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This turned out to be a mighty long posting, but I don+t think that any
apology is warranted since exchange of info is what this is all about!
Even though we have been moving away from the original issues - we are
mainly discussing the "oxygen window" and "thermodynamic models" rather
than "nitrox decompression" - I choose to retain "nitrox decompression" as
a heading. I think it+s prudent to look into some of my background material
and assumptions:
There are two major approaches to decompression theory, the Haldanian and
the thermodynamic approach. Haldane worked with the supersaturation concept
- he believed that the body could hold and retain an excess amount of inert
gas provided that the supersaturation didn+t exceed certain limits. He
found a 50% pressure drop to be acceptable and postulated that this would
not cause any bubbles to form in a diver upon surfacing. In effect, he
postulated a general m-value of about 70 feet absolute pressure.
Note that Haldane+s approach was a practical one - if his "guinea pigs"
felt fine upon ascent he assumed no gas separation. When we use an array of
half-times and m-values today, we are employing a refined version of
Haldane+s decompression principle. A dive to the noD limit is thought to
bring at least one tissue-group up to its supersaturation limit. Staged
decompression means that at least one tissue has exceeded its surface
supersaturation limit and one must wait at a ceiling depth until the
tension in the governing tissue has dropped sufficiently. Two important
assumptions: 1) As long as m-values are not violated, there will be no gas
nucleation. 2) The higher the offgassing gradient the more efficient the
inert gas elimination. All tables and computers of today assume that inert
gas is lost in the same way as it is acquired. They are basically Haldanian
and aim at preventing all gas nucleation. In theory a well conducted dive
should not cause any bubble formation.
Enters modern technology with the doppler and tells that we form
microbubbles even on correctly performed table dives. We+re still far from
symptomatic DCS and conclude that even though the supersaturation model
fails to some extent, it still works if we impose some safety factors.
Factors include: limiting the number of repetitive dives, avoiding sawtooth
profiles, longer surface intervals, deepest dive first etc.
The thermodynamic model takes the opposite stand. Supersaturation without
any gas nucleation does not exist. Minute bubbles form (or gas nuclei are
present in the first place) and the tidy picture of dissolved gas tension
as the driving force for elimination is challenged. Rather than a maximum
gradient for diffusion, the emphasis is shifted towards minimum gas
separation. Note that "gas separation" is not the same as getting bent
since the bubbles are too small to clog even the finest of capillaries.
But, they slow down the ideal tension-driven offgassing that could have
been since they have to redisolve before they can be eliminated. A
thermodynamic decompression is in effect the reverse of a Haldanian
decompression: Rather than undergoing a long haul up to shallow stops, the
diver starts to decompress at depth. Short, small increment stops are
carried out. The gradient for elimination is low, yet still optimal since
gas separation is kept at a minimum. From 30 feet the diver surfaces
directly, accepting the microbubbles formed - he/she is thus skipping the
depths where Haldanian stops would begin! The rationale is that tension is
lost/ bubble growth checked to an extent where the final haul is
acceptable. Conversely, traditional tables would allow a lot of
microbubbles to form before one reaches stop depths. Wienke says that
ordinary stops probably work as therapeutic treatment for microbubbles
rather than as a preventive measure.
This is my stand:
I believe that Haldanian m-values give practical limits for acceptable
amounts of microbubbles, but I don+t think that they assure bubble-free
dives.
The supersaturation concept works in most cases, not because it+s correct,
but rather because it limits gas separation and bubble growth. I don+t
believe that it+s possible to be supersaturated without an inherent
initiation of gas separation.
Obviously I+m not a confessing Haldanian, but it+s a good starting point if
we expose the flaws of the supersaturation concept. If an adherence to
maximum supersaturation (m-values) really prevented all bubble formation,
sawtoothing and rep dives would not present any problems. I don+t believe
in further fine-tuning of m-values and diffusion gradients without taking
into account that we+re always dealing with an interplay between tension
and separated gas, not a trouble-free tension that can be lost in the same
way as it was acquired.
David is right in stating that the oxygen window works across bubble
interfaces. The thermodynamic approach is one of checking / managing gas
nuclei rather than assuming that it+s possible to avoid them. Again, note
that gas nuclei are a far cry from what most people would think of as
bubbles - they are, however, minute bubbles and follow the physics of free
gas. Luckily, gas nuclei have a high surface tension that must be overcome
if they are to grow. Larger, "true" bubbles have a low surface tension and
will expand readily in a supersaturated solution. Once a critical radius is
attained, surface tension doesn+t retain growth. The practical difference
between a true Haldanian supersaturation and a thermodynamic combination of
tension and gas pockets is that the physics of free gas must be included in
the latter. It doesn+t make any difference to a diver whether he/she is
thought of as holding an acceptable amount of dissolved gas or a
combination of/interplay between dissolved gas and gas nuclei. But, the
physics of dissolved gas differs considerably from that of free gas. If we
take the Haldanian stand, we don+t expect to deal with any category of
bubbles unless something goes wrong. In thermodynamic decompression, the
management of gas nuclei is implicit. This is where the "oxygen window"
comes in - it gives the driving force for elimination and a means to
balance gas nuclei without further phase change.
I guess the main issue here is how to define the synonomus terms "oxygen
window" and "inherent unsaturation". The equations I presented are not
products of my imagination. If they don+t match David+s "oxygen window"
there must either be a case of wrong application, various definitions or an
unclear elaboration of equations. Wienke lists an example of a dive to 150
feet for 30 minutes, in order to compare a thermodynamic decompression with
standard ones. He says that, "In the thermodynamic scheme the inherent
unsaturation is rapidly taken up by direct ascent to 115 feet..."
The equation gives: ( 1 - f )P + f2.04 - 5.47 fsw = (1 - .79)185 +
.79(2.04) - 5.47 fsw = 38.85 + 1.61 - 5.47 fsw = 34.99 fsw
Subtract 34.99 feet from 150 feet and you get 115.01 feet.
This is exactly what I did for other depths and other nitrogen percentages.
When I coined the more colloquial term of "taking in the slack" I did
exactly what Wienke called "taking up the inherent unsaturation". So, if
the inherent unsaturation = oxygen window is 34.99 feet at 150 feet
according to Wienke I seem to be doing the right thing.
What I do not understand is how the venous unsaturation, as a result of
oxygen consumption, can explain a pressure dependent unsaturation. If
oxygen consumption and carbon dioxide production were the only factors, the
inherent unsaturation should be independent from ambient pressure. David
says the inherent unsaturation doesn+t come anywhere near the values I
presented. I say that even though I don+t see why it does, I+ve got
equations saying that this phenomenon has more to it than a
pressure-independent consumption of oxygen.
The two significant factors are inert gas fraction and ambient pressure -
the only variables. I will check this with a physicist to see if he can
share some light and present it to friends in hyperbaric medicine. I will
also read what Vann has to say. In the meantime, please read Wienke+s
proceedings and we+ll hopefully get to the bottom of this. In the parlance
of physics, a "window" denotes a space/time in which an action will take
place, or a space/time in which it must take place in order to be
successful/proceed in a desirable direction.
David Story writes:
>I think there are two things going on here. First, there is
>terminology confusion. "Oxygen window" as Julius' sources define it
>is not the most common definition. "Inherent unsaturation" was
>brought up by me as an alternate term for the common "oxygen window,"
>not as another name for Julius' phenomemon. I would call Julius'
>phenomenon "the nitrogen gradient" and not "the oxygen window" since
>there are other gases aside from oxygen which contribute to the
>offgassing gradient of nitrogen.
I have used the term mentioned in the same way as Wienke used them; it
comprises the partial pressures of all gases + the tension of nitrogen.
>> The higher the O2 percentage, the lower is the inert gas percentage and
>> more +slack+ is provided - the inert gas tension will never rise beyond the
>> partial pressure of the inert gas.
>
>Unless there is a breathing media change, there can be no "slack" by
>the definition of saturation: equilibrium with atmospheric pressure &
>composition. If you reduce the ambient pressure, a saturated diver
>will immediately become supersaturated.
>
>> Nitrox 32 at 130 feet gives a P N2 of .68( 130 + 33 ) feet absolute = 111
>> feet absolute. What pressure does it take to keep a full load of N2 in
>> solution? 111 feet absolute, or ( 111 - 33 ) feet = 78 feet
>> Ascending from 130 feet to 78 feet takes the nitrogen load up to the
>> pressure that balances the tissue tension. From there on we+re dealing with
>> supersaturation and the inevitable nucleation of microbubbles. At 78 feet
>> we+re dealing with 100 % saturation - Nitrox 32 ( 68 % N2 ) at 130 feet
>> corresponds to 100 % N2 at 78 feet.
>
>But neither air nor EANx32 are composed of 100% N2, so your "fully
>loaded" (saturated) diver is already supersaturated when he ascends
>even the slightest amount. Are you suggesting that a diver who is
>saturated ("full load") at 130fsw on EANx32 can ascend to 78fsw on
>EANx32 and not be supersaturated?
>
>If so, you are missing an important issue here: the diver is
>supersaturated whenever she is breathing media at an atmospheric
>pressure which results in the breathing media containing less total
>pressure of inert gas than her tissues.
When I used the term "saturation", or rather meticulously avoided
"saturation", I was focusing on the maximum attainable inert gas tension at
a given depth for a given breathing gas percentage. I was then asking, what
ambient pressure does it take to balance this inert gas tension?
David is right in stating that the slightest amount of ambient pressure
reduction will cause supersaturation, provided that the oxygen fraction is
counted in. Physiologically, oxygen does not count, primarily because
excess tension will be called upon in metabolism. Moreover, oxygen has a
low lipid solubility. Raising the PO2 does not lead to a significant rise
in total body tension. The main effect is one of occupying all of the
hemoglobin. Oxygen in solution is consumed before oxygen bonded to
hemoglobin; metabolism ensures that oxygen tension is a non-issue in
decompression. Theoretically, oxygen-bends is possible (as evidenced by
Haldane+s goats taken to 130 feet on pure oxygen), but one would have a
serious toxicity problem long before a bubble problem.
The principle of ignoring oxygen is the fulcrum of "equivalent air depth"
or "equivalent table depth" for heliox. Enter oxygen percentage (or by
indirection: inert gas percentage) and one gets the appropriate table
depth.
There seems to be a problem of semantics here; I relate saturation to
solubility while David relates saturation to breathing medium. If switching
from air to nitrox at constant depth (or: air to oxygen at the surface?)
leads to supersaturation, this definition does not connect to risk of
bubble formation. To me "supersaturation" means a tissue tension exceeding
that which can be acquired at a given ambient pressure; that is, a
situation where one is at/approaching a risk of gas separation. "Saturated"
is ambiguous while "supersaturated" is always related to solubility. That+s
why I used "a full load of N2" rather than "saturated with N2".
Nitrox 32 doesn+t provide the maximum amount of nitrogen that can be
dissolved at 130 feet - at that depth it provides the maximum amount that
can be dissolved at 78 feet (That+s where the theoretical 100% and my
"slack" come in.) True, in diving we call it "saturation" when a diver has
equilibrated with the breathing medium at a given depth. One week of
breathing heliox at 300 feet is saturation. But, the maximum amount of gas
that could be dissolved (from a physical point of view) would be attained
by permeating the solvent (body tissue) with the solute (helium). Since the
diver needs some oxygen as well, this theoretical greatest amount of helium
that could be dissolved in body tissues at 300 feet is never experienced. A
gas dissolves according to its partial pressure, not as a function of the
total pressure. "Supersaturation" means holding an amount (of inert gas)
which exceeds the maximum that could dissolve at a given pressure.
(Webster: Saturate: to dissolve the maximum amount of a gas in solution at
a given temperature and pressure; -ed: Containing the greatest amount of
solute that can normally be dissolved.
Supersaturate: To make more highly concentrated than in normal saturation)
The following sentences are not what I meant to say:
>> One might ask, why am I talking saturation - according to m-values SCUBA
>> diving will never saturate anything but the fastest tissues?
>> If one +takes in the slack+ of the
>> oxygen window, the time he+ll have to spend at the ceiling in order to be
>> able to proceed and still avoid nucleation will depend on tissue m - values.
>
It should be "half-times", not "m-values" and I apologize for the
confusion. Apart from picking the wrong word, I don+t think there+s any
confusion of basic (Haldanian) decompression theory. I should add, in that
first sentence I used "saturate" to mean equilibrium with a given medium,
not as the maximum solubility of nitrogen in body tissues.
>> The same
>> physical laws apply for any tissue - more than 100 % saturation will
>> initiate gas nucleation whether that tissue be blood or cartilage.
>
>Actually, a saturation ratio greater than 1.0 is not the only issue.
>I assert that, I can cause a non-diver to form bubbles, and I can take
>a diver to saturation and back without ever causing her to bubble.
>This has been proven again and again; I assert that although
>supersaturation is the primary contributor, supersaturation does not
>*always* lead to bubble formation.
According to Haldanian decompression models you+re right - according to the
thermodynamic model - nope! Supersaturation is always believed to cause
(further) gas nucleation. I think what has been proven again and again is
that it+s next to impossible to avoid gas nucleation (microbubbles/silent
bubbles) while it+s certainly possible to avoid even the slightest hint of
decompression sickness. I define DCS as a situation where gas pockets
attain a size sufficient to obstruct vessels, bend nerve endings or in any
other way interfere with normal functions. If one advocates that even the
smallest of gas pockets should be termed DCS, I say we probably get bent -
transitorily, at a microscale - on every dive.
>> In practice we+ll probably break the oxygen window anyway, and accept some
>> amount of microbubbles. The oxygen window gets increasingly critical as one
>> approaches the surface - it+s practically impossible not to violate it from
>> 30 feet and up.
>> However, breaking it close to the surface is no big deal
>> since the microbubbles formed won+t undergo too much further decompression
>> and won+t grow to attain obstructive sizes.
>
>Here I must strongly disagree. Decompression does not stop at 0fsw!
>Bubbles grow after surfacing by inward diffusion of supersaturated
>nitrogen from tissues.
I+m not saying that it+s a good idea to skip decompression. I+m saying that
in choosing the lesser of two evils:1)compromising it at an early stage of
the ascent versus 2)at the final stage of the ascent, number two is
preferable. I guess you see the relevance of this point since it+s actually
a principle of thermodynamic decompression. Provided that there+s no
productive change in breathing medium, the model gets increasingly critical
as pressure drops, and a practical compromise is advised. Keep it straight
from the very beginning and some slop is acceptable at the end, as opposed
to: accept a lot of microbubbles on the first long decompression haul and
try to straighten it out at 10 feet!
I try to keep it straight at both ends of the scale - first by looking to
the thermodynamic model for the deeper part and then by switching to a
nitrogen poor mix and remain at medium depths till I+m cleared. I guess
that some of this controversy relies on the fact that I didn+t explain that
my line of thinking challenged well established Haldanian "truths".
Hans Petter Roverud (using Julius+ mail account)
PS Why is it that all apostrophes come out as plus signs in replies?
Do we use a different standard of characters?
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