SAR Question
 

The recent Seaside Rescue programme showed the SAR helicopter carrying a diver with a "bad" case of the bends at what looked like about 1000-1500 feet.

Surely it would be better to fly as low as possible to prevent the condition from becoming worse?

Whenever I flew a bends patient I was always requested to fly as low as possible by the medical staff. Obviously you can't do that if there are other restrictions - night, wx, terrain etc. I had an interesting conversation with a doctor from the RAN hospital in Sydney who explained the reasons to me in great detail with lots of long words (the gist of which I can't remember) but 'tis important apparently.

Bends and Low Altitude
 

That is correct. The bends has different forms but is basically a problem with nitrogen which has bubbled out of the tissues or blood rather than being returned to the lungs and expired normally.

This will have occurred with a reduction in ambient pressure as the diver ascends from depth. Ambient pressure is further reduced (at an additional 1mb per 27ft agl) which could permit the offending nitrogen bubbles to further grow and therefore cause further damage.

Thus, it is preferable when transporting bends victims to fly them at the minimum altitude that the flight can be still be conducted safely thereby minimising bubble growth.

Compare the effects of altitude/depth verses pressure.

At the surface atmospheric pressure is 1 atmosphere (1 bar) by definition.

Consider a descent under water. At 10m below sea level the pressure is 2 atmospheres & at 20m it is 3 atmospheres. This continues in a roughly linear fashion due to water being incompressible.

Now consider a climb in an aircraft. 1 millibar (a thousandth of an atmosphere) = 27 feet at sea level (air of course is compressible so the pressure change is not linear but can be considered so over small altitude changes). So at 1500' the pressure will have changed by 55 millibars, ie 0.055 atmospheres.

For the bends to have been a concern a diver would need to go down to 20m & for a serious case probably significantly more. So having already had a 2 atmosphere pressure change another 0.055 is insignificant, & indeed the kind of altitudes a SAR helicopter would normally transit at will have little additional effect. That said obviously the lower you can safely fly the better.

As an aside it is normally recommended that leisure divers allow 24 hours before flying. This is not because normal cabin altitudes are likely to be a problem but a cabin decompression at 39000' could cause real problems for a diver.

Thanks. That was my understanding as well.

Yes - you should fly bendy divers as low as possible but, if you're on an overland transit you should not follow the contours. Repeated climbs and descents can be very detrimental to the patient and it's better to pick one altitude and stick to it until the final descent to the decompression chamber LS.

Very happy to show my ignorance and gratitude all at the same time.
never knew Napoleon was a this smart.

spent oodles of time trying to figure out the relationship betwixt millibar and PSI for a motor car tyre.
Thanks, learn something everyday, what eh?
cheers tet

Merely trying to highlight the vast difference between the units involved. I don't mean to insult anyone's intelligence.

Having discussed this question at length several times and consulted the experts who run the hyperbaric chambers, the crucial issue is time to treatment. If you have a choice between a protracted low level transit and a quicker but higher level one, then the quicker option is the one to choose.

The sooner the casualty can be recompressed and given oxygen therapy the better the prognosis. I know it seems to fly in the face of common sense but that is what the experts recommend and by taking a long time to get the cas to hospital you can worsen, rather than improve, the situation.

Having run a chamber or two in my day, the treatment protocols are to get the patient back to depth as rapidly as possible, so crab is correct.

Depending upon the type of DCS (Decompression Sickness, Type I is simple pain such as joint pain, Type II involves neurological damage), tissue is busily dying the longer that gas not in solution (bubbles, essentially) is in the tissues. Recompression forces gas back into solution so that it can come out of solution via the alveoli in the lungs. Open a bottle of beer to see the process in reverse.

A common myth is that there is a requirement to go to a great depth to contract DCS. Not true. The final 3m in the water column to the surface are the most dangerous because of the steeper pressure gradient.