Temperature and decompression sickness: Is cold a risk fact

DIVE MEDICINE

Of course, everybody appreciates that the depth-time profile of a dive is of paramount importance in determining this risk. But what divers are frequently referring to when they ask questions about risk minimisation are those factors other than depth-time profile that may be relevant. For example, a diver may ask “assuming I adhere to the no decompression limits prescribed by my dive table or computer, are there any other things I can do that will materially lower my risk”? This question has become even more relevant to the activities of technical divers performing deep decompression dives. In this setting we sometimes see DCS despite adherence to the decompression prescribed by the divers’ computers or planners, and the divers naturally want to know what they can do to try to minimise the risk of it happening again.

Over the years various risk factors for DCS have been proposed. Examples include exercise during diving, dehydration, cold water, and obesity. Although some of these are discussed with great enthusiasm and conviction in diving courses, the data supporting their status as risk factors are often surprisingly weak. Indeed, there are probably only three risk factors that are very strongly supported in the diving medicine literature. One of these, patent foramen ovale, is a physiological risk factor which I have discussed in a previous article. The other well supported risks are exercise during the bottom time, and cold. I will discuss exercise in a future article. It is cold that I will focus on here.

It has long been taught that diving in cold conditions increased the risk of DCS. This was certainly the message when I learned to dive. One plausible explanation for a detrimental effect of cold was its potential effect on blood flow if body tissues became cold during a dive. To understand this we have to briefly consider how dissolved nitrogen reaches tissues when we breathe compressed air at depth. Since we are respiring air at greater pressure, more nitrogen can be dissolved in the blood, and ultimately in the tissues as the blood carries the nitrogen to all parts of the body. The most important determinant of how fast any tissue accumulates nitrogen at depth is the tissue’s blood flow. The greater the flow, the faster the nitrogen loading.

"…The most important determinant of how fast any tissue accumulates nitrogen at depth is the tissue’s blood flow. The greater the flow, the faster the nitrogen loading…"

Importantly, the same is true for nitrogen elimination during ascent and arrival at the surface. As the pressure of inspired nitrogen in the air falls at shallower depths, the nitrogen moves from the tissues into to the blood which carries it back to the lungs for elimination. Once again, the greater the blood flow to a tissue, the faster it will eliminate nitrogen.
This gives us a potential explanation for an effect of cold on risk. If for example, a tissue is warm and well supplied by blood early in the dive when nitrogen is being absorbed it would tend to take up a lot of nitrogen. But then if the same tissue became cold during the dive and its blood flow decreased (cold tends to cause blood vessels to constrict thus reducing flow), it would eliminate nitrogen more slowly. This hypothetical scenario of warm tissue during nitrogen uptake and cold tissue during nitrogen elimination potentially creates an increased risk because there is rapid gas uptake but slower gas elimination. If you think about it, this is certainly the most likely scenario during a typical dive in cold water; we start out warm and become progressively colder over the course of the dive.

A recent study by the US Navy Experimental Diving Unit has effectively confirmed this hypothesis. They conducted a remarkable experiment involving 400 dives by humans, in which clinical DCS was the outcome measure. [1] They performed dives mostly to 36m with a standard approach to decompression. To increase the risk of DCS the divers performed moderate work throughout their bottom time. The researchers were able to arrange what were effectively two separate pools of water adjacent to each other in their depth simulation facility. In one pool they had warm water and the other contained cooler water. The divers wore no thermal protection so that their exposure to the ambient water temperature was standardised and repeatable.
This arrangement allowed them to vary the temperature conditions of the dive, either running the whole dive at one temperature, or getting the divers to change pools at the start of the decompression and thereby changing the temperature conditions between the bottom and decompression phases. It was a complex study, but for simplicity, it was the latter experiments which are of greatest interest to us. Basically, they ran a series of dives to 36m for 30 minutes in which the conditions were either warm on the bottom and cool during the decompression (warm-cold series), or the opposite (cool on the bottom and warm during decompression (cold-warm series)). In the warm-cold series they had seven cases of DCS in 32 dives (22%), and in the cold-warm series they had zero cases of DCS in 80 dives (0%).

"…Getting cold after being warm during the bottom phase of a dive is a bad thing. There is
no doubt about it…"

These numbers speak clearly for themselves. Although the experimental conditions are difficult to interpret in terms of real world diving, there is a clear signal that what we have always suspected is true: getting cold after being warm during the bottom phase of a dive is a bad thing. There is no doubt about it. However, there is equally no doubt that this disadvantageous state of affairs is more typical of what happens to divers in the real world than starting the dive cold and ending it warm.

So what is the practical application of this finding? Well, no one is suggesting that one intentionally starts a dive cold and tries to warm up during decompression. That would be a hard thing to achieve for a start. I suppose the most plausible practical message is fairly simple one: try to minimise the process of getting cold during a dive. This means having good thermal protection. If you are doing a lot of diving, particularly deeper longer dives in temperate water, then it is worth considering using a dry suit. If you are already a dry suit user, and you use actively heated undergarments, then it would be reasonable to consider not turning the heating on until the decompression phase when you actually want to be warm. Turning your heating garment on at the bottom may promote inert gas uptake.