wayne2 once bubbled...
All of my formal dive training has preached no flying after diving, for some period of time. Some divers use a dive-computer and many others simply use a rule of thumb like; no flying within 12 to 24 hours after diving. I don't remember any mention of different flight altitudes above sea level being a factor.
Is there any correlation to flying @ low altitudes such as 500'ASL to 1000'ASL and below as apposed to flying commercial aircraft that may have cabin pressures in excess of 5000'ASL.
My question is for any Doctors out there that may be able to clear up some misconceptions with regards to this subject. What are the hard facts and rules on this subject? Would you consider it safe for divers to fly @ lower altitudes within a few hours of diving?
Thanks.
This assumes no formal 02 decompression and diver just does no stop limit, NSL, dives.
Yes, but it depends on the duration and depth of previous dives, and surface interval time, SIT, between previous dives.
A bit more detail in diving training curricula may be helpful to divers as its common to take small non-pressured aircraft to a major airport, and this entails lower altitudes.
Another poster mentioned the FAD table in the NOAA manual. The FAD chart is taken from the 2000 USN Diving Manual and can serve as a more aggressive guide beyond the simple 12h or 24h FAD SIT rule [or its recent compromise, a midpoint of 18 hours]. You must know that bulk of the chart is an extrapolation from very few data points [ a personal communication with DAN researcher who did the table.] It measures your inert gas exposure as inert gas groups in the USN Air tables and gives a SIT before ascent for various altitudes.
http://www.ndc.noaa.gov/pdfs/AscentToAltitudeTable.pdf
By 'safe' a certain degree of risk is taken for flying before the 24 hour period: the sooner the ascent after a dive, the higher the ASL altitude, the higher the risk for DCI. Recent data has reduces the FAD SIT to 12 hours, but in multiday dives to maximum exposure on NSL [3-6 dives a day to table limits], a 24 hours SIT is still recommended.
The rule you mention is for flying in commercial planes pressurized at 8000 ASL ~ 0.7 ATA
The good news is that most divers suffered joint bends after a short FAD interval.
The bad news, is that diver in flight has to suffer with the discomfort until landing, but this can be mitigated if not treated in flight by 100% 02. The longer joint bends are left untreated, the more possible one may suffer from dysbaric osteonecrosis later on in life.
In a diving analogy, a subject at "0' ASL = 1 ATA" taken to higher altitude is like going shallower [flying] after a saturation dive [your surface time without diving], if ascent is rapid and high, inert gases in tissues may go from dissolved to bubble phase and cause DCI even without diving or in bubble theory, feed existing micronuclei into further growth. This is a common problem for pilots ejecting from planes or in sudden cabin depressurization. A solution is to desaturate tissues from inert gases by pre-breathing 100% 02 before flight, and Dr. Deco has excellent discussions on this scattered throughout this website.
After diving, there is an increase in inert gas tension depending on the depth and duration of a dive. NSL tissue surfacing tensions are calculated so that for most people, on surfacing to 1 ATA, tissue gas tensions are low enough to keep gases dissolved rather than in the bubble phase, note people may bubble more than others. If the diver surfaces at a lower ambient pressure, such as commonly taught in diving at altitude without sufficient acclimatization, then different mechanisms are in order, and corrections are needed to the tables.
The recreational FAD SIT is based on few studies, expert judgment and epidemiological review. Because the data is controversial, continued research in refining the optimal interval is ongoing. Its very possible to fly substantially before the 12 hour limit, but there is little data to make such a recommendation to the general diving population.
For those willing to be more aggressive, general guidance can be had from the USN NOAA FAD table. Most dive computer algorithms take their cue from the UHMS recommendations, the USN FAD table, and proprietary rules of thumb such as in the SUUNTO Vyper, which increase the FAD SIT to past 30 hours.
Aggressive models use Buhlmann's algorithm to set a flight as the end point of a dive: it calculates a SIT based on an inert gas desaturation rate at 1ATA such that a diver can surface at 0.5 ATA without troubles [ with 8000' ASL ~ 0.7ATA the differences are margins of safety]. These algorithms are especially aggressive when part of formal 02 decompression, it literally can takes a diver into altitude immediately.
Take Home Message:
Know that a FAD USN table exists to give you the range of FAD SIT possibilities
Single Rule of Thumb:
Target flights ~ 18-24 hours after your last dive. Avoid flying before 12 hours FAD SIT. A > 24 h FAD SIT duration is ideal as possible.