Is a long safety stop beneficial?

[Alban]

Please excuse my ignorance on this subject but I am a little confused . I know it is well documented that slow ascents are benificial but I am unsure how ? if I am diving to NDL to 30 mts and I do a slow ascent 4 minuets wouldn't the amount of N2 offgassed during ascent be minimal in all but the fastest compartment and if there is a delay in inspired air PP , blood flow , expired air and off gasing would I not be allmost on the surface before I started to off gass ?

Alban

 

[The Iceni]

Hi Alban,

Can I jump in here at risk of teaching my grandmother to suck eggs?

I can well understand why you are confused. Most divers take a while to understand DCI and the concept of tissue compartments. Let me try and explain how I see it?

A bottle of carbonated drink contains gas "forced" into solution by the high pressure in the bottle. Because it has been sealed for a long time it is at equilibrium - it is saturated and no more gas can dissolve.

If you (shake the bottle and then) quickly take off the stopper (crack the can?) it will fizz all over the place. This is because at the new, lower, pressure it contains more gas in solution than it can hold at that pressure ( it is super-saturated) so the gas comes out of solution throughout the whole body of the liquid.

If on the other hand you take the top of slowly, there will be a more gradual reduction in pressure within the bottle and the gas can diffuse out of solution from the surface of the liquid alone (provided the rate of pressure drop does not exceed the diffusion rate of the gas in solution). Consequently, no bubbles form in the body of the liquid.

Hence slow ascents and deco stops - It is all to do with the rate of change in pressure.

Recreational divers do not become fully saturated, like the bottle of beverage, and only the faster tissue compartments will become close to saturation during any dive. I think this is what has confused you. The fastest tissues such as the blood - with a half time of seconds - will be saturated but the slower compartments will not.

Only saturated tissues are at risk of causing DCI, so if the ascent is slow enough to allow surface diffsusion for all saturated tissues there is no risk of DCI and if no tissue "compartment" is saturated at any particular depth there is no risk of DCI.

I think you will now appreciate that the very slowest tissues are likely to be effectively ongassing at all stages of an NDL dive, but does this really matter as they will never approach saturation in any case?

Did you note my comment about shaking the bottle? Consider this is what you would be doing with post-dive exercise. You are generating micronucleii on which new bubbles form. (I think that's right, Dr Deco?)

As, I said above becuse the body is a dynamic system the risk of DCI is determined not by just the rate of ascent but by the rate of change in pressure and for a given rate of ascent this is greatest as you approach the surface.

Alban, I do hope this helps and does not cause even more confusion!

 

[Alban]

Only saturated tissues are at risk of causing DCI, so if the ascent is slow enough to allow surface diffsusion for all saturated tissues there is no risk of DCI and if no tissue "compartment" is saturated at any particular depth there is no risk of DCI.

Thanks Dr Thomas this is what confused me .I did not realise it was only saturated tissues that where at risk , so diving within NDL will never saturate our tissues therefore we do not risk DCI unless the pressure change is to great.
The question I think I was trying to ask was , if I refer back to your anolgy of the drink bottle the gas inside is instanly supersaturated but there is no presure gradient holding it back so it bubbles .If I have a slow ascent is there not some delay in the time I breath in a lower PP to the time this circulates around to my tissues ? Is the pressure in the tissues reduced considering the pressure gradient all the way up over a 3 minuet period ? Or as you said , is it the pressure change which will make more difference ?
Sorry for asking so many questions Aban

 

[The Iceni]

Hi Alban

I was not quite correct in my post. Blood, with a very fast half time will rapidly saturate on most dives but equally well with a fast half time it very rapidly desaturates and is not a major problem in diving - unless you perform a rapid ascent, such as an uncontrolled bouyant ascent. Then bubbles will form in the circulation after even the shortest of dives.

The answer to your question is very easy!

Scuba diving is at ambient pressure at all times. Because your body is largerly soft tissue, liquid or in direct contact with liquid (blood) every part of the body is at ambient pressure at all stages of a dive.

There is no delay. Any change in the external pressure is instantly transmitted to all of the body parts at that depth.

You may be interested to know there was some discussion on the forum a while ago about the benefits of a horizontal as opposed to a vertical position to adopt during deco stops.

If you adopt a vertical position with your head at 3 metres the brain is at 1.3 bar but the feet are about 1.5 metres deeper so are at 1.45 bar.

If you lie hoizontally during the stop the whole body is effectively at 3 metres and therefore at 1.3 bar.

(I wanted to use a nickname but most already know me as that odd British doctor!)