DCS hits: "deserved" vs. "explained"

[PfcAJ]

Tell me what you disagree with, Victor

 

[MXGratefulDiver]

Put me in the "all DCS hits are deserved" category. It's physics. What people mean when they say they took an undeserved hit is that they followed all the rules and got bent anyway. Yes, that happens ... and there shouldn't be a stigma associated with it. And people need to understand that your dive computer doesn't know a damn thing about you, and that there is no "line" where on one side you're safe and on the other you're not. Something about your dive exceeded your body's ability to offgas properly, and even if you followed all the rules, that's what caused your DCS to occur.

What tables and computers really do is provide you with a reasonable risk factor ... meaning simply that based on mathematical computations they perceive these parameters to be "safe enough".

It's a lot like getting struck by lightning ... as long as you expose yourself to thunderstorms there will always be an element of risk. All you're doing when you take precautions is reducing the risk ... you can never completely eliminate it.

We still don't understand all the variables associated with DCS, much less how they interact together to increase or reduce our risk. It's particularly difficult to predict when some of those variables will change, even with the same diver, from day to day. Every time we go diving we risk getting bent ... best diving practices are intended to manage the risk, not eliminate it.

... Bob (Grateful Diver)

 

[victorzamora]

AJ, like I said, your comment was so random I'm really not sure if I disagree with it because it could be from my taking it out of context. However, I'll try anyway.

If you violate the your ascent parameters, you definitely deserve it.

This I agree with. Well, not "deserve" like "you deserved it, so I wish you wouldn't have gotten away with it" but deserved the way we're using it. It was expected. It was caused by diver error. Even within PADI's RDP, if you ride your BCD up like a rocket ship you are likely to get hit. If you DID ride your BCD up like a rocket and DIDN'T get a hit, I wouldn't say you DESERVED one and should've gotten one....but I'd say you weren't within ascent protocol and I would've expected one. So, this I mostly agree with....as long as your connotation is what I think it is, "expected" vs "deserved".

But "underserved" and "unexplained" etc etc simply don't exist.

This, again, I'm not sure if I disagree with it. I think I agree with the first part, and I think I disagree with THIS part. If you mean that diving has an inherent risk, and the "unexplained" doesn't make sense because the explanation is "you went diving" then I agree. From that perspective, anybody that goes diving has the possibility of getting bent. There's no mystery there. When you expose your body to >1ata, there are effects that could lead to consequences, like DCS. If someone got bent sitting on their couch, never having gone diving, then yeah....I'd call it truly "unexplained."

Where I think I disagree is that I definitely think people get hit while diving within known, accepted parameters/theories. If you're diving within current deco theories and take a hit anyway, I'd call it "undeserved" or "unexpected" or whatever. I'm not trying to add any connotations to those words, despite the strong subtext to all of them. I just mean people get bent when they statistically "shouldn't" and don't when they statistically "should."

I think there is a distinction to be made, and I think we can learn a lot in making that distinction.

 

[PfcAJ]

I think the confusion comes from not really having an understanding of statistics.

There is NO model with a 0% chance of DCS, even when followed to the letter. None. The PADI tables might come close, but its NOT a 0% chance. All the known, accepted models are non-zero chance models. Therefore, getting bent when following these models IS expected, just not often.

Now, add in some uncontrolled variables (hydration, fitness, exertion, nutrition, BMI, etc etc), and your model quickly becomes an approximation.

Moral of the story: DCS happens. Get DAN.

 

[Akimbo]

I might be onboard with foreseen and unforeseen. I have been lucky and avoided DCS symptoms a few times when DCS was foreseeable but unanticipated circumstances compromised the dive.

 

[victorzamora]

I think the confusion comes from not really having an understanding of statistics.

There is NO model with a 0% chance of DCS, even when followed to the letter. None. The PADI tables might come close, but its NOT a 0% chance. All the known, accepted models are non-zero chance models. Therefore, getting bent when following these models IS expected, just not often.

Now, add in some uncontrolled variables (hydration, fitness, exertion, nutrition, BMI, etc etc), and your model quickly becomes an approximation.

Moral of the story: DCS happens. Get DAN.

I agree with every shred of this. My point, though, is that if we track the anomalous hits (good phrase??) we might be able to track patterns. That's why I think it's important to make the distinction.

Akimbo, how about "expected" vs "anomalous"? Maybe something other than "expected" but I think "anomalous" is good in that it describes the fact that, while not unexplainable, definitely not within the norm of deco profiles.

 

[PfcAJ]

If you had a normal 6 sided die, but all sides were black except 1 (1/6), would you really be shocked if you rolled so that the 1 white side was up? No, of course not. Nor would you be very surprised if you rolled 6 of them, and they were all black except 1. Or even if there were 2 white, and 4 black. All are pretty reasonable because the chances are at a familiar scale. You wouldn't say that rolling white was "anomalous" or "unexpected", even though the chances of rolling black are much much higher.

1/100, or 1/1,000 or 1/10,000 are unfamiliar scales. DCS chances for rec dive is somewhere around 3/10,000. 3/10,000 is not something we deal with in our normal lives. Humans do well within a certain range. Things that are too big, too small, too fast, too slow, or in this case, too rare are tough to work with. Hence looking for explanations beyond "well, it was bound to happen eventually", which is actually the case.

 

[Akimbo]

…Akimbo, how about "expected" vs "anomalous"? ...

Expected is too strong because there are so many expected incidents that don’t manifest into actual DCS symptoms. Anomalous, in my view, also implies a far greater understanding than we actually have.

The whole model of using algorithms to determine decompression is flawed, though necessary at this point. It is like determining your temperature by asking 100 statisticians what they think. Ultimately we will be able to directly measure physical parameters in each diver’s body to determine safe ascent profiles for that moment.

---------- Post added December 12th, 2013 at 02:57 PM ----------

… My point, though, is that if we track the anomalous hits (good phrase??) we might be able to track patterns…

“They” track divers that get bent bad enough to take a chamber ride. Then “they” make wild guesses at how many divers don’t get bent, even if breaking all the rules. Clinically bent is the only thing that matters for tracking patterns. Even then, decompression algorithms are going to ignore the outliers because they will always have an “acceptable” hit rate. Kind’a sucks if it is your day to be the outlier. Always know where the closest chamber is.

 

[uncfnp]

If I could jump in, I think, by your analogy PFCaJ, that you are looking at DCS as a random roll the dice event and therefore not worth categorizing as such. That there is no value in assigning any DCS event a label because it is not controllable, there will always be the risk as long as we dive.

I think victorzamora is agreeing that we all are at risk and as such the hits are "deserved" because we choose to dive but that the hits are not truly random, there is "something" that made that individual susceptible to that hit that day. And that by determining if the dive profile fell outside the statically area in which we might "expect" a hit and thereby label it as "unexpected or undeserved or anomalous" we could then try and determine the "something" that made that diver more susceptible to DCS on that dive.

I'll butt out now...

 

[MXGratefulDiver]

I might be onboard with foreseen and unforeseen. I have been lucky and avoided DCS symptoms a few times when DCS was foreseeable but unanticipated circumstances compromised the dive.

I'd be on board with "understood" and "ununderstood" ... or would that be "derstood" ... in the respect that sometimes we know what caused someone to take a hit, and sometimes we just don't.

Richard Pyle put it best in an article titled "Diving Physics and Fizzyology" ...

"If you ask a random, non-diving person on the street to explain what's really going on inside a diver's body that leads to decompression sickness, the answer is likely to be "I don't know".

If you ask the same question of a typical scuba diving instructor, the answer will likely be that nitrogen is absorbed by body under pressure (a result of Henry's Law); and that if a diver ascends too quickly, the excess dissolved nitrogen in the blood will "come out of solution" in the blood to form tiny bubbles; and that these bubbles will block blood flow to certain tissues, wreaking all sorts of havoc.

Pose the question to an experienced hyperbaric medical expert, and you will probably get an explanation of how "microbubbles" already exist in our blood before we even go underwater; and that ratios of gas partial pressures within these bubbles compared with dissolved partial pressures in the surrounding blood (in conjunction with a wide variety of other factors) determine whether or not these microbubbles will grow and by how much they will grow; and that if they grow large enough, they may damage the walls of blood vessels, which in turn invokes a complex cascade of biochemical processes called the "complement system" that leads to blood clotting around the bubbles and at sites of damaged blood vessels; and that this clotting will block blood flow to certain tissues, wreaking all sorts of havoc.

You will likely be further lectured that decompression sickness is an unpredictable phenomenon; and that a "perfect model" for calculating decompression schedules will never exist; and that the best way to calculate the best decompression schedules is by examining probabilistic patterns generated from reams of diving statistics.

If, however, you seek out the world's most learned scholars on the subject of decompression and decompression sickness, the top 5 or 6 most knowledgeable and experienced individuals on the subject, the ones who really know what they are talking about; the answer to the question of what causes decompression sickness will invariably be: "I don't know". As it turns out, the random non-diving person on the street apparently had the best answer all along."

... Bob (Grateful Diver)