Flying After Shallow Dives

[gxsr_sarge]

I understand that the common advice given is to avoid flying for at least 12 hours after diving (24 hours for deep and/or consecutive days of diving). I further understand that the purpose of this is to give more time for our tissues to off-gas and equalize at around 1ATA prior to being subject to the relatively high aircraft cabin altitude/lower atmospheric pressure of around 8,000ft and thereby subjecting the body to further expedited off-gassing thereby increasing the risk of DCS.

When you think about it, controlling the risk of DCS is all about controlling the relative change is pressure. One atmosphere at sea level is defined as 14.78 pounds per square inch. But at 8,000 feet above sea level (approx cabin pressure) the atmospheric pressure is roughly 0.74 or 74 percent of sea level pressure. Thus, we would be at .75ATA. So for argument's sake, there's really not THAT much of a relative change in pressure. As opposed to ascending from 33 FSW to sea level where there is a 2:1 change in atmospheres.

The "founding fathers" of deco theory initially came up with a 2:1 "safe" relative change in ATA's which was later narrowed down to approx. 1.5:1. These are the bases behind some of the most popular Buehlman/Navy model deco tables we use today. Accordingly, the relative pressure change that one would experience in flying after diving would still fall within these ratios.

My question is: If one limits diving to two repetitive sub-2 ATA (i.e., 33 or less FSW) dives and then flies within a relatively short period of time (say around 6-10 hours after surfacing after the second dive), why is there a risk given the relatively minor change in ATA's? For that matter, why can't one just jump right on a plane after diving shallow? Can any risk be mitigated by using EAN36 or higher? What am I missing?

I'm sure that most of the answers will be "we don't know that much about it so why risk it" - which in the absence of concrete science - I agree with. I'm also not trying to promote or condone any unsafe practice. I'm just curious about the science/rationale behind this often repeated advice in the context of shallow dives.

Cheers

 

[knotical]

Reasonable question.

This table from NOAA:
http://www.ndc.noaa.gov/pdfs/AscentToAltitudeTable.pdf
allows you to climb to 8000 ft altitude with no wait time if you are no greater than a C-diver, as defined by NOAA tables here:
http://www.ndc.noaa.gov/pdfs/NoDecoAirDiveTable.pdf
and
http://www.ndc.noaa.gov/pdfs/ResidNitroTable.pdf

So according to NOAA, it could well be “permissible” to do as you suggest, depending on your dive durations and surface intervals.

 

[gxsr_sarge]

knotical, thanks for the tables. The "Ascent to Altitude" table is very helpful.

 

[eponym]

I understand that the common advice given is to avoid flying for at least 12 hours after diving (24 hours for deep and/or consecutive days of diving). [snip] When you think about it, controlling the risk of DCS is all about controlling the relative change [in] pressure. [A]t 8,000 feet above sea level (approx cabin pressure) the atmospheric pressure is roughly 0.74 or 74 percent of sea level pressure. So for argument's sake, there's really not THAT much of a relative change in pressure. As opposed to ascending from 33 FSW to sea level where there is a 2:1 change in atmospheres.

The "founding fathers" of deco theory initially came up with a 2:1 "safe" relative change in ATA's which was later narrowed down to approx. 1.5:1. These are the bases behind some of the most popular Buehlman/Navy model deco tables we use today. Accordingly, the relative pressure change that one would experience in flying after diving would still fall within these ratios.

I'n not clear whether or not you are correctly focusing on the key ratio: between the pressure of dissolved nitrogen in your tissues (accumulated at depth, decreased somewhat during your surface interval) versus the pressure of nitrogen in your lungs while aboard the plane. Perhaps that's just my misreading.

Regardless, in your analysis you don't account for the possibility of the cabin losing pressure at altitude, which could put a diver at substantial risk of DCS. The inevitable delay in getting to the ground and getting proper medical attention thereafter are a major factor in my thinking on this issue. In risk/reward terms, the extra time spent ashore before flying home is not a great burden considering the additional safety it affords.

My question is: If one limits diving to two repetitive sub-2 ATA (i.e., 33 or less FSW) dives and then flies within a relatively short period of time (say around 6-10 hours after surfacing after the second dive), why is there a risk given the relatively minor change in ATA's? For that matter, why can't one just jump right on a plane after diving shallow?

I asked a similar question some years ago--although I phrased it not as "why is there a risk" but as "how great is the risk." I had been asked to conduct some shallow training on the morning of the same day I was scheduled for an evening commercial flight. I asked SB's own Dr. Deco, if I remember correctly, and his opinion was that in my case (limited to fifteen feet) there was no significant risk associated with flying.

In the years since then I've found myself teaching diving and leading dives locally, including several high-altitude lakes (3000 to over 6000 feet above sea level), some of which require travel over even higher mountain passes. In this regard I use the same table Notical referred to: NOAA Diving Manual table 4.3 (in my edition), titled "Required Surface Interval Before Ascent to Altitude After Diving." It includes altitudes to 10,000 feet, so as you've already responded it could be used in your case if you don't care to consider loss of cabin pressure a possibility. I use it to determine minimum surface interval before driving back over the passes, but loss of pressure in the truck cabin is not an issue.

-Bryan

 

[Dr Deco]

Hello gxsr sarg:

Altitude Depressurization

Whether or not you have been diving, depressurization to altitude can result in decompression sickness. Whether it is frank DCS or just a few Doppler-detectable bubbles depends on the height to which you ascend , i.e., the final pressure. This is the result of microbubbles being present in your tissue and some amount of dissolved gas. [In altitude, we must count not only nitrogen, but also oxygen carbon dioxide and water vapor.]

the micronuclei result from musculoskeletal activity as we move about in our daily activities. Depressurization to altitude that was my major research focus at NASA, and in addition was the role of the activity-generated micronuclei. It is possible to choreograph depressurizations to render very repeatable results, but this regularity is not found in the real life of humans on this planet, generally. This includes SCUBA divers.

Scuba Diving

When divers ascend to altitude, they take an additional gas load in addition to a random concentration of micronuclei. This results in a problem that can not be easily solved in advance, Only general guidelines can be given. Laboratory studies have shown that DCS is possible with altitude and diving, and it is not easily determined by ratios and loads.

To this we must add an unexpected depressurization of the cabin. You would not want to have abbreviated you surface interval when this occurred!

Dr Deco :doctor:

 

[gxsr_sarge]

I'n not clear whether or not you are correctly focusing on the key ratio: between the pressure of dissolved nitrogen in your tissues (accumulated at depth, decreased somewhat during your surface interval) versus the pressure of nitrogen in your lungs while aboard the plane. Perhaps that's just my misreading.

I was referring to the change in pressure in ones tissues relative to sea level and subsequently on board an aircraft. Sorry for the confusion. Thanks, by the way, for sharing your experience on the 15' shallow dive and the tables.

When divers ascend to altitude, they take an additional gas load in addition to a random concentration of micronuclei.

To this we must add an unexpected depressurization of the cabin. You would not want to have abbreviated you surface interval when this occurred! Dr Deco [/I][/color]:doctor:

Dr., Thank you for your response as well. However, I have a follow-up question: How does one take on additional gas when they ascend?? Doesn't the decrease in pressure call for the opposite (i.e., the off-loading of gas)?

Also, regarding both of your points on sudden cabin depressurization .... maybe I'm glossing over a very important safety point. While I have flown literally over 2 million miles since 1995 (thanks to my work - I guess), this has never happened to me (knock on wood)? Do either of you have any personal knowledge of this occurrence?

Cheers