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tunnel vision?

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BatRay

BatRay

Well-Known Member
Nov 1, 2002
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I was discussing freediving with a coworker, and he mentioned a story that he'd heard from a recovery diver. The diver was holding his breath while swimming underwater and got tunnel vision. One of his buddies had to pull him up to prevent him from drowning. There was also a drowning at one of the nearby lakes, and the victim was said to be a strong swimmer who enjoyed swimming underwater. The diver suggested that the victim may have experienced the same tunnel vision, followed by unconsciousness and drowning.

Is he describing shallow water blackout, or simply hypoxia? I don't tend to push my limits that much, so I can't say that I've experienced tunnel vision. My coworker spoke of it as if it was lurking out there, waiting to seize anyone who dares to hold their breath underwater. I tend to be skeptical of such boogymen.
 
Generally, tunnel vision is a result of hypoxia and means the diver is right on the edge of a B0, often accompanied by loss of color vision. I had a couple of doses of this long ago, hyperventilating and diving with extremely short surface intervals plus staying down as long as possible. Scared me bad enough to change my ways.

You are unlikely to see such symptoms unless you are pushing things hard.

Connor
 
indeed, as Connor says, tunel vision and a general progressive loss of what is going on around you is Hypoxia, which can be followed by a BO, and of course death if the driver is unsupervised.

Real life example: a freediver on a Dynamic swim slowly drifts out of his lane, and eventually just grinds to a halt, totally blacked out, on the bottom of the pool. He will just stay there until he dies unless saved. This is exactly what you are describing.
 
BatRay said:
Is he describing shallow water blackout, or simply hypoxia?
Click to expand...

It's all hypoxia, SWB is caused by hypoxia anyway. Tunnel vision is usually just a few moments away from blackout. If the guy you know had to be pulled to the surface he probably had a SWB too.

Unfortunately being an expert swimmer does not help at all (probably is worse actually) unless one understands the basic concepts between hyperventilation, blackouts etc.

In many places unfortunately hyperventilation is still even taught as the way to freedive, especially amongst non-trained freedivers..
 
just to be clear: it's very easy to blackout with no hyperventilation too. It's just a harder work!
 
azapa said:
just to be clear: it's very easy to blackout with no hyperventilation too. It's just a harder work!
Click to expand...

Very true, thanks for making it clear Simon as my post could have been essily misinterpreted...

Just to add to your comment for anyone that should come across this post, it's particularly easy to blackout without hyperventilation when depth is involved because the pressure essentially falsely tricks the body into thinking there is plenty of O2 left when there isn't...

I don't spearfish myself but I would imagine this is particularly relevant for techniques like aspetto and so on...
 
Its already been mentioned, but tunnel vision is a classic symptom of hypoxia, no matter the cause of loss of oxygen to the brain. When I was a jet pilot making hi-G turns, the first indication that blood was not making it to my brain was tunnel vision. Next was grey vision, with no color. Then I would go totally black and be unable to see the instruments. If I had to continue turning, I would just ease up on the back pressure on the stick so that my heart was able to overcome gravity, getting more oxygen to the brain, and restoring some vision.

When I was a student in US Navy diving school at Pearl Harbor, we were sitting next to the pool on a break, and some Navy kid challenged me to see who could swim the most laps underwater. He went first, and I wasn't going to let him beat a Marine, so I went a bit further. But then I got tunnel vision, started to see stars. and an instructor who had been following me up and down the pool saw me start wavering out of my lane, so he jumped in a rescued me. I was lucky I was being watched.

And one more example before you go to sleep. For at least 5 years in the late 70s and early 80s, I had this thing that when I burped, I would get tunnel vision, maybe grey out for a few seconds, and then be fine. Finally one time I blacked out and fell down, so I decided maybe it was time to confess to a doctor. Turned out I had orthostatic hypotension, and when I put pressure on my vagus nerve by burping (or by passing gas) my heart would quit beating for a while and I'd lose oxygen to my brain. I let them talk me into a pacemaker. My resting heart rate is about 38 to 40 bpm, so the pacemaker is set at 35. If my natural rate falls below 35, it fills in.

To sum it all up, I'm a veteran of hypoxia and tunnel vision.
 
Bill McIntyre said:
Turned out I had orthostatic hypotension, and when I put pressure on my vagus nerve by burping (or by passing gas) my heart would quit beating for a while and I'd lose oxygen to my brain.
Click to expand...


Although I have never blacked out from passing gas, its a common thing for those around me. Especially after eating a burrito. :hungover
 
Thanks for the responses. I have not pushed myself to the point of near-blackout, so the symptoms were foreign to me. I think my coworker meant well, but I he tends to accept certain things at face value. He also shows no interest in diving.

Question about SWB then. Doesn't it occur as the result of surfacing from a deeper dive, with little or no warning? The accounts I've read indicated that the victim was unaware of what was happening.
 
good question Batty. a depth related SWB and a pool blackout have the same end result: hypoxia and brain shut down.

the route to each is different:
- pool: the depth is constant, the hypoxia comes on progressively.
- coming up from depth: the effects of hypoxia come on real fast, in fractions of a second.

The big difference is the partial pressure of Oxygen: at depth it is increased, leading to a general "feeling if well being" (as it is often described). The problem is on the way back up the partial pressure rapidly drops off (remember, the biggest pressure change is in the last few meters).

It could be said then, that a black out is easier to anticipate in the pool, harder from depth. The whole thing is so relative though. The only way to do it is with good, trained safety.
 
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Not sure if it's at all related but you can get tunnel vision in cycling too. Couple with loss of colour in your vision, everything starts to grey out. Only happens when you're pushing above and beyond. So it seems to be the same result.
 
Reactions: podge
Poida said:
You're cycling underwater Reefy?:blackeye
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Just saw yesterday that this gets offered at my lokal high-diving-Hall for a mere of 80Euro ><

should buy some led and sell a course on how to walk underwater like I did some times :king
[long leg tipping ftw]
 
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Wierder and wierder.

Somewhere I've got a video of my son standing straight up and walking on flat surface. Looks perfectly normal except he is upside down and underwater and walking on the keel of the boat. it was a pretty good trick to make it look natural. Maybe somebody should teach that.
 
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