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Thread: Scrubber Breakthrough - How do you define it?

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    Scrubber Breakthrough - How do you define it?

    Hi guys -

    We all takl about breakthrough in the scrubber and it often becomes a hot slinging match .... not interested in that here.

    What I want to undersgtand is what are we generally defining as the point of breakthrough?

    Is it when we start to get a headache?
    Is it when we start to get rapid breathing?
    Is it just a feeling??

    I have been reading some old US navy testing data an this and they have done extensive testing both manned and unmanned at lots of depths and workload situations.

    They are defining breakthrough technically as the point at which the CO2 SEV reaches 0.5% of SEV on the inhale side of the loop.

    They have run many of the tests out to a point where the SEV is 2.0% but generally they are defining 0.5% at the limit and that is where they define the scrubber limits at various water temps etc.

    SEV is from the definitions page Surface Equivalent Value

    Any thoughts??

    Steve L

    ps it was also interesting to note that packing density was a significant factor

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    Re: Scrubber Breakthrough - How do you define it?

    Quote Originally Posted by SteveL  View Original Post
    ...what are we generally defining as the point of breakthrough?

    Is it when we start to get a headache?
    Is it when we start to get rapid breathing?
    Is it just a feeling??
    In the past (when I was young and foolish ), I have tried to find the limit of the CK scrubber emperically so I done a lot of mix dives on the same scrubber.

    After about 7 accumulated hours (bottom and deco) of doing multiple 72-92m wrecks, I chickened out and changed the sorb. This was done quite a few times.

    The only symptom I experienced was random slight headache once back on the boat, but it didn't happen systematically at any accumulated time period so could be a function of work load on different dives.

    However, I have over-breathed the unit a number of times when having to swim very hard against current with stages chasing the anchor line. The symptom in the form of hyperventilation came more rapid, but also went away relatively quick when rested or bailed to OC.

    That'd a very good time to realize that you need a BOV since you can't stop breath long enough to remove the mouthpiece. Even rotating the switch on the BOV was difficult.

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    Re: Scrubber Breakthrough - How do you define it?

    Quote Originally Posted by decoweenie  View Original Post
    That'd a very good time to realize that you need a BOV since you can't stop breath long enough to remove the mouthpiece. Even rotating the switch on the BOV was difficult.
    This seems to be common theme - but I suspect when we are at this point from over breathing or breakthrough we are "close" to buying the farm so to speak.

    Dont want to side track the thread but for this reason along I have been considering a BOV -

    So getting a headache like the one you get from skip breathing on OC (in an effort to be the last out of the water - and say look how little gas I used.....air hogs....) might be the result - but if your're in the water and start to get a raging headache do you assume breakthrough - or bad gas - or did you just remember about the visa bill that is overdue:o

    I guess if you go to oc and the symptoms dissapear realativly quickly and you are pretty sure about the gas quality then it is starting to get conclusive.

    For the medical boffins out there - if the navy are stipulating 0.5% sev and test to 2% anyway what is the point from a medical viewpoint that is starting to get dangerous?? Im sure everyone is a bit different but there must a range that says if CO2 is between x and y then you are in the "shiza"

    Steve

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    Re: Scrubber Breakthrough - How do you define it?

    From what I understand.

    0.5 SEV% is about the point that the human body can notice the effects of CO2 i.e. slight headache, dizzyness..., ect and this is the point as previously mentioned is considered "breakthrough" as far as the US Navy is concerned.

    2 SEV% is a decent headache

    4+ SEV% prolong exposure can cause unconciousness

    10 SEV% can instantly cause unconciousness

    Jeff Bozanic's book Mastering Rebreathers talks about CO2 exposure.

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    Re: Scrubber Breakthrough - How do you define it?

    I guess its a hard thing to say something about in absolute terms.

    I have also read that >10% CO2 SEV is lethal if the exposure is long enough, like trapped inside a sub w/o working scrubbers.

    I have had a try at breathing 10/90 CO2/O2 as a test once for a few minutes. Not very confy and nice but it did not feel catastrophic.

    On the other hand I have had a few CO2-hits UW and at least one of them felt like imminent death and decent to hell was upon me. It gave me a long lasting headache too.

    This is an interesting article on things like: the subjective sensation of CO2, the ability to detect CO2, and CO2-retention. The test subjects are 40 Israeli Combat swimmers.

    Response to CO2 in novice closed-circuit apparatus divers and after 1 year of active oxygen diving at shallow depths -- Eynan et al. 98 (5): 1653 -- Journal of Applied Physiology

    Some more articles:

    Training improves divers' ability to detect increased CO2.
    Entrez PubMed

    This one I think had a rather odd result, but a small set of individuals:
    CO2 detection in closed-circuit oxygen divers with and without a distracting task.
    Entrez PubMed
    Last edited by jaap; 16th November 2006 at 19:41. Reason: Added a few articles

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    Re: Scrubber Breakthrough - How do you define it?

    Breakthrough is described as a sudden manifestation of an event. In this case we are talking about CO2 in a circuit. The attached powerpoint slide shows the sudden rise in CO2 that occurs at a 0.05% breakthrough. I could not quickly put my hand on one that went all the way to 2% but you get the idea of a continued fast rise in CO2 at the point in which the scrubber becomes saturated. This data was collected as a part of the DAN REMO2 scrubber duration trials. The top graph shows the times of the "air breaks" and the O2 percentage in the loop (so the scrubber does have some time to recover and it is room temp for this phase). Thanks to Neil Pollock and Mike Natoli for allowing me to use this slide (from a manuscript under review in a peer-reviewed journal).

    The question of CO2 measures is one we are currently tasked with addressing. We are looking at differences in alveolar to arterial measures of CO2. The physiologic effects of CO2 are hard to quantify and knowing what impact diving once breakthrough is reached is impossible to determine because of this. ( Duke Center For Hyperbaric Medicine And Environmental Physiology :: Research)

    Quote Originally Posted by Moon et. al.
    Duke IRB Protocol # 1C347B
    Title: Effects of exercise, gas density, and static lung loads on end-tidal vs. arterial PCO2 differences breathing N2O-O2 mixtures during immersed prone exercise.
    Research Summary

    1. Purpose of Study - (US Navy Need Addressed): The Navy Experimental Diving Unit is responsible for all USN underwater breathing apparatus (UBA) evaluation. As part of that evaluation manned dives are done under immersed exercising conditions where end-tidal CO2 (PETCO2) measurements are made as a direct approximation of arterial CO2 levels. These estimated arterial CO2 levels are then used as a measure of the UBA design adequacy. While it is often assumed that hypercapnia occurs as a direct result of poor UBA design (i.e., high breathing resistance), hypercapnia can occur even when breathing resistance is low. Several factors are believed to contribute to hypercapnia during diving, including level of exertion, pulmonary ventilation, efficiency of pulmonary gas exchange, breathing resistance (both external and internal), mixed venous PCO2, cardiac output and static lung load. However, no study has thus far measured these variables simultaneously during moderately heavy exertion during diving conditions. The proposed study would evaluate the effect of UBA performance vs. physiological factors on arterial PCO2 (PaCO2), and significantly improve the Navy’s ability to evaluate UBA performance by providing a non-invasive method to assess arterial PCO2 from PETCO2.

    Aims/Objectives: The aims of this study will be to determine, during immersed prone exercise at 122 fsw (while the chamber pressure is 120 fsw, the additional depth incurred by water immersion adds 2 fsw): (1) the predictors of PaCO2 and (2) the relationship of PETCO2 to PaCO2, as a function of the various breathing environment parameters and physiological variables. These variables will include O2 consumption, flow resistance, inspired PO2, static lung load, cardiac output and mixed venous PCO2. We propose a series of experiments that will define how the various conditions encountered during prone, immersed exercise will affect PaCO2 and PETCO2-PaCO2 differences while breathing various N2-O2 mixtures (including air). We will measure the magnitude of the differences, and devise ways that PETCO2 measurements can be corrected to more accurately estimate PaCO2.

    2. Background and Significance - During normal exercise increased CO2 production is matched or exceeded by a parallel increase in alveolar ventilation. Thus, during exercise, despite a large increase in PCO2 in the blood returning to the lung (mixed venous PCO2), PaCO2 tends to remain constant or even decrease1-2. In diving, however, there is mild resting hypercapnia, which is accentuated during exercise. Hypercapnia in diving can be a significant problem, and has been implicated in impaired judgment and loss of consciousness underwater3. Hypercapnia occurs in the absence of significant external breathing resistance, but is made worse by breathing apparatus that adds respiratory load. This is particularly important for Navy missions, in which there may be especially heavy exertion, but it also applies to commercial and recreational diving. In recreational diving, enriched-oxygen breathing mixtures, which are increasingly popular, may contribute to hypercapnia (see below).

    Hypercapnia in diving is due to (1) relative hypoventilation4 and (2) pulmonary gas exchange abnormalities manifested as increased deadspace4-5. Hypoventilation tends to occur because of higher gas density and the ensuing increase in flow resistance in the tracheobronchial tree, as well as in the underwater breathing apparatus (UBA). The increased respiratory load and possible depression of central respiratory drive due to inert gas narcosis and a higher PO2 may contribute to relative hypoventilation. The second factor, increased respiratory deadspace, has been described in both shallow4 and deep5 diving, and may vary with gas density and static lung load (‘SLL’), i.e. whether the breathing gas is delivered to the diver at a pressure higher or lower than the pressure at the midpoint (‘centroid’) of the lungs. A positive SLL occurs when the diver is in a head-down position; a negative SLL occurs with the diver head-up.

    Usually, PaCO2 is assumed to be close to the end-tidal PCO2 measured non-invasively at the mouth. Most studies of respiratory gas exchange in diving have relied upon this assumption3. Unfortunately, during exercise and diving there are several factors that may cause PETCO2 to either over- or underestimate PaCO2. The increased gas density breathed during a dive causes a generalized increase in airway resistance, similar to increases in the general population due to obstructive lung disease. In patients with COPD, gas from under-perfused/over-ventilated gas exchange units dilutes the alveolar gas, leading to a reduction in end-tidal PCO2 relative to the arterial value6. On the other hand, during exercise in normal subjects, end-tidal PCO2 can exceed arterial PCO26, because during exhalation CO2 continues to diffuse from the capillary blood into the alveoli, to a tension that may approach the PCO2 in mixed venous blood. Mixed venous blood is in turn determined by the degree of exertion, cardiac output and lactic acid accumulation in the blood (buffering of hydrogen ions by bicarbonate causes venous PCO2 to rise). Cardiac output is affected by immersion (increase), exercise level (increase) and both external and internal breathing resistance (decrease). There is evidence that water temperature may be important, in that blood redistribution from the periphery to the core that normally occurs during immersion may be minimized in warm water due to vasodilatation7.

    Mixed venous PCO2, typically 43-45 mmHg at rest, has been measured as high as 75 mmHg in this laboratory during submerged moderate exercise. Hence, during exercise in divers PETCO2 could be either higher or lower than PaCO2. Measurements in upright subjects in a dry hyperbaric environment at an equivalent depth of 60 feet of seawater (fsw) revealed that PETCO2 tended to be lower than PaCO2 at rest, but tended to exceed PaO2 during light exercise4. No simultaneous measurements exist of arterial and end-tidal CO2 during heavy exercise at deeper depths

    Thus, during exertion underwater at a given depth, the variables that may contribute to elevated end-tidal PCO2 include the breathing resistance of the UBA, breathing gas PO2, gas density, inert gas narcosis, SLL and cardiac output. In addition, the required increase in alveolar ventilation during exercise may be attenuated by higher respiratory deadspace. Increased deadspace means that, for a given level of exercise, in order to maintain the appropriate alveolar ventilation, total ventilation must be even higher.

    The less a UBA and other variables impede CO2 elimination, as reflected in a lower PaCO2, the less exercise magnitude, exercise duration, and mental performance will be impaired. Direct measurement of PaCO2 requires arterial blood samples, and this measurement is not made during routine UBA evaluations at facilities such as the Navy Experimental Diving Unit (NEDU). Having predictive equations that give accurate estimates of PaCO2 from the easily obtained non-invasive measurement of PETCO2 would greatly improve evaluation of exercise CO2 elimination during UBA evaluations, and provide guidelines to minimize the risk of hypercapnia in military and recreational divers with regard to depth, exertion and inspired O2 level.

    Measurements of PETCO2 have been made over a wide range of gas densities in dry and immersed exercising divers9,10. We are not aware of any studies in immersed exercising subjects that have been reported where PaCO2 and PETCO2 measurements and the other physiological variables have been made simultaneously, at any gas density.

    Preliminary Work: Studies done in this laboratory using submaximal exercise at 130 fsw have demonstrated that mixed venous PCO2 values are approximately 10 mmHg higher at depth than at the surface, and deadspace 100% higher. In a recent study at 60 fsw under dry conditions, we observed that the reliability of PETCO2 as an estimate of PaCO2 is imperfect during rest and exercise, but worse during exercise at depth.

    10. References

    1. Wasserman K, et al. Anaerobic threshold and respiratory gas exchange during exercise. J Appl Physiol 1973; 35:236-43.
    2. Wasserman K, Whipp BJ. Exercise physiology in health and disease. Am Rev Respir Dis 1975; 112:219-49.
    3. Warkander DE, et al. CO2 retention with minimal symptoms but severe dysfunction during wet simulated dives to 6.8 atm abs. Undersea Biomed Res 1990; 17:515-523.
    4. Mummery HJ, et al. Effects of age and exercise on physiological dead space during simulated dives at 2.8 ATA. J Appl Physiol 2003; 94:507-17.
    5. Salzano JV, et al. Physiological responses to exercise at 47 and 66 ATA. J Appl Physiol 1984; 57:1055-1068.
    6. Liu Z, et al. Comparison of the end-tidal arterial PCO2 gradient during exercise in normal subjects and in patients with severe COPD. Chest 1995; 107:1218-1224.
    7. Kurss DI, et al Effect of water temperature and vital capacity in head-out immersion. In: Underwater Physiology VII. Bethesda, MD: Undersea Medical Society, 1981, pp. 297-301.
    8. Moon RE, et al. Pulmonary hemodynamics and gas exchange at 40 m. Undersea Hyperb Med 1998; 25(suppl):26.
    9. Thalmann ED, et al. Effects of immersion and static lung loading on submerged exercise at depth. Undersea Biomed Res 1979; 6(3): 259-290.
    10. Hickey DD, et al. Respiratory function in the upright, working diver at 6.8 ATA (190 fsw). Undersea Biomed Res 1987; 14:241-262.
    11. Thalmann ED, et al. Chamber-based system for physiological monitoring of submerged exercising subjects. Undersea Biomed Res 1978; 5:293-300.
    This is an excerpt from our current protocol. Courtesy of Dr. Moon.

    Quote Originally Posted by E-man  View Original Post
    0.5 SEV% is about the point that the human body can notice the effects of CO2 i.e. slight headache, dizzyness..., ect and this is the point as previously mentioned is considered "breakthrough" as far as the US Navy is concerned.

    2 SEV% is a decent headache

    4+ SEV% prolong exposure can cause unconciousness

    10 SEV% can instantly cause unconciousness

    Jeff Bozanic's book Mastering Rebreathers talks about CO2 exposure.
    There is a graph of this info in the 1971 US Navy Manual but in looking through the Thalmann papers, I have not been able to locate the original work to see how it was designed. I will keep looking. Ed kept notes on every revision he had anything to do with. I need to check the JAP references in the paper below.

    In addition to the papers Jaap listed, this paper describes the concerns of higher CO2 pretty well.
    Rubicon Research Repository
    Physiologic Basis for CO2 Limits Within Semiclosed and Closed-Circuit Underwater Breathing Apparatus.
    Rubicon Repository ID#: 123456789/3304
    and yes, you can get this one at DTIC. <g>

    Work by Schaefer is well worth looking into if you have further interest in elevated CO2 effects on the body (DISSUB situations). Lanphier also did some GREAT work on respiratory physiology that is worth looking at.

    We hope that our current work will have some impact on the interpretation of the information previously collected and reported.
    Attached Images

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    Re: Scrubber Breakthrough - How do you define it?

    Quote Originally Posted by decoweenie  View Original Post
    In the past (when I was young and foolish ), I have tried to find the limit of the CK scrubber emperically so I done a lot of mix dives on the same scrubber.

    After about 7 accumulated hours (bottom and deco) of doing multiple 72-92m wrecks, I chickened out and changed the sorb. This was done quite a few times.

    The only symptom I experienced was random slight headache once back on the boat, but it didn't happen systematically at any accumulated time period so could be a function of work load on different dives.

    However, I have over-breathed the unit a number of times when having to swim very hard against current with stages chasing the anchor line. The symptom in the form of hyperventilation came more rapid, but also went away relatively quick when rested or bailed to OC.

    That'd a very good time to realize that you need a BOV since you can't stop breath long enough to remove the mouthpiece. Even rotating the switch on the BOV was difficult.
    that is the most beatiful illustration of the value of a BOV I've heard yet...I hadn't thought of that, but certainly, when working to that point, it's time to make the switch and that sounds like the best way to do it....hmmmm. now i'm officially jealous

    Gill Envy

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    Unhappy Re: Scrubber Breakthrough - How do you define it?

    I've had two severe episodes of hypercapnia. In the first case, I had a solid state scrubber that was partially used and had been sealed for 2 weeks. According to my calculations, the scrubber still had at least another 2 hours on it. just to be sure, I geared up with the 100 pound RB on my back, and ran up and down the stairs in my home to simulated a severe case of over-exertion. I was totally asymptomatic of hypercapnia. The next day I was on a wreck dive, and when I got down to about 30fsw without warning, and instantly (even though I wasn't even exerting myself) I experienced the worst headache of my life, went totally blind, and had severe vertigo, I was unable to even remember how to bail-out! To this day I have no recollection of how the Hell I managed to survive and get back to the surface. The other occasion, was also on a dive with a scrubber that was partially used and supposedly also had at least 2-3 hours of use still left in it. This time the hypercapnia hit at 56fsw, and initially my breathing on the RB felt very 'hot' and then there was again a rapid escalation of symptoms from headache, to vertigo, to severe confusion.
    LESSON LEARNED: hypercapnia's onset is nearly instantaneous with little of no warning. when your scrubber breaks through, its scrubbing efficiency does not gradually decline, it just goes to zero within seconds! This is very scary stuff; we spend thousands of dolars on our machines, yet want to save a few bucks by re-using a scrubber, even if this seriously endangers our lives! It happened to me, and I've done hundreds of dives on a CCR over twenty years -- you think I would have known better

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    Re: Scrubber Breakthrough - How do you define it?

    Quote Originally Posted by TechWreck  View Original Post
    I've had two severe episodes of hypercapnia. In the first case, I had a solid state scrubber that was partially used and had been sealed for 2 weeks. According to my calculations, the scrubber still had at least another 2 hours on it. just to be sure, I geared up with the 100 pound RB on my back, and ran up and down the stairs in my home to simulated a severe case of over-exertion. I was totally asymptomatic of hypercapnia. The next day I was on a wreck dive, and when I got down to about 30fsw without warning, and instantly (even though I wasn't even exerting myself) I experienced the worst headache of my life, went totally blind, and had severe vertigo, I was unable to even remember how to bail-out! To this day I have no recollection of how the Hell I managed to survive and get back to the surface. The other occasion, was also on a dive with a scrubber that was partially used and supposedly also had at least 2-3 hours of use still left in it. This time the hypercapnia hit at 56fsw, and initially my breathing on the RB felt very 'hot' and then there was again a rapid escalation of symptoms from headache, to vertigo, to severe confusion.
    LESSON LEARNED: hypercapnia's onset is nearly instantaneous with little of no warning. when your scrubber breaks through, its scrubbing efficiency does not gradually decline, it just goes to zero within seconds! This is very scary stuff; we spend thousands of dolars on our machines, yet want to save a few bucks by re-using a scrubber, even if this seriously endangers our lives! It happened to me, and I've done hundreds of dives on a CCR over twenty years -- you think I would have known better

    Thanks for This

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