Varying Deep Stop Times

diveoceanos

For some time now evidence suggests that deep stops might improve the overall diver's decompression stress and thus decrease the likelihood of DCS. The question is how deep and how long should those stops be?

Evidence about the beneficiary effect of deep stops is becoming widedly known within the diving community when Dr Pyle described that he was feeling much better when he was diving to collect fish samples. Contrary to the dives he was making without bringing fish samples from the deep, where the aascent was linear, he was obliged to make staged deep stops when he was ascenting with fish samples from deep dives.

Dr Pyle's observation had defintely played a catalytic role in the development of various decompression strategies where deep stops have been introduced. Particularly in technical diving deep stops are considered by the great majority of divers as a must. Hence we sparingly see nowdays divers to plan any dive without using at least one program that employs a method of adding extra deep stops as a means of conservatism. We have seen straight Buhlmann models to employ Gradient Factors, and other approaches like RGBM and VPM which all resulted in the addition of deep stops.

The success of staged compared to slow linear ascents has been demonstrated by some experiements with measurement of precordial doppler bubble detection in relatively shallow dives. The results of this study have been discussed in various forums.

Last November there was a publication by that same group of people, in the Journal Undersea and Hyperbaric Medicine. They again tested different deep stops duration times to show that the best bubble scores are obtained when deep stops are at least 2,5 minutes in duration. The results confirm that deep stops of 1 minute are too short. The dive depth was 25 msw.

In practice the most serious form of decompression is that involving the CNS. According to the above set of publications the spinal cord is a tissue with a half time of 12,5 minutes which is considered as a relatively fast tissue. Therefore it makes sense to add short Deep stops to facilitate off-gassing from the relatively fast tissues. Bennet, Maroni and there colleagues suggest that 1 minute deep stop (for 25msw dives) are not long enough as they can tell by the results of end-dive doppler bubble detection scores.

Let's consider the whole body now full of microbubbles, that are potential nuclei for large bubble formation. We are now begin ascending. We are now making a deep safety stop. All the blood in our body including that present in distal parts of the body with limited perfussion rates should have enough time to pass through the lungs before we leave for the next stop. Lungs are filtering the microbubbles. Considering that every minute there is approximately so much blood passing through the lungs as the total blood volume, I would expect that the blood of the distal areas will not have the time to pass by. Let's then give it another minute! Therefore I am tending to accept the results of the above study where time of 2,5 minutes for deep deco stops is more effective in elimiating microbubbles.

Ben Field

Hiya,

I'm no deco expert but in my experience few people just do 1minute stops... surely decompression should be considered as a whole not just "bits" of it?

On OC most people I've dived with do do 1minute stops... in between much longer stops- Gas switch stops which are longer to make best use of the highest allowable PPO2/pressure gradient (3-6minutes typically) longer stops at ascent checks and to bag-off or mess around with the shot/deco station.

Now on CCR my group all use GF deco, as applied by the Vision and Shearwater this ends up being more of a constant upward drift than specific stops... as an MCCR user I tend to stop periodically when I'm bringing the loop back up to make the most of it.

While I agree that research is a very good thing- research into just one element is IMO questionable- I'd rather see research into the best total approach to decompression from max depth to surface.

__________________

Beware Fridge Suck

diveoceanos

Hi Ben,

Remember pre-Pyle's era? The whole logic was to get as close as possible to your calculated ceiling. The idea was that by doing this you accelerate off-gassing (least possible inhalation pressure and maximum possible pressure gradient). However the traditional decompression models failed to explain the occurence of DCS in certain cases and indeed nowadays are considered aggressive! Despite the fact that changes occured in general deco strategies, no one can claim that the no-DCS recipe has been found.

Various component research evidence has been used at times, to develop all these new approaches. Different ascent rates, deep stops, minimisation of pressure gradients and so on, resulted. The above work is just a component that some day when sufficient evidence will be accumulated might be applied in some way.

Nothing more true with what you wrote above though about Decompression which should definitely considered as a whole, and not bits.

1-minute stops to my knowledge, are still the case in many decompression models. Even if you are using GFs you might get a lot 1 minute stops at the deepest part of your deco. So the question is if there is any relevance of the above findings to our style of diving. I would be very naive to beleive that an answer exists under the current evidence.

My personal feeling though without any scientific base except from simple logic is that there would be no harm if we would set as a minimum deep deco stop time the 2,5 minutes for the last 21 meters to the surface. So if the planning software allows it I would increase the 21, 18, 15, 12 or whatever stops in this depth range to 2,5 minutes if the software gives them to be 1 or 2 minutes. Of course less shallower stops could be resulted and the plan would have to change according to the new calculations of the deco software. Having said that i maybe full of Sh*t and be completely wrong here!

I am using a deco software that is self developed and based on Buhlmann's and GFs. This allows me to play with deep stops and add 1 minute or 2 to certain stops.

Cheers,
Sotos