I'm only a new freediver (40m PB in CWT) but I come from a tech diving background and have had a long held amateur interest in decompression and hyperbaric effects on humans. In my short experience talking to freedivers, they of course know the term of shallow water blackout, but I'm not sure there is a wide understanding of the actual difference between a straight forward hypoxic blackout, versus one resulting from the mechanism of falling PO2 driven by reduced hydrostatic pressure. I actually writing a piece on this and humbly offer the following for 'peer review' by the experts on this forum. I would appreciate all feedback, especially clarifications/corrections. Bear in mind this is to be published in a magazine for tech and cave divers, hence some of the explanatory notes.
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Since the ‘bad old days’ of scuba diving many long-held practices have rightly fallen into disrepute. Being good at Deep Air was once a badge of courage and toughness, whereas today you’d be ridiculed for suggesting say a dive to 100m on a single 80cuft bottle. I learnt to plan an ascent utilising US Navy tables which were based on decades old data and long before the first wearable dive computers (Yes I am THAT old) Today we can pick and choose our algorithm of choice and tune it to our liking.
Likewise we now have a better understanding of how breath hold diving works in practice though there is still much to learn. Indeed, getting hard data on freediving is much more difficult than compressed air diving as the commercial imperative to fund solid scientific research on freedivers simply is not there. With compressed gas, there was great interest from the various fields of military, scientific, commercial and recreational communities to know with a high degree of certainly whether a certain dive profile could be conducted safely. So funding research had an easily justifiable ROI (Return On Investment).
Freediving however is more of a niche recreational activity and it’s a tough sell for research scientists to prove an ROI on expensive studies for a recreational pursuit like freediving, other than for reasons of pure research. Nonetheless progress is being made [quote example papers and work by DAN Europe]
Hyperventilating for instance, once a mainstay of breath-hold diving is now understood to be not only counterproductive but fatally dangerous. Counterproductive, because the boffins now understand that not only do elevated levels of CO2 drive a stronger MDR, but the presence of elevated levels of CO2 in the blood trigger the body to release extra oxygen molecules that would otherwise be still bound to the haemoglobin, thus realising an increase in diver performance. Sure, increased CO2 and the resultant contractions don’t feelvery nice. But like a lot of things in life, sometimes those things which is good for us in the long term, can cause us discomfort in the short term!
But the most deleterious effect of hyperventilation is far more sinister. Humans have evolved so that under normobaric conditions in the average person, the rising levels of CO2 reach a point where the urge to breathe becomes undeniable beforeoxygen levels fall to a point unable to sustain consciousness.
By hyperventilating before the dive, the diver blows off CO2, artificially lowering the starting point from which the CO2 starts to build. But the hyperventilation does not result in any big increase in O2. So, as your O2 level drops as per usual towards the danger zone, the CO2 is building from a much lower base. Since the vast bulk of urge-to-breathe comes from the level of CO2, there is every chance you will ‘feel’ fine even though your O2 levels are perilously low, you haven’t yet got that big urge to breathe and you may black out.
All the above in relation to hyperventilation is still true in a pool, without any depth change, but diving vertically to depth introduces ambient pressure changes, ramping up the danger logarithmically by the insidious effect of partial pressure. Air of course is largely comprised of O2 and N, but the nitrogen can be dismissed in this discussion so really, it’s all about the O2. Nitrogen as an inert relates to DCS for sure, but not applicable to this discussion of Shallow Water Blackout (SWB).
PP02 is a subject well understood by tech divers but at this early stage of free dive instruction, I don’t think it as universally understood by students or perhaps even some instructors, at least not to the extent that we do as tech divers. In my experience, there is tendency to lump it all under ‘hypoxia’ without a proper appreciation of the crucial difference between normobaric hypoxia resulting from a simple denial of urge to breathe, versus SWB, driven by falling hydrostatic pressure and resultant PO2.
So, consider a breath hold dive to depth after hyperventilation. The CO2 floor has been lowered reducing the urge to breath but oxygen levels start from more or less the same point and are consumed as usual. BUT, at say 30m the PO2 of 21% is under 4ATA of pressure, resulting in a PO2 of 0.84. It actuality, it is never that high by the time the freediver has arrived at 30m because unlike SCUBA, the freediver is metabolising the O2, from the second they leave the surface, but for the purposes of this discussion let’s leave the numbers at that.
As the freediver who hyperventilated is at depth consuming that O2, the higher concentration or PP02 driven by the partial pressure allows them to function up to a point. But as they return to the surface, the falling hydrostatic pressure reduces the PO2, most dangerously in the final seconds of the dive when the diver Is the most stretched on their breath hold. Between 10m and the surface the PO2 drops by a whopping 50%, when they are only 10 seconds from safety. All of this, possibly without yet a single strong recognisable urge to breathe because of hyperventilation.
Note that the following numbers are not scientific but anecdotal, to illustrate the problem. Even if our diver arrives at 30m with an effective arterial PO2 of .84 he hangs out for a while, or even if he turns the dive and commences his ascent, he has consumed a large % of his accessible O2. How much by the time he leaves the bottom … maybe 25%? Which means they will need at last 25% more to get bac to the surface. In practice this might be more as the effort to get down is less than the effort to fin up, against the negative buoyancy at depth. But if we say a consumption of 50% during the dive, then has he approaches the surface his PO2 of 0.84 if half consumed at depth would leave the diver with an arterial PO2 of 0.42 on the bottom. But let’s break down the ascent and PO2 in stages of ascent.
30m/4ATA 0.42
20m/3ATA 0.32
10m 2ATA 0.21
0m/1ATA 0.11
Since the average person cannot endure a concentration of O2 lower than about 13%, that diver is in danger of blackout.
It will not surprise you then to learn that the clear majority of blackouts occur in the last 10m, and indeed on the surface. After ‘completing’ the dive. This phenomenon explains almost all breath hold deaths in the ocean, of which in the USA there are 50+ per year. Almost all of those fatalities are untrained divers such as long time spearos who scorn structured training, do not follow ‘one up, one down’ freediving practice, and fail to perform basic safety procedure such as meeting buddy at 6 – 10m then observing them for minimum of 30 seconds post-dive. Those of you familiar with the early deaths in cave diving n the 60s and 70s will immediately note the similarity of untrained divers entering an environment, about which they don’t know, what they don’t know. Truly a recipe for disaster and frustratingly easy to avoid if simple protocols were followed which have been categorically proven to almost eliminate than chance of a fatality.
Talking to survivors of shallow water blackout, the overwhelming consistency in their recounting is they had ZERO warning of the event. They felt fine right up until they blacked out with no recollection of the incident. As yours truly has experienced what was later explained to me as a 20 second blackout in pool training, I can likewise assure you, there was no panic, or impending sense of doom. I felt stretched for sure with a very strong urge to breathe, but as I got within 5m of the wall, I felt “Yeah, I got this”. The next thing I recall was being on my back over the lane rope with people saying loudly; “BREATHE DEAN!, BREATHE DEAN!” Wow, what just happened ? Absent a buddy/ trained in how to recover a blacked out diver, I was dead.
As WKPP pioneer George Irvine was fond of saying; “Diving solo removes any second chance you ever had” Wiser words have never been spoken whether recreational, tech, cave, OC, RB or freediving. We must respect the fact that whilst we retain a biological connection to our ancient aquatic past, that is not our native domain. Any journey into water, whether by compressed gas or breath hold, must be treated with the full respect for our underwater domains.
As it is with technical and cave diving, we freedivers owe it to those payed with their lives, to learn from their mistakes and dive safely in the future.