Rebreathers and Co2 – Fact, Fiction and Voodoo
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REBREATHERS AND CO2 – FACT, FICTION AND VOODOO FACT 1. Increase ventilation to maintain CO2 at nominal limits. Closed-circuit rebreathers 2. Conserve energy and let CO2 rise. (CCRs) are popular in advanced recreational and military diving Our ability to ventilate correctly is a function of depth (gas density) and the breathing owing to advantages such as resistance within the breathing loop. minimization of gas consumption, To illustrate the effects of this, in a study¹ the optimizing (reduction) of tests subjects were exposed to varying decompression and the ability levels of inspired CO2 with and without Figure 1 - CO2 retention as a function of workload to provide a low signature. breathing resistance. Three results were Rebreathers recycle expired gas summarized as: around a closed loop circuit using n Elevated CO2 with breathing resistance Work of Breathing added, produced an increased one-way valves. Expired carbon ventilation that moderated (but did Work of breathing (WOB) put simply is the dioxide (CO2) is removed as it not stop) CO2 increase. effort required to ‘push’ gas around the breathing loop. passes through a ‘scrubber’ canister n No CO2 but elevated resistance added containing CO2 absorbent. produced no rise in ventilation rate It is a combination of the resistive and CO2 retention occurred as in (restrictive) elements of the loop (small The most commonly used absorbent is a working (functioning scrubber) hoses etc.) and the pressure difference soda lime (a mixture of sodium hydroxide rebreather with excessive breathing (distance) between the counterlungs and and calcium hydroxide). resistance. the lung centroid. This distance adds to the resistive element and changes as the Rebreathers are more complex than n Moderate depths (18 meters) was open-circuit scuba equipment and can be shown to reduce the effectiveness diver rotates in the water. prone to operator errors. Some of these of ventilation in removing CO2 due WOB increases with depth and gas density. relate to the CO2 scrubber and its use. The to gas density, lung stiffness etc. absorbent material has a finite capacity and therefore the ability to control As previously discussed, elevated WOB can and must be changed regularly. CO2 through increased ventilation is lead to CO2 retention. reduced with depth. Errors include failing to replace the Linked to this, gas density trials have been absorbent material when required, In summary, in considering the conducted where groups of divers were incorrect packing of the absorbent canister, effectiveness of CO2 removal, a secondary exposed to increased gas density and then incorrect installation of the canister in the factor related to the breathing resistance their cognitive function was tested. rebreather and, rarely, forgetting to install (Work of Breathing/WOB) of the rebreather The following graph³ (figure 2) details a it (or the material) entirely. Such errors may must be considered and this increases series of tests at elevating gas density allow expired CO2 to enter the inhaled gas with depth. in grams/litre (g/l). Looking at the first which may in turn cause symptomatic CO2 column where the gas density was 2 toxicity (hypercapnia). The effects of CO2 retention were further expanded on in a study² referenced in to 3 g/l, 439 tests were completed. 393 Hypercapnia is also known to speed the figure 1. The red line records the CO2 completed successfully, 31 failed for onset of oxygen toxicity and the narcotic passing through a scrubber canister reasons other than perceived CO2 issues effect of nitrogen breathed at higher (the spikes are analyzer calibration runs) and 15 failed because of ‘recognized’ CO2 partial pressures. and clearly show that the scrubber is issues. Conversely the last column where How do we deal with increases in CO2? functioning correctly (almost no CO2 the gas density was 6 to 7 g/l, only 35 breakthrough). The blue line references completed successfully and 31 failed due Put simply, as a mechanism, with increased CO2 measured at the end of each breath to CO2, almost 50/50 (ignoring the ‘other workload the body generates more CO2. (end tidal CO2) recorded at three work reasons’). Therefore, the elevated gas For analysis purposes this is recognized rates (80watts, 125watts and 100watts). density which in turn created an elevated as arterial CO2. Arterial CO2 is detected The dotted black line is the maximum work of breathing generated high failure by the brain. Arterial CO2 will rise as CO2 allowed limit of end tidal CO2 and the rates. production rises or ventilation (breathing) graph clearly shows that while the falls. The study suggests that ideally gas density scrubber is functioning perfectly, the end should be below 5.2 g/l or air at 31m and The brain’s mechanism to deal with tidal (retained) CO2 rises above acceptable definitely not exceed 6.3 g/l or air at 39m. CO2 can be summarized as a two-fold limits as workload (as a function of gas approach: density or WOB) increases. make this a dangerous assumption. do, in that it uses the following parameters While there have been tests using as a test requirement; new absorbent at differing storage conditions⁴, this procedure should not be n 1.6l/min CO2. relied on. n 40l/min ventilation. 6. Advanced CO2 absorbents such as n Water temperature 40°C. lithium hydroxide can be safely used in an underwater rebreather. Author’s n Depth 40m (an additional test at 100m note: Lithium hydroxide (while very is used for heliox/trimix rebreathers). efficient as an absorbent) is an extremely n Diluent air. volatile chemical composition and reacts aggressively with water. It should not be The current reality is that test results Figure 2 used in rebreathers. obtained using these parameters do not - Test failure due to increased gas density make some rebreathers ‘look good’ from 7. Out of date absorbent, as long as it a performance perspective in that they is in a sealed container, can be used. FICTION do not live up to their manufacturers Author’s note: Again, this procedure initial design expectations, this creates a cannot be relied on. Out of date The following items can marketing issue. medicines would not be used, so the be considered as common same applies to absorbent material. Often the end result is that within misinformation and should be 8. In a breathing loop the CO2 scrubber operating instructions, while testing to treated accordingly. In short, all is the most important element for EN14143 will be referenced in a manual, of the below are NOT true. managing CO2 around the loop. there will be a caveat that references a Author’s note: Obviously the scrubber change in the parameters such that the 1. All CO2 absorbent is of similar design is vital for removing CO2 but even a endurance figure is increased. and generates similar performance. partly used or poorly packed scrubber Therefore, I can put any absorbent While the CE parameters are arguably is not as dangerous as a failed extreme in relation to sport diving they do in my rebreather and it will perform mouthpiece mushroom valve. The first provide a good safety net and especially within the manufactures published test of any rebreather setup should be for for technical/military diving profiles and/ limitations. Author’s note: There is a mushroom valve integrity. large selection of CO2 absorbents used or when high work rates are experienced across the medical, diving and other VOODOO towards the end of a scrubber’s life. industries. Each has unique properties The end result is users must beware when ranging from shape and construction Comparing endurance comparing rebreather performance and through to chemical makeup and water Divers obviously need to know should check with the manufacturer if content. Only Diving Grade absorbent there appear to be any ambiguities at all in should be used for diving. how long their rebreather will continue to remove CO2 from the the quoting of endurance figures. 2. The published CO2 endurance times for my rebreather can be conducted at loop in use and manufacturers The 5 minute pre-breathe any depth, in any water temperature would ideally want to quote Historically, a scrubber integrity test and at any work rate. Therefore, these a duration as guidance. This is conducted by divers is to breathe an three variables do not have any affect assembled unit for 5 minutes in order on the ability of the scrubber to difficult to quantify because to generate symptoms of hypercapnia remove CO2. Author’s note: Scrubber a rebreather scrubber’s ability experienced as a result of a failing endurance can be massively affected to remove CO2 is a function of scrubber or sealing system that in turn by these variables. In addition, each several variables: may create a bypass of CO2. This process rebreather’s specific design can affect the has several issues such as ‘how large’ is any endurance so a duration obtained with n Work rate (metabolism and therefore bypass and can we physiologically detect an absorbent type in one rebreather will CO2 generation). that level? not be the same in another rebreather. n Ventilation rate (primarily as a function In a trial completed⁵ in New Zealand, the 3. One type of scrubber design (be it of work rate). following observations were noted: radial, axial, cross flow etc.) will always perform better than another. Author’s n Water temperature. The five-minute pre-breathe is an insensitive note: Historically it was thought that n Absorbent type. test for CO2 scrubber function in a diving radial designs always gave better rebreather, even when the scrubber canister performance, but this is not universally n Rebreather design (primarily as a is absent. A pre-breathe is nevertheless true and in reality, axial designs are often function of heat loss).