The wealth of information in this thread is a good example I why I read, and not just write on, scubaboard. Thanks guys for a very useful discussion with lots of good information!
DivemasterDennis
DivemasterDennis
Descend to Decompress???
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SkimFisher
Contributor
The reason in-water recompression is frowned upon is because once bubbles are formed, the depth you have to attain in order to re-dissolve them is immense. It's not a matter of "max depth was 100', so go back to 100'." Once the phase change occurs it does not work the same way in reverse. I'm currently taking Adv. Nitrox/Deco and somewhere in the course materials it discussed IWR, and I want to say the depth you would have to attain is something like in excess of 165'. I'm going from memory, so I could be off.
Going that route obviously introduces other problems and risks into the equation.
Going that route obviously introduces other problems and risks into the equation.
I think that before you could make your decision you would need to add a little more detail to your original question. I am NOT and expert on decompression, but having just finished reading both Deeper into Diving by DAN and Deco for Divers by Mark Powell from cover to cover I also wondered about this question and came away with the following impression of what the advice was saying and the reasoning.
First are you talking about a dive where you have a decompression obligation, or a fast ascent from depth, when you were near your no decompression limit, and you missed your safety stops?.
And second I would ask whether the cause of the ascent could be immediatly rectified to allow a descent back to depth (at least safety stop depth) within a minute at most.
If the answer to question 1 is that it was a no decompression dive, and you have simply missed a safety stop(s) - and you could immediately descend to the depth of your missed stop within about a minute, and you have NOT suffered any symptons of pulmonary barotrauma due to the fast ascent - then as has been said above you can descend to your safety stops before significant overloading of bubbling starts.
This is because bubble formation happens on all dives, even on NDL dives - not just if you ascend too fast, but so long as the bubles in the tissues are below a certain size and quantity then the bubbles are asymptomatic (they have no significant adverse effect). Bubbles only becomes symptomatic when the size and quantity of bubbling exceeds what they call the M value.
You reach/exceed the M value by allowing the tissue pressure gradient to become too high - this is linked to the difference between ambient pressure and the pressure between the gas disolved in your tissue/cells and the gas in your circulation. (sorry for a better explanation you will have to read the book which I haven't got to hand). Because it takes time for gas to come out of the tissues this change is not instant even with a fast ascent. So you can go back down and the increase in ambient pressure will stop you exceeding the M value, then you can make a slow controlled ascent extending your safety stops by an additional margin if allowed by your available air. You then have a high chance of preventing DCI occuring in the first place as your have increased your external partial pressures before the level of bubbling can reach the M value, the level of bubbling is then unlikely to become symptomatic.
However if it was a dive with significant deco obligations, or you have any symptons of barotrauma or DCI, then in line with all the agency advice, in water decompression/recompression AS A TREATMENT for DCI is an absolute last resort if you have no other option available to you. And by no options they included access to oxygen and a chamber within an hour.
I can recommend both books - they provide a much better explanation than I can, and if you are really sad they tell you how to do the equations to work out bubble size, volume and M values !
All the best - P
First are you talking about a dive where you have a decompression obligation, or a fast ascent from depth, when you were near your no decompression limit, and you missed your safety stops?.
And second I would ask whether the cause of the ascent could be immediatly rectified to allow a descent back to depth (at least safety stop depth) within a minute at most.
If the answer to question 1 is that it was a no decompression dive, and you have simply missed a safety stop(s) - and you could immediately descend to the depth of your missed stop within about a minute, and you have NOT suffered any symptons of pulmonary barotrauma due to the fast ascent - then as has been said above you can descend to your safety stops before significant overloading of bubbling starts.
This is because bubble formation happens on all dives, even on NDL dives - not just if you ascend too fast, but so long as the bubles in the tissues are below a certain size and quantity then the bubbles are asymptomatic (they have no significant adverse effect). Bubbles only becomes symptomatic when the size and quantity of bubbling exceeds what they call the M value.
You reach/exceed the M value by allowing the tissue pressure gradient to become too high - this is linked to the difference between ambient pressure and the pressure between the gas disolved in your tissue/cells and the gas in your circulation. (sorry for a better explanation you will have to read the book which I haven't got to hand). Because it takes time for gas to come out of the tissues this change is not instant even with a fast ascent. So you can go back down and the increase in ambient pressure will stop you exceeding the M value, then you can make a slow controlled ascent extending your safety stops by an additional margin if allowed by your available air. You then have a high chance of preventing DCI occuring in the first place as your have increased your external partial pressures before the level of bubbling can reach the M value, the level of bubbling is then unlikely to become symptomatic.
However if it was a dive with significant deco obligations, or you have any symptons of barotrauma or DCI, then in line with all the agency advice, in water decompression/recompression AS A TREATMENT for DCI is an absolute last resort if you have no other option available to you. And by no options they included access to oxygen and a chamber within an hour.
I can recommend both books - they provide a much better explanation than I can, and if you are really sad they tell you how to do the equations to work out bubble size, volume and M values !
All the best - P
There was a story of a science team. Three divers came up at different times. Last one didn't have enough air for her deco stop. So she took her Buddy's BC unit and descended with it, still wearing her BC. Shortly later the bubbles stopped and the buddy's BC popped to the surface. I don't think they were able to find her.
I personally would want to take a DCS hit on land.
So short answer, it'd be wiser to go on O2 and stay topside, rather than trying to get back into the water to deco.
I personally would want to take a DCS hit on land.
So short answer, it'd be wiser to go on O2 and stay topside, rather than trying to get back into the water to deco.
Interesting - I have just looked into my copy of deeper into diving, which I did bring away with me (as there were big chunks I want to reread) and this deals with in water recompression (IWR) as a treatment for DCI on pages 457 - 460 and actually recommends IWR in certain scenarios where symptons of DCI have already manifested themselves on the strict proviso that you have a number of things in place - such as an adequate oxygen supply, FULL FACE mask, a weighted descent line, a means to keep the diver warm, and a means of connecting/fixing the injured diver to the decompression/treatment stage.
It discusses M values on pages 56/57 but the description in Deco for divers is better in my opinion. - P
It discusses M values on pages 56/57 but the description in Deco for divers is better in my opinion. - P
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For a recreational diver, diving within recreational limits (130 feet no deco) IWR will NEVER be necessary regardless of the ascent rate. If I wasn't clear, let me be more so. IWR will NEVER be required regardless of ascent rate. For dives outside of recreational limits, It is not necessary to go to extreme depths to "collapse the bubbles" as has been suggested here. Navy Table 6 recompression tables for chamber use have you at 60 feet. If you're bent, you're going to get a chamber ride on a Table 6. No matter how badly a recreational diver is bent, they will get a Treatment Table 6. Treatment Table 6 occurs at 60 feet, not 100, or 165, or where the diver got bent, but 60 feet. Now, the problem with doing an IWR (as has been described before) is that the victim is cold, a situation non conducive to off gassing and Oxygen may cause convulsions at 60 feet. Convulsions while using a regulator second stage are fatal in the water. There is no other way around it, if you convulse in the water on a normal second stage, you will drown. So, regardless of the status of support divers, you must complete the IWR on a full face mask. You still need air breaks, so now you have to switch from full face mask to some other way of getting air for 20 minutes every hour. If I were in Truk, I'd try IWR long before I ever went to the chamber there, but if the plane were waiting to take me to Guam, I'm getting on the plane.
Absolutely - I am sure when IWR was discussed in deco for divers there was also a requirement that there was a buddy with the decompressing diver, in this case she redescended alone. I think the times when you will have the logistical arrangements available to do IWR safely will be very rare indeed. - P
if no symptoms of DCS are present, and you can re-descend to your first required stop within 2 minutes, you complete that stop and all stops up to and including your 40ft stop as scheduled, and extend the rest of your stops by 1.5 times, and stay at your last stop for as long as you can. once on the surface, get on O2, monitor for DCS, and start making arrangements for EMS just in case.
on a recreational dive, where no deco obligation exists, I would re-descend to 15ft (within a couple minutes) and stay as long as you can (again, as long as no DCS symptoms are present). if signs or symptoms of DCS are present, get out of the water immediately, get on O2 if possible, and seek EMS as soon as possible.
it is not considered recompression unless DCS is present. in water recompression takes hours, and requires the diver to be on 100% O2 at very high PO2s, making CNS toxicity a major threat. it can only be done with high volumes of O2 available, adequate thermal protection, and a full face mask. which makes it virtually impossible on a recreational dive boat.
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on a recreational dive, where no deco obligation exists, I would re-descend to 15ft (within a couple minutes) and stay as long as you can (again, as long as no DCS symptoms are present). if signs or symptoms of DCS are present, get out of the water immediately, get on O2 if possible, and seek EMS as soon as possible.
it is not considered recompression unless DCS is present. in water recompression takes hours, and requires the diver to be on 100% O2 at very high PO2s, making CNS toxicity a major threat. it can only be done with high volumes of O2 available, adequate thermal protection, and a full face mask. which makes it virtually impossible on a recreational dive boat.
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waterpirate
Contributor
For me I would only attempt IWRC if all other options were not possible and death was immenant, i.e. exibiting signs of dcs and no medivac or ems for hours.
I have seen many a diver thrown back in the water due to a bone head move, missed stop, abreviated stop. Those divers were not showing any signs of DCS, and were guilty only of aggresive profiles vs. a conservative one.
On a lot od tech charters wether the diver is aware of it or not the crew checks or glances at the divers computer on exit, and or checks the log for the divers planned RT. More than once I have seen divers exit with blinking or beeping computers and be asked for an explanation, and told to get back in to ten feet till that thing clears.
Again not a life and death, just a safety move.
Eric
I have seen many a diver thrown back in the water due to a bone head move, missed stop, abreviated stop. Those divers were not showing any signs of DCS, and were guilty only of aggresive profiles vs. a conservative one.
On a lot od tech charters wether the diver is aware of it or not the crew checks or glances at the divers computer on exit, and or checks the log for the divers planned RT. More than once I have seen divers exit with blinking or beeping computers and be asked for an explanation, and told to get back in to ten feet till that thing clears.
Again not a life and death, just a safety move.
Eric
DevonDiver
N/A
Lets be clear on some terms:
In-Water Recompression = A treatment method for diagnosed/symptomatic DCI victims, requiring extensive pre-planning, logistical support and expert medical/diving supervision.
Missed Decompression = A pre-formulated response, taught on technical diving courses, to minimize risk of DCI, should the diver otherwise miss planned and mandatory decompression stops on ascent.
Safety Stop = A voluntary/optional extended pause at shallow de-saturation depth, to enable additional off-gassing and further reduce DCI risk, before surfacing from a dive.
What the OP suggests is; in no way, shape or form, 'in-water recompression'.
On a dive where no mandatory, staged decompression has been planned, then missed decompression procedures are largely irrelevant. You can't adopt a calculated response for missed deco, unless you have calculated the deco in the first place. That said, even on a recreational dive, with no scheduled or mandatory stops - the ascent rate itself is a calculated form of deco. Exceeding an ascent rate is, arguably, 'missed deco'.
Where no formal stops are required, the 'standard' response (as adopted by many dive computers) is to apply a 'mandatory' safety stop for instances where the ascent rate has been substantially violated. By switching the emphasis of the stop from 'voluntary' to 'mandatory' it essentially becomes a deco stop.
Re-descent following an accelerated ascent sounds very risky to me. If the rapid ascent was significant, then there is good chance that bubbles have already started to form. Re-descending will initially compress those bubbles - thus allowing them to migrate within the body. Ascending once those bubbles have migrated offers a significantly higher risk of DCI.
In order to 'achieve' some resolution, the diver would have to re-descend and then simultaneously de-saturate the bubbles at depth. This won't happen, because the diver is on-gassing at depth, not off-gassing. The existing bubbles can attract more volume from the additionally saturated nitrogen - causing a more severe problem on eventual ascent.
Using nitrox will reduce the absorption of nitrogen, at any given depth, but it will not reverse it. The same issue with additional saturation at depth remains.
The important issue to understand is that re-descent (increased ambient pressure) compresses bubbles. Compressing a bubble is not the same as dissolving a bubble. Desent, in itself, would relieve DCI symptoms, and thus prevent/minimize physical injury, but wouldn't resolve the issue. If DCI symptoms are not presented, then descent is utterly unnecessary.
The focus, on non-symptomatic divers, needs only to be on de-saturation. Descending is never a solution for de-saturation.
In-Water Recompression = A treatment method for diagnosed/symptomatic DCI victims, requiring extensive pre-planning, logistical support and expert medical/diving supervision.
Missed Decompression = A pre-formulated response, taught on technical diving courses, to minimize risk of DCI, should the diver otherwise miss planned and mandatory decompression stops on ascent.
Safety Stop = A voluntary/optional extended pause at shallow de-saturation depth, to enable additional off-gassing and further reduce DCI risk, before surfacing from a dive.
What the OP suggests is; in no way, shape or form, 'in-water recompression'.
On a dive where no mandatory, staged decompression has been planned, then missed decompression procedures are largely irrelevant. You can't adopt a calculated response for missed deco, unless you have calculated the deco in the first place. That said, even on a recreational dive, with no scheduled or mandatory stops - the ascent rate itself is a calculated form of deco. Exceeding an ascent rate is, arguably, 'missed deco'.
Where no formal stops are required, the 'standard' response (as adopted by many dive computers) is to apply a 'mandatory' safety stop for instances where the ascent rate has been substantially violated. By switching the emphasis of the stop from 'voluntary' to 'mandatory' it essentially becomes a deco stop.
Re-descent following an accelerated ascent sounds very risky to me. If the rapid ascent was significant, then there is good chance that bubbles have already started to form. Re-descending will initially compress those bubbles - thus allowing them to migrate within the body. Ascending once those bubbles have migrated offers a significantly higher risk of DCI.
In order to 'achieve' some resolution, the diver would have to re-descend and then simultaneously de-saturate the bubbles at depth. This won't happen, because the diver is on-gassing at depth, not off-gassing. The existing bubbles can attract more volume from the additionally saturated nitrogen - causing a more severe problem on eventual ascent.
Using nitrox will reduce the absorption of nitrogen, at any given depth, but it will not reverse it. The same issue with additional saturation at depth remains.
The important issue to understand is that re-descent (increased ambient pressure) compresses bubbles. Compressing a bubble is not the same as dissolving a bubble. Desent, in itself, would relieve DCI symptoms, and thus prevent/minimize physical injury, but wouldn't resolve the issue. If DCI symptoms are not presented, then descent is utterly unnecessary.
The focus, on non-symptomatic divers, needs only to be on de-saturation. Descending is never a solution for de-saturation.
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