DCI and freediving

[sebastien murat]

Since I've been queried now several times about the relative risks of of succumbing to DCI with respect to diving on full compared to empty, I thought to lay my thoughts on the topic for those who may be interested, namely deep divers.

Several factors help limit the rate of (inert) gas uptake on `empty´ versus full lungs, thereby reducing the risk of DCI:
- A rapidly diminishing alveolar surface area, which decreases more quickly on empty versus full lungs
- An increasing alveolar membrane thickness, since it is un-stretched compared to the full lung state
- Cardiac output: (a component of the DR) is decreased more quickly in the empty state, which results in a shallower alveolar-capillary gas concentration gradient. This would suggest that a prompt and strong DR may be an important requisite for limiting gas uptake. It would also be an important factor in reducing the risk of SWB, but that another conversation altogether.

With respect to gas uptake through the upper (respiratory) cavities, of the sinuses (including middle-ears and mastoid air-cells) and nose, it is entirely feasible that copious amounts of gas are absorbed (cf. Einer-Jensen on the ability of gases and low molecular weight substances to pass through into the cerebral circulation and, subsequently, enter the brain, for example). This is rendered all the more plausible if we consider that the volume of these cavities are on the order of maybe 150-200cc and equalization (with gas) would increase this substantially, more or less in direct proportion to the partial pressure experienced; at a depth of 20 atm the gas load could be in the order of 20 x 150-200cc! A way to circumvent this problem is of course is to switch to water equalization early in the dive, which also potentiates the DR. Furthermore, since a prompt and accentuated DR results in peripheral vasoconstriction (which also limits uptake by the lungs), gas uptake via this uppermost route would be further limited. This is readily noticeable as a stuffy/congested nose becomes patent during a breath-hold maneuver.

In short, therefore, there is I would suggest a pretty strong positive correlation between the amount of gas that is taken on board and the risk of DCI that can be expected. It’s not just that a large lung volumes increase the risk because of the sheer amount of gas (about 5: 1 between full and empty), but also because of the effect that such a large ventilatory load this has on the dynamics of gas uptake in relation to blood perfusion and flow through ventilated areas, which is how the gas gets in, in the first instance. Even in seals, considered well adapted for venturing to depths in excess of 300m, the risk is very real as was eloquently, albeit accidentally shown by Scholander in the 1940s, when a seal was submerged on full lungs to 300m and promptly died from CAGE upon surfacing. I appreciate that exhaling into a bottle at 15m may mitigate this risk, as might jumping on O2 post-dive, but the issue of gas uptake via the uppermost routes of the respiratory tree is not lost. Perhaps freedivers should take heed and do their homework first before blindly venturing, especially with repeated venturing with inadequate surface intervals or worse deep warm-ups!

Now, I haven't spoken to Herbert about his dive or his future plans, and so cannot speak on his state of mind, but as our approaches seem to be at least superficially widely different, the notion of stepping up to 300m with a full lung approach is simply a case of playing Russian roulette with a fully loaded chamber. Eitherway, this approach will require serious rethinking. One suggestion I might offer is to at least to slow the (speedy) descent, at least until the DR has kicked-in. Staying on the surface, under breath-hold, for 20-30 seconds wouldn't be bad dive planning.

Best
S

 

[fcallagy]

Thanks Seb, its not just deep divers who are interested I think we all are to a point whether we are competitive or recreational.

"With respect to gas uptake through the upper (respiratory) cavities, of the sinuses (including middle-ears and mastoid air-cells) and nose, it is entirely feasible that copious amounts of gas are absorbed (cf. Einer-Jensen on the ability of gases and low molecular weight substances to pass through into the cerebral circulation and, subsequently, enter the brain, for example)"

I never was aware of this. Fascinating. On the total gas uptake is there also a correlation between gas uptake and the physical size of the person and therefore their ability to absorb gas, so a small body with a larger lung volume would be at a distinct disadvantage from a dcs point of view? Even though we would think that those physical characteristics would make for a "better" freediver.

 

[Fondueset]

Very interesting. Thank you for taking the time to post.

 

[efattah]

Personally my risk of DCS did not change when diving with VC=4.0 to 5.0L versus VC=10.5L, but perhaps truly empty lungs would be a different story.

I ran Herbert's dive plan through the Xen before his dive and the Xen insisted that the first stop must be at -22m (which I suggested to Herbert via e-mail in the weeks before the attempt).

I have many times publicly stated my opinion that for current no-limits diving, an enormous static apnea is required, and in my mind a 250m dive on full lungs with air equalization would last around 7-9 minutes at least, due to the diver stopping on the ascent at 22m (or deeper) and making numerous 1 minute stops from 22m, 19m... to 10m, followed finally by surfacing, finishing the protocol, then descending immediately to 22m (or deeper) again on 50% nitrox and completing a minimum of 16 minutes of deco (according to the Xen).

Considering Branko's 12:11 static (and much longer in training), no-limits might actually become a difficult 'athletic' discipline if the dive time required is more than 10 minutes due to all the deco stops. For full lungs diving with air equalization it is clear that the 'bend and mend' approach no longer works (i.e. doing almost all the deco after the dive is too late; DCS has already started).

Considering that seals and sperm whales do apnea deco stops on the ascent from deep dives, even diving with empty lungs, then even an empty-lungs no-limit dive might need to last 5 or 6 minutes (even with reduced gas uptake).

However I do strongly agree with Seb that water equalization is critical to avoid gas uptake through the sinuses-- especially since that gas is fed straight into the brain, likely to cause a stroke when bubbles form.

When Herbert is back online, the first question I have for him is what depth did he start to feel bad, during the ascent.

 

[sebastien murat]

A large lung volume to body size would be a distinct disadvantage. Getting int the deco game isn't the solution, because O2 uptake dynamics and, therefore, breath-holding ability are not the same at depth, so that a 12min surface breath-hold that hinges on lung capacity wont help you much. Better dividends are to be had by lowering O2 consumption rate, i.e., metabolic rate, and result in tisue cooling. Cold exposure, when presented in the right way not only lowers metabolic rate and, hence, inert gas uptake, but also allows for better gas solubility and more gradual off-gassing. This means focusing on those factors that potentiate the dive response.

Eric, I've looked a lot of animal dive profiles of the years and many of these have been really deep, long, repetitive and strictly U-shaped, with no indication of reduced ascentt rate or deco stops. The problem of DCI seems always to have been when these animals have been diving on full versus empty lungs; they dived on full when they were forcibly dunked or harpooned. Either way they were surprised and caught out with a lung full of air when they sounded. I would strongly recommend reading early accountsand research reports from pre 1940s, going back the 1800s for anyone seriously interested.

 

[sebastien murat]

And to sum up: Little gas, no troubles. Big gas, big troubles.
Gas has no/little place in the business of freediving. Better dice with scuba if thats the case!

 

[sebastien murat]

Eric, please excuse, I know little about the Xen, but does/can it incorporate uptake parameters that more realistically reflect metabolic state, e.g., cardiac output using heart-rate as a proxy? Reason I ask is because, unlike with (breathing) diving, freediving generally results in a dive response, which in turn significantly affects regional blood-flow and, hence, uptake dynamics. That being said, using an off-the shelf algorithm would over-estimate the amount of uptake and release.

 

[efattah]

Eric, please excuse, I know little about the Xen, but does/can it incorporate uptake parameters that more realistically reflect metabolic state, e.g., cardiac output using heart-rate as a proxy? Reason I ask is because, unlike with (breathing) diving, freediving generally results in a dive response, which in turn significantly affects regional blood-flow and, hence, uptake dynamics. That being said, using an off-the shelf algorithm would over-estimate the amount of uptake and release.

The Xen algorithm is experimentally fitted to freediving DCS data, and adds three levels of conservatism. If you believe you are absorbing less gas, you can run it on the most aggressive setting. I need to run it on the most conservative setting.

 

[Simos]

Really sorry if this is the wrong place to ask perhaps a newbie question but it is related to the subject - I never really understood why apnea deco (on the way up) works. Are we sure it is actually effective to stop and wait at a certain depth in apnea?

From my little understanding, the reason O2 is usually works in deco (say in scuba) is because you are breathing a gas that does not contain the culprit (nitrogen) so the diffusion of nitrogen is much more rapid and hence you get quicker/better results.

In apnea deco stop, you still have in your lungs presumably a mix of gases which is very close to what caused the issue in the first place (CO2? Nitrogen?) and you are not breathing in/out anything so I would just guess that this apnea stop on the way up would be very ineffective at best.

The bend and mend approach where you breathe a bit on the surface before you go down for apnea deco seems to me that would have some results as you are at least starting with a different mix of gases in your lungs but of course if you've surfaced it's already late in some ways.

So assuming the above is true, would it not make sense not to make an apnea stop at a certain depth on the way up but instead to use up the time to surface slowly at a given rate?

Since pressure difference seems to be what matters, this would suggest that for the most dangerous part (say last 30m-40m) you should ideally try to adopt a rate of ascent that is decreasing proportionally with depth so that the pressure differential you experience is constant.

Apologies if this does not make any sense or is factually wrong, my knowledge on the subject is very limited but hey, we are all learning!

 

[Fondueset]

If I get the time I'd like to consolidate all this info on the Xen for a review. It's such an awesome and unique unit.

 

[efattah]

A scuba diver can dive with air (21% O2, 78% N2), go to 60m, incur huge amounts of deco, and do deco stops on the way up, all on air. It just takes longer to deco on air.

Consider at 60m the pressure is 7ATM. So the tissues absorb N2 at a pressure of 7 x .78 = 5.46ATM.

Now, at a deco stop at 22m, the pressure is 3.2ATM so the N2 pressure is 2.496ATM.

So, at 22m, the nitrogen in the tissues is at around twice the ambient pressure, and N2 leaves the tissues and enters the lungs.

Apnea deco stops are extremely effective; in my own personal experience I can violate any freedive table as long as I do apnea deco stops on the ascent. The big problem with apnea deco stops is blackout. Especially if you are doing your last stop at 5m, you are right in the blackout zone. Normally you want to speed through the last 10m as quickly as possible to spend as little time as possible in the 'vacuum blackout' zone. In no-limits, with little effort, lots of apnea time is available. However on 'working' dives to 40m+, there is often little O2 available for apnea deco stops, thus requiring a bend and mend approach, which can also be done on apnea.

Someone else on this forum also reported great success with apnea deco, I can't recall who it was.

 

[Simos]

A scuba diver can dive with air (21% O2, 78% N2), go to 60m, incur huge amounts of deco, and do deco stops on the way up, all on air. It just takes longer to deco on air.

Consider at 60m the pressure is 7ATM. So the tissues absorb N2 at a pressure of 7 x .78 = 5.46ATM.

Now, at a deco stop at 22m, the pressure is 3.2ATM so the N2 pressure is 2.496ATM.

So, at 22m, the nitrogen in the tissues is at around twice the ambient pressure, and N2 leaves the tissues and enters the lungs.

Apnea deco stops are extremely effective; in my own personal experience I can violate any freedive table as long as I do apnea deco stops on the ascent. The big problem with apnea deco stops is blackout. Especially if you are doing your last stop at 5m, you are right in the blackout zone. Normally you want to speed through the last 10m as quickly as possible to spend as little time as possible in the 'vacuum blackout' zone. In no-limits, with little effort, lots of apnea time is available. However on 'working' dives to 40m+, there is often little O2 available for apnea deco stops, thus requiring a bend and mend approach, which can also be done on apnea.

Someone else on this forum also reported great success with apnea deco, I can't recall who it was.

Thanks for the info Eric - does make sense, I just thought it'd take ages to get any meaningful offloading of nitrogen while doing apnea deco and there would be a limited amount you could get rid of since the air in your lungs is not circulated but looks like this is not the case.

One thing I still need to get my head around is what the composition of the air in the lungs would be on the ascent. Presumably nearly all O2 would be gone and it'll be full of nitrogen and CO2 - which would make a less favourable gradient for offloading those two gases. Am I wrong in my thinking?

Also, in terms of CO2, I thought your lungs would be quite 'full' after a dive so you wouldn't get significant offloading but perhaps this is easily offset by the difference in pressure.

Finally, relating perhaps more to this thread - am I right in thinking that if you are diving on empty or FRC, apnea deco stops would be less effective? (I appreciate you'll have less problems with deco in the first place)

 

[chrismar]

Considering Branko's 12:11 static (and much longer in training), no-limits might actually become a difficult 'athletic' discipline if the dive time required is more than 10 minutes due to all the deco stops. For full lungs diving with air equalization it is clear that the 'bend and mend' approach no longer works (i.e. doing almost all the deco after the dive is too late; DCS has already started).

Bend and mend is used all the time with deep scuba. Why would it not work here if you did it right (ie deep nitrox, long deco, ending on O2 from 9 to surface)? Also, the overwhelming opinion here is that despite severe DCS and hospitalisation, Herbert's dive was a success. No apnea deco needed.

 

[efattah]

Simos,

On the ascent, the gas mix in your lungs would be more favorable for DCS than on the descent. On the descent, at 20m you would have 3atm x 78% N2 = 2.34 ATM N2. On the ascent from a very deep dive, a huge fraction of the N2 in the lungs will have dissolved into the tissue and fluids in the body. This means that on the ascent at 20m, the N2 pressure in the lungs will be way less than 2.34 ATM. The exact amount would be hard to quantify.

You are getting mixed up in terms of percents. The partial pressure is all that matters; even if the lungs fill up with CO2, that does not change the partial pressure of nitrogen in the lungs at all. The lung volume may change during the dive as gases are absorbed or released, but the fraction of each gas is not relevant, only the partial pressure.

The gas in the lungs is circulated even in apnea; if it were not, you would black out even with tons of O2 in your lungs.

Apnea deco stops would be effective on an FRC dive; although if the cardiac output is lower, then the rate of off-gassing would be lower too.

Keep in mind Herbert did do apnea deco on his 214m and 249m dives, 1-minute at 10m. If he had skipped this stop he may not have survived the dive at all.

 

[Will]

And to sum up: Little gas, no troubles. Big gas, big troubles.
Gas has no/little place in the business of freediving. Better dice with scuba if thats the case!

You couldn't expect such a broad and uncorroborated statement would go unnoticed!

I won't weigh in on sled-diving, which I know little about. Perhaps it's true that when you don't require oxygen for transport then reducing LV in order to slightly counter DCS risk is a good pay-off. In any case any relevance there doesn't by default carry over to the self-powered disciplines.
The same might be said for wet equalising. Again I'm no expert on the subject, but Seb, it sounds like in France you had difficulty with wet equalisation past 70m, in a head-up and static position. You have been wet-equalising for some years. Is it plausible to expect that others might be able to safely go deeper than this mark in constant weight, when compounded with the extra difficulties of being head-down and having to effect their own descent?


Regarding Dive Strategy
I think that a lot of caution must be taken in using other animals as models to emulate. J. Ponganis states "before considering the anatomical, physiological, and biochemical adaptations underlying gas exchange and dive capacity, it is important to emphasize that diving mammals are a diverse group and that they exhibit a wide range of diving behaviors"(1)
Some diving mammals exhale, some inhale. Some are much more buoyant than others. Their foraging depths, habitats and swimming techniques are very different.
Pelagic spotted dolphins (Stenella attenuata) dives can be up to 5 minutes in duration and as deep as 200m(2) Bottlenose dolphins have been recorded to dive to 390m.(3)
The Elephant Seals are obviously the champions if you look at performance as a depth v.s. body size ratio, diving to 1600m. However, in the same way that human sprinters can't run faster on all fours (like cheetahs) than on two legs, the best system for human diving might not replicate that of the deepest diving mammals.

A central and revealing fact is that Elephant seals store 97mL of oxygen per kilo of bodyweight, but 93mL/kg of that is in their blood and muscle, and only 4mL/kg in the lungs.(4) So whether they inhale or exhale is only going to make a difference of 2% to their total oxygen stores! Such a negligible difference means that benefits like reducing gross energy expenditure through less buoyancy change because of smaller initial lung volume become more relevant, encouraging them to exhale.
By contrast, a human freediver with full lungs might store 25mL of oxygen per kilo bodyweight, but since about 18mL/kg of that is in the lungs then exhaling would dispose of 36% of their oxygen stores! Clearly it is a decision of far greater consequence to the human freediver than to the Elephant Seal!
In terms of where oxygen is stored in the body and how much there is of it available, humans are much more similar to dolphins than pinnipeds. They are also more similar in terms of key physiological parameters such as fat content (which determines the body's buoyancy in water). Dolphins and other cetaceans have developed a dive strategy that allows them to inhale and reach depths of 200-400m, repetitively and without DCS. I submit that their greater similarities with humans make their species a more appropriate model for the development of human freediving abilities.


Regarding the rate of inert gas uptake on empty v.s. full lungs
For the factors listed (DR, alveolar collapse, greatly reduced alveolar surface area, increased membrane thickness and lower cardiac output), the difference between an inhale and exhale approach becomes less significant with increased depth, and I can't see that difference being at a level greatly relevant to DCI at any depth.
Even after inhaling, RV will be reached at around 30m, meaning all these factors will already be kicking in. At most this is 15-20m (15-20 seconds) behind when similar processes or values would have been reached if starting with an exhale.
Alveolar collapse, more than any other factor, seems requisite to reduction of DCS risk in diving mammals. The Weddel seal's arterial PN2 peaks at 2.7ATM and doesn't increase with depth after that, which would indicate alveolar collapse (hence no more N2 absorption) at 17m or shallower.(5) If the same were true of an exhale human diver (collapse at ~17m) then after inhaling alveolar collapse would occur at ~35m instead. Would this extra 15-20 seconds of exposure to Nitrogen between 17-30m make a significant difference in DCS risk?
For the sinuses etc, as you mention, the DR's peripheral vasoconstriction in the tissues that adjoin these chambers (which incidentally leads to a less stuffy nose during a breath hold as the airways dilate - hence paper bag treatment for asthma attacks) will render inert gas uptake even less significant in these chambers at depth. Again this effect can be only slightly delayed on inhale as compared to exhale dives.


(1) Ponganis PJ, Kooyman GL, Ridgway SH. Comparative diving physiology.
(2) Scott MS, Chivers SJ. Movements and diving behavior of pelagic spotted dolphins.
(3) Mate BR, Rossbach KA, Nieukirk SL, Wells RS, Irvine AB, Scott MD, Read AJ. Satellite-monitored movements and dive behavior of a bottlenose dolphin (Tursiops truncatus) in Tampa Bay, Florida. Mar Mamm Sci 11: 452-463, 1995.
(4) Kooyman GL, Ponganis PJ. The physiological basis of diving to depth: Birds and mammals. Annu Rev Physiol 60: 19-32, 1998.
(5) Falke KJ, Hill RD, Qvist J, Schneider RC, Guppy M, Liggins GC, Hochachka PW, Elliott RE, Zapol WM. Seal lungs collapse during free diving: Evidence from arterial nitrogen tensions. Science 229: 556-558,1985.

 

[sebastien murat]

Hi Will,

In France I had plenty of issues to contend with, including one sticky tube. The issue wasn't with wet equalizing per se, but with keeping the tube open to permit water equalization. The problem started when I received a bit of an ear hit in going from 1 bar to 2 bars in about one second. Indeed, simply keeping the tubes open and equalizing with air demonstrated to me that even with air such a pressure change is difficult to handle without at least initially over-pressurizing the ears (Fabrizio's suggestion). The hit made it more or less unrecoverable for the remainder of my time there.

The diving on `empty´lungs strategy is a universal strategy employed by a diverse group of animals, ranging from mammals, reptiles (crocodiles, iguanas, sea-snakes), to birds. Its essence is to reduce the cost-of-transport (COT); it has other beneficial side effects. If however, the aim is to get from point A to point B quickly, in which case a high burn rate is needed, then there is a conflict with energy conservation. Then of course you'd be better off filing the lungs. I'm aware that many diving animals dive on full lungs also.

The exhale strategy makes sense only from several points of view:
- one has relatively small lungs or relatively weak inspiratory muscles such that the lung O2 stores on full versus empty is not so large
- one has a poor exercise response
- one has the possibility to recover and lower metabolic rate between dives (i.e, keeping dives aerobic)
- one has a strong DR
- one has a stronger or more promt DR on exhale than inhale
- one has poor swimming technique or employes a stroke that expends lots of energy (breast-stroke) compared to simply sinking to depth
- minimizing narcosis
- reducing the risk of DCI
- minimizing the risk of ascent-induced hypoxia or SWB.... to do with, in part, the prevention of redistribution of cardiac output to peripheral tissues and sudden fall in pCO2, associated with the lung re-inflation reflex


We could get into the various nuances of DCI risk between exhale and inhale...it would take a long time.

One salient factor is that if the DR cannot at least be maintained during ascent, because the lung re-inflation reflex, which is present only when diving on lung volumes above FRC, then this means that cardiac output will be redistributed towards peripheral tissues, i.e., less blood to vital organs. I see this constantly when diving on full versus empty. Apart from the increased risk of SWB, this means that the ability to adequately off-gas the vital organs becomes problematic. This is problematic for at least two reasons: one, blood-flow to vitasl organs becomes reduced, restricting the amount of N2 that can come out of solution and, two, brain temperature increases (the brain cooling effect, part of the DR, is progressively lost) so that the solubility of DCI causing gases decreases.

In short, you can't simply look at exhale versus inhale from a simple mechanistic point of view relegated to one of mere alveolar collapse, there are important cardiovascular differences. And, its not just about what happens when you dive but also what happens when you ascend.

 

[sebastien murat]

I think you're calculations might be a bit off. I'm basing this on Fitze-Clarke's work (2009). If you have a look at these papers, they predict collapse at something like 200-300m on full lungs (depending on how much above VC a diver packs); not much different than Scholander's predictions in the 1940s actually. On exhale, collapse is predicted to occur probably at around 40m or so. In short a big difference in time and depth. Indeed, if you don't reach collapse depths gas-uptake continues, which is most likely since for the average man (TLC=8L) collapse depth would only occur at about 180m or so, i.e., out of reach. Furthermore, assuming you did get to collapse depth, the depth at which the lungs would likely re-inflate, thereby proving a sink for off-loading the incurred N2 load is not the same as the collapse depth. Indeed, this depth is much shallower. Moreover, this difference is much greater diving on full lungs compared to empty, so that at an initial lung volume of, say, 10L (conservative by today's elite freedivers) the difference between collapse and re-inflation depth would require going from about 220m all the way back to about 130m, i.e., a substantial pressure difference. At a lung volume of 3L (my volume for example) there's essentially no difference between between collapse and re-inflation depth. What this means is that once I pass about 40m I stop picking-up N2 and when I need to release it, it comes out at the same depth it went in. Once I release the pressure, my tap is open and N2 can flow out at the same ppN2 as it went in, i.e., no sudden and dramatic pp difference.

Now,this is all academic of course, since looking at Fitze-Clarke's curves, diving on constant-ballast, on full lungs, you'll never reach collapse depth anyway; to say nothing of packing. In other words, you'll always be picking up N2 on the way down for as long as there's a partial pressure difference, which there will be considering the the large lung volume and high fractional concentration of N2. In other words, you'll never benefit from lung collapse. And, if you factor in the effects I mentioned earlier about the DR being reversed during the last 30m of the ascent, especially on hyper-inflated lungs, things can only get worse for you.

Back to reality and theory aside, from a purely practical point of view, I already know all this you could say, because my ppN2 never gets so high that I experience even the most minute sign of narcosis, at any depth, on exhale, which could be considered to be a tell-tale sign of what could be install for you if you deep dive on full lungs and should you linger too long.

 

[efattah]

When ascending during a dive with packing, I do a major exhale at 25m on the way up (I have an alarm set at 25m on the ascent). This prevents the lung expansion related problems and allows me to finish the dive with the effectiveness of an FRC ascent (which I found by the way, to be greatly more resistant to ascent hypoxia).

As I mentioned before, I'm highly unconvinced about DCS risk changes with different lung volumes.

If I do a single FRC dive to 70m in constant weight, and I sprint to the surface at maximum speed (circa > 1.8m/s), and I don't do any post dive deco, then I get minor DCS. I also know that a single 70m dive with packing does not cause me any DCS (note: I ascend a lot slower when doing 70m with packing). However, with packing, two consecutive 70m dives with 12 minutes in between causes the same minor DCS as one FRC dive to 70m with a sprint ascent.

It takes around three consecutive 55m FRC dives with moderate ascents and 10 minutes intervals to give me minor DCS, which is the same number as with packing. With post dive apnea deco I can extend that to around five or six dives before the niggles set in.

In all cases, I start the FRC dives with one monofin kick, and on the packing dives I need to kick down to around 18m-30m depending on how buoyant I am diving.

When comparing humans to marina mammals, undoubtedly the sea otter is the best example; not well adapted, extremely buoyant, with a constant weight depth limit of around 97m; inhales to the limit before diving and stores most of the O2 in the lungs.

 

[sebastien murat]

Very interesting Eric.

What's your typical HR response during a dive? I have a pretty strong and prompt response which drops to mid-low 30s typically within 10-20 seconds, but only on empty.

In the past I've done some borderline DCI profiles on full: 2 x 110m with as much as 6 hrs apart and feeling pretty shitty and anxious afterwards.

On the other hand, on empty, I've also done 3 x 6-70m preps with maybe 5-8 minute breaks in between, followed by a deep 150m dive (10 minute break), with absolutely no ill-effect.

Personally, I think that without a strong DR and lowered metabolic rate the outlook is pretty bleak, including increased risk of narcosis and SWB

This is all hardly scientific, but at least it put us in the ball-park where we can find some clues and trends

 

[sebastien murat]

Eric, you realize that exhaling at depth to get you to FRC has its own problems: you drop your volume and lose the O2, which in turn drops the ppO2, since the HbO2-suction pump drains it more quickly.

Guess your in the trade-offs game....risky strategy trying to keep account of all this. I prefer a clean sheet with no guessing.