Medical Issues of Diving at Altitude

[cougar]

I am an EMS instructor. I have been teaching about the body's response to high altitude vis a vis climbing. In Colorado there are several dive sites well above 5,000 feet. One in particular is at 9,000 ft with a depth of 68 ft. Assuming the diver lives at sea level and does things stupidly. What happens physiologically when he stay down too long, blows his safety stop and pops up like a bubble at 9,000 ft? Because there is less air pressure at 9,000 feet...is 68 ft less than 68 feet? What happens to the rules as I know them and what happens to the human body?
Thanks in advance for any input you may offer.

 

[knotical]

The short answer:
From a DCS perspective, essentially the same thing happens at altitude as at sea level, just more extreme.
Because of lower atmospheric pressure, ratios between ambient at a certain depth and at the surface are greater.
i.e. 68 feet is “more than” 68 feet.

For a diver who lives at sea level, there is also the issue of acclimating to elevation. Arriving at elevation is similar to having just surfaced from a very long shallow dive.

One of many articles available:
Diving At Altitude

 

[Duke Dive Medicine]

Cougar,

The U.S. Navy Diving Manual has a pretty good explanation of it. It's available for download here. See Chapter 9, starting on page 9-46.

Best regards,
DDM

 

[Glenn08]

Supposedly dive computers are now available that can do altitude compensation calculations and profiles. The simple way to go is a capillary depth gauge, which by design will compensate for altitude.

 

[Montana Diver]

At 9,000 ft, the ATA is .73. The true depth will be greater than the 68' due to the lower pressure at the surface. The possibility of bubble formation could be greater since the tissues need to off-gas more to reach equilibrium.
The other factor that might be a little more troubling is that normally we experience a doubling(200%) of absolute pressure in the first 33 feet of descent, however at this higher altitude, the effect will be greater. At 34 ffw
the pressure will be 1.73 or nearly a 240% increase in pressure. Greater awareness of frequent equalization is imperative.

While most computers will compensate for the altitude differential, the computer between our ears needs to analyze and adapt to this environment.

Dennis

 

[Duke Dive Medicine]

For clarification's sake, 68 feet of fresh water is 68 feet of fresh water. The pressure change due to the water is no different at altitude. It's all about the pressure that the diver returns to once he/she finishes the dive. The Navy decompression tables and recreational no "D" tables are calculated assuming that the diver will be at sea level after ascent. The altitude depth correction is a way to adjust printed decompression tables to compensate for the fact that the diver will be returning to an ambient pressure that is lower than that at sea level. If you have a dive computer that adjusts for altitude, you don't need to make any further depth corrections. Also, commercially available decompression software like V-Planner and Deco Planner both have altitude diving options and will generate tables that don't require altitude depth corrections as long as the diver's depth gauge is calibrated for use at altitude.

 

[Glenn08]

68 feet of fresh water is 68 feet of fresh water. But it's not that simple at altitude. At 9000 ft altitude, the absolute pressure is 0.715 atmospheres. Expressed in feet of sea water, 9000 ft altitude is 23.6 ft. Decompression is about pressure, especially pressure ratios. For decompression calculations, 23.6 ft at 9000 ft altitude is comparable to 33 ft at sea level. Spend some time studying decompression theory. And, just for a second opinion, take along a capillary depth gauge and brush up on your tables. And dive conservatively.

 

[rossh]

68 feet of fresh water is 68 feet of fresh water. But it's not that simple at altitude. At 9000 ft altitude, the absolute pressure is 0.715 atmospheres. Expressed in feet of sea water, 9000 ft altitude is 23.6 ft. Decompression is about pressure, especially pressure ratios. For decompression calculations, 23.6 ft at 9000 ft altitude is comparable to 33 ft at sea level. Spend some time studying decompression theory. And, just for a second opinion, take along a capillary depth gauge and brush up on your tables. And dive conservatively.

Internally inside a program, that are arranged to compute in units of equivalent water depth pressure, what you say is true. BUT, none of that information format is displayed to the user.

A good computer has the smarts to measure surface pressure at the start of a dive. The user will see only real depth (using a modern computer). To the diver, 68ft is 68ft real depth.

The need for table corrections and depth meter adjustments, is for older capillary depth gauges only. For most divers today with computers, its not required. The computer does all the calculations needed for elevation diving.

 

[cougar]

Here's what I am wondering about. I have seen enough people go to the chamber in Cozumel due to sloppy profiles, skipping deco stops and too many days of repetitive diving that my question is from the emergency responder point of view. If they dive up here at at altitude (I just found out we have a lake at 7,500 feet with a max depth of 350ft.) Are they in any WORSE trouble than the same guy who is sloppy at sea level? My gut says that even if the deco tables on their computer takes the altitude into consideration...when they surface...they aren't at sea level and most of them are going to stay at altitude. I wasn't asking what the computer adjusts for... What I am wondering is, if things go south...is the fact that you don't come out of the water at sea level a factor that makes things significantly worse or does it not matter?

 

[lambroskol]

Actually i was wondering about that too.

If you finish a dive at 3000ft altitude and the dive had max depth 65 ft is it safe to return after the dive at sea level or it is dengerous?