Loss of consciousness at altitude

[Winkiepinkie]

This is probably a stupid question, but one that I have never got a good answer to.

Why is it, that when there is a sudden decompression at altitude (say 37,000 ft) a person will lose consciousness almost immediately. Clearly my train of thought "ha, I can hold my breath for at least 60 seconds" holds no value.

Even with no air in lungs, at 1 bar (i.e. seal level) , you do not lose consciousness straight away (you would stay conscious for at least a few seconds) so why does it occurs at altitude? What is it that makes the difference?

Thanks in advance for any replies, W.

[MobiusTrip]

You don't suddenly conk out if you experience rapidl decompression at 37K, you have a (small) period of useful consciousness followed by a period of (I suppose) 'useless' consciousness before you finally expire (assuming you are not hooked up to appropriate oxy). You could hold your breath - but you can't hold the same mass of air in your lungs at 37 K as you would at SL, you could try, but you'd prolly blow up.

If you do a Google search for hypoxia, Rapid Decompression and the like then you'd most likely find a bunch of gen. I am sure there are a bunch of people who know much more about this subject that I do that will respond to this question - I just know from experience, I don't know the whole medical shennanegons about it.

If you go through (most countries) mil flying training, you get the delight of experiencing the decompression chamber. This fun machine can simulate RD and you can experience hypoxia and horrendous stomach cramps cos you didn't follow the advice to avoid the beer and curry the night before.

Cheers,

MT

[teeteringhead]

IIRC (and not from Googling) it's because the blood's ability to accept and transport oxygen is dependant on the "partial pressure" of the oxygen, derived from the proportion of oxygen in the air (same at 370 as on the ground) and the ambient pressure(a lot less at 370). So, simplistically, the brain is fairly rapidly starved of oxygen, as the body was designed (or evolved) to function at normal Ts and Ps.

When I did my military flying training, we were decompressed while being invited to write names and addresses on a piece of paper. The "drunken" deterioration in the writing certainly confirms Mobius' theory of "useless consciousness".

And the beer and curry the night before could be quite noticable - even if it was the other guys who had indulged!!:ugh:

[gingles]

but you'd prolly blow up

you actually have a massive uncontrollable fart

[QDMQDMQDM]

One reason you go unconscious immediately with explosive decompression is you may well end up with something like this:

http://www.uam.es/departamentos/medicina/anesnet/journals/ija/vol3n3/answer2.htm

As the air in your chest expands rapidly due to the massive drop in ambient pressure you can literally blow out the chest wall. This is much as happens to a diver who surfaces too quickly. Speed of decompression is important.

In a decompression chamber, they turn the pressure down gradually. The same would happen in a Jumbo with one window out. The cockpit of a fast jet, however, or cabin of a Lear is a different kettle of fish. There, you may well get explosive decompression and your emergency O2 mask is about as relevant as a lifejacket off Greenland.

Any true aviation medics please pick me up if I have this at all wrong. Bad Medicine, FD?

QDM

[Bad medicine]

Both QDM and teeteringhead have part of it. In an explosive decompression to high altitude, if one holds one's breath then the trapped gas in the liungs will expand according to Boyle's law. The air in the lungs may rupture into the chest cavity causing a pneumothorax (which may compress the heart enough, or reduce blood return to the chest enough to cause a rapid loss of consciousness). If the air ruptures into the blood supply of the lung, it may travel to various parts of the body. If it goes to the brain (cerebral arterial gas embolism - CAGE), it may cause instantaneous loss of consciousness.

The partial pressure of oxygen in the inspires air (Dalton's law) will also decrease significantly, and as teeteringhead says, this is what drives the oxygen onto the haemoglobin in the blood. There may also be some off-gassing of the oxygen off the haemoglobin, back into the atmosphere. Therefore the amount of oxygen available to the brain is reduced very rapidly.

There is also a risk of decompression above 18000ft.

If one simply holds one's breath at sea level, the partial pressure of oxygen drops fairly slowly, as there is a large reserve of oxygen in the lungs.

Having said that, time of useful consciousness at 37000ft is still around 1 min. 45000 and up, it is about 9-12 sec. Remember, as QDM said, that the cabin altitude might take some time to reach aircraft altitude (and may never reach it). In fast jets, after loss of a canopy, the cabin altitude may exceed the aircraft altitude due to the aerodynamic shape of the top of the fuselage. Time of useful consciousness, and the risk of decompression illness depend on a number of factors, including rate of decompression, altitude, duration of exposure, exercise at the time, etc.

Cheers,

BM

[Milt]

What happens above about 60,000 ft.

At the low air pressure up there and above you've had it. Your blood boils

Pressure suit is mandatory.

Also at about 35.000 ft you need to breath pure oxygen to get the same oxygen into the blood as at sea level. Above that you are being deprived unless you wear a pressure mask which will take you up to about 45,000.