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Is there any scientific evidence that safety stop decrease DCS risks?

Discussion in 'Advanced Scuba Discussions' started by nohappy, Oct 20, 2017.

  1. clownfishsydney

    clownfishsydney Loggerhead Turtle

    # of Dives: 2,500 - 4,999
    Location: Sydney Australia
    From http://www.divingmedicine.info/Ch 14 SM10c.pdf by Dr Carl Edmonds et al:

    Following experiments and reviewing of the established decompression tables, between 1960 and 2000 a whole series of innovations and modifications were introduced. Dr Bruce Bassett (a USAF physiologist) concluded that the US Navy Tables resulted in an excessive incidence of about 6% DCS, when pushed to the no-decompression limits. Merrill Spencer in Seattle verified this observation and supported it with extensive Doppler monitoring, showing that bubbles developed in many routine dives – implying inadequate decompression. Many others observed similar inadequacies and in an attempt to cope with this new information, tables Chapter 14 — 2 based on modified Haldane principles were developed by Bassett, Huggins, NAUI, PADI, and many others. The main alterations to improve safety were in;
    • reducing the acceptable no-decompression times by 10-20%
    • reducing ascent rates from 18 m/min to 9-10 m/min (at least in the top 30m)
    • Adding a “safety stop” of 3-5 min at 3-5 m.
    nohappy likes this.
  2. Dr Simon Mitchell

    Dr Simon Mitchell Medical Moderator Staff Member


    As mentioned by someone else, Donna's masters thesis addressed this issue. The abstract is as follows:

    Venous gas bubbles in the blood of human divers may be indicative of several incipient diving pathologies, such as decompression sickness and air embolism. the present study compared the presence of venous gas emboli in the bloodstream of human divers detected with an ultrasonic Doppler device. Two dive profiles were conducted for the study, one with direct ascent to the surface and one with a reduced rate of ascent, accomplished by making a safety stop. Doppler monitoring was conducted at 20, 40 and 60 minute intervals post-dive. Dives with safety stops showed significantly fewer venous gas emboli. Other factors, such as, depth and bottom time influenced the efficiency of safety stops. Overall, safety stops are capable of significantly reducing venous gas emboli detected by Doppler in the blood of human divers. Research was conducted in conjunction with the National Undersea Research Center/ University of North Carolina at Wilmington (NURC/UNCW) as an observational study of operational diving.

    The abstract is a little parsimonious with information. The link to a page where you can download the entire document is:

    Doppler Detection of Silent Venous Gas Emboli in Non-Decompression Diving Involving Safety Stops

    It is reasonably convincing, and why Donna never published it properly is a mystery to me.

    On another topic that has arisen, the depth threshold for DCS symptoms after direct ascent from a saturation exposure appears to be around 6m (20') [1]

    Simon M

    1. van Liew and Flynn. Direct ascent from air and N2-O2 dives in humans: DCS risk and evidence of a threshold. Undersea & Hyperbaric Medicine. 2005;32(6):409-419
    2airishuman, nohappy, Ayisha and 2 others like this.
  3. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    @Dr Simon Mitchell 's post above also explains why it's a good idea to do a safety stop, even if the reduction in DCS risk is minor. Although we - AFAIK - don't have hard evidence that asymptomatic bubbles (AGE) are associated with post-dive lethargy, it's both very likely and quite plausible. If we believe that, both a proper safety stop and a slow ascent from that safety top is very good practice. Even if it won't reduce the DCS risk measurably, it may reduce the risk of falling asleep at the wheel on your way home :)
  4. bada3003

    bada3003 Solo Diver

    # of Dives: 200 - 499
    Location: Indiana
    1 fpm sounds about right. If conditions allow. :)
  5. bada3003

    bada3003 Solo Diver

    # of Dives: 200 - 499
    Location: Indiana
    Indeed. Another good practice, IMO, is to do a "how do I feel" check at the completion of deco. It's obviously subjective but in my case the symptom is heaviness in my head such as what one may experience before the onset of a headache. If so, I try to tag on 10-15 minutes of additional deco at 10 ft. If conditions allow.

    In the times I've ignored it, I've suffered the consequences, mainly in the form of extreme lethargy. Whether this is a weak form of DCS I do not know. The physiological mechanisms of DCS are not well understood and that's not likely to change in the forseeable future. Be conservative and stay safe.
  6. Kharon

    Kharon Solo Diver

    # of Dives: 200 - 499
    Location: Upstate NY
    I thought it was advised to go straight up from depth at a proper ascent rate to the safety stop depth. This is to prevent further ongassing of the slow compartments. What say you tbone1004?
    Bob DBF likes this.
  7. Wookie

    Wookie Secret Field Agent ScubaBoard Business Sponsor

    I stole this from @RayfromTX who stole it from @MaxBottomtime. It seemed appropriate to the discussion.
    nohappy, doctormike and RainPilot like this.
  8. RayfromTX

    RayfromTX Student Of Gas Mixology Staff Member ScubaBoard Sponsor

    # of Dives: 200 - 499
    Location: Hill Country of Central TX
    To me it seems appropriate to every discussion.

    I'll soon add part two of the signature line which is. "nobody wins an argument on scubaboard"
    RainPilot, nohappy and Wookie like this.
  9. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    depends on their definition of "gradually" and what you're doing. If you're doing square profile, ocean diving, i.e. on a wreck with an upline, and there is nothing to see between the bottom and the hang line, then go up at 30fpm ish and chill out. If you're doing a shore dive and there's fun stuff to look at, then just go up at whatever rate you want to to keep looking at the pretty fish
    Dr Simon Mitchell likes this.
  10. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    xkcd: Duty Calls

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