Freediving After Scuba (long)

[BillP]

Hi Dr. Deco:

Scuba divers are often told not to freedive after scuba. I've heard a number of arguments for the taboo along with rebuttals for the arguments. I'd like to hear your take on the subject.

Argument #1-If you freedive after scuba, venous gas emboli that formed as a result of the scuba dive(s) but were trapped by the capillaries will be compressed and allow to pass to the arterial side of the circulation. On ascent, these bubbles will re-expand in the arterial circulation and cause problems.

Rebuttal #1- If this is a problem with freediving after scuba diving, would it not be even more of a problem to SCUBA dive after scuba diving? The same people who prohibit freediving after scuba with this argument don't seem to have the same problem with repetitive scuba dives. If the only problem with freediving after scuba is allowing already-formed bubbles to pass into the arterial circulation, wouldn't repetitive scuba dives be just as dangerous? Since you seem to be in the "bubble modeler" camp of decompression theory, I'm particularly interested in your take on this.

Argument #2- Super-aggressive professional freedivers like the Ama of Japan and the Polynesian pearl divers of the Tuamoto Archipelago dive deeply enough, long enough, and with enough repetitions that they are able to absorb sufficient nitrogen to exceed the no decompression limits (NDL's) for their dives and develop DCS. Recreational freedivers after scuba also absorb nitrogen in addition to their residual nitrogen load from their scuba dives, and can develop DCS by exceeding the NDL's.

Rebuttal #2- Most recreational divers will dismiss this argument as not being applicable to them. They don't dive as deep as the professional divers and don't stay down nearly as long. They'll say that they weren't that near the NDL's on their scuba dives, and that they had enough of a surface interval that there's no way they could surpass the NDL's on their freedives. Therefore it is not a valid reason for them to avoid freediving after scuba.

Argument #3- Freediving after scuba will cause the diver to absorb additional nitrogen and will slow the offgassing of their residual nitrogen from their scuba dives. This additional nitrogen will alter the diver's position on the dive tables and make future calculations of NDL's for subsequent scuba dives inaccurate increasing the risk of DCS.

Rebuttal #3- Same rebuttal as #2. The divers will say they don't freedive that deep or long. They spend most of their time at the surface. They will also say that they do their freediving at the end of the diving day and won't scuba dive again until the next morning- more than an adequate enough surface interval to start them fresh on the tables the next day. Besides, all they'll say they have to do if it is a "problem" is take their dive computer along on the freedives to keep track of the nitrogen. (I understand that some newer computers even have a "freedive" mode.)

Now this brings me to the argument that you seem to see the least often in the recreational diving world, but the one that seems to me to have the most validity.

Argument #4- Rapid ascents are associated with an increased risk of DCS, even when the diver has not reached the NDL's. (Using an old PADI RDP table for the examples.) A scuba diver who makes a dive to 40' for 115 minutes is an "X" diver. If upon returning to the boat, this diver immediately makes a freedive for a few seconds down to 40' to free the anchor and then ascends, he is now an "X" diver who has made a rapid ascent from 40' and is at increased risk of DCS- even though he didn't absorb significant additional nitrogen on the single freedive. A scuba diver who makes a dive to 90' for 18 minutes is a "K" diver and if that diver makes a rapid ascent at the end of his scuba dive he would be at increased risk for DCS- even though he's not at the NDL. But if a second scuba diver makes a dive to 90' for 25 minutes (a "Q" diver) and then has a surface interval of 30 minutes he is also a "K" diver. This second "K" diver would also be at some increased risk for DCS by freediving and then making a rapid ascent at the end of the surface interval- even if he doesn't dive deep or long enough to absorb significant additional nitrogen or surpass the NDL's.

Rebuttal #4- I've heard various rebuttals for this argument. One is that the diver has already made a slow ascent from his scuba dive(s) to slowly off-gas nitrogen so any nitrogen that would bubble would have already done so. Since the diver ascended slowly from his scuba dive and did not develop DCS he is now "safe". On his freedive, the diver's nitrogen will somehow "remember" those previous slow ascents and not be affected by the rapid ascents from the freedives. As the diver is not at or near the NDL after his slow ascent on scuba, he can't get bent from his freedive. You've already answered in my questions earlier in this forum why that rebuttal isn't valid. Another rebuttal that I've heard is that the freediver will do his scuba dives in the morning and then freedive in the afternoon after off-gassing. Since he's off-gassed, he can't get DCS. But while the risk may indeed be LESS after a surface interval, how much surface interval is "enough" for freediving after scuba? Other rebuttals are, "that's just a theoretical argument and unproven; I dive in the real world", "divers often freedive after scuba; if it were a problem we'd have cases of DCS after freediving all the time"; "I've been freediving after scuba for years, and I've never gotten bent", etc. Of the arguments against freediving after scuba, the problem of rapid ascents seems to be the most valid argument for the taboo, but at least in the recreational diving world it seems to be the least often used.

I'd be interested in hearing your comments on freediving after scuba.

Thanks for your answers to my questions. I promise that I won't be bugging you forever. I'm just "clearing my desk" of pent up questions now that I have someone to answer them.

Bill

 

[Dr Deco]

Bill:
Please do not consider that the questions are excessive. You obviously are not a novice by any means in decompression concepts. My answers today are much shorter than yours were….
Michael Powell

1. As far as the first argument goes, Doppler bubble detection work for several decades does not indicate a passage of gas bubbles from the venous side to the arterial with recompression. That study was one of my first efforts in the field of barophysiology [MR Powell. Leg pain and gas bubbles in the rat following decompression from pressure: monitoring by ultrasound: Aerospace Med., 43, 168-172 (1972); MR Powell. Gas phase separation following decompression in asymptomatic rats: visual and ultrasound monitoring: Aerospace Med., 43, 1240-1244 (1972)]. Your argument that what holds for SCUBA would also fold for breath hold diving would appear to be true.

2. As for the second and third points, I would doubt that a SCUBA diver would do that much free diving to add much dissolved nitrogen.

3. The fourth point would seem to indicate that some individuals believe that gas “remembers” or partitions itself differently as you dive - - if I get the gist of this. I doubt that gas is that smart. Anytime gas is added or subtracted, it does so without knowledge of previous dives being safe. Free diving could, however, form new micronuclei because of the body’s physical activity - - if that activity were strenuous - - or it could promote off gassing by increasing the blood flow (from physical activity). Very strenuous activities (such as pulling to free an anchor) are known from laboratory studies to promote gas bubble formation/growth. In addition, the last activity is often associated with a Valsalva-like maneuver, and it could promote arterialization of bubbles should they be present in the venous return.

 

[BillP]

So looking at your point 3, is the main problem with freediving after scuba the strenuous nature of the activity, or do the rapid ascents associated with freediving also play a valid role in the admonition to avoid freediving after scuba? If a scuba diver could freedive without performing a strenous activity like freeing an anchor, is it OK to freedive immediately after scuba?

(This harks back to my earlier question about the role of rapid ascents and DCS when not at or near the NDL's.)

 

[Dr Deco]

I would venture to say that the strenuous nature of the dive is the greater concern. Activity is good in that it can promote blood flow and gas elimination, although probably no gas is lost during the free-dive portion itself.

While at surface, blood flow is elevated from the physical activity during the breath-hold dive, and it remained elevated for ten to twenty minutes. This “carry over” is termed the “post exercise oxygen deficit.” It is a part of the reason that people are encouraged to exercise daily if only for twenty minutes; as a consequence, the metabolism is elevated (and blood flows are also elevated.)

The rapid ascent would be a problem only if you took on much gas during your free dive (unlikely). STRAINING of the musculoskeletal system is always bad, as it provokes micronuclei formation.

 

[BillP]

Well, now I guess that I'm confused. Under my question heading "Rapid Ascents and DCS" I got the impression that an effect of the pressure release from a rapid ascent is to increase the local supersaturation of nitrogen around the bubble micronuclei leading to bubble growth, even if you were not at or near the no decompression limit. And while the risk of bubble growth might be greater if you had higher levels of nitrogen supersaturation in the blood/tissues, the risk of bubble growth and DCS was still present at lower levels of nitrogen supersaturation if you make a rapid ascent with increased nitrogen in your blood/tissues.

I guess what I don't understand is the difference between a rapid ascent on scuba and a rapid ascent with freediving after scuba with residual nitrogen still on board. If the effect on the residual nitrogen when making a rapid ascent freediving after scuba isn't of significant concern for increasing the risk of DCS, why is a rapid ascent on scuba when you're not at near the NDL's a problem? (Ignoring for a moment the risk of AGE or hitting your head on the boat prop.) How does the nitrogen in your blood/tissues "know" whether you have a snorkel or a regulator in your mouth? I'll agree that you don't take on much additional gas from a freedive. But getting back to my earlier example of the "X" diver who freedives immediately after scuba- if he were to passively ride a sled to depth and have a lift bag carry him to the surface (instead of exerting himself to free the anchor as in my earlier example) his rapid ascent wouldn't be a significant problem? His nitrogen would somehow be stabilized by his slow ascent on scuba immediately before the freedive and somehow "know" that he's freediving when he makes his rapid ascent? See where I'm getting confused?

People often exert themselves to some degree on repetitive scuba dives (gearing up, swimming against current, climbing back on the boat in full gear, etc.) and yet if they back off their NDL's for particularly strenuous activity, repetitive scuba dives are considered "OK". Your typical recreational freediver might not exert himself on his freedives after scuba any more than the repetitive scuba diver, and the freediver isn't taking up significant amounts of additional nitrogen. If the "problem" with freediving after scuba is the strenuous nature of the sport, it would seem that NONE of the typical prohibitions about freediving after scuba are valid if you don't exert yourself any more strenuously when you freedive than when you do repetitive scuba dives. Even the effect of the rapid ascents associated with freediving on the residual nitrogen from the scuba dives are of no significant concern?

TIA,

Bill

 

[Dr Deco]

I believe I see one source of confusion, and it lies in my assumption for my answer. That is, I assumed that the free-dive ascent rate was no faster than the safe, scuba ascent rate. Additionally, I assume that some interval had passed and the tissue gas loads during the free ascent were less than the gas loads during the last scuba ascent (naturally). It is not necessarily a fair assumption that the free ascent is slow. In addition, I assumed that we were not talking about any significant degree of gas phase separation in the SCUBA dives. If one were to have some degree of gas phase growth, this gas would return to solution during the free dive and the rapid ascent could provoke the growth of these bubbles and other micronuclei. Thus a slow ascent, even in a free dive following scuba, is always a good idea.

The problem is really quite complicated when you deal with repetitive compressions and decompressions in quick succession. This was treated a few years ago in a study which showed that, in truth, gas phase separation occurs even in asymptomatic dives, and a repetitive dive could “amplify” this free gas phase and cause problems [Gait D, Miller KW, Paton WD, Smith EB, Welch B The redistribution of vascular bubbles in multiple dives. Undersea Biomed Res. 1975 Mar;2(1): 42-50].

I believe that the case you presented might be quite dependent upon the initial conditions. We always return to the rule that, for any given sequence of dives, shorter bottom times are safer than longer bottom times and longer durations between dives are better than shorter intervals. Therefore, in the absence of a gas loading analysis (which would really require a bubble dynamics model in this specific instance), I would say that the longer the surface interval between SCUBA and free dive, the better. Beyond that, it would take some computation to sort it out. Even then, one might really need some laboratory testing, since these models are still in their infancy. Wow - - - this then becomes complicated…..

Mike Powell

 

[old_mike]

we typically shoot to the surface as fast as we can when freediving, especially after chasing mantas or other photographic subjects. I've never gauged our ascent rates but 4 to 5 feet per second is my guess. Add to this, the sheer number of repetitions in a short amount of time (anywhere from 5 to 30) and the fact that we are generally within 25 feet of the surface (where of course, the proportional change in pressure is the greatest) and i can't help but wonder if we are playing Russian roulette?

By the way, i make it a hard fast point to avoid freediving after a deep dive (90'+) i have no scientific basis for this arbitrary cut off point, just an intuitive hunch.

 

[BillP]

I think mike makes a good point. The typical scuba diver will ascend at 30 feet per minute then make a 3-5 minute "safety stop" at 15' before completing the ascent to the surface- taking several minutes to ascend to the surface from 30'. They typical freediver will stay at depth as long as he can hold his breath and then ascend to the surface as fast as he can kick- taking only a few seconds to ascend to the surface. Except for the occasional accidental buoyant blow-up ascent on scuba, I don't think scuba ascent rates often approach those used when freediving.

Unfortunately I don't think that funding will be forthcoming for a freediving after scuba study, so we won't have the definitive "answer" to my question any time soon. Right now it's just mental, uh, gymnastics. I've asked this freediving-after-scuba question of Michael Lang, diving director for the Smithsonian, and Charlie Lehner of the Univ. of Wisconsin (they organized the big symposium on Reverse Dive Profiles last year) and they both gave answers similar to Dr. Deco's. There's no scientific data showing the risk of DCS from freediving after scuba after whatever surface interval, but both advise against freediving after scuba (and Michael Lang prohibits it for the Smithsonian divers) because of the potential risk.

Bill

 

[Dr Deco]

I believe that when it comes down to the wire, the answer is that there is no definitive answer. As BillP indicates above, both Mike and Charlie feel roughly the same way – it could be a risk and there are no data. Best play it safe with a considerable surface interval

When dive computers came out in the 80s, I was afraid that the multilevel nature of the dives could lead many divers into trouble. This does not appear to have happened. My guess is that it is because we are not actually seeing free-gas phase separation to the extent that problems could arise. Naturally, this is good.

We appear to have gone through quite a few iterations on this question, starting with the simplest reply and proceeding to the more complex. {Several years ago, in another forum, someone questioned my remarks on adding nuclei to a carbonated beverage by dropping in salt crystals. I stated that the crystals have small air pockets on their surface, and that these are nucleating sites and resulted in effervescence. The reader wrote back something to the effect that bubble formation was more likely the result of a change in the activity coefficient of the water because of the ionization of the sodium chloride molecules and the hydration of the ions by the solvent (water molecules) and hence their inability to solvate the dissolved gas. This was to be coupled with the electrostrictive effect of the dissolved ionic species. I wrote back that the physical chemistry was indeed correct, but that I still believed the major player was the gas motes on the crystal surface. Now in my classes, I use sugar.}