Freediving after scuba diving

[JoshNZ]

Hi guys/girls,
Hopefully you don't mind a quick stop to resolve a subject that's come up several times for me and some over the years!

I know you're not supposed to freedive with nitrogen dissolved in blood after diving, but I don't exactly understand why, despite numerous explanations. I do have a reasonable understanding of gas laws, I know nitrogen is dissolved in blood and ascents must be slow to allow this gas to offload without coming out of solution and causing illness. I also understand after a safe ascent when sitting in an appropriate pressure group at the surface, the pressure gradient between gas tension in blood and partial pressure in atmosphere is low enough that bubbles will not form. So, what is it about a moment at depth before returning to this state that can unfurl this situation?

It would make sense (in my uneducated mind, at least!) if your dissolved nitrogen at the surface is at a safe concentration, momentarily increasing pressure and then decreasing back to this same surface pressure, should change nothing.. I know some gas will be dissolved at depth from the one lungful of compressed gas you are holding but I thought this was insignificant.

Why does going from a safe ambient pressure, to depth, and back to the same safe pressure without loading more gas cause issues?

Not here looking for an excuse to be an idiot, it's just something I've wanted to understand and have finally gotten around to asking the question!
Thanks in advance as always,
Josh

 

[hxtk]

The freediving itself is more or less irrelevant. It's just another rapid ascent without a safety stop, which is advised against for the usual reasons. I suspect, though I do not have data to make a recommendation, that there would not be any additional risk associated with a freedive on which one observed an ascent rate limit and performed a safety stop, but that's not a realistic picture of recreational freediving in practice.

 

[Esprise Me]

There's also the exercise involved in freediving. Generally you don't want to exercise after scuba.

 

[O-ring]

I remember at one time (this was probably 20 years ago or so) the big fear was that venous gas emboli from diving that had been trapped by the capillary beds would get recompressed by subsequent freediving thereby allowing them to pass to the arterial side of the circulatory system where they would then re expand. If I recall correctly there was a study or two that largely debunked that and the major concern was just the strenuous exercise factor since bubbles are generally thought to be at their peak between 30 and 45 minutes post-dive so it is important to take it easy during that period. I suppose, technically, you would halt or reduce your off-gassing by returning to depth for those brief periods you were freediving so that would cause minor inaccuracies in your surface interval since you weren't offgassing at surface pressure during the entire SI. Again, it's probably so small it doesn't matter, but hopefully someone smarter will come along and straighten us out.

Eggheadery aside, I have done plenty of freediving after diving and never had a problem. One of the boats I used to dive off frequently had surface supplied deco gas dropped over the side at the 20' so after removing my gear I would pull myself down the anchor line, across the hangbar and over to the regulators and just hang out under the boat breathing the surface supplied gas with just a swimsuit, mask and fins on. I probably wouldn't recommend that, but if you try it make sure to exhale before you head to the surface.

 

[JoshNZ]

I'm aware the rapid ascent is the reason it is advised not to freedive but my question is, why is this rapid ascent harmful if your gas tension is already at a safe level - at the surface. Surely a rapid ascent to the surface where your level of dissolved nitrogen is already proven safe, is harmless (without adding more of course).

From my understanding a slow ascent on scuba allows dissolved gas to off-gas, presumably from tissue to blood then blood into lungs and then exhaled, without coming out of solution in your tissue. A rapid ascent to the surface would put you in a state where the pressure gradient across your blood and atmosphere would be great enough that gas may come out of solution in body tissue. But once surfaced and pressure gradient across blood and atmosphere is at a safe level, will momentary increase in pressure and return to ambient atmosphere where it was safe, actually do anything?

Exercise is a good point I know it is not advised either. Can we assume a relaxed freedive as I am more interested in the blood-gas science haha.

I was told the probably over simplified soda bottle analogy once, if you cracked the lid slightly and allowed the gas to hiss out slowly you would not have bubbles or fizzing form in your soda, akin to ascending slowly on scuba. Eventually you could remove the cap and no bubbles would form - like being surfaced and stable. So my thoughts are if you then recapped the soda, stood on it for a moment, then let off, would bubbles form or not?

 

[JoshNZ]

Was that to lengthen your safety stop O-ring?

 

[johndiver999]

Hi guys/girls,
Hopefully you don't mind a quick stop to resolve a subject that's come up several times for me and some over the years!

I know you're not supposed to freedive with nitrogen dissolved in blood after diving, but I don't exactly understand why,
Josh

First, freediving does add to the inert gas load. So much so that very good freedivers will get bent from freediving alone if they don't have sufficient surface intervals.

Second, a freediving ascent is not normally going to be as slow as is assumed for scuba diving. So making rapid ascents with tissues that are supersaturated with nitrogen, is going to be stressful. You will absorb the same amount of nitrogen on a dive to 40 feet for 1 minute regardless of whether you are scuba diving or freediving.

Third, - and I don't really understand this, but doing a bunch of aggressive freedives is "shaking the bottle" and promoting bubble formation - is what i understand.

 

[CuriousRambler]

I've been reading a ton on gas physiology lately (somewhere near 400 pages of various journals and publications this week). That is to say, I'm somewhat read on the topic but have no formal education or any real knowledge, so take this with a grain of salt.

First, understand the fact that we honestly don't really know what's going on with decompression (we're going to go ahead and classify *all* offgassing as decompressing for our purposes here). Nobody truly knows what causes DCS, or the best way to avoid/minimize it. That's just fact.

Now, we obviously have some idea how to reduce DCS, as the low instance of DCS hits resulting from diving within NDLs proves. There are essentially two "camps" trying to fit formulas to the problem of gas transfer. One is the dissolved gas model (Haldane) which assumes gasses in the blood stay dissolved in the blood and transfer out primarily through respiration. An extension of this logic is that the formation of bubbles is bad, and some theorize bubbles equal DCS, period.

The other side of things are bubble models, which assume inert gasses WILL bubble in the blood under normal diving conditions. The fact that bubbles form in the blood of many divers who never show any DCS symptoms has been repeatedly proven through post-dive analysis with various instruments, including some fancy Doppler audiogram thing I can't recall the name of. Again, the existence of these bubbles in blood is fact.

One of the ideas is that slow ascents and safety stops serve to limit the size the bubbles grow to. By keeping them relatively small as they come out of solution, the first place they're going to hang up is most likely in the lungs, as there are many capillaries so small they are literally only wide enough to pass a single blood cell at a time. If you create a blockage of nitrogen bubbles behind these, the net effect is you have an abundance of nitrogen piling up waiting to be exhaled. Even if the bubbles grow to "dangerously large" sizes in this location - you're still just stressing the lungs to expel more nitrogen.

As o-ring mentioned, there are theories that diving down during this bubble phase (which my reading suggests is on the order of 4-6 hours for most divers, not 30 minutes, but ABSOLUTELY varies depending on individual characteristics, and your horoscope) can recompress some of these bubbles, making them small enough to pass by the lungs and wind up on the arterial side of your circulatory system. You surface after a quick freedive, the bubbles expand back to "dangerously large," but now on the oxygenated side of your lungs, and your heart pumps a relatively large bubble of nitrogen into a joint, or into a constricted blood vessel and creates a blockage, and you have a recipe for DCS.

 

[JackD342]

As I expected, DAN has something to say on the subject. My takeaway after skimming the article is "possible but not proven and not likely, especially if the free diving is moderate and not extreme."
But read it for yourself and see what you think.
Alert Diver | Could Breath-Hold Diving after Scuba Cause Decompression Sickness?

 

[O-ring]

Was that to lengthen your safety stop O-ring?

No. Strictly to harass people on deco.