Question-non-pressurized flying after diving

[Dr Deco]

Hello Artimas:

Limits

Flying after diving should really be approached conservatively by most divers. Duke University showed that short surface intervals of only a few hours produced what might be termed random occurrences of DCS without a clear pattern. In my experience, this is what is found when one approaches a “DCS limit/range,” where the free-gas phase in the body is beginning to grow sufficiently large to cause obvious problems. [Before then, the gas phase was not large enough to result in clinical symptoms.]

The NDLs in tables are at much reduced loads and DCS does not appear under normal conditions.

Reduced Pressure DCS

What we are finding in reduced pressure [altitude] cases is that the body has large [a few microns] tissue microbubbles already present – it is not a matter of forming new bubbles [as, for example, in the “metastable limit].” These microbubbles have come from gas loads and musculoskeletal activity prior to altitude depress resulting in hydrodynamic cavitation. This is a random [mathematically termed as chaotic] process and cannot be statistically treated unless one is a very low gas loads and/or no microbubbles present.

The number of these bubbles formed during activity can be different for each diver. Some people may be very active, hauling suitcases and gear, sometimes even shortly after surfacing when the propensity to bubble formation is greatest. Surface activity was not included in the Duke University study.

This is handled by advising divers to allow a sufficiently long duration between diving and flying. I would definitely not cut short the surface interval in the dive and fly situation.

Rapid Depressurization

Unfortunately, in airplanes, rapid depressurization is a problem that could – though rarely – occur. As I wrote a few weeks ago, depressurization at high altitude should not result in DCS problems if the plane descends. This conclusion is based on (historical) statistics from NASA laboratory studies where slow depressurization is the norm. Rapid depress would not be performed on human test subjects because of the possible great risk.

A recent paper [1] indicates that repaid depress can result in DCS that is more traceable to barotrauma and gas embolism than tissue bubble formation/growth. Note that if these individuals also had a sizable tissue gas load, their outcome might have been more severe as the emboli could grow and interfere considerably with local circulation.

Direct formation of gas bubbles in the arterial circulation with rapid depressurization did not occur in studies with sheep that I performed several decades ago [2]. The chamber was modified to allow assents of 10 ft/sec. We can assume therefore that the aircrew experienced barotrauma.

I would definitely not cut short the surface interval in the dive and fly situation and not ascend to altitude any more than necessary.

Dr Deco :doctor:



References :book:

[1] Johnston MJ. Loss of cabin pressure in a military transport: a mass casualty with decompression illnesses. Aviat Space Environ Med 2008; 79:429-32.

Presented here is the sudden cabin depressurization of a military C-130 aircraft carrying 66 personnel. They suffered a depressurization from 700 ft (2,134 m) to 24,000 ft (7,000 m), resulting in a potential 66-person mass casualty. The aircrew were able to descend to below 3049 m in less than 5 min. They landed in the Kingdom of Bahrain—the nearest hyperbaric recompression facility. Three cases of peripheral neurologic DCS and one case of spinal DCS were identified. Limited manning, unique host nation concerns, and limited available assets led to difficulties in triage, patient transport, and asset allocation. These led to difficult decisions regarding when and for whom to initiate ground level oxygen or hyperbaric recompression therapy.

[2] MR Powell, MP Spencer, MT Smith. In situ arterial bubble formation and atraumatic air embolism. Undersea Biomed. Res. 9, (1), Suppl., 10 (1982).

 

[ChrisA]

Here is what I don't understand about this: I've only done a few altitude dives but I think I could plan a safe dive in a 10,000 foot lake. So, if it can be made safe, with proper dive planning to dive and then step right out of the water onto a 10,000 foot mountain. I would think that if we planned our ocean dive as if it were in a 10,000 foot lake that we should be able to fly right after a dive.

About the only reason I can think of the above could not work is the possibility of explosive de-compression but in this case the airplane is not pressurized so it can't decompress.

Ok. Either I'm missing something (likely) or I just figured a way to fly after diving.

 

[Riger]

8< snip >8 I would think that if we planned our ocean dive as if it were in a 10,000 foot lake that we should be able to fly right after a dive.... 8< snip >8

Well I am not an altitude diver, but on a theoretical level, that makes absolute sense. Here's a question, how much different (decomp I guess) is there between altitude diving and Sea Level Diving ?

Best Regards
Richard (Riger)

 

[Charlie99]

Ok. Either I'm missing something (likely) or I just figured a way to fly after diving.

What you missed is to include all the time you are "diving" on the surface, sitting on the boat, walking around town, and sleeping at night.

If you are planning your dive as if you are at 10,000' altitude, then all of the time that you are at sea level pressure needs to be treated as if you are diving at 15' equivalent depth at 10,000'.

This is because the 0.69ata pressure of 10,000' is what you are now using as "0'", and the 1.0ata of sea level is 1.0 / 0.69= 1.45 local atmospheres. That 0.45 local atm extra is the equivalent of about 15' equivalent depth.

Include the very long periods that you are at sea level (such as sleeping overnight in your seaside hotel) as being submerged at 10' (15' equivalent depth) at altitude, and your system will work.

 

[ChrisA]

What you missed is to include all the time you are "diving" on the surface, sitting on the boat, walking around town, and sleeping at night.

Thanks. Of course it is obvious once it is pointed out. They never cover this scenario in training because if you are diving in a lake you can't then walk around at sea level post dive

I'll actually run the dive profile. It may work because you have a really long NDL at 15 feet.
Oddly there is likely a window. It may be OK to jump right on a plane while dripping wet and OK after 24 hours but but at some point between. But your observation is good - that you are still "diving" when you get out of the water.

Now I can see why they say "just don't do it" it is way to complex for most divers and only works in a few odd ball cases.

 

[Dr Deco]

As commented earlier, diving at 10,000 ft has a much lower tissue nitrogen load than being at sea level and then ascending to 10,000 feet.

Best not to mix this up!

 

[Artimas]

Is there a numerical (e.g.- mathematical) way to compare the DCS risk for flying at 8,000 vs. 12,500 feet after diving?

BTW, Thanks for the great info!

 

[leapfrog]

Very interesting and useful thread. As a SCUBA Diver and a Parachutist, I often get asked by divers who don't have a military background if they can do a skydive AFTER SCUBA diving. Of course the answer is "no". This is one of the key difficulties of military airborne insertion. We can HALO into the water, from say 30,000 feet, reach the objective but then we have to leave by sea or helicopter. The max altitude for extraction is 1,000 ft ASL.

 

[MikeRDar]

As a diver and a Coast Guard helo pilot I have to deal with this all the time. The Coast Guard (and I believe the other services as well?) have a very simple rule: 24 hours. Even if, like us, you rarely plan to fly above 1000 feet, you never know what might be required after you take off, so we have to be prepared to go up to 12,500 (our limit for the same reasons discussed above). Hence the full 24 hours regardless of number of dives.

I'd say the best solution by far is to just fly low... there's nothing much to see way up there.