Loss of conciousness:

By Rik Rösken

Introduction

Hypoxia, a condition with lowered oxygen levels, is a situation often met by the average freediver. It is thus not a coincidence that on a regular basis questions arise on this subject.

One of the concerns, partially discussed in Loss of consciousness: oxygen as rescue gas was the role of reactive oxygen species (ROS) and reperfusion. In this article we will explain this matter simplified. If you want have the whole detailed picture, you are encouraged to consult the cited articles below.

Energy Metabolism

The main pathway how body produces its energy is by the metabolism of glucose and oxygen towards water and carbon dioxide. This metabolism takes places in specialized parts of our cells, the mitochondria, and is vital for the cells survival.

Under normal circumstances, the amount of oxygen available from the blood is more than the cells need to function. Especially in the brains, often a topic of concern, the normal amount of oxygen is more than once sufficiently. The whole string of effects that is required to turn glucose and oxygen into energy is working easy.

Hypoxia, Ischemia and Acidity

While breath-hold diving, we expose our body to lower levels of oxygen. This state is also called hypoxia. Prolonged hypoxia activates various protective mechanisms to protect our vital functions. Some of those are activated by the diving reflex as experienced in freediving. Others are caused by hypothermia and still others are purely activated by the low amount of oxygen present in our circulation.

It is important to know the difference between hypoxia and ischemia. Hypoxia suggests that a small trickle of oxygen is still present. Ischemia suggests that there is no oxygen at all, mostly because there isn't any circulation in the effected area. There is not only a acute lack of oxygen, but also an acute build-up of waste productions as carbon dioxide and water.

When the lack of oxygen continues, the string of effects where the metabolism exists of becomes affected. This causes the build-up of wast products as lactic acid and other compounds who are produced midway the metabolism. This product affects the acidity and if not dispersed, damage as with ischemia.

When the oxygen levels are restored, one of the first effects is the diminishing levels of lactic acid. The role of the acidity of tissues, including that of the brain, seems to be two fold. Sever acidity, especially in the brains, can lead to profound cell damage, but mild acidity seems to have a protective role. How this is related to breath-hold diving is yet to be researched.

Reactive oxygen species

Reactive oxygen species (ROS) are the predecessors of the more known free radicals; aggressive molecules that bind and damage molecules they come in contact with. ROS are products of inflammatory cells in damaged tissue, blood outside the blood vessels and the disrupted string of metabolism. Together with oxygen it can form the feared free radicals.

It is important to understand that ROS plays a more important role in ischemia compared with hypoxia and that the predecessors of free radicals form during the hypoxic period. Further more ROS and free radicals seem to play a important role in the decreased sensitivity towards hypoxia after intermitted hypoxic training.

Especially for freedivers, ROS seems to be a two edged sword. Too much will cause damage, but low levels can cause adaption to the hypoxic environment.

Conclusion

While there is no doubt that high levels of ROS and Free radicals can cause significant damage, it is yet to be seen what the function of ROS and Free radicals are in the use of oxygen as a rescue gas and in freediving in general. Hypoxia, as experienced in freediving, is a different entity as ischemia, on which most research is focused.

Especially with the current emerging effects of reactive oxygen as a body messenger, further research might be needed to determine to which extend it affects freediving training and freediver safety.

Used Sources

Zauner, Alois M.D.; Daugherty, Wilson P. M.S.; Bullock, M. Ross M.D., Ph.D.; Warner, David S. M.D. Brain Oxygenation and Energy Metabolism: Part I-Biological Function and Pathophysiology. Neurosurgery. 51(2):289-302, August 2002.

Vollaard NB, Shearman JP, Cooper CE; Exercise-induced oxidative stress : myths, realities and physiological relevance. Sports Med. 2005;35(12):1045-62.