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>
>>When teaching on a BSAC Oxygen Administration course we tell our students
>>to expect a transient worsening of symptons to a casualty shortly after
>>starting to administer 100% O2. This is due to two reasons. Firslty, if
>>there is a lodged bubble some of the oxygen in solution will diffuse into
>>the nitrogen bubble. Secondly, the body reacts to the higher partial pressure
>>of O2 and reduces the blood flow due to vasoconstriction. This can be read
>>in the BSAC Safety and Rescue manual.
>
>Although this appears in the BSAC manual, I am not sure that there is
>any data to support that either of these phenomena occur in a clinically
>significant (read as symptomatic) fashion.  Increased FIO2 does decrease
>blood flow in at least some capillary beds but not at the cost of O2
>delivery as far as I know (since the reduction in blood flow is, in
>principle, due to the increase in avaialable oxygen).  Under normal
>conditions, there is a fair excess of O2 available to tissues so a 15%
>or so reduction would not cause symptomatic change.  The issue of oxygen
>diffusing into the bubbles may occur but this should be offset by
>nitrogen diffusing out (which is the reason for using O2 in the first
>place).  There may be some interesting transient phenomena but I have
>seen nothing documenting them.  Maybe John Crea or prime Rat have some
>data on this.  Is there a reference given in the BSAC manual for this
>statement?
>
There is evidence for transienty increase in bubble size with the
administration of oxygen.  The work was done in exposed fat pouches in 
hamsters, visualizing and measuring bubble diametre by microscope.  
Bubbles were produced by air diving then decompression and their 
resolutuion was followed with the hamster breathing oxygen only, or 
with nitrogen, helium, or nitrous oxide.  With oxygen, bubbles 
transiently increased in diametre then shrunk.  In case your 
interested, nitrogen caused a longer period of bubble growth before 
shrinking, helium caused slow shrinking, nitrous oxide caused rapid 
growth.  Don't let an ambulance attendant give a diver enternoix 
(N2O/O2) for pain relief!!!
Work done by Hyldegaard, published in Undersea Biomedical Research, 
two articles that I can recall.  I can dig up the reference if 
anyone wants.

David Doolette
ddoolett@me*.ad*.ed*.au*

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