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April 2016    Download the Entire Issue (PDF) Vol. 31, No. 4   RSS Feed for Undercurrent Issues
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That Deep Stop Option on Your Computer

forget about it

from the April, 2016 issue of Undercurrent   Subscribe Now

Richard Pyle, the famous deep diving ichthyologist, discovered that if he stopped during ascents at deeper depths than those mandated by contemporary and conventional decompression table or algorithms, he felt a lot better during his surface interval. These deep-stop depths, despite being mainly anecdotal, became formalized at half the deepest depth of the dive -- i.e., if you go to 150 feet, make your first stop at 75 feet -- and became popular within the deeper diving technical diving community as 'Pyle Stops.'

Dr. Mitchell's presentation makes a compelling argument that it may be more hazardous to make deep stops.

Today, most of the algorithms (such as VPM) used in technical diving computers currently include deep-stop. Even popular sport diving computers nowadays give the option to include deepstops, such is the belief in their efficacy in providing a healthy decompression regime.

That is the key word -- belief. Deep stops are based on belief, not science.

Enter Simon Mitchell, a professor of anesthesiology at the University of Auckland in New Zealand, an expert on diving physiology and an experienced deep diver. Recently he has been making presentations to interested parties about deep diving and 'deep-stops' and is turning conventional thinking on its head. Even Suunto has added deep-stops only an option for diving with its top-of-the-range Eon Steel.

His presentation (based on an NEDU study and available on-line) compares gas-content models against so-called bubble models and makes a compelling argument that it may be more hazardous to make deep stops. "There is no reliable data that supports the view that deep-stops approaches are superior and the available human data available suggests caution." In fact, the U.S. Navy has declined to adopt these modern bubble-model decompressions for routine operations.

Apparently, making long pauses at depth during an ascent from a deep dive can load the slower tissues with nitrogen, leading to the greater possibility of a DCI. Prof. Mitchell appears to propose that the older conventional method of heading to the shallowest stop might be safer.

Simon Mitchell made a convincing argument in his presentation to a meeting of the UK Sports Diving Medical Council last October. You can download it by searching 'Simon Mitchell Deep Stops.'

Conclusions of Prof. Simon Mitchell:

• The data is not presented as conclusive proof that deep stop decompression is inferior, or does not work across the range of technical diving depths and gas combinations, but.....

• It makes a strong case for arguing against the current perception that deep stop decompression is superior or safer

• Deep stops may fail because super-saturation in slower tissues may be more important in DCS than faster tissues in at least some time/depth combinations

• On the basis of available evidence, divers may consider de-emphasizing deep stops in configuring their decompression algorithms.

(Doolette DJ, Mitchell SJ. Decompression from technical dives. Diving Hyperbaric Med 2013:43(2):96-10)4

For a sport diver diving to, for example, 100 feet, it probably makes little difference whether he pauses on the way up or not. However, the deepstop option has been added to computers more to enhance their sale than as a practical aid to decompression. The typical leisure diver meandering up a reef might be subject to a stop for sixty seconds or so at sixty feet, but otherwise, would be making natural pauses anyway. You could do just as well following the PADI mantra of coming up slowly from every dive!

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