Blackwood
Contributor
A while back, Bob (NWGratefulDiver) posted a thread about a questionable reply to a member letter found in Alert Diver magazine. Flipping through the latest edition (March/April 2009), I came across another such reply.
Rick Reimann from Washington related a story about missing his safety stop after an 80 foot dive, and returning to depth after surfacing to complete the stop. DAN Medical Information Specialist Marty McCafferty's reply contained the usual discussion that there is little agreement about safety stops.
The procedure described is what's often called bubble pumping, and depending on the situation, it has the potential to cause disastrous problems.
A Safety Stop is generally short, on the order of 3-5 minutes. Re-descending after a dive during which bubbling has occurred (on an no-stop single gas profile, they're most likely to appear at the highest gradient, i.e. the surface, and they will certainly be at their largest there) introduces the potential for those bubbles to be compressed and passed from the venous side over to the arterial side in just enough time to re-ascend, turning a benign bubble into a potential type II neurological hit.
Like everything in decompression mechanics, the above scenario isn't a definite, but it is entirely possible. There have been cases (I don't have easy access now, but I'm sure if Thalassamania shows up he can help) of DCS hits in shallow water attributed to bubble pumping.
I encourage all divers (particularly new ones who are still learning to control their ascents) to remember what a Safety Stop is: a way to slow the ascent on a dive which theoretically allows a no-stop profile. There are procedures for re-descending to complete missed mandatory stops, but none are simply to go back down and do it. They all involve substantially longer stays.
If after a planned no-stop dive you feel that you need some more decompression time (due to a runaway ascent or a missed safety stop), stay out of the water longer. Bouncing back down likely won't help, and despite what DAN's Marty McCafferty says, has the potential to cause substantial problems.
Rick Reimann from Washington related a story about missing his safety stop after an 80 foot dive, and returning to depth after surfacing to complete the stop. DAN Medical Information Specialist Marty McCafferty's reply contained the usual discussion that there is little agreement about safety stops.
Though not a physiologist, I'm in that camp.
Though not sure I've seen any physiologists suggest following that procedure, I disagree with it (for reasons to be briefly discussed below).
While he may be right that some people think it's a good idea, the suggestion that there is consensus from all parties that it won't cause problems couldn't be further from the truth.
The procedure described is what's often called bubble pumping, and depending on the situation, it has the potential to cause disastrous problems.
A Safety Stop is generally short, on the order of 3-5 minutes. Re-descending after a dive during which bubbling has occurred (on an no-stop single gas profile, they're most likely to appear at the highest gradient, i.e. the surface, and they will certainly be at their largest there) introduces the potential for those bubbles to be compressed and passed from the venous side over to the arterial side in just enough time to re-ascend, turning a benign bubble into a potential type II neurological hit.
Like everything in decompression mechanics, the above scenario isn't a definite, but it is entirely possible. There have been cases (I don't have easy access now, but I'm sure if Thalassamania shows up he can help) of DCS hits in shallow water attributed to bubble pumping.
I encourage all divers (particularly new ones who are still learning to control their ascents) to remember what a Safety Stop is: a way to slow the ascent on a dive which theoretically allows a no-stop profile. There are procedures for re-descending to complete missed mandatory stops, but none are simply to go back down and do it. They all involve substantially longer stays.
If after a planned no-stop dive you feel that you need some more decompression time (due to a runaway ascent or a missed safety stop), stay out of the water longer. Bouncing back down likely won't help, and despite what DAN's Marty McCafferty says, has the potential to cause substantial problems.
