Understanding Decompression Sickness

[boulderjohn]

The PADI tables were created when the standard ascent rate was 60 FPM, and that is what was used as the standard when they were doing their research. If you were to go back a dozen years of so on ScubaBoard, you will find posts by me in which I incorrectly said that ascending slower than 60 FPM was not an accurate use of the table. The table instructions said to go "no faster than 60 FPM." When I did my research for an article I wrote on ascent rates several years ago, I learned from PADI headquarters that the research found no harm with slower ascent rates. The tables were still valid.

But at some point, if you go too slowly, you violate NDLs and go into deco. How can you tell when? That is the key problem with using tables. There is no way of knowing how slow is too slow. If you are using a computer, the computer will tell you if you are hanging out too long.

 

[Duke Dive Medicine]

So if you're ascending slower than the normal ascent rate, you can actually on-gas?

1. What are you considering the normal ascent rate to be sure?

2. Is this the harder, more dense body tissue (slower absorbing & releasing) that is still absorbing nitrogen as you ascend very slowly or all body tissue (whether slow or fast absorbing)?

3. Let's say you ascend slower than the normal rate for the entire ascent. When the tissues are still on-gasing nitrogen, does that on-gasing occur during the entire ascent or just during the first part of the ascent?

1. Current normal recommended ascent rate is 30 feet per minute, though as @boulderjohn pointed out, that has changed over time.
2. The tissues you're referring to are known as slower tissues, and the short answer is yes. We mentioned pressure gradients earlier in the thread in the context of the pressure gradient of nitrogen in the lungs vs. in the body. Your question takes that one step further in that there are also pressure gradients inside the body. The blood is a very fast tissue and will be off-gassing on ascent, but if the partial pressure of nitrogen in the blood is still greater than that in the tissues it supplies, those tissues will on-gas.
3. Great question. At some point on the ascent all of the tissues will be off-gassing, but depending on the rate of ascent, they may be loaded with inert gas to the point that direct ascent to the surface would be unsafe - the pressure gradient between those tissues and the surrounding faster tissues / lungs would be too great and bubbles could be produced. The diver would then have to make a decompression stop to let all of those tissues catch up on their off-gassing. There's a good practical demonstration of this in the U.S. Navy Diving Manual. You can download it from the SupSalv website here. Lookup the section on variations in rate of ascent.

Best regards,
DDM

 

[tursiops]

1. Current normal recommended ascent rate is 30 feet per minute, though as @boulderjohn pointed out, that has changed over time.
2. The tissues you're referring to are known as slower tissues, and the short answer is yes. We mentioned pressure gradients earlier in the thread in the context of the pressure gradient of nitrogen in the lungs vs. in the body. Your question takes that one step further in that there are also pressure gradients inside the body. The blood is a very fast tissue and will be off-gassing on ascent, but if the partial pressure of nitrogen in the blood is still greater than that in the tissues it supplies, those tissues will on-gas.
3. Great question. At some point on the ascent all of the tissues will be off-gassing, but depending on the rate of ascent, they may be loaded with inert gas to the point that direct ascent to the surface would be unsafe - the pressure gradient between those tissues and the surrounding faster tissues / lungs would be too great and bubbles could be produced. The diver would then have to make a decompression stop to let all of those tissues catch up on their off-gassing. There's a good practical demonstration of this in the U.S. Navy Diving Manual. You can download it from the SupSalv website here. Lookup the section on variations in rate of ascent.

Best regards,
DDM

For those not wishing to download and search the entire US Navy Diving Manual, here is the section mentioned above, on Variations in Ascent Rate. It would be nearly impossible to follow on a dive! Another reason we have and use computers.

 

[Duke Dive Medicine]

For those not wishing to download and search the entire US Navy Diving Manual, here is the section mentioned above, on Variations in Ascent Rate. It would be nearly impossible to follow on a dive! Another reason we have and use computers.

Thank you for doing that, and agree. The procedure is primarily for surface-supplied diving where the divers' ascent rate is monitored and controlled from topside. The hope was for the OP to have the perspective that there are circumstances in which a slow ascent rate needs to be compensated for. The Diving Manual procedure is one way of doing that, and considering that it's bulky and not waterproof, most divers prefer computers

Best regards,
DDM

 

[tursiops]

I would have thought it would make more sense if you asked how to avoid getting decompression sickness.

To whom are you addressing this post?

 

[Duke Dive Medicine]

I would have thought it would make more sense if you asked how to avoid getting decompression sickness.

The OP is doing that, indirectly. Please be reminded that this is a learning zone, and that posts that resemble trolling / disparaging remarks will be removed.

Best regards,
DDM

 

[ginti]

@RXTdiver @kaylee_ann this is usually a suggestion for those entering the tech realm, but given the type of questions you are giving, maybe you want to have a look:

Deco for Divers, by Mark Powell (Amazon)

My version is a bit old, and some concepts are not up to date, but the basic principles are there - I think most of us read it.

Disclaimer: more advanced stuff than that of an OW course, so be sure you are ready to understand it otherwise, you'll get confused. And don't overthink decompression because there are more severe threats in rec diving (panic, poor basic skills, etc.).

 

[kaylee_ann]

@RXTdiver @kaylee_ann this is usually a suggestion for those entering the tech realm, but given the type of questions you are giving, maybe you want to have a look:

Deco for Divers, by Mark Powell (Amazon)

My version is a bit old, and some concepts are not up to date, but the basic principles are there - I think most of us read it.

Disclaimer: more advanced stuff than that of an OW course, so be sure you are ready to understand it otherwise, you'll get confused. And don't overthink decompression because there are more severe threats in rec diving (panic, poor basic skills, etc.).

I actually already have it and have read it several times thanks for the advice though!! I also have the technical diving introduction book. They’re interesting and I learn a lot

 

[ginti]

I actually already have it and have read it several times thanks for the advice though!! I also have the technical diving introduction book. They’re interesting and I learn a lot

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!

 

[steinbil]

So if you're ascending slower than the normal ascent rate, you can actually on-gas?

1. What are you considering the normal ascent rate to be sure?

2. Is this the harder, more dense body tissue (slower absorbing & releasing) that is still absorbing nitrogen as you ascend very slowly or all body tissue (whether slow or fast absorbing)?

3. Let's say you ascend slower than the normal rate for the entire ascent. When the tissues are still on-gasing nitrogen, does that on-gasing occur during the entire ascent or just during the first part of the ascent?

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.