There is so much light and dark grey between casually holding one's breath for "shorter" periods of time to "longer" periods of time, but the reality of blacking out is "only takes once" to die if you don't have an attentive dive buddy at the time...
There's also some physiological realities (such as euphoria is a symptom of hypoxia...) that make wet breath-hold activities so dramatically safer with a buddy than without. At the end of the day the worst case consequences of dry solitary use of a rowing machine alternating between breathing and not almost certainly non-fatal even if one were to pass out and blacking out on any solitary breath-hold in the water is almost certainly fatal...
My personal rule, which I don't advocate and almost never break, is no more than one-third to one-half (trust myself but verify with dive computer) of my personal bests if i don't have a trusted buddy watching me.
GirafeMarineSalvage,
I like your cautions, and by what you say I should have died in my teens. I had blackout in a pool when doing an underwater swim during swim team practice. We were in a 20 yard pool, and my buddy Tommy had just completed 4 lengths underwater. I was determined to beat his time, and experienced some of that euphoria on the third length. I was swimming the fourth length, and told myself that I’d hit the wall, make an underwater turn, take one stroke and swim to the side, which is what I did. However, I did not remember anything after that last turn. Suddenly, I was holding onto the side of the pool, just like I’d preprogrammed myself to do, and a bit dizzy. I’m pretty certain that if I had not told myself to surface after one stroke, I would have continued swimming underwater until I died.
I’ve since done a lot of investigation of Shallow Water Blackout, and the one thing that stands out is extreme hyperventilation prior to the swim underwater. The hyperventilation does not increase the oxygen in the lungs, but it does “blow off” the carbon dioxide (CO2) in the blood stream. And, guess what? It is an excess of CO2 in the blood stream that is the signal to breathe, the “must breathe” signal. If that signal is postponed beyond the time that the level of oxygen in the blood stream becomes insufficient for consciousness, the diver will unexpectantly black out.
The mechanism is slightly more complicated for deeper breath-hold dives, in that the lessening pressure can lead to oxygen actually traveling backwards, from the blood stream back into the lungs, with the lessening partial pressure of oxygen in the lungs, also leading to blackout in shallow water, the notorious shallow water blackout.
The technique I’ve developed I call “rhythm breath-hold diving,” which does not rely upon hyperventilation, but on gradually increasing breath-hold time over about half an hour. The “must breathe signal” is still there, and comes in prior to low blood oxygen level. But over that half-hour time period, the body does naturally adapt. This is similar to how the Japanese Ama divers dive, although without the whistle. This rhythm breath-hold diving does not try to set any records, just to adapt to breath holding. I explained the technique in the post above.
The problem I have with a buddy being there is not that the buddy is there, but that the breath-hold diver is pushing himself, using hyperventilation, to go further without adaptation, and may actually black out prior to ceasing body swimming motions. It is this attempt to break records, stay further, or go deeper, that is extremely hazardous and I do not recommend. The idea is to prevent the blackout entirely, using sound physiological concepts, rather than a buddy trying to resuscitate an unconscious diver after having to bring that diver to the surface.
SeaRat
PS1, a DAN report:
PS2, 1994 research article on Ama divers:
Arterial blood gas tensions during breath-hold diving in the Korean ama
J. Qvist, W. E. Hurford, Y. S. Park, P. Radermacher, K. J. Falke, D. W. Ahn, … See all authors
01 JUL 1993https://doi.org/10.1152/jappl.1993.75.1.285
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Abstract
Korean female unassisted divers (cachido ama) breath-hold dive > 100 times to depths of 3–7 m during a work day. We sought to determine the extent of arterial hypoxemia during normal working dives and reasonable time limits for breath-hold diving by measuring radial artery blood gas tensions and pH in five cachido ama who dove to a fixed depth of 4–5 m and then continued to breath hold for various times after their return to the surface. Eighty-two blood samples were withdrawn from indwelling radial artery catheters during 37 ocean dives. We measured compression hyperoxia [arterial PO2 = 141 +/- 24 (SD) Torr] and hypercapnia (arterial PCO2 = 46.6 +/- 2.4 Torr) at depth. Mean arterial PO2 near the end of breath-hold dives lasting 32–95 s (62 +/- 14 s) was decreased (62.6 +/- 13.5 Torr). Mean arterial PCO2 reached 49.9 +/- 5.4 Torr. Complete return of these values to their baseline did not occur until 15–20 s after breathing was resumed. In dives of usual working duration (< 30 s), blood gas tensions remained within normal ranges. Detailed analysis of hemoglobin components and intrinsic oxygenation properties revealed no evidence for adaptive changes that could increase the tolerance of the ama to hypoxic or hypothermic conditions associated with repetitive diving.
