You may already be a fan of deep stops or you may be like a lot of divers, you have heard of them but have been diving successfully for years without them so don’t see why you need to change. Here is some food for thought (my sources for this blog are the NAUI Journal of Underwater Education, Vol 20, Issue 2 and the published journal on the “Effect of Varying Deep Stop Times and Shalo Stop Time on Precordial Bubbles” by Bennett, Marroni, Cronje, Cali-corleo, Leonardi Bonucelli, Balestra, Undersea and Hyperbaric Medicine 2007; 34(6).)
NAUI (as an example which is not meant to imply that other agencies do not already recommend this) had a recommended deep stop of 1 minute at a depth that is half that of the deepest depth reached on the dive. They have now changed that stop from 1 minute to 2 or 3 minutes ( the ideal being 2.5 minutes). This is based on the research by Bennett et al (for us more technically minded divers this ‘deep’ stop should be half the absolute depth and not half the actual depth…and yes, they really mean at least 2.5 minutes…read on)
What does the research say ? Well, it has already been proved that a deep 5 minute stop at 15 m (on a 25 meter dive) in addition to the typical 3 to 5 min shallow stop (6 m) significantly reduces the number of bubbles and fast tissue compartment gas tensions (this research also indicated that the optimal ascent rate from 25 m was 10 m/ minute). This led to a number of agencies introducing the 1 minute ‘deep’ stop. BUT, the new research indicates that 1 minute is too short and actually increases the bubbling rather then decreasing it. Dives with 2.5 minute deeps stops showed the highest bubble reduction. Interestingly the research also found that if you have a deep stop, you could drastically reduce the time you spent shallow without changing the bubbling.
Some underlying theory - most decompression computer algorithms and dive tables are based on the original ideas of Haldane or Hill. Haldane modelled gas uptake and elimination on 5 tissue compartments that were either fast or slow to uptake/ eliminate gas (the compartments are virtual entities into which the various parts of the body are assigned a group, for example, brain tissue and nerves are fast tissues, fat is a slow tissue…I will blog on the basics of decompression next week).
Buelmann increased the number of tissue compartments to 16 / 18 creating an algorithm that is perhaps more accurate and safer as a result. Most models are based on the common premise that as long as none of these compartments become supersaturated beyond a certain critical threshold, decompression sickness (DCS) is avoided. Haldane also (and critically) introduced the concept that it is as safe to come from 6ATA to 3ATA as it is to come from 4ATA to 2ATA, or a 2:1 ratio of absolute depth. This ratio of one half the absolute depth has been modified over the past decades and now ranges from 4 to 1 for fast tissues to less than 2 to 1 for slow tissues. Interestingly though, most tables adopted Hills approach of a linear ascent of 10m/ min with a safety stop at 3 to 5 meters. This meant that the concept of staged decompression at one half the absolute depth was eliminated… and so were the positive effects there-of. In essence this is what Bennett et al are doing, re-introducing a stop at one half your absolute depth. Their research indicates that such a deep stop (for their 25 meter dive) often completely eliminated type 3 and 4 bubbles (bubbles were graded as follows 0 (none), 0.5 (sporadic), 1 (15 bubbles over 1 minute with bubble showers), 2 (30 bubbles), 2.5 (>30 bubbles with showers), 3 (virtually continuous bubbling), 3.5 (continuous bubbling) and 4 (continuous bubbling with continuous showers).
What did they find across their various profiles ?
- After a 2.5 min deep stop, decreasing the shallow stop time from 5 min to 1 min had no significant difference to bubbling. They are do not come out and quite say it, but the evidence seems to indicate that the safety stop at 5 meters can be seriously reduced to as little as 1 minute although they still recommend it for recreational diving to prevent pulmonary barotrauma.
- Without a deep stop increasing the shallow stop time did not give you the same effect as a profile with a deep stop. So basically, without the deep stop you had significant bubbling and nothing you do changes that. This was supported when they increased the shallow stop to 10 minutes (from 5 ) with only a slight reduction in bubbling being reported
- If a deep stop is used, any times less than 2.5 minutes ACTUALLY INCREASES YOUR BUBBLING as compared to not having the deep stop at all. So basically either do not do the deep stop or do it properly spending at least 2.5 minutes, otherwise you are making it worse.
I find this research quite interesting as deep stops have always been part of my profile. In fact, I remember quite clearly asking Dr Cronje what advice he had to prevent decompression and him stating “prevent bubbles from forming” (well his actual answer was do not do the dive J ), which is what this research seems to indicate.
If you want to stop bubbling you have to stop it occurring, which is normally deep in your ascent. Staying shallow is there to remove bubbles that have already formed.
Dives that produce bubbles tend to produce DCS!