I just received a reply about dcs and freedivng from a senior medical officer at NEDU. Here it is:
Tim,
I'm not familiar with "Non-clinical DCS", but I have heard of "Sub-clinical DCS". This is somewhat of a pet peeve of mine. DCS is a clinical diagnosis, so it is impossible to have either a "non-clinical" or "sub-clinical" clinical diagnosis. It's possible that damage may be going on that we can't detect, that is, the damage is below our clinical observation/diagnostic threshold, but it tain't DCS until we diagnose it as such. This may be one of the few times in which DCI might be appropriate (I don't like that term either since it lumps everything together, instead of trying to cluster the symptoms or problems. Oh well.) Naturally it goes
without saying, but I'm going to say it anyway, that not everyone agrees with me about this, but equally naturally, I'm convinced that they are wrong.
Now free divers and skin divers, who are not breathing compressed gas during the dive, and so only breathe on the surface, can get DCS, although it is somewhat rare. The general group of divers that everyone uses as an example is the Ama divers of Japan/Korea. It appears that the key requirement is that the diver must make repetitive breathhold dives. The
theory (well, one theory at least) is that if one makes enough deep breathhold dives, the body will eventually take up enough of the nitrogen from air in the diver's lungs to give a sufficient nitrogen load as to require decompression. Remember that the lungs are compressed, but not collapsed when the diver is at depth. Therefore, gas in the lungs is also compressed to ambient depth.
I'm not aware of any good models for predicting DCS in breathhold divers. One can take one of the many models and play around with them, but there's not a good diver database for such dives, so you'd just be winging it. I'd also think that if there were sufficient large database, it would be pretty noisy, since there can be some variation in blood shunting across
the pulmonary vasculature with altered exposure to the alveoli, especially since the lung volumes will be decreased due to compression.
Another potential problem that is not considered by many people is that it is possible have an arterial gas embolism during a breathhold dive. There are several reports of such. The ones of which I'm aware involved performing at depth, either a hard val salva, or heavy lifting/straining, such as in picking up boulders underwater to move them. This rare situation
can also be confused with the rare DCS in breathhold diving.
The bottom line for the Navy at the present is that we don't have any special tables for breathhold diving, and we generally shy away from it for repetitive dives, since we lose a diver every now and then who is practicing breathhold diving and dies due to breathhold blackout.
Generally, I'd think that if one was going to be doing a lot of deep
repetitive diving, you'd be better off just going ahead and using scuba. Most people's ears will appreciate it. Now if you're only diving down to 30 fsw or so, I wouldn't worry about DCS. I'd be more concerned about AGE or ear/sinus barotrauma.
Feel free to contact me if this was insufficient. (However, I'm going to be
on travel for most of this week and the next week. If you need to talk with
someone here before then, just send another e-mail to
publicaffairs@nedu.navsea.navy.mil and we'll get it routed to someone else.)
David Southerland, CAPT, MC, USN
Senior Medical Officer
NEDU
321 Bullfinch Road
Panama City, FL 32407
Please guys, i've already questioned his comment about SCUBA diving instead of freediving, so don't mail him anything Please?
Tim,
I'm not familiar with "Non-clinical DCS", but I have heard of "Sub-clinical DCS". This is somewhat of a pet peeve of mine. DCS is a clinical diagnosis, so it is impossible to have either a "non-clinical" or "sub-clinical" clinical diagnosis. It's possible that damage may be going on that we can't detect, that is, the damage is below our clinical observation/diagnostic threshold, but it tain't DCS until we diagnose it as such. This may be one of the few times in which DCI might be appropriate (I don't like that term either since it lumps everything together, instead of trying to cluster the symptoms or problems. Oh well.) Naturally it goes
without saying, but I'm going to say it anyway, that not everyone agrees with me about this, but equally naturally, I'm convinced that they are wrong.
Now free divers and skin divers, who are not breathing compressed gas during the dive, and so only breathe on the surface, can get DCS, although it is somewhat rare. The general group of divers that everyone uses as an example is the Ama divers of Japan/Korea. It appears that the key requirement is that the diver must make repetitive breathhold dives. The
theory (well, one theory at least) is that if one makes enough deep breathhold dives, the body will eventually take up enough of the nitrogen from air in the diver's lungs to give a sufficient nitrogen load as to require decompression. Remember that the lungs are compressed, but not collapsed when the diver is at depth. Therefore, gas in the lungs is also compressed to ambient depth.
I'm not aware of any good models for predicting DCS in breathhold divers. One can take one of the many models and play around with them, but there's not a good diver database for such dives, so you'd just be winging it. I'd also think that if there were sufficient large database, it would be pretty noisy, since there can be some variation in blood shunting across
the pulmonary vasculature with altered exposure to the alveoli, especially since the lung volumes will be decreased due to compression.
Another potential problem that is not considered by many people is that it is possible have an arterial gas embolism during a breathhold dive. There are several reports of such. The ones of which I'm aware involved performing at depth, either a hard val salva, or heavy lifting/straining, such as in picking up boulders underwater to move them. This rare situation
can also be confused with the rare DCS in breathhold diving.
The bottom line for the Navy at the present is that we don't have any special tables for breathhold diving, and we generally shy away from it for repetitive dives, since we lose a diver every now and then who is practicing breathhold diving and dies due to breathhold blackout.
Generally, I'd think that if one was going to be doing a lot of deep
repetitive diving, you'd be better off just going ahead and using scuba. Most people's ears will appreciate it. Now if you're only diving down to 30 fsw or so, I wouldn't worry about DCS. I'd be more concerned about AGE or ear/sinus barotrauma.
Feel free to contact me if this was insufficient. (However, I'm going to be
on travel for most of this week and the next week. If you need to talk with
someone here before then, just send another e-mail to
publicaffairs@nedu.navsea.navy.mil and we'll get it routed to someone else.)
David Southerland, CAPT, MC, USN
Senior Medical Officer
NEDU
321 Bullfinch Road
Panama City, FL 32407
Please guys, i've already questioned his comment about SCUBA diving instead of freediving, so don't mail him anything Please?
