DECOMPRESSION SICKNESS

– IS THERE SUCH A THING?

and pressure, there are a multitude of factors that can potentially impact divers’ decompression, as the author explains. Once divers reject the escapism that accompanies the “undeserved” label, they can get on with the important business of diving and giving adequate consideration to their deco planning.

Dgenerally speaks more from an emotional perspective than a rational one. The driving factors are typically faith in imperfect tools and a desire (conscious or unconscious) to shift responsibility.

Decompression algorithms rely almost exclusively on pressure and time data to predict effects. Enticing pictures can be painted on the authority of any algorithm, but the reality is that all rely on limited input to interpret complex situations for people who are not uniform. Modern decompression models are important constructs that can help us to dive safely, but the products are rudimentary from a physiological perspective, without sufficient sophistication to deserve unquestioned trust.

The dive profile is almost certainly the most important determinant of gas uptake and elim- the variables that can influence outcomes (Pollock 2016). Instead, algorithms rely on simple measures and mathematical bracketing with the hope of covering the contributing factors. The problem is not in doing this; the problem is in being surprised when the outcome is not what was expected.

Decompression safety is influenced by a multitude of factors, variably related to the dive and the diver.

Decompression factors

Accepting that the dive profile is the most important determinant of decompression risk, there are additional factors that can also have dramatic effects. Exercise is one of these. Predive exercise may have complicated effects on the subsequent diving exposure. Exercise during the descent and bottom phase will increase inert gas uptake and the resulting decompression stress. Mild exercise during the ascent and stop phase can promote inert gas elimination and decrease the resulting decompression stress, but excessive exercise can promote bubble formation and increase decompression stress. Post-dive exercise is likely to increase decompression stress in all cases. Practically, while the concepts are clear, the definition of meaningful thresholds for “mild” and “excessive” exercise is difficult at best, and quantifying real-time effects far exceeds current capabilities.

Thermal state is another potentially dramatic factor (Gerth et al. 2007). Being warm during the descent and bottom phase can substantially increase blood flow and delivery of inert gas to the periphery and increase the subsequent decompression stress. Being cool during the descent and bottom phase can decrease inert gas uptake and decrease the subsequent decompression stress. Being cool during the ascent and stop phase will inhibit inert gas elimination and increase the subsequent decompression stress. Being moderately warm during the ascent and stop phase can promote blood circulation to the periphery and increase inert gas elimination, but excessive heating of peripheral tissues in this same phase can promote bubble formation as heating decreases the solubility of inert gas, effectively increasing the decompression stress.

Moderate exercise during stops aids inert gas elimination and reduces decompression stress, while excessive exercise promotes bubble formation and increases stress.

Again, as with exercise, it is extremely difficult to identify meaningful thresholds for thermal state at different points in a dive, and quantifying real-time effects is not within current capabilities. It is certainly clear that the ambient temperature measured by a dive computer can have little correlation to the thermal status of the diver, and any thought that this information informs decompression models in a meaningful way is misplaced.

Scapegoats

The wild card of individual (“predisposition”) factors further highlights the challenges unmet in current decompression models. Not only are these parameters not measured, but it is also unclear how the information could practically guide the risk assessment at this time if available. While the importance of these factors is hard to assess, it is also noteworthy that some, most often dehydration, may be used as scapegoats to explain away decompression sickness (DCS).

A state of dehydration can adversely affect circulation, potentially impeding inert gas elimination, but this almost certainly has much less impact than the dive profile, exercise, or thermal state in many cases. The impact is also not