Concerning DCS, why is nitrogen gas bad but not oxygen?

[Flynn Colorado]

Hello,

I've long understood the concept of decompression sickness. That is, if you ascend too quickly without properly off gassing you build up nitrogen bubbles in the tissues of the body. I accept this without question of course, but I admittedly don't know why oxygen isn't equally bad.

Can someone explain why oxygen build up in the tissues isn's similarly bad for the body? Perhaps the simple answer is that oxygen is vital for the tissues, and nitrogen, an inert gas, is not. Perhaps its prevents oxygen from reaching the tissue.

But again, I don't know necessarily why nitrogen (and not oxygen) build up from DCS is harmful.

Anyone want to take this on who is knowledgeable?

 

[caruso]

It's my understanding that since the air we breathe is about 80 percent Nitrogen so first off there's a lot more Nitrogen than Oxygen and because we cannot metabolize Nitrogen like we do Oxygen it can reach dangerous levels that don't typically happen for Oxygen unless a diver is breathing Oxygen enriched gas.

 

[Dr Simon Mitchell]

Hello,

Putting it as simply as possible, nitrogen causes problems because its nitrogen is breathed at increased pressure when breathing air at depth, and so it is carried in the blood and diffuses into tissues. The partial pressure of nitrogen in tissues increases, and the deeper you are and the longer you stay there, the higher the pressure gets. Then, during ascent, and depending on the tissue, it can take some time for nitrogen to leave the tissues. Therefore as the ambient (surrounding) pressure decreases, you can get a situation where the pressure of nitrogen in the tissues exceeds the ambient pressure. This is known as supersaturation, and it is the fundamental condition required for the nitrogen to form bubbles.

This does not happen with oxygen, because even though we breathe it at increased pressure during the dive, the delivery of oxygen in the blood effectively does not exceed the tissues' metabolic requirement for oxygen and the partial pressure of oxygen in the tissues rises very little. This means that during ascent we don't get supersaturation of oxygen and so it does not form bubbles. We would have to breathe oxygen at dangerously high partial pressures in order to have it significantly accumulate in tissues.

Simon M

 

[Flynn Colorado]

Hello,

Putting it as simply as possible, nitrogen causes problems because its nitrogen is breathed at increased pressure when breathing air at depth, and so it is carried in the blood and diffuses into tissues. The partial pressure of nitrogen in tissues increases, and the deeper you are and the longer you stay there, the higher the pressure gets. Then, during ascent, and depending on the tissue, it can take some time for nitrogen to leave the tissues. Therefore as the ambient (surrounding) pressure decreases, you can get a situation where the pressure of nitrogen in the tissues exceeds the ambient pressure. This is known as supersaturation, and it is the fundamental condition required for the nitrogen to form bubbles.

This does not happen with oxygen, because even though we breathe it at increased pressure during the dive, the delivery of oxygen in the blood effectively does not exceed the tissues' metabolic requirement for oxygen and the partial pressure of oxygen in the tissues rises very little. This means that during ascent we don't get supersaturation of oxygen and so it does not form bubbles. We would have to breathe oxygen at dangerously high partial pressures in order to have it significantly accumulate in tissues.

Simon M

Thank you Simon, your second paragraph nailed it for me. Much appreciated! That also leads in to why Nitrox has its depth limitations due to partial pressures of oxygen causing oxygen toxicity.

 

[cneal]

A high nitrogen parial pressure for to long will equal bends it decompression is not done.

A high oxygen partial pressure for to log will equal convulsions. (See nitrox class for oxygen toxicity.)

 

[TMHeimer]

Yes, it's all fairly simple physiology (if I can understand it). Do remember of course that DCS and Oxygen Toxicity are still things not totally understood, and that vary from person to person and day to day in the same person. You can go deep enough on Air to get Ox. Toxicity and get DCS staying within depth/time limits. Rare as these things happen.

 

[CT-Rich]

Nitrogen doesn’t get metabolized, it stays in your tissue as N2. Oxygen gets metabolized to CO2. Gases always diffuse from areas of high concentration to areas of low concentration. As you descend, pressure forces N2 to diffuse into your tissue. O2 does the same thing, but is converted to CO2. The CO2 diffuses ou because, despite the high pressure, there is very little CO2 in your breathing mix. When you ascend O2 and CO2 continue to move in and out of tissue because they are being metabolized N2 diffuses only because of the differences pressure, so it much slower. Go up too fast and the N2 forms bubbles and you get a DCS hit.

Ox tox is caused by a change in the way O2 reacts in your body when it is under pressure. While they are both pressure related, the ways they affect you are very different.

 

[kmarks]

CT-Rich gives a great answer - by far the best. Even without ascending, your body is naturally reducing the amount of oxygen it has taken on just by running metabolic processes.

His answer also ventures a little ways towards an explanation for why EANx uses higher concentrations of oxygen to replace the nitrogen and doesn't just replace the nitrogen with another inert gas like argon, which would have the same problems of not enough of a pressure differential to aid diffusion (not to mention increased work of breathing and heat retention). Deep tech divers use tri-mix or even heliox to balance the narcotic effect of nitrogen and the toxic effect of oxygen with helium, which is pretty inert in the body because it is so small and lightweight, so it diffuses easily, and it is also fairly insoluble in body tissue, so it off-gasses quite readily. The biggest problem with helium is that it is expensive and we're running out of it because we keep filling giant balloons with it for parades.

 

[Lake Hickory Scuba]

A lot of great answers. Maybe these video will help you understand how we metabolize certain gases, and how decompression sickness occurs.

 

[GJC]

Hello,

I've long understood the concept of decompression sickness. That is, if you ascend too quickly without properly off gassing you build up nitrogen bubbles in the tissues of the body. I accept this without question of course, but I admittedly don't know why oxygen isn't equally bad.

Can someone explain why oxygen build up in the tissues isn's similarly bad for the body? Perhaps the simple answer is that oxygen is vital for the tissues, and nitrogen, an inert gas, is not. Perhaps its prevents oxygen from reaching the tissue.

But again, I don't know necessarily why nitrogen (and not oxygen) build up from DCS is harmful.

Anyone want to take this on who is knowledgeable?

The answer is that your body consumes the oxygen really really FAST. In only 5-10 minutes, if you stopped breathing at the surface, your body will cosume all the O2 disolved in your tissues and all the O2 attached to your hemaglobin in your blood. Under pressure, the disolved O2 doesn't add a lot more time before it is all used up. Most of the O2 in your body is attached to hemaglobin and that amount of O2 does not change under pressure. Only the relatively small amount dissolved in tissues increases with pressure, and again, the body uses it up faster than bubbles can form on ascent.