DCS and altitude

[broncobowsher]

So many variables. This is why I like the tissue graph on the Shearwater. You can look at it and decide for yourself what your risk looks like. Instead of going with a canned generic covers the worst case answer.

For myself, I'll do a weekend of San Diego technical dives. Then to head home there is a ~5000' mountain pass. By the time I am done diving, there is a boat ride back, unload the gear, load up the truck. Go find a place to eat, sit down take your time place. Now start the drive several hours after getting out of the water. What does the tissue graph look like? The fast stuff is all back to normal. The medium and slow tissues are the ones I look at. Start the climb, it isn't a fast climb. Then down the back side back to nearly sea level (Colorado river) and a few more hours across AZ with almost no elevation

I know dive shops in Phoenix that have trips to San Diego that tell you that either go up through LA because the mountain pass on I-10 isn't as high as I-8, or spend the night in San Diego before heading home. It is a generic one size fits one one but is blindly applied to everyone rule.

 

[boulderjohn]

In case the point was missed in my earlier post, an ascent to 2,000 feet can be done at any time, so the OP's ascent of a little over 1,000 feet is not an issue.

 

[Fishdip]

I would like to give a more detailed explanation of these two posts.

It is extremely hard to say anything definitive about driving to altitude because there is no good way to measure such trips, which are all different, and because it has not been studied much at all. When one is flying, the ascent is very rapid. When one is driving, it can be like doing a staged decompression ascent. What jvogt describes is something I have done countless times, following the same route at about the same time after diving. For a full description of the dive, go to this thread. Here is a shortened version.

1. Begin with the understanding that both DAN and the US Navy say an ascent of 2,000 feet is acceptable at any time after a dive.
2. He took about an hour at the site to pack his gear before leaving.
3. He had been breathing pure oxygen underwater for maybe 40 minutes, and his fastest tissues had off-gassed beyond saturation for that altitude. That means that those tissues were on-gassing as he breathed the local air while packing his truck.
4. After leaving his 4,600 foot altitude dive site, he had an ascent of about 800 feet, which took about 15 minutes.
5. He then stayed at that elevation for about a 20 minute decompression stop.
6. He then climbed another 600 feet (to 6,000 feet) and then another slow climb, reaching 6,800 feet after about an hour of total driving from the dive site. Note that this is almost exactly the limit for an immediate ascent to altitude, but he did it over two hours, with the first hour decompressing at the site and the second ascending with a couple of decompression stops along the way.
7. He then did a 1.5 hour decompression stop as he drove, still at about the limit for an immediate ascent to altitude.
8. He then did a fairly rapid ascent of around 1,000 feet to the top of the mountain pass.

All that assumes he was just breathing the air as he drove. If it were me, I would be breathing oxygen for at least the first hour of that drive.

As you can see, that dive and that drive have characteristics that make it probably safer than most drives to that elevation, but there is no research to support it. As I said earlier, I have decided that this drive is safe enough for me, at least while breathing oxygen, but I will not tell you that it is OK to do it. That is a decision you have to make for yourself.

So what about your drive to altitude? You will have to analyze it the way I have analyzed mine, and then you will have to decide for yourself.

A final note: when I researched this, I talked with someone at DAN who told me that all ascents are not the same. Ascending from sea level to 2,000 feet has a greater impact than ascending from 4,600 feet to 6,600 feet.

So to see if I am understanding this correct when I drop down from the dive site its acting as a deco stop?

 

[Akimbo]

So to see if I am understanding this correct when I drop down from the dive site its acting as a deco stop?

Not sure I understand the question but here goes: Whenever a person ascents (goes to a lower pressure) they outgas diluent, usually Nitrogen, which is decompression. That continues until they reach saturation (equalization) at the new pressure. Outgassing fast enough for bubbles to form in your blood stream and interfere with blood flow is when you could have DCS. It doesn't matter if the pressure change is in a water or gaseous atmosphere.

Info - Decompression & Treatment Simplified

1: Introduction Everything should be made as simple as possible, but not simpler — Albert Einstein (paraphrased) I probably violated the "But Not Simpler" part, but I tried to include enough Hyperlinks to make amends. Clicking on them should allow any interested diver to "drill-down" as far...

scubaboard.com

 

[boulderjohn]

So to see if I am understanding this correct when I drop down from the dive site its acting as a deco stop?

The descent you do is pretty much inconsequential, but, then, so is your ascent.

Let me explain the idea of a decompression stop, and that may help. When you ascend, whether in water or in air, your tissue pressure of nitrogen is greater than ambient pressure. That is called supersaturation. You need to make sure that your tissue pressure is not too high in comparison to ambient pressure. When you finish a normal dive, the pressure in all your tissues will be higher than ambient pressure, but it will be in a safe range of supersaturation. As you hang around on the surface, you are breathing in nitrogen at a lower pressure than the nitrogen pressure in your tissues, so the tissues begin to equalize (come into balance) with the air being breathed. Eventually the nitrogen in your body is balanced with the nitrogen you are breathing.

When you ascend during a dive, you go to lesser ambient pressure, and as you do, the nitrogen in some of your tissues will be greater than ambient pressure--those tissues are supersaturated. They will then start to become balanced with the ambient pressure you are now under. You have to make sure that the amount of supersaturation is not too great, so you ascent slowly. On a typical NDL dive, you will likely do a safety stop to give the tissues a further chance to come to balance before you surface.

On a decompression dive, the diver's tissue nitrogen pressure is too great to allow a direct ascent to the surface, and even on a typical safety stop, the amount of supersaturation may be unsafe. The diver therefore makes a deeper stop to give the tissues a chance to get close enough to balance to enable another ascent to lower pressure. On a serious decompression dive, the diver makes a series of stops to give the tissues repeated opportunities to shed nitrogen so that the diver will be safe to go to the next level.

The same thing can happen under the right circumstances when driving to altitude. As you ascend to lower atmospheric pressure, your tissues will become more supersaturated. You do not want them to become too supersaturated, so you wait for a while before beginning to ascend. If you ascend to an altitude that is safe and stay there for awhile, you will essentially be doing a decompression stop, and you will be speeding up the rate at which the nitrogen leaves your body. The ideal is to reach a point where you are at the highest safe altitude and stay there while your nitrogen levels drop, and then go to the next highest safe altitude.

That means your descent during the trip does not help you at all. It even hurts a little if you are then going back up afterward.

The problem is that none of this has been studied, and we have no way of knowing for sure what the safe ascent will be.

 

[crofrog]

A final note: when I researched this, I talked with someone at DAN who told me that all ascents are not the same. Ascending from sea level to 2,000 feet has a greater impact than ascending from 4,600 feet to 6,600 feet.

The Barometric Formula

Assuming a standard atmosphere:

0ft msl to 2000ft msl= 0.067 bar delta
4600 ft msl to 6600ft msl= 0.059 bar delta

 

[J-Vo]

Did you breathe oxygen on the way?

This time it did it a bit differently.

I spent an extra 30 minutes beyond deco above 10'. You can see below in the heat map, when I went down to 20' to pick up my bailout tanks all tissues went back to on-gassing. I felt after that I was fairly safe to go over the pass.

 

[boulderjohn]

This time it did it a bit differently.

I spent an extra 30 minutes beyond deco above 10'. You can see below in the heat map, when I went down to 20' to pick up my bailout tanks all tissues went back to on-gassing. I felt after that I was fairly safe to go over the pass.

View attachment 732800

Yep.

I think computer manufacturers, including Shearwater, are afraid (because of potential liability) to do what it would take to calculate ascents to altitude based on your actual dives (etc.) a part of the programming. I wrote to Shearwater about this a couple of years ago, and they told me to refer to the DAN flying after diving guidelines for all ascents to altitude. They also will not allow you to input the gas you are actually breathing during your surface interval, forcing the computer to calculate based on air only.

 

[broncobowsher]

This time it did it a bit differently.

I spent an extra 30 minutes beyond deco above 10'. You can see below in the heat map, when I went down to 20' to pick up my bailout tanks all tissues went back to on-gassing. I felt after that I was fairly safe to go over the pass.

View attachment 732800

What software is that?

 

[J-Vo]

What software is that?

Subsurface