Skin Bends - Lessons learned

[Ken Kurtis]

It's interesting how readily you arrive at this conclusion, despite DAN's more measured opinion and despite the aggressive profiles, which you seem to relegate to a near-afterthought

Perhaps I didn't 'splain it clearlty enough so let me try again.

First of all, I had three phone conversations with DAN (two during the incident and one post-incident a few days later, one 30-minute conversation with Karl Huggins at the Catalina Chamber, some further chats with Audrey, and ruminated on it for a week. That's not a rush-to-judgement or knee-jerk reaction in my book.

I emphasized the dehydration because it's something people frequently don't think about. I'm not saying it's the main casue. If I had to rank them in order, it would be:

1. Aggressive dive profiles (in other words, diving the computer to the limits) over multiple days.
2. Body fat composition that likely puts her outside of the computer model predictions (nitrogen stores more readily in fat).
3. Dehydration.
4. Bare minimum (3 minutes max) safety stops.

They all factor in with #1 being the main culprit, #2 & #3 adding to the problem, and #4 being a potential solution (in other words, her "safety stop" - had it been extended - really becomes a "deco stop" that takes her out of decompression and perhaps avoids the hit).

The other thing I discussed with Audrey was just generally how to use the graphic nitrogen-load pixels on the left side of the PP2 as a guide. You've got a bunch of green ones, then two yellow ones (caution zone) and then it goes red (deco). She never got into the red but frequently lit up both yellows.

I advised her that she should stay at her safety stop long enough to clear BOTH yellow pixels so she surfaces at the top of the green. (Years ago, Oceanic had that recommendation in their computer manuals but I couldn't find it in any of the current versions.) You can generally clear the top yellow pixel pretty quickly. It's that first yellow pixel that may take a while when you're trying to off-gas some more.

But again, the take-away is that isn't it better to spend ten minutes longer in the water waiting for that second pixel to clear than to spend six hours in a chamber wishing you'd spent ten minutes more in the water????



- Ken

 

[vladimir]

Perhaps I didn't 'splain it clearlty enough

Well, since your #1 factor was described as "The other thing that may have factored in," and your #3 factor was described as "a big factor in all of this" and "probably what pushed her over the edge," I agree that perhaps some more explaining was called for. Thanks for clarifying, Ricky.

It is perhaps worth noting that obesity, your #2 factor, is also considered by DAN to be a risk factor that lacks conclusive evidence.

 

[TSandM]

I don't know what her profile looked like, but another thing to think about is the ascent after the safety stop. All too often, people "do their time" at 15 feet, and then basically go from there to the surface in the minimum time possible. You're still in high proportional pressure change territory there, and that last ascent should be slow, if it is safe to do so (sometimes, I realize, with high boat traffic and free ascents, you just can't spend time in that shallow zone). We try to take at least two minutes to go from ten feet to the surface. It makes me, at least, feel way better.

 

[redacted]

The concept of specific body tissues corresponding to specific time compartments is largely a theoretical one. With the exception of some fairly remote studies done by the Navy (the results of which, to my knowledge, are not generally available) I'm unaware of anybody instrumenting divers with anything that could measure nitrogen levels in real time during a dive.

Skin would be a particularly problematic tissue to assign to a compartment, because blood flow to the skin is highly variable. It's markedly decreased by cold, volume depletion, or high adrenaline levels, and markedly increased by surface warming.

I got to thinking about that after I posed the question and your answer regarding the variability makes complete sense. In fact, I suppose that areas of skin could actually change during a dive as blood flow changes as a response to body temperature changes. Perhaps that could be why skin bends seems to be somewhat common. Someone who starts a dive nice and warm with good (or even excessive) blood flow to the skin may load nitrogen at a higher rate at the beginning of a dive and a lower rate later in the dive and during ascent when the body may have responded to the extended exposure to cooler water.

I see I will have to stop giving my wife/buddy a hard time about wearing a 5mm full suit in tropical waters.

I was reading an article the other day that suggested the brain and other nerve tissues were in the faster compartments and that bone and fat were in the slower compartments. But I saw Ken's post above that suggests fat is also one of the faster tissues. But, I guess fat is another tissue that may experience a high variability of blood flow. I also saw an article that said blood gas imbalance has a half-life in the single digit seconds which I am trying to still wrap my head around.

 

[TSandM]

A lot of authors draw a parallel between "fast" compartments and highly perfused tissues, and "slow" compartments, and less highly vascular structures. It gets confusing, though -- for example, although calcified bone and synovium aren't highly perfused, periosteum and bone marrow ARE. So is bone a slow tissue, or a fast one? Connections are drawn between profiles which seem to put "fast" compartments at risk (deep dives, fast ascents) and apparently high rates of injury to central nervous system and spinal cord, but some of those injuries are actually embolic, which doesn't have anything to do with the speed of offgassing of the tissue.

All the references I have urge caution with trying to interpret the mathematical models in terms of actual body structures, and I think that is wise.

 

[Ken Kurtis]

But I saw Ken's post above that suggests fat is also one of the faster tissues.

I would sugest that fat is actually a SLOW tissue.

The way I've always understood (and taught) the terms is that it's all relative to the half-time of the theoretical tissue. Fast tissues have short half-times which means (remembering that 6 half-times is considered saturation/desaturation) that a "fast" tissue fills up quickly but it also empties quickly. A "slow" tissue may fill up more slowly, but - perhaps more importantly - it also empties more slowly. So my perception of fat has always been that it's a "slow" tissue since it tends to hold on to the dissolved gas longer.

The other thing to think about is margin-of-error between you and the model. I always tell my people that dive tables & computers are "mathematical models that may or may not pertain to your particular body physiology." So let's assume, for the sake of illustrration, that your body's tissue half-times are just 5% higher than what the model is predicting.

If we take a "fast" tissue with a 5-minute half-time (which is actually the fastest tissue group in the PP2), that difference is only 15 seconds per half-time or 1.5 minutes (6 half-times) for saturation/desaturation. Probably not a big deal over the course of a "normal" recreational not-square-profile dive. However . . .

The slowest tissue group in the PP2 is 480 minutes. A 5% error there skews it 24 minutes per half-time or a whopping 144 minutes (almost 2.5 hours) for saturation/desaturation. Even if you look at the PADI tables where the slowest half-time is 60 minutes, a 5% difference there is still 18 minutes on a dive that goes to the tissue compartment limits.

No matter which numbers you use, the point is that as you account for the personal differences between you and the computer model, the gap between what the model says and what your body says - especially when it comes to "slow" tissues - could be quite significant and it's something you need to keep in mind when you're diving. And that's simply another reason I'm personally a big believer in long safety stops because if you're body's gone into deco even though your computer hasn't, a long safety strop may be what prevents you from adding a Chamber ride to your list of dives.

So - show of hands - how many of you want to keep pushing your computer limits because you think it's "safe"?????

- Ken

 

[Mr Carcharodon]

The issue with fat is solubility more than time constant. Fat rich tissues dissolve a greater volume of gas for a given pressure. I remember a paper that had micrographics of the spinal cords of subjects that had been compressed/decompressed. The bubbles present were all concentrated around the fatty tissue in the spinal cord. Water in contrast is a lousy medium to dissolve nitrogen in. If I recall correctly you gain an additional ~12 ml of nitrogen per atmosphere of extra pressure per liter of water at 40C. Being dehydrated by a liter would be fairly significant and would only shift your nitrogen load by less than one percent.

Personally I believe focusing on dehydration is a distraction from paying attention to depth, time and mix. Recreational divers were be better served if they were told how to decompress from dives that ran close to the limits they are working with and how to add time appropriately for risk factors like getting chilled, working hard, being overweight/obese and maybe even dehydration.

Some of the early physiologists did measure nitrogen tension in venous circulation returning from various organs in order to get an explicit time constant. So there is some physical basis for the time constants. Even then though the values are aggregates for multiple types of tissue in an organ.

 

[Hatul]

Personally I believe focusing on dehydration is a distraction from paying attention to depth, time and mix. Recreational divers were be better served if they were told how to decompress from dives that ran close to the limits they are working with and how to add time appropriately for risk factors like getting chilled, working hard, being overweight/obese and maybe even dehydration.

Some of the early physiologists did measure nitrogen tension in venous circulation returning from various organs in order to get an explicit time constant. So there is some physical basis for the time constants. Even then though the values are aggregates for multiple types of tissue in an organ.

Unfortunately the reaction of some divers to their computer going into deco is to look for a more liberal algorithm. I see people looking for the most liberal computer, which is really saying, I want to increase my bottom time and sacrifice my margin of safety.

 

[redacted]

Unfortunately the reaction of some divers to their computer going into deco is to look for a more liberal algorithm. I see people looking for the most liberal computer, which is really saying, I want to increase my bottom time and sacrifice my margin of safety.

Again, NO. It is not saying that. At least not with everyone seeking safe, liberal computers. It is saying I want to know where the edge is and I'll manage how close I'll get to it and how careful I will be when I approach it.

 

[Garuland]

Could someone explain PFO?

Thanks!