Understanding Decompression Sickness

[kaylee_ann]

Great reading! Look at the fast vs slow tissues thing - I don't remember which chapter honestly... Anyway, you'll find the explanation why you may on-gas even during the ascent ("slow" tissues do that). If you don't get it, feel free to send a pm (or better open a new thread and tag @Duke Dive Medicine ).

PS I don't log in that often, so don't be surprised if my answer takes time.

Dive safe!

Thanks! If anything I’ll shoot ya a PM later asking if my understanding of the concept is right.
Will do- you too!

 

[RXTdiver]

When you are at sea level and ypu dodn't dive yet, are your body tissues absorbing nitrogen just from normal breathing?

 

[boulderjohn]

When you are at sea level and ypu dodn't dive yet, are your body tissues absorbing nitrogen just from normal breathing?

The act of breathing followed by the perfusion of blood through the tissues means you are both absorbing and releasing nitrogen at all times. When you have been at sea level long enough, your tissues are at equilibrium, meaning you are absorbing and releasing at about the same rate. (It's just random.)

Go to 99 FSW, and you will be inhaling 4 times as many molecules as you were at the surface. You continue to absorb and release nitrogen, only now you are absorbing 4 times as many molecules as you release.

 

[RXTdiver]

Try thinking about it this way:
- Your tissues want to be saturated (in equilibrium) to the ambient pressure
- On-gassing happens when the ambient pressure is higher than the gases in your tissues
- Off-gassing happens when the ambient pressure is lower than the gases in your tissues
- This process takes time, the speed depends on the type of tissues
- By changing the ambient pressure (i.e. changing depth), you control how these processes happen

At 20m / 60 feet (3 ATA), the ambient pressure is tripled from sea level. You actually have to stay at that depth for a long time before all your tissues are saturated (some are slower than others). So if you do a quick bounce dive to 20m, and then ascend to 10m, your tissues will not have time to get saturated to 20m or even 10m. So that means when you arrive at 10m your tissues will still be on-gassing. As you ascend, you will hit a depth where some tissues are saturated (no longer on-gassing) and others are still on-gassing, and if you continue to ascend you will hit a depth where some tissues are supersaturated (higher pressure of gases in your tissues than ambient pressure) which means they will start off-gassing. So you can have different tissues in any combination of these 3 different states: on-gassing, saturated, off-gassing.

Also:
Since off-gassing the slower tissues takes time, it's not realistic to stay underwater long enough and have a slow enough ascent that your tissues will be fully saturated to sea level when you surface. Some of your tissues will stay supersaturated (off-gassing) for several hours after an NDL dive. The amount of supersaturation is what the NDL tables/computers control, so that you can ascend directly to the surface at any point in the dive without unecessary risk of DCS. With deco diving it's the same thing, you do decompression stops to make sure your tissues never exceed a certain predetermined level of supersaturation on the ascent. But some tissues will still off-gas for hours after a dive. Which is one of the reasons for surface intervals, time before flying, exercising etc.

Awesome explanation. Thank you.

What about this? Let's say you go down to some depth. At some depth your body absorbs enough nitrogen to the point where the pressure in the tissues equals ambient pressure. OK. Then you descend deeper and your body absorbs more nitrogen until it equals ambient pressure. If you keep doing this, wouldn't you reach a point where your tissues just can't absorb anymore nitrogen?

If so, if you reach such a point, and you keep breathing, where does the nitrogen go? It has to go somewhere right?

 

[boulderjohn]

Your tissues are not like a bucket. They don't fill up. There is no limit to what they can absorb. Nitrogen molecules are pretty small.

 

[harvalen]

At some point the pressure will be high enough that the nitrogen will be liquid and then solid. At that point we can probably consider the bucket full.

Of course the the same thing would happen with the gas you breath, so you'll have problem finding a breathable gas long before you hit this pressure.

 

[boulderjohn]

Your tissues are not like a bucket. They don't fill up. There is no limit to what they can absorb. Nitrogen molecules are pretty small.

Another point is that if you are going that deep, you should be breathing different gas mixtures which greatly decrease your nitrogen intake. See Equivalent Air Depth.

 

[Eric Sedletzky]

At some point the pressure will be high enough that the nitrogen will be liquid and then solid. At that point we can probably consider the bucket full.

Of course the the same thing would happen with the gas you breath, so you'll have problem finding a breathable gas long before you hit this pressure.

I think you’d be narced out of your skull and possibly dead way before the nitrogen turns to liquid and then goes solid.

 

[John C. Ratliff]

The way I understand it is in terms of partial pressure. (ppN2= N2%*pressure)
Say you dive to 100 feet, so 3 atm of pressure using nitrox 32. 0.68*3= 2.04%

Naturally, the deeper you go, the higher the atmospheric pressure. For example, a shallower depth of 66 feet would give a ppN2 of 1.36%. So, the inspired inert gas pressure increases as you descend.

Question 1- it happens when descending
Question 2- by the inspired inert gas pressure increasing as ambient pressure increases

You are forgetting to use absolute pressure rather than gauge pressure. When you are at 100 feet, yes it is 3 times the pressure at the surface, but it is 4 atmospheres absolute pressure (add the pressure at sea level). You have to use absolute pressure in the calculations.

SeaRat

 

[John C. Ratliff]

it can, but it's not a concern in recreational diving. for recreational, when you ascend, you're always off-gassing the excess N2. but you're still going to be supersaturated relative to your normal tissue N2 pressure at the surface. this mainly affects your NDLs, which has been discussed further up in the thread.

when you're carrying out decompression stop dives, if you do stops that are too deep, or don't produce an efficient pressure gradient, the "slower tissues" (ones with a longer N2/He on and off gassing halftime) will absorb nitrogen. or helium too if you're using trimix. which is why you set your gradient factors so that your first stop isn't dangerously above your bottom depth (giving a tissue tension/pressure gradient that's too high), but not too close to it either.

Hopefully, the diver will never have ”supersaturated” N2 in the bloodstream, as that is when bubbles form. There is a pressure gradient in the diffusion of nitrogen into the tissue, or out of it, depending upon the amount of dissolved nitrogen there is in the bloodstream. So the idea of using the tables and/or the computer to calculate no-decomression times or the decompression stops is to prevent supersaturation.

SeaRat

PS, I may have a different idea of “supersaturation” than others. Here’s what I meant:

…During decompression, the tissues contain excess numbers of dissolved gas molecules taken up during compression, which means that the tissues are supersaturated (the sum of dissolved gas pressures in a tissue exceeds the ambient pressure). The excess dissolved gas must leave the tissues and return to the lungs. If a diver ascends slowly, so that ambient pressure is reduced gradually, the partial pressure of gases in the alveoli and hence in arterial and capillary blood decrease proportionately. The tissues have higher partial pressures of dissolved inert gas (usually nitrogen) than capillary blood and the dissolved inert gas diffuses out of the tissues into capillary blood down the concentration gradient and is carried in venous blood back to the lungs to diffuse into the alveoli. Gases that are largely biochemically inert in humans (e.g. nitrogen, helium) are sparingly soluble in blood. Therefore if decompression is more rapid, the gas that has dissolved in the tissues will come out of solution to form bubbles in the tissues and in venous blood. Echocardiography shows that venous bubbles are transported to the lungs where, in most cases, the gas passes out of the bubbles into the alveoli down the concentration gradient as the bubbles pass through the pulmonary capillaries. Usually this decompression process does not result in illness. If the rate of decompression is too rapid, so that the number of bubbles or the site where they lodge causes injury, the diver suffers DCS…

The pathophysiologies of diving diseases - PMC

www.ncbi.nlm.nih.gov

This brings up the concept of micro bubbles, which is what I was talking about with the term “supersaturation.”