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Posted by Kevin on January 06, 2001 at 07:59:59:

In Reply to: RBCR posted by John Walker on January 05, 2001 at 23:43:45:

John, a very simple question and an excellent point. I actually had to start digging through my old ANDI and IANTD textbooks. Not for the information to respond to your question, but rather thats how I filed away an old responce by Bill Mee. VERY GOOD POINT ABOUT DEEP AIR JOHN.

The following is long, but worth reading twice to get the full impact. It has made me think about my own use of deep air . . . .

"Effects of Deep Air on The Body

The reality may be a lot worse than any of us may have imagined. Fortunately, for
vertebrates, the apparent viscosity of whole blood decreases in tubes with
diameters less than 0.1cm. This observation is known as the Fahraeus-Lindqvist
effect and implies that the shear stress at the capillary wall decreases as the tube
diameter decreases. On the contrary suspensions of rigid particles display no
such behavior. Mammalian capillaries range in diameter from 10 micrometers
down to 3 micrometers so that blood cells have to flow down them in single file.
For those capillaries whose diameters are less than that of a cell (7 - 8
micrometers) Red blood cells must deform to pass through the vessel. High
pressures must be generated locally within the thin layer of fluid around the cell in
order to deform it and therefore depend considerably on the elastic properties of
the cell. Anything which impairs the elastic characteristics of the blood cell may
have a profound mechanical effect on both the cell and the blood vessel.
Abnormal cells which are hardened or almost spherical cannot be deformed so
easily and therefore cannot pass through the smallest vessels. In experiments
performed to determine the size of the smallest vessel through which a red blood
cell can transit it was found that anything smaller than 3 micrometers caused
damage to the cells. Cells which are artificially hardened or made to swell into a
spherical shape (osmosis), impairing flexibility, cannot pass through pores less
than 8 micrometers in diameter without suffering haemolysis (cell destruction).
In view of the above scenario it would seem that any action which would
artificially "stiffen" blood cells could have a potentially negative impact in terms of
both the mechanical damage to the cells and capillaries themselves as well as
decreased perfusion of the surrounding tissues. Where critical tissues are
concerned, such as those subserving neurological processes (your brain for
instance) the effects of diminished mass transport could be profound. Since the
capillary micro-circulation plays an essential role in gas transfer during
compression and decompression one might be further concerned as to the effects
of red blood cell rigidity associated with hyperbaric exposures to air.
Blood is forced to flow through capillaries as little as 3 micrometers across in the
splenic pulp. Normal cells can do this with little haemolysis; however abnormal
spherical cells cannot, such as what occurs in the disease known as hereditary
spherocytosis (sickle cell anemia is another disease which distorts blood cells).
The spleen filters out blood borne debris as well as housing a large population of
B cells, the immune soldiers which manufacture antibodies in response to an alert
by the T cells. There was some speculation recently in the DAN journal regarding
"Divers Disease", a flu-like illness which sometimes accompanies hyperbaric
exposures. I wouldn't be at all surprised if the cause of this problem turns out to
be the aftereffects of red blood cell rigidity and the consequent build up of
haemolized cells in the spleen and lymph nodes.
Maybe its just a coincidence, but deep air diving carries obvious and subtle
neurological effects and it poses very problematical decompression issues for
long exposures (empirically and anecdotal observations). I personally have
experienced fatigue and other flu-like symptoms following exposures to air diving
and attribute this to some form of immune system response.
Eventually, physiologists and hematologists will understand the specific
underlying effects of excessive hyperbaric air. For the mean time I think its a safe
bet to avoid deep air at all costs.
Regards, Bill Mee

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