wazza once bubbled...
Concerning hypothermia, in what way can effect the diver and what can it do to him while diving. What I am trying to say is that if I am very cold in the water, of course I have to adored the dive, but if I am doing some deco stops what could happen to my body concerning off gassing if I was very cold. Or could it affect something else.
Hi again Wazza,
Forgetting about hypothermia for a moment I am sure you will realise no-one really fully understands what happens to gas transfer overall when a diver's body temperature changes.
What I do know is this.
1) The solubility of gasses is decreased with increasing temperature. Thus if the diver is already cold at the start of a dive his tissues can hold more gas molecules before they become saturated. Once he has rewarmed this extra dissolved gas must come out of solution. Hence the need for increased conservatism when diving in very cold water. This is also another reason why post-dive hot tubs and heavy exercise are not recommended as rewarming reduces gas solubility.
2) On the other hand, the rate of diffusion is increased with temperature, reducing the tissue half times for the affected tissues and theoretically lessening total deco obligations.
3) I am aware that in the '70s, at the London Hospital, Professor Keating researched hypothermia using pigs carcasses. Not suprprisingly his findings could not be directly applied to live animals because they have a circulation. An unregulated circulation in a live animal serves to speed up the heat transfer from the periphery to the core and visa-versa. However, as you know living animals have reflexes to prevent this. The most important is peripheral vasoconstriction in response to cold. This reduces peripheral tissue perfusion to preserve core temperature. The down side is reduced perfusion directly reduces offgassing, thus increasing the half-times for the affected tissues with an attendant increase in deco obligations.
If my understanding is correct, recreational deco shedules assume a constant body temperature.
Now to hypothermia
Hypothermia occurs when the heat conserving mechanisms are unable to prevent the core temperature from falling any further. Even on land this will happen if an individual is not wearing sufficient protective clothing. The heat loss in an unclothed and submerged diver is over twenty times faster than in the air and even in tropical waters this can lead to true hypothermia. (The life expectancy of an uprotected man-overboard in the North Sea in winter is less than 15 minutes.)
The danger is this. Once the body's core temperature falls below a certain level the reflexes I mentioned cease to work effectively so heat loss is accellerated. (These reflexes require energy, which is no longer available as the enzymes needed for its generation are less efficient at lower than normal temperatures.) The victim tends to feel better because he has stopped shivvering, for example, but is actually in a very dangerous situation made worse by the fact that the brain no longer functions properly. He is much more likely to make dangerous mistakes in judgment, made even more likely bacause hypothermia causes a state of euphoria or a feeling of well-being. In the case of a fell walker he may be lulled into a false sense of security and stop to sleep - never to wake up.
I am sure there are other factors in diving but to me the one that matters the most is, like narcosis, the inability to think properly and to make the right decisions in an alien environment. This is why, in Britain at least, the mantra is
plan the dive (on the surface when you are thinking straight)
and dive the plan (because any decision to change the plan made underwater is likely to be based on false assumptions.) Technical divers frown on dive computers for this reason alone.
I have tried to make this simple, Wazza but in reality it is very complex.
Hypothermia is a killer and must be avoided at all costs
