In praise of shallowness

[BryanW]

I'm not talking about paying more attention to the fit of Tanya Streeter's wetsuit than her conservation work. On a recent freediving course, the instructor made the usual point that freediving was a personal journey, a way of discovering oneself. Well, on a trip to Greece recently I discovered – or finally accepted – something about myself as a freediver.

I DON’T ENJOY DEPTH!

For years I’ve struggled to get down to any depth at all, mostly due to problems equalising. But now that I’ve solved those problems to some degree, I realise I was wanting to get deeper for the wrong reasons – mostly about achievement, being able to impress others, and wanting to see myself as a “freediver” rather than a “snorkeller”. On my recent holiday I was happiest not when getting down to a gloomy 20m (I only had scuba fins, plus I was alone ) feeling uncomfortable and coming straight up again, but when cruising round all the rocks and fish at 5-10m down, where there was more light, more life, less effort getting down there in the first place and less worry about when to come up. Because I was much more comfortable, I seemed to have much more air – once when finning around at 7m, I saw an interesting fish in an underwater valley and popped down to have a look, realising afterwards that I’d got to 15m without even noticing, whereas if I’d gone down there straight from the surface that would have been almost it for the dive – I would have spent so much effort and stress equalising to get down there, I would have felt the need to come almost straight back.

Don’t get me wrong – I love being underwater far, far more than being on the surface. But trying to notch up metres has lost its allure for me. I am shallow, and I don’t care. Anyone else?

[fcallagy]

Brian I think most here would agree with you. i like to try and go deep on ocassion to test myself no one else. Almost all of my own recreational diving is done less than 10m and same for most spearing and photography accross the board. there are exceptions of course but less than 10m is where most of the life is in any case so if thats what you enjoy looking at thats where you will find it.

[ILDiver]

I agree. My goals for depth are actually aimed at increased comfort at lesser depths. I figure if I can comfortably dive to 30m - 40m, it will make dives to 15m a breeze.

However I think I am more concerned about the more shallow depths and time. If I'm more comfortable, I *may* not think about what my body is telling me. Also, considering where SWB most likely occurs and why. I actually think more about when to come up on the shallow dives. Deeper dives I spend more time planning the depth and thinking about the dive itself. With "shallow" dives there are other factors, like the urge to return to the bottom sooner because "It's only XX deep, that's an easy dive!" Or if I see something and cut my surface interval too short.

I do have "dreams" of competing someday, but my main goal is to enjoy the underwater environment on one breath.

[BryanW]

I'd agree that shallow diving might seem more "casual" and might therefore lead to more risk by the diver not taking it so seriously. However, I've always found that one minute underwater feels like two or three at least, so the risk of overstaying is quite low. The SWB point is interesting. I thought that it occurs in the top 10m because that's where the partial pressure difference is greatest, but if you don't go down to any depth, there isn't so much pressure acting on the blood gases in the first place - is that right? I'm a bit sketchy on the theory.

Of course there's great value in being able to go deeper - I've been to several places where there was nothing above 15m, so if I wasn't comfortable at that depth I wouldn't have got close to anything. And if i see a turtle at the surface and it dives to 20m I want to be able to follow it comfortably. But it's heartening to know that the general freediving community's apparent fixation with depth was probably mostly in my imagination.

Cheers, Bryan

[ILDiver]

I'm not sure I'm one to be talking about this theory. But since the change of PP is the greatest in that range, it makes it the most likely place for SWB. Regardless of the max depth. Whether you went to 40m or 10m, the amount of oxygen could get to the same point at the 10m mark on either dive. Since your body is using the oxygen and the partial pressure is reducing the concentration of oxygen at the same time.

Time, depth, exertion, etc... A lot of factors play into a SWB.

[salibandy]

I thought PP was only a factor in deep water black outs?

[ILDiver]

It's based on Dalton's Law. I don't want to offend any instructors by putting part of a course I took, out online. In short Partial Pressure would be a reason why you are less likely to have a deepwater blackout.

[sciencemike]

All this is based upon confused terminology. Some people call a BO when its shallow a SWB. Some people call DWB a black out in shallow water when you resurface from a deep dive. But i tend to stick with the definition i was taught that a SWB is when you come up from a deep dive where your blood PP02 and you lung PP02 are low, and upon entering the shallows again the PP02 in your lungs negatively and DRAMATICALLY changes, decreasing the pulmonary vein blood PP02 which will then immediately be fed from the heart to the brain, therefore causing a black out. Im sure there are better definitions out there but this is the one that i use.

As for a SWB happening in shallows without the concomitant deep dive experience. And most likely it can happen even in shallow water as i illustrate at the end of the post. You are also correct that a the PP02 shift is the greatest/meter near the surface. Changing that last 10m changes the PP02 by 50%, but if surfacing from 20 meters it will change it by 66% and from 30m by 75%. So although the change in PP02 is the greates, its not the greatest PP02. Now im not sure what the diffusion rate is for the 02 across the PCB, but you must remember that its not just the shifting PP02. There are other factors that amplify this change in PP02 to make it cause a SWB.

First, 02 must diffuse across the pulmonary capillary bed (PCB) and into the lungs. We find that higher differences in PP02 result in much higher diffusions. Secondly, diffusion is also a time dependent function so that a faster diffusion is also multiplied by the longer time per ascent So we see a faster rate of diffusion as well as a longer diffusion time resulting in a much greater dPP02 and therefore the greater chance of SWB from deep water. But in reality i can definitely agree with you and see that its really a function of the initial PP02 in the lungs and blood if they are in that danger zone.

Example.....if you are about to black out at -15m, a SWB is more likely vertically than if you were doing that last 15m horizontally in a pool at 2m. But in this example the person is about to black out any ways so it would probably be called a BO vs. a SWB.

Please if anyone sees anything wrong with this let me know, so i can update my knowledge, and change the post so others dont follow the blind.

[Bill]

sciencemike
First let me make it clear that we agree on the ratio of partial pressures. My only observation is that by expressing the difference in reference to depth, you are understating the problem. No question that when you go from 10 meters to the surface, the PP changes in a ratio of 2 to 1. If you view that one way, it is a 50% decrease. If however you view it from the surface, where you end up, it changes from 200% to 100% and that isn't 50%.
Or is it??

[sciencemike]

I do see what you are saying. Let me see if i can summarize it.

When going from -10 meters to 0m, the volume of air in your lungs will double. So, if your reference point is -10 meters and you take that to be 100%(volume), then when you surface your lung volume percentage is now 200% of what it was.

That is true, and i like that for lung volume. Now if you look at the pressure, which is a different story. I will cut and paste but replace with pressures.

When going from -10 meters to 0m, the volume of air in your lungs will double. So, if your reference point is -10 meters and you take that to be 100% (Pressure), then when you surface your lung pressure percentage is now 50% of what it was.

Thank you. It does make it more understated to use pressure. I probably shouldn't say this but there are parts of Henry's law that i haven't worked out fully in my head. I would like to actual numbers put into the equations, ill do some searches on DB. Im sure Trux will have some java script calculator or reference article i can find.

[fleshy]

Shallow water blackout short and simply is this. Not blacking out from lack of oxygen, blacking out from having to much CO2 in your blood. It happens the most with younger and middle experience ranged divers because they are pusing their limits. Building up more CO2 in their systems than they are used to, and at the same time taking lots of fast repitition dives. Best way to avoid SWB, is to have good breaks between dives.

[trux]

Originally Posted by fleshy

Shallow water blackout short and simply is this. Not blacking out from lack of oxygen, blacking out from having to much CO2 in your blood. It happens the most with younger and middle experience ranged divers because they are pusing their limits. Building up more CO2 in their systems than they are used to, and at the same time taking lots of fast repitition dives. Best way to avoid SWB, is to have good breaks between dives.

Sorry, fleshy, but this is a completely wrong statement. Shallow water blackout (as commonly understood), or depressurizing blackout (as sometimes referred to) happens when the oxygen level drops suddenly during the ascent due to the fastly decreasing PAO2 (partial alveolar oxygen pressure). Due to the expaning lungs during the ascent, the PAO2 drops below the level of PaO2 (partial arterial oxygen pressure) or even PvO2 (partial venous oxygen pressure), so the diffusion through the aleolar wall decrease, halts, or even reverses (sucking out the oxygen from the blood back to the lungs).

The SWB often happens after self induced hypocapnia (low level of CO2) through hyperventialation. The low CO2 level suppresses urge to breath, and decreases tolerance to hypoxemia.

[Pastor]

Originally Posted by BryanW

Don’t get me wrong – I love being underwater far, far more than being on the surface. But trying to notch up metres has lost its allure for me. I am shallow, and I don’t care. Anyone else?

Absolutely! Whatever make you smile, it's about having fun for me not about numbers, just for a change

[sciencemike]

Hey Trux- or anyone else,
Sorry to derail, but there is soo much bad information out there on SWB. So much confusion, and it is the most serious problem spearos face(as someone wiser than me said).

I have often wondered a couple of things. Maybe i should just use the search forum or start a new thread. Upon surfacing from a very deep dive gas bubbles can come out of your blood stream, even when just free diving (as evidenced by free divers even getting the bends, which is an extreme case of bubbles). When these bubbles (assuming they are larger than 8 um) get lodged in your pulmonary capillary bed to be destroyed, they have been shown to dramatically increase the blood pressure in the pulmonary artery. My question to you is have you ever seen reports of this adding to the SWB? Lets say if someone did 20 or so deep dives.


Secondly, blood pH affects heme/O2 binding. When acidic (muscle capillary bed), O2 is released from Heme and when its basic (pulmonary capillary bed) it binds - well lets not say it binds but is more sequestered out of solution into a complex with hemoglobin, thusly reducing its partial pressure. This dramatically affects the partial pressure of O2 in the lungs and in the muscle. Can anyone correlate this with changing blood pH when you dive? Does the blood pH drop at the end of the dive in the lungs, whereby reducing the pH gradient allowing for O2 to be released back into the lungs? Because otherwise O2 will never be allowed to backflow like we have talked about.

[Mr. X]

I used to climb a lot and after that did triathlons. I progressed quite far with both but many of my most vivid/happiest memories are of the early days, learning and making mistakes - it was such a big adventure. You can sometimes loose sight of that as you progress. You don't have to push it all the time to have a good time, in fact you'll often have more fun if you don't (although it's also good to take on big challenges sometimes).

I don't dive deep at all, almost always less than 10m. Plenty of fish and sea life in that top 30 feet - and enough daylight to see them. I think you made an important discovery - enjoy yourself