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> With all condition the same,,,,,which is better ? 10 foot or 15. I
> would = think 15 is better do to the higher PP02
I would say 15 is better - but not because of the higher PO2. People
often make the mistake of thinking that higher PO2 = less deco. This is
not strictly true. It is more correctly: lower inert gas partial
pressure = less deco. Because O2 is considered a "freebie" gas for deco,
it's easy to think that these two things are the same. But, they are not
- and this particular issue (10' or 15' .... or 20') serves as a good
example.
It is true that breathing 100% O2 at 15 feet results in a higher inspired
PO2 than breathing 100% O2 at 10 feet. The relevant physiological
consequences of this that come to mind are:
1) Hypoxic tissues are better-treated at 15' on O2 than 10' on O2,
because the total concentration of O2 in the blood is higher, affording
an increased potention for O2 diffusion into hypoxic tissues. This is a
GOOD thing for a decompressing diver, because a decompressing diver
likely has enlarge4d bubbles in the bloodstram, and these enlarged
bubbles may be blocking blood flow to certain tissues causing tissue
hypoxia (whether or not this hypoxia leads to symptoms that are
detected by the diver as DCS symptoms).
2) Internal diameters of capillaries are likely to be smaller when
breathing O2 at 15' than when breathing O2 at 10'. This is because
high PO2 is a vasoconstrictor. This is a BAD thing for a decompressing
diver for two reasons. First, vasoconstriction means reduced perfusion
(blood flow through the tissues), which means N2 and He are
less-efficiently removed from tissues and transported to the lungs (i.e.,
offgassing is slower). Second, smaller internal capillary diameters means
that these capillaries can be blocked by smaller obstructions. In other
words, bubbles/clots that would pass through more dialated (more opened)
vessels might get stuck and obstruct constricted vessels. Such
obstructions lead to tissue hypoxia, which is likely a primary causal
factor of DCS symptoms.
These two issues notwithstanding, the reason I would rather do my O2 time
at 15' than at 10' has to do with *ambient pressure*, not PO2 per se.
Let me explain:
The gases that we (as decompressing divers) want to get out of our bodies are
(usually) nitrogen and/or helium. The rate at which those gases leave our
bodies depends (for the most part) on the concentration *gradient* across the
alveolar membranes (the membranes that separate the gas in our lungs from the
blood in our circulatory system). The idea of breathing 100% O2 on deco is
that there are no N2 or He molecules in the inhaled breath, so the
"steepness" of this concentration gradient is maximized (which means that the
rate at which N2 and/or He leave our blood through our lungs is maximized).
The thing to consider here is that the "steepness" of this gradient is
essentially the same regardless of whether a diver is breathing O2 at 10', or
breathing O2 at 15' (or 20'). In other words, when we consider only the rate
at which N2 and He leave our bodies (i.e., the rate at which we off-gas),
there is essentially no difference whether the diver is at 10 feet or at 15
feet when breathing 100% O2.
So why would I breathe O2 at 15' instead of 10'? The answer lies in the
fact that the rate at which N2 and He leave our bodies is only *part* of
the decompression story. The other part of the decompression story is
avoiding the bends.
We're reasonably certain that bends symptoms are caused, at least partly,
by the presence of gas bubbles within our blood. There is good reason to
believe that these bubbles exist on every dive (and in fact, that they
exist all the time - regardless of whether a person did a dive at all).
So, it is reasonably safe to conclude that bends is caused when these
bubbles grow large enough to lead to restriction of blood flow (either
through blockage by the bubbles themselves, or due to direct
or indirect biochemical responses to the presence of these bubbles).
Therefore, the way to avoid bends is to prevent these bubbles from
growing large.
The size of the bubbles depends on a wide variety of factors, but one thing
that is certain is that bubbles will grow if more molecules enter them, and
bubbles will shrink if more molecules exit them. As decompressing divers,
we want to do everything we can to encourage these bubbles to shrink,
rather than grow.
I won't go into all the highly complex dynamics of bubble physics
(largely because I do not fully understand them), but the important thing
to consider is that gas molecules play by the same rules at the
blood/bubble interface as they do at the blood/lungs interface. Whereas
the interface at the lungs is the alveolar membranes, the blood/bubble
interface is the surfaces of the bubbles. As with the alveolar membranes,
the way to get the N2 and He molecules out of the bubbles and into the
blood (so the bubbles shrink in size) is to maximize the concentration
gradient across the bubble surface. How can we do that? Getting back to
the 10' on O2 vs 15' on O2 question, we'll remember that in either case
(10' or 15') the concentration of N2 and/or He in the blood will be the
same. By breathing O2, we are reducing the concentrations of these gases
in the blood as much as possible, but it really makes no difference
whether the diver is at 10' or 15' - the concentrations of N2 and He in
the blood will be the same regardless of whether the diver is at 10' or
15'.
HOWEVER, the concentration of N2 and/or He *inside the bubble* will be
greater at 15' than at 10'. Why? - because the ambient pressure is
greater. If the ambient pressure is greater, this increases the pressure
inside the bubbles, becasue the pressure transmits straight through the
water and the blood to the bubble (there are other effects which cause
the internal bubble pressure to be even higher than ambient pressure, but
I won't go into those now). The bottom line is that the pressure inside
the bubbles will be greater at 15' (and greater still at 20') than it
will be at 10'. Because the pressure inside the bubble is greater, the
cocnentration of N2 and He in the bubbles is higher. This means that the
"steepness" of the gradient from inside the bubbles to the surrounding
blood is greater, so gas molecules will diffuse from the bubbles to the
blood faster, so the bubbles will shrink faster.
Of course, there are two major caveats: The first is that the bubble
physics are MUCH more complicated than this (I don't fully understand
them myself); and the second is that this is all theoretical, and may or
may not apply to the real world of decompressing divers.
Nevertheless, I have done my best to understand the factors involved, and
have decided that the theraputic effects of high PO2, combined with the
theoretical benefits of increased internal bubble pressures, outweigh the
potential disadvantages of vasoconstriction and toxic effects of
increased exposure to higher PO2; so I do my 100% O2 deco stops between
15' and 20', rather than 10'.
Hope that helps.
Aloha,
Rich
P.S. I want to make a point here that I have made in the past, but feel
needs making again. Quite often, discussions of bubble physics and deep
decompression stops are intertwined. I want everyone to understand that
bubble physics and associated theories are **NOT** the reason I do deep
decompression stops. The **REASON** I do deep decompression stops is
because, in my experience, they work. The discussions of bubble physics
in association with deep stops is just to provide a possible theoretical
explanation for why they work. Even if all the bubble physics stuff is
hogwash, that does not negate the growing empirical support for the
practice of deep decompression stops.
Richard Pyle
Ichthyology, Bishop Museum deepreef@bi*.bi*.ha*.or*
1525 Bernice St. PH: (808) 848-4115
Honolulu, HI 96817-0916 FAX: (808) 841-8968
"The views are those of the sender and not of Bishop Museum"
--
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