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So what happened, did you die or what?

   Jim
 -------------------------------------------------------------------
 Learn About Trimix at http://www.cisatlantic.com/trimix/

> From: "Doug Chapman" <dougch@at*.ne*>
> Date: Sat, 21 Apr 2001 16:36:32 -0400
> To: <techdiver@aquanaut.com>
> Subject: Re: Counter-Diffusion
>=20
> I recall a number of years ago we had a visiting diver during dinner stat=
e
> that his expert friends in the Pacific northwest said that if he switched
> from air at 200ft to trimix, it would kill him =96 because of isobaric iner=
t
> gas counter-diffusion. Not wanting to pass up this opportunity we mixed s=
ome
> gas and did a dive to see if that was true. So about midnight that night =
we
> dropped to 200ft, switched to trimix and the rest is left to posterity.
> Needless to say nothing happened; although he was bit by a turtle the nex=
t
> day during deco at DiePolder. I almost died of laughing too hard. End of
> story.
>=20
> We must recognize that until recently, the prevalent theories used for
> constructing decompression models were limited to the classical
> decompression theories of Haldane, Buhlmann, and Workman. These theories
> assumed either the body was bubble free, or it wasn=92t. If bubbles were
> present, then the subject was bent. Even though Doppler studies are not
> without their own set of problems (e.g. false readings, inability to
> distinguish between dislodged bone or fat particles, or overall correlati=
on
> to DCS), it became apparent through the use of this technology that simpl=
y
> the presence of bubbles did not necessarily mean that DCS was imminent. A
> second contributor to DCS was identified to include free-phase bubbles. I=
n
> fact the notion of bubble seeds has been known for quite some time, but
> until recently this phenomenon has not received much attention in diving
> applications. More modern theories suggest we must not only account for
> dissolved gases (classical theories) but must also consider free-phase
> bubble growth (Hill, Gernhardt, and Weinke). Therefore the excitation of
> free-phase bubbles into growth and collapse must also be accounted for in
> decompression modeling.
>=20
> Digressing a bit, I recently read the following passage by a hyperbaric
> specialist on diver physiology who writes for Underwater Magazine, the
> journal for The Association of Diving Contractors International that furt=
her
> illustrates the complexity of decompression theory:
>=20
> =93=85. We also find that the correlation to circulating bubble emboli is
> imperfect. Large numbers of detected bubbles are not necessarily accompan=
ied
> by decompression illness, and the absence of detected bubbles does not
> necessarily accompany a symptom free experience. In surface decompression
> using oxygen (a pervasive commercial diving practice), decompression illn=
ess
> cases occur without any observable bubbles more frequently than in
> decompression procedures that do not use oxygen. Saturation dives also
> exhibit a weaker correlation when compared with typical non-saturation
> diving. The correlation between Doppler detection and decompression illne=
ss
> in saturation diving is so bad that some investigators have concluded tha=
t
> Doppler monitoring is useless in saturation diving=85..=94
>=20
> Hummm!
>=20
> Bubble growth/collapse mechanics appear very complex. If we are to consid=
er
> this phenomenon as part of decompression procedure, and the indications a=
re
> we should, we must evaluate this process, even when it tends to provide a
> contradiction to conventional theories [dissolved gas mechanics]. Bubbles
> will grow (or collapse) though various mechanisms that include, but not
> limited to, ambient pressure, osmotic tension, and size/surface tension.
> There is a critical size at which the bubble may grow or collapse dependi=
ng
> on the parameters. Risking the possibility of placing my foot in my mouth=
, I
> recall discussions on the effects of osmotic pressure on bubble growth.
> Specifically a bubble may be stimulated into growth if subjected to an
> certain osmotic pressure differences. Therefore a bubble excited at depth=
 by
> one particular gas mix, may tend to grow if subjected to a deco mix
> containing a different gas (in addition to the lowering ambient pressure
> effects and surface tension reduction as bubble radius increases). In
> essence, N2 or even O2 may diffuse into the bubble thereby increasing its
> size, or total distributed volume?? This effect may reinforce an argument
> for using more helium during decompression for long and deep dive profile=
s
> that tend to excite small bubble seeds not considered a factor in shallow=
er
> or shorter duration dives??? While classical Buhlmann theory may suggest
> that decompression using increasing N2 fractions, and O2 is most benefici=
al,
> free-phase bubble mechanics may indicate otherwise for some instances. We
> have already alluded to this possibility when it became understood that g=
as
> bubbles in the circulation system have an effect on the body=92s efficiency=
 on
> off-gasing thereby further indicating the classical exponential
> ongas-exponential offgas computations to be over-simplified. Suffice to s=
ay
> there is more to the process than satisfying Buhlmanns equations as
> provided.
>=20
> Getting back to the subject of isobaric inert gas counter-diffusion, and
> speaking strictly in regard to the dissolved gas phase dynamics (no
> consideration for free-phase bubble growth) on which Haldane, Buhlmann, a=
nd
> Workman based their work, it may be concluded that a technical diver is n=
ot
> at risk from isobaric inert gas counter-diffusion. I include this discuss=
ion
> based only on their work and not to include more modern approaches to
> demonstrate that even with their findings that isobaric inert gas
> counter-diffusion as they define it does not appear to pose a risk to
> technical divers.
>=20
> The concept of counter-diffusion is applied and demonstrable in the
> technical diving community. Hans Keller and Albert Buhlmann in their work
> demonstrated that counter-diffusion can be used constructively to
> significantly reduce decompression times [1]. Their application illustrat=
ed
> the principle that inert gases of differing molecular weights will diffus=
e
> into and out of tissues at differing rates (comparable half-times based o=
n
> the reciprocal of the square root of their molecular weights). Furthermor=
e
> Buhlmann=92s [and others] theory assumes the decompression obligation, or
> degree of super-saturation above ambient pressure, is based on the total =
sum
> of all the inert gas partial pressures of the constituent inert gases.
> Appling this theory which inherently embodies the counter-diffusion
> principle he was able to demonstrate that decompression times can be
> shortened by switching to inert gases of greater molecular weight (N2,Ar)
> during deco. In effect, the helium would diffuse out of the tissues at a
> faster rate than the N2 or Ar would diffuse into the tissues. This in eff=
ect
> would create a degree of sub-saturation that proves beneficial to the
> decompression event. For example, Hans Keller in an open ocean dive to
> 1000ft, with 3 minutes of activity at depth, was able to decompress in 61
> minutes.
>=20
> Buhlmann further noted that on relatively short duration dives (<2 hours)=
, a
> high helium content breathing medium would typically result in longer
> decompression time than one done on a N2-O2 mix [1]. However after about =
2
> to 3 hrs duration a He-O2 dive would result in shorter decompression time=
..
> This may be explained by counter-diffusion as during the longer duration
> dives the N2 inert gas tensions in the tissues would be somewhat diminish=
ed
> and that during deco, the He inert gas would flush out of the tissues abo=
ut
> 2.6 times faster than if the tissues were otherwise loaded with N2. Buhlm=
ann
> also commented on the potential problem of super-saturation at an isobari=
c
> state if one switches to a lighter inert gas at depth.
>=20
> Very good discussions of isobaric inert gas counter-diffusion can be foun=
d
> in C. Lambertsen, M.D. [2], and C.A. Harvey and Lambertsen [3]. However o=
ne
> must keep in mind that the applications of these papers, as well as many =
of
> Buhlmann=92s [et.al.] on this subject are directed to commercial saturation
> diving applications (e.g. long duration exposures, great depths, high
> helium/hydrogen content bottom mixes, and chamber/bell environments). Thi=
s I
> believe is where a bulk of misinformation occurs as sometimes information
> garnished from this sort of research is incorrectly applied to technical
> applications. =91Like comparing apples and oranges.
>=20
> As George I. mentioned, some of the research on the isobaric
> counter-diffusion principles involved farm animals. For example one
> experiment showed that pigs will develop venous gas embolisms after about=
 30
> minutes of breathing a N2-O2 mixture at 1 ATA while surrounded by helium.=
 In
> this experiment, the blood vessels contained more gas than blood [2].
> Similar experiments with rabbits while breathing air with a helium
> atmosphere (@200 ft ambient pressure) indicated death will occur in about
> 1.5 to 2 hours. What does this mean? Don=92t take your pet pigs and rabbits
> diving? Perhaps, but a key factor outlined in these sorts of tests almost
> always includes the test subject being enclosed in a light inert gas
> environment while breathing a heavier inert gas mix.
>=20
> Human experiments have indicated that itching of the skin and vestibular
> effects (middle-inner ear diffusion) may occur as a result of isobaric in=
ert
> gas counter-diffusion under similar parameters as the animal testing. Aga=
in
> this form of superficial isobaric inert gas counter diffusion is primaril=
y a
> product of commercial diving procedure involving chambers or bells where =
it
> is possible for the occupant to mask breathe a heavier gas mix while bein=
g
> surrounded by a much lighter heliox mixture for whatever reasons we may o=
r
> may not understand. However one should note this phenomenon will not occu=
r
> in a submerged diver wearing a wetsuit, but may occur if the diver is
> wearing an inflated suit (drysuit) that is filled with a lighter inert ga=
s
> than what is breathed. This latter consideration provides additional
> incentive to not use helium or high helium (backgas) as an inflation medi=
um
> for your drysuit, beyond the thermal considerations that hopefully common
> sense would indicate.
>=20
> According to conventional theory, there is the possibility of a =93deep
> tissue=94 problem resulting by breathing a high-helium content gas followin=
g
> a prolonged exposure to a N2-O2 mixture. For example at 200ftsw constant
> ambient pressure with tissues saturated, or near-saturated, with N2, a 50=
ft
> supersaturation gradient will be achieved after about 480 minutes of
> switching to a high helium content mix (for the 480-N2/240-He minute tiss=
ue
> half-times). It takes several hours after reaching this supersaturation
> level for it to diminish to ambient (for the selected tissue halftimes).
> Depending on the tissue compartment in question, this may pose a scenario
> for a supersaturation above the allowable to prevent DCS. Again, however,=
 we
> must be reminded that this example requires a near-saturation of the tiss=
ues
> with N2 (near-saturation occurs roughly after a period of 4 to 5 halftime=
s =96
> for this example about 40 hours). This is not a scenario that is remotely
> likely to occur in a technical diving application. It however may be an
> issue for some commercial applications.
>=20
> The only potential problem area I have noted for technical divers after l=
ong
> duration N2 mixes is if a diver is being treated for DCS at a recompressi=
on
> depth considered unsafe for air or N2 mixes (narcosis, O2 toxicity) and i=
s
> switched to He mixes. However an increase in the ambient pressure
> (compression) at the switch will reduce the likelihood of bubble growth o=
r
> development according to conventional theory [2]. This should not be an
> issue with experienced chamber operators or people willing to call DAN (D=
uke
> University hyperbaric research) for assistance.
>=20
> Given the duration, depths, light inert gas fractions, and environment, i=
t
> just doesn=92t appear likely that a technical diver is at risk due to isoba=
ric
> inert gas counter-diffusion [IMHO] even considering conventional theory.
> With the remote exception being a situation where the diver is breathing =
air
> or N2-O2 mix during deco while being surrounded in a high helium medium i=
n
> his or her drysuit. However, using a high helium inflation gas in a drysu=
it
> would be a very stupid and ignorant thing to do considering the purpose o=
f
> the exposure suit is to keep the diver warm. So this should be a non-issu=
e
> unless the diver is an idiot and inflates the drysuit from a backgas
> containing a high helium fraction =96 again not wise. Since a more acceptab=
le
> practice is to use a heavy inert gas for drysuit inflation, such as Argon=
;
> if any isobaric counter-diffusion process is occurring, it would tend to
> create a sub-saturation condition thru the skin and therefore produce a
> desirable effect. Again posing no problems for the technical diver. As fo=
r
> switching to a trimix or heliox mix at depth from air or N2-O2 mixes, it =
is
> unlikely to result in a problem (re:deep tissue problems) because the deg=
ree
> of N2 saturation at this point (v.s. ambient) for technical divers is ver=
y
> low and any increase of the total inert gas loadings due to the switch at=
 a
> constant depth (pressure) should not prove to be significant. Furthermore
> most technical diving applications involving a gas switch from air/N2-O2 =
to
> trimix or heliox also involve a continuation of pressurization that furth=
er
> renders the phenomenon inconsequential.
>=20
> According to conventional theories, a switch to a heavier (slower) gas fr=
om
> a lighter (faster) gas on ascent is desirable as it tends to create a
> sub-saturation condition during deco and therefore shorter deco time.
> However this switch must be done in such a way that it does not contribut=
e
> significantly to the decompression obligation (switching too deep). Howev=
er
> this dissolved gas theory does not account for free-phase bubble mechanic=
s
> and its effect on decompression directly, and indirectly on its modifying
> effect on the conventional theories. It also does not account for other
> factors one may wish to consider during decompression such as the likelih=
ood
> of less long-term tissue damage if DCS occurs with helium mixes compared =
to
> N2 mixes. In any case it appears that isobaric inert gas counter-diffusio=
n
> is just not a consideration worth worrying about in technical diving, IMH=
O.
>=20
> I would like to read more references on free-phase bubble mechanics as
> applied to decompression diving if anyone can suggest them.
>=20
> Take care,
>=20
> Doug
>=20
>=20
> References:
> [1] =93Deep Diving and Short Decompression by Breathing Mixed Gases,=94 H.
> Keller and A.A. Buhlmann, J. Appl. Physiology,20: 1267-1270 (1965).
> [2] =93Advantages and Hazards of Gas Switching: Relation of Decompression
> Sickness Therapy to Deep and Superficial Isobaric Counterdiffusion,=94 C.J.
> Lambertsen, M.D., Institute for Environmental Medicine, University of PA.
> [3] =93Deep-Tissue Isobaric Inert Gas Exchange: Predictions During Normoxic
> Helium, Neon and Nitrogen Breathing at 1200 FSW,=94 C.A. Harvey and C.J.
> Lambertsen, Proceedings of the 6th Symposium on Underwater Physiology,
> FASEB, Bethesda, MD (1978).
>=20
> --
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>=20


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