues is one possibility. This principle applies to heavy exercise soon after ascent; so surfacing for a game of tennis is not the best of plans.

Despite this concern about heavy exercise decompression modellers and researchers would agree that there is probably some advantage in maintaining gentle exercise during decompression, particularly if you are performing a dive involving decompression stops. So, for example, instead of just hanging on the decompression line, you might maintain a gentle finning motion with your legs, not with the object of moving yourself anywhere, but rather just to keep your legs moving. There is some evidence that this is beneficial.

So, to summarise our thoughts on exercise so far:

Heavy exercise during bottom time: = bad
Gentle exercise during decompression: = good
Heavy exercise after diving: = bad

Exercise before diving
So what about exercise before diving? Beyond a vague impression that it was a good thing for diving safety if you were physically fit, there has never been a more specific edict on exercise before diving. That may soon change.

Dr Alf Brubakk, a colleague from Norway, has been researching the pathophysiology of decompression illness using various animal models for some years. The description of his work that I am about to provide is a gross simplification of a complex program, but you will get a general idea about the sequence of events.

About a year or so ago, using a model of severe decompression illness in rats, he made the observation that rats subjected to a pre dive exercise training program (over a substantial period) seemed to be considerably less susceptible. This appeared to support the generally held notion that being fit and trained was somehow an advantage.

However, then came the much more intriguing finding that rats performing only one bout of heavy exercise prior to the diving exposure had the same "protection" as those that had completed the longer training program. Even more fascinating, was the discovery that the timing of that single bout of exercise was critical. Only those rats that exercised 20 hours prior to the dive benefited. There was no benefit for exercise 48 hours prior, or intervals shorter than that. Indeed, as Alf reported at the Undersea and Hyperbaric Medical Society Annual General Meeting at Sydney in May, exercise just a couple of hours prior to diving actually makes outcomes worse.

While all of the preliminary work was performed using animal models, there is now some evidence that these findings may translate into human diving. Just recently a European group collaborating with Alf published a study in The Journal of Physiology in which 12 divers performed dry chamber dives to 18m for 80 minutes with a 9m/min ascent rate and a decompression stop at 3m for 7 minutes. Each diver performed this dive twice; once with no exercise prior, and once with a single 40 minute bout of exercise 24 hours before the dives. The divers were monitored for venous nitrogen bubbles using Doppler ultrasound following all dives. As is common, bubbles were detected after most dives, but significantly fewer bubbles were detected following the dives that were preceded by exercise.

Why would a single bout of exercise at a relatively precise interval prior to a dive protect the diver from decompression illness? Initially, it looked like the exercise might be causing cells in the blood vessel walls to produce a substance called nitric oxide that in normal physiology signals the blood vessel wall muscle to relax. It seemed that the nitric oxide was conditioning the vessels in some way to reduce bubble formation. However, Alf has since shown that properly timed exercise is still protective even when the enzyme that produces nitric oxide is inhibited by a drug. This seriously questions any link between exercise protection from decompression illness and nitric oxide. There are a number of other theories, but none of them are published yet and their owners might get grumpy if I chose to do so here, so we will just have to wait for experimentation and publication to catch up with these bright ideas.

What we can say at this stage is that this is the most exciting material to appear in diving medicine for quite some time. The protection afforded the rats in a very severe model of decompression illness was not subtle. If a similar effect can be demonstrated in humans, then the risk of decompression illness could be substantially reduced. The exercise strategy would be particularly applicable to decompression dives and technical diving operations. Stay tuned and I will update you with any new advances in this exciting area.