Posted by Steve on July 05, 2001 at 18:17:02:
In Reply to: isobaric counter diffusion is a PADI, IANTD, TDI scare tactic posted by MHK on July 05, 2001 at 14:48:19:
Do you disagree with the entire theory of "Vestibular Bends."?
ISOBARIC COUNTER GAS TRANSPORT
(The 'Vestibular Bend')
ISOBARIC COUNTER GAS TRANSPORT
(When we learned of Rob Parker’s death, Fred Winstanley – the Cave Diving Group’s Technical Officer wrote the following:)
It was with great sadness that I learned of Rob’s Parker’s untimely demise. Although I only met him a couple of times I gained the impression of a modest, pleasant, genuine person full of the thrill of exploration.
Over a period of time more facts have come to light regarding the details of his dive profile and this may give some clues as to the cause of death. As I understand it Rob was completing a trimix dive and was on his way up when his accident occurred. He became distressed after a switch to air at 60m depth. The symptoms displayed were classical symptoms of those of someone suffering what is incorrectly known as a vestibular bend.
In the early to mid sixties commercial diving in the North Sea depended upon a lot of heliox bounce diving as opposed to saturation. The tables devised for this procedure called for a switch from heliox to air at eighty feet. At this switch some divers would show signs of loss of balance and extreme vertigo and vomiting. The symptoms where identical to those of someone suffering from an infection in the inner ear, specifically the semi-circular canals which are involved in control of balance. For many years it was thought that the bubbles causing the damage occurred in the vestibule part of the ear, hence the name given to that specific type of decompression sickness. It is now thought that the bubbles do not form in this region but in the cerebellum of the brain. This part of the brain controls muscles and receives the impulses from the semi-circular canals of the ear, hence the symptoms displayed. Wherever the bubbles occur the result is the same, severe disability which can leave survivors quadriplegic. It is also thought that the symptoms shown are only the most noticeable and in fact the brain is suffering massive trauma with huge bubble formation. The divers in the North Sea where decompressing in a chamber and so did survive, anyone suffering such an occurrence in the water, especially on SCUBA, would stand little chance of survival. So what causes these bubbles to form? The answer seems to lie in an occurrence known as Isobaric Counter Gas Transport. To understand the mechanics of this phenomenon it is essential to understand gas solubility in the body’s tissues. In fatty tissues helium is twice as soluble as oxygen but nitrogen is twice as soluble as helium. This means that the body’s tissues become saturated to different levels dependent upon the inert gas breathed. Henry’s law states that the solubility of a gas in a liquid is directly proportional to the pressure exerted on that liquid. In other words the deeper you are the more inert gas you have in your body, but remember that gases are soluble at different rates. So where does this leave us in Rob’s case?
As you ascend, gas comes out of saturation and is removed from the blood by the lungs. If, however, you switch to a different gas, specifically a nitrogen rich gas like air, the nitrogen dissolves into the tissues quicker than the helium can come out, creating a super saturation situation - hence bubbles of helium are formed in the blood. This bubble formation can occur with no change in depth, hence the name Isobaric, meaning “same pressure”.
How do we prevent this situation arising? The answer is quite simple, during decompression never let your partial pressure of nitrogen rise. Come as shallow as is safely possible on your bottom-mix, and avoid switching to air at any stage of the decompression, using nitrox or pure oxygen instead. This procedure will result in a slightly longer decompression profile but, remember, - you’re an awful long time dead.
Technical Officer – Cave Diving Group
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