Last Minute Exhale...

[Aquagenic]

I have this terrible habit of exhaling within the last 10 to 15 seconds of a breath hold, then inhaling when the time is up.

I was just wondering if I exhaled within the last 15 feet of surface on a dive is it dangerous for me? Should I be worried about SWB or anything like it?

As always, any thoughts comments and suggestions are welcome, because I am clueless...

~Picksmith

 

[Adrian]

Hey Picksmither...

Starting to exhale 15 feet below the surface seems too extreme. Many people start exhaling around 3-4 feet from the surface so their first action is to inhale to help keep the O2 pressure up and not black out.

Adrian

 

[Ben Gowland]

15 feet is OK to start if you are ascending fast. The problem is that exhaling itself gives a quick burst of oxygen to the brain followed by a drop in oxygen. So the forcefulness of your exhale is more important than the depth or length of time (within reason).

Conversely, holding onto your breath can have the same effect as the lungs expand they compress the heart in the same way that you would on a forceful exhale.

The optimum is to get this quick squeeze effect for the pulse of oxygen, but to breathe in as fast as is possible after that squeeze.

Taking the above into account, the best practice on a speedy surface is to relax your glottis and lips so that air comes out as it expands, and then actively purge just before you hit the surface. Then take a quick breath in, hold it for a second and at the same time 'squeeze' your lungs to 'up' the pressure. Then do another few breaths in the same way.

Hope that helps and makes sense.

Ben

 

[sumpa]

I have been taught never to exhale before you break the surface, since the pressure in the lungs sinks so quick/much that it can, in extreme cases, "suck" oxygen FROM your blood TO your lungs (a drop in oxygen, as Ben said).. If you are "on the limit" when ascending you might induce a SWB.

If i were you i would try to "train away" that breath out-habit.. If you can't wait another 10-15 feet, then you dive above your ability anyway..

 

[Ben Gowland]

Sumpa - I disagee with the advice you have been given. I think that when you take the overall picture into account it shows that it doesn't make much difference if you breath out or not.

This is why:

Yes, the oxygen offloading from your blood to your lungs is due to pressure drop but offloading is also affected by the volume of air in your lungs. By breathing out you lower the volume (good) and the pressure (bad), so the net effect will probably be close to neutral.

Ben

 

[sumpa]

But there are also other things to consider. Like, you loose boyancy, and if you misjudge how far you are to the surface and how much you breath out, you might end up 1 meter below the surface thinking "hey, where's the air??" or be forced to make an extra hard kick at the end which MAY kick you into a SWB (i am talking about max-dives, when you are close to your limits at the end of the dive.. i agree that on "normal" dives you probably have so much error-margin that exhale or not exhale makes little difference)

I do this sometimes (i admit, i do not always practice what i preach.. bad me) when doing dynamics with much weights (>3 kg to be aligned in the water), and when i hit the wall i exhale, turn around, and go up all the same time... but sometimes i forget how much weight i have on and sinks when exhaling.. it's only 1-2 meters deep, so no problem, just embarrasing/annoying to misjudge the surface and be another 2 seconds without air (and empty lungs) when you are already tired from the dynamic..

In my opinion it is a matter of judgement. Sure, you might add a meter on your depth, by adding a slightly increased risk for SWB. If you know what you are doing and know your body, then exhale.. But if you are clueless (as stated in the first post) then you better keep the extra safety and use some other technique to add that one meter (cardio training or mental training for example).

 

[cdavis]

Interesting thread, I had not realized how much difference of opinion existed in this area. In my PF clinic, they were quite strong on exhaling in the last few meters to reduce lung volume and the chance of SWB. Hook breathing thereafter to get as much 02 in the system as fast as possible.

Connor

 

[Adrian]

Connor,
I understood it was the last feet, perhaps a couple of meters max. If you are ascending fast at aound 1m/sec we are only talking about a couple of seconds difference and perhaps then it's the change in pressure that might affect more the possibility of SWB than anything else.

I find it interesting what Ben says about how exhaling will give a burst of O2 to the brain. How does that work? If exhaling through the nose does the "nose-brain" connection come into effect? But if it's through the mouth, what happens? Please excuse my ignorance of physiology!

Adrian

 

[Ben Gowland]

A forced exhale compresses the thoraic cavity and thus the heart. This inhibits the flow of blood into the thoracic cavity and aids the heart in pumping out 1 heartbeat's worth of blood.

As soon as that heartbeat of blood leaves the thoracic cavity - the inhibition of venous blood returning to the heart is left and thus reduced blood flow into the arterial system.

There is still some residual effect of the thoracic pressure on the aorta and carotid artery in pushnig blood up into the brain (increased cerebral blood flow AND pressure) but this will consequently reduce too.

This is the 'pulse' I refered to.

As soon as the 'squeeze' is released (by breathing or relaxation) things start to return to normal.

So - as you can see any extra pressure on the heart from lung re-expansion in the last 5-10m is not beneficial - for this reason as well as the oxygen 'sucking' effect.

 

[Ben Gowland]

By the way - an extra hard kick at the end will have no effect on blackout or not as the deoxygenation of the blood in the muscle as a result of that kick will take at least 15 seconds (more like 2 minutes!) to make it's way back to the heart and then another 3-10 seconds before it gets to the brain.

 

[sumpa]

Originally posted by Ben Gowland
By the way - an extra hard kick at the end will have no effect on blackout or not as the deoxygenation of the blood in the muscle as a result of that kick will take at least 15 seconds (more like 2 minutes!) to make it's way back to the heart and then another 3-10 seconds before it gets to the brain.

hm.. then i stand corrected

 

[Ben Gowland]

No worries, Sumpa.

BTW - where does that weird 'niples' quote come from? Is that a Swedish joke/fable/saying?

 

[sumpa]

My worries are that i may teach something incorrect to some newbie.. :/ anyway, better to stand corrected and learn than to not learn and black out

The nipples joke i just stole from somewhere on the internet.. But since it is me who is telling it i guess that makes it a swedish joke.. not a traditional one though.
I have more strange quote if you want some

 

[cebaztian]

There is a cardioologist that has a "new" theory on SWB.

He says it has to do with overpacked lungs (heart squeeze). Like you faint when overpacking.

When returning to the surface air expands and due to Bloodshift there is no room for the air.

 

[Aquagenic]

So many great responses. I love the forum.

Ben, sounds like you took the argument with your Physiology 101. Correct me if I am wrong, you are suggesting to exhale SLOWLY and naturally within about 15 feet of the surface. Then hook breathe once you are on the surface? Are you recomending to do this, or simply saying that it's ok but not beneficial.

Does anyone else have any recommendation on this one?
Exhaling slowly close to the surface seemed like the most natural thing for me to do.

Seb, that new theory seems to make sense to me, is it conflicting with the previous theory of SWB?

Thanks for the info guy's.

~Picksmith

 

[cdavis]

Adrian,

I can't remember the depth Kirk was suggesting to begin exhaling, but am pretty sure it was deeper than a few feet, because that is what I normally do and was having trouble changing my habit. I went back to the PF clinic book and could not find any detail .

Does anybody else have a good number?


Connor

 

[Ben Gowland]

Well - the original question was 15 feet - which is when I would expect people to start exhaling if they want to. And yes, exhale 'naturally' rather than forcefully.

What I would say though is to exhale forcefully just before you hit the surface and then hook breathe the moment you break the surface.

That 'new theory' is hardly new....it has been around for years. What's more, SWB will (of course) be a combination of the classic factors as well.

 

[Adrian]

Gracias Ben, Connor,

Talking about speedy surfacing, yesterday I went diving with Oscar, (BlueIcarus) and he forgot to bring a weightbelt, thinking that I would have an extra. So we simply shared the belt beween us. For the first time I tried going down with a 3mm suit and no belt. It was pretty tough the first few meters but after, when I started to come up, and streamlined my shape, I was going so fast my hips even cleared the surface! Quite exhilarating! It felt like I was an express train and that I covered the last 15 feet in 2 seconds.

Adrian

 

[Aquagenic]

Thanks for the advice Ben. That did seem the most natural thing, and as with most things in life that is normally the right thing.

Just wanted this to get that cleared up before I went on a dive trip over the weekend. I will be diving with inexperienced divers(not that I am experienced) and I wanted to play it real safe and avoid SWB anyway that I can. So, thanks again!

~Picksmith

P.S. Your hips cleared the water??? Dang, that is some serious speed. Actually, sounds like fun, surface next to your partner and scare the living crap out of him!