So..here's my physician's take on CO2 and nitrox in relation to breath holding...
Breathing is not stimulated by lack of oxygen, but rather, by presence of increased levels of CO2. That is why you hyperventilate at the surface - but not more than a few breaths - before doing a dive. Taking deeper breaths or breathing more quickly than usual causes you to decrease your CO2 levels in your lungs. If you breathe off ALL your CO2, then you put yourself at increased risk of passing out, because the reflex to breathe that is triggered by increasing CO2 levels will come too late, and you will have metabolized all your O2 down to a dangerous level, putting you at increased risk for blackout.
Now...breathing nitrox certainly increases your oxygen level, but it does nothing to decrease your CO2 level...it only decreases your nitrogen exposure. And, since oxygen is metabolically active - as in, your body burns it up over time, I don't think it would cause you to be "super saturated" with oxygen for a significant enough period of time that it would affect your breathing reflex - particularly since oxygen doesn't trigger the reflex. Finally, any degree of O2 super saturation would be beneficial for preventing blackout, if a persisting saturation actually existed, because it would keep your hemaglobin in your blood more saturated, and thus, able to sustain a breath hold for a longer period of time before your oxygen saturation dropped to a dangerous level, resulting in blackout. That is why I put patient's on 100% oxygen before I give sedation or anesthesia - it allows them to be apneic safely for a longer period of time in case I need a few extra seconds to secure an airway.
For these reason, I don't think that nitrox use before a breath holding dive would predispose someone to CO2 accumulation or CO2 washout. If there really is a risk to nitrox use in relation to shallow water blackout, I am having a hard time coming up with a physiologic basis as to why that would be. I COULD, however, see the potential of increased risk of decompression related problem with freediving post scuba diving, as you are recompressing then decompressing an increased gas load by doing things in that order...maybe increasing risk of an air gas embolism or DCS event as the cause of blacking out?
---------- Post added December 3rd, 2015 at 11:20 PM ----------
I'll admit to speculating but you are doing the same with your blackout hypothesis (which would not technically be a shallow-water blackout since he went deep).
Oh...and just to clarify...argument over the the depth in regards to what caused the blackout is a little silly. Shallow water blackout has less to do with how deep someone dives, and more with the mechanism by which they end up passing out. It simply refers to the phenomenon of passing out after ascending from a breath hold dive due to a critical decrease in O2 tension prior to developing the reflex need to breathe. Wikipedia will tell you that shallow water blackout only occurs when all stages of the dive are shallow, and there is
no factor of depressurization involved. I challenge you to explain exactly how shallow that is, then, since the most rapid degree of depressurization your body will experience occurs in the 1st 33 feet. That's why they say "deep water" blackout occurs at greater than 10 meters (32.8 feet). So really, anything other than putting your head just under the surface probably risks deep water blackout, too. My understanding of the mechanism for the role of depressurization involved in this process goes a little something like this...and it will sound just like the stuff you learn in a nitrox class.
As oxygen goes under increased pressure, it has greater effect on the body. Typically, we plan for pO2 of oxygen to be at 1.4 or below at the high end, as at above that point, there is some increasing risk of seizures related to oxygen toxicity, particularly at 1.6 at above. If the same rules hold true for breath hold diving, then you would also experience increase partial pressures of oxygen, though you would be nowhere near the toxic limit. In our breath holding example, the gas you breathed in at the surface was 21% (pO2 0.21). If you dove to 50 feet, you now have an effective pO2 of 0.5. As you can see, the toxic limit is not what we are worried about here...it's the minimum required amount to keep your brain awake.
Deep divers know that they can take advantage of the increased partial pressures of oxygen to function at very deep depths. In fact, it's common practice to use hypoxic mixes (mixes with LESS than 21% oxygen) at very deep depths, because the increased effect of oxygen there will still keep you alive and functioning. There is a "danger zone" of breathing mixes, where you want the effective pO2 to be above 0.15 (or 15%) at all times. If you go below this, you risk passing out.
Still with me in this novel? Good...here's how this all comes together. Remember, at all times, your body is using O2 and making CO2. If you are breath holding, that means O2 levels in your blood are dropping, and CO2 levels in your lungs are rising. As CO2 increases, is stimulates your breathing reflex...which is what causes that spasm like sensation in your lungs when you are holding your breath. There IS NO SENSATION that is tied to low O2 levels, though. That means that if you are getting below a pO2 of 0.15, you don't really realize it...you just start to pass out. Now remember, gasses like oxygen exert a greater force at depth. So going back to our 50 foot example...let's say you hold your breath, and your body continues to use oxygen, down to a pO2 of 0.15 while you are at 50 feet. Now, you quickly head up to the surface. OK...if NOTHING FURTHER changed in your body, what is your pO2 at the surface? It's now 0.06. That's not enough to keep your brain running, so you pass out.
