Time of lowest O2 level & BO risk?

[baiyoke]

A question about the lowest O2 level during/after apnea...:

I have heard several times that after apnea, when you breath again, it takes fx 20-30 seconds before the O2 level has hit the lowest level, after you started breathing again.

Fx Freediving safety

Your lowest level of oxygen is 20 seconds after surfacing.
Keep breathing (or observing the freediver).
Focus on inhaling when you break the surface, only a litre of new air is enough to oxygenate the brain and keep you conscious.

But watching (countless ) videos of people resurfacing, it seems like by far most confusion, sambas, LMC's, BO's etc. happens close to the re-breathing point, or 5-10 seconds after. I don't think I have seen a single video actually of someone who got BO after 20 seconds... So the theory doesn't fit reality as far as I can tell... Most people seem to recover within the first 15-20 seconds (it seems to me).

So what's going on?
- When IS the lowest level of O2 in the brain bloodflow?
- Doesn't the body reoxygenate the blood in just a few seconds, so that it can hit the brain in say 5-10 seconds? (Wich it looks like on many videos).
- If the 20 sec. delay aplies, why don't we se a lot of 20 sec. delayed BOs?

Can someone clarify this?

 

[cdavis]

good point, I've wondered the same thing.

 

[Bill]

I think that there are two or three things involved here. The first is, where the measurement is taken. Blood to the finger sensor might take that long but one on the ear is probably more realistic for O2 to the brain. The second is when the diver shows that he has blacked out. I have seen a diver grab the float and not move. If he doesn't breathe, it may take a while but he will black out.
Back in the bad old days during spearfishing meets, we also had two strange blackouts where the divers did not come out of it but went straight to sleep for ten or fifteen minutes. We were taught to clear the snorkel before hitting the surface in the fifties.

 

[baiyoke]

I think that there are two or three things involved here. The first is, where the measurement is taken. Blood to the finger sensor might take that long but one on the ear is probably more realistic for O2 to the brain.

I have had the same thought since yesterday. I had a chance to play aroynd with a finger-oximeter for the first time, and this is what I experienced: In the final 30 seconds of the breath holds the O2 percentage stopped dropping down more or less. Then after the first breath, it started to drop again quickly, as if it had to "catch up" with general blood levels, and after app. 20 sec. then quickly rose again (O2 69 stopping, then jumping to 65 suddenly app. 15-20 sec. after breathing, puls 44 bpm at lowest).

Now it doesn't make sense to me that the drop in O2 should be so very highly irregular (dropping gradually, then stopping, then dropping quickly). I interpret it more as a function of something regarding perhaps the bloodstream. Perhaps blood is not moving around in the fingertip, and the fingertip is using O2 more slowly than muscletissue... or something like that. It just seems like it has something to do with bloodflow, because of the highly irregular readings in the last 30 sec., and first 30 sec. after apnea. I'm thinking it has maybe something to do with the release of vasoconstriction.

Is the fingertip oximeter perhaps to blame for at false interpretation, that O2 levels are lower 20 sec. after apnea???

Does anyone know of studies measuring O2 levels directly in the core/brain bloodflow?

On a more anecdotal note I remember from my time as a smoker, that the time from inhale to feeling altered sensations can be very short, a few seconds sometimes. Wich suggests that O2 might also be able to reach the brain in a few seconds I think.

 

[Rik]

It is true that the finger oxymetry is about 20-30 seconds delayed compared with the oxygen in the brain. The measurements can also be different (probably lower) compared with an real-time measurement due:
a. the delay in distance between lungs-brain and lungs - finger.
b. the effect of (diving related) vasoconstriction of the limbs in relationschip with blood flow and saturation
c. the effect of cold related vasoconstriction.

Re-oxygenation on the level of the brain is within 5 seconds after breathing. However...:

Some divers are not able to do proper breaths after a dive. This can be caused by loss of motor control of muscles (LMC) relating to breathing or to insufficient deep breathing. Those divers could surface consciously, but could get a black-out after 20 seconds due prolonged hypoxia. They might have LMC during this time, but this doesn't have to be the case. While this phenomenon has become rare due proper training, it is still something to look for. This is the reason why you as an buddy should pay attention for at least 30 seconds before continuing to do something else after a dive.

 

[baiyoke]

Ok Rik.

From visiting your profile, you seem quite informed about diving and medicin, activ on DP since 2002, and following papers about diving related studies...

So unless someone comes around and convince me otherwise, I personally regard the 20 second delay a myth or misconception, created buy the use of fingertip oximeters earlier in freediving history...

Question is if ear oximeters will do the job...? Probably more accurate anyway...

 

[Rik]

The 20s delay before an BO would hit is an misconception, but that misconception should not be an reason not to watch out for your buddy for at least 30 seconds after the end of his/her dive.

Ear-oxymetry would be better but more difficult / expensive to get your hand on.

 

[cdavis]

Thank you all, very informative.

Connor

 

[trux]

The 20s delay is definitely not a hoax with the sole purpose of promoting fingertip pulse oximeters. It is true that fingertip sensor for pulse oximetry is not well suited for freediving, because the blood flow is strongly infulenced by vasonstriction. And as Rik wrote, the delay at fingertip sensors is also considerable, and that the gas exchange after the start of breahting is relatively quick.

Nevertheless, the time after the surfacing remains the most critical moment of the dive anyway. Whether it is 5s, 10s, 20s, or 30s is highly individual, depending on many factors, and it is irrelevant to the actual oximetry. As Rik wrote, the gas exchange depends especially on the proper ventilation, which is not always correct, especially at freedivers who do not drill proper the rescue brething under deep hypoxia regularly. And from experinece I know that it is practically the totality of recreational freedivers, spearfishers, and many competitors as well.

The next factor omitted in this thread is the sudden drop of arterial pressure, and the PaO2 due to the release of vasoconstriction, and due to the drop of hydrostatical pressure. The drop of arterial pressure may (and likely will) continue well beyond the moment when the gas exchange starts to catch up.

Hence, again as Rik already wrote, the 20s are in no way a myth. It is true that at competitions most athletes black out (or recover) quicker, but in no way it means you are perfectly safe after just 5 or 10s.

 

[baiyoke]

again as Rik already wrote, the 20s are in no way a myth.

The way I read Riks post, he's saying the exact opposite: Quote: "The 20s delay before an BO would hit is an misconception". To me everything points in that direction, except yor point about bloodpressure (but see below).

About the other points from both Rik and Trux, offcourse one should keep watching the freediver in question, but that was not what was questioned when talking about the false fingertip readings. Safety first, allways...

But good point about the bloodpressure. However it still doesn't fit with actual BO's with a delay of 20 sec. as I perceive it. Also bloodpressure drop effect seems to be faster than the 20 sec. delay, since BOs happens faster. And since we don't seem to get these 20 sec. delayed BOs, it seems plausible, that perhaps drop in bloodpressure after the initial 10-15 seconds is counter balanced by fresh O2 in bloodstream. Just a guess, but it fits reality.

Honestly, I see no reason in not clarifying old myths/misconceptions... that does not mean safety is not an issue, just the opposite: Knowledge is key for safety in my opinion.

Offcourse it's not a Hoax it's a misconseption/misinterpretation it seems...

But I can see how "lowest level of O2 and BO risk" is in fact a very broad subject, however is is, I hope, specified in my initial post, that it is not safety, but the physiology/chemistry I'm targeting.

What I see as a misconception is probably better described as "time before re-oxygenation of blood in brain".

But nonetheless, the 20 sec. delay in actual BO's is also as a general conception, as I see it, a misconception and should be clarified and aborted (not the safety procedure, just the conception regarding O2).

So I really don't understand how you can write that
quote
"the 20s are in no way a myth."..?
followed by
"It is true that at competitions most athletes black out (or recover) quicker".

???

Quote Trux: "but in no way it means you are perfectly safe after just 5 or 10s." This I and probably everybody totally agree on, but no one has stated that. The topic circles about the initial mentioned 20 sec. delay of O2 level.

What happens in the individual diver is offcourse allways conditioned by the individual, and safety should be conservative and put in front at all times.

 

[trux]

So I really don't understand how you can write that quote
"the 20s are in no way a myth."..?
followed by
"It is true that at competitions most athletes black out (or recover) quicker".

I think you misinterpret both my conclusion and the initial statement. I do not think that the original claim was that the O2 is the lowest exactly at 20s after surfacing, but rather that the O2 is at the lowest levels during the 20s after surfacing, and that's the reason it is very important to watch the diver for at least 30s. Yes, most competitors probably recover or black out quicker, but it is very individual and depending on many factors, and it does not mean that you cannot black out at 20s.

 

[baiyoke]

I do not think that the original claim was that the O2 is the lowest exactly at 20s after surfacing, but rather that the O2 is at the lowest levels during the 20s after surfacing,.

Ok, I think we agree.

I can see now that the sentence "Your lowest level of oxygen is 20 seconds after surfacing" can be understood as both AT/AROUND 20 seconds, and DURING the 20 seconds. I've allways understood it as a delayd low, the way it is written. and I think I've been told the same thing. Hmmm...

Anyway, nice to get more clear on that now...

 

[Mullins]

I think you misinterpret both my conclusion and the initial statement. I do not think that the original claim was that the O2 is the lowest exactly at 20s after surfacing,

That is how it was written, though. There's no ambiguity.

 

[growingupninja]

A lot of newer fingertip O2 have a pulse graph display. For a freediver this is extremely useful since as Trux mentioned, vasoconstriction figures so greatly into the equation. From training with mine, many of the sensations I feel during and after a hold seem to related mostly to vasoconstiction, and likely blood pressure. They are all inter-related of course and anything I 'feel' is subjective...

 

[trux]

That is how it was written, though. There's no ambiguity.

Then it was not formulated properly, but I do not see any problem. The message is short and perhaps not scientifically perfect, but quite clear - you are not safe when you surface, and that before you start recovering the hypoxia deepens. Whether the most critical time is 5s, 17.8s, or 20s depends on diverse factors, but is little important. It is not excluded that you can black out 20s after surfacing, and that's on my mind the message Sebastian tried to pass.

 

[baiyoke]

Well, I guess we just prefer different levels of information then. I would very much like to know if it actually was at that 20 point app. the lowest O2 would hit, and especially offcourse if people got BO at that point, because then I would do some really strong hook-breathing up until and beyond that point. But knowing it's more important much earlier, I'm not gonna do 30 sec. of hook-breathing after all my dives, but instead focus on the very important first 5-10-15 seconds...

I think expanding knowledge in our young sport is important also, because this is just one piece that might fit in another puzzle another time. My impression of you Trux was actually that ever expanding knowledge about freediving was equally important to you, you seem to embrace the science part pretty much in your DB activity...

 

[baiyoke]

...and that's on my mind the message Sebastian tried to pass.

Btw I have great respect for Sebastin, it's not about that at all. He's done a lot of interesting stuff and articles, I would actually like to believe that he would be just as interested in getting to the bottom of this.. In fact I'm quite sure he allready has most likely..

 

[trux]

I have never heard of someone doing 30s of hook breaths or recommending it. It is indeed important to do a few of them at the very beginning to limit the hypoxia, better told the blood pressure drop, just after surfacing. However, if you don't do any, and do not ventilate properly, it is not at all excluded the hypoxia will be at its lowest at 20s (though it may be quicker too).

And I do not understand why you are presenting the 20s rule in Sebastian's list as some official echelon. It is not presented as such, and I am not aware of any official statements or studies claiming that the hypoxia will be lowest exactly at XY seconds after surfacing. As I wrote it depends on many factors, so such prediction would not make any sense. The only thing that is sure, is that the time after the surfacing is the very most critical time of the dive, and that's all what you should take from it.

 

[baiyoke]

And I do not understand why you are presenting the 20s rule in Sebastian's list as some official echelon.

I guess it just shows how much I respect the guy and his work. I have been reading a lot of his stuf and it's great

 

[monkeyhatfork]

A couple years ago a great free dive photographer and fellow PFI classmate Sergio Goes died free diving a wreck off oahu the initial report I had read said that upon surfacing his buddy thinking he was fine dove after 20 sec or so and when he surfaced Sergio was gone had blacked out and drifted off his body found later 6 miles away. So in unlucky circumstances u can black out a bit later than the initial surfacing.