> I always remember being taught physiology by Professor Keating (a
> good name to search on if you want to find out more about this
> area) who had developed the temp - time curves for immersion. In
> UK waters in the summer the immersion with exercise curve
> (swimming) had you hypothermic in under 30 minutes.
Making the speed of cross channel swimmers about 50mph. Impressive.
Depends a lot on fat thickness. When thinner I spent hours at a time mainly
in the water surfing while wearing a wetsuit (and certainly I know people
who've done 1 hour dives in wetsuits- presumably without significant drop in
core temp). Surfing wetsuits are rarely over 6mm even for winter; diving
semi dry suits 10mm even for Arctic diving; and compress with depth. Fat
insulates as well as neoprene, and I've got over 10mm of that (spent a lot
of time and money building it up in case I fall in the water...), so should
be OK swimming for a couple of hours in summer.
Not just speculation. There is evidence (can't remember where I put it-
sorry) that each 1mm of fat is the equivalent of raising water temp by 1.5
K.
Even taking this into account, some swimmers (cross channel, Lake
Windermere) stay in the water for theoretically non survivable times.
There's a lot we don't know about physiology.
> (Interestingly, if you keep still rather than swim you take longer to
> cool - which is why the person who trys to swim for the shore rather
> than clinging to the upturned boat usually drowns. It is amazing how
> often I have read news reports of the survivor found clinging to the
> boat, while his companion, the supposedly strong swimmer who
> struck out for the shore, doesn't make it).
I've seen this attributed to increased convective loss from waving about
rather than tucking arms into your sides. Anyone got a better explanation?
In answer to Mike Bjarkoy's original question: the DGH should be able to
provide critical care, lavage etc. Not bypass, but the case series where
bypass is used don't seem to have better results than the case series where
it isn't (I accept a RCT would be difficult- nobody gets the numbers needed
to hit significance even assuming a 10% absolute difference in survival.
Also generalisation between mountain, cold water and urban hypothermia could
be criticised). The only comparative study I've found showed better results
starting lavage rather than bypass. On that basis, go to the closest centre.
Also the arguments about starting rewarming 10 mins earlier; reperfusion
damage if the patient starts an output in your vehicle etc. Not a lot
(?shred) of evidence either way, though.
I would not assume someone with 1 hour immersion and VF to have a temp under
30. Cooling times vary. The temperatures at which arrythmias start vary, so
are very indirect evidence. Lots of other potential causes of VF (fell into
cold water causing MI, attempted suicide by tricyclic OD and drowning).
Matt Dunn
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