Rebreathers and Co2 – Fact, Fiction and Voodoo

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). recommended for purposes such as checking better performers. n Depth (primarily as a function of gas the function of the oxygen addition system 4. Post dive, rebreather absorbent can be density). before entering the water, but a duration less than five minutes should be adequate for removed, dried and refitted to improve This often creates a dilemma, especially that purpose. performance. Author’s note: This is for manufacturers. The ideal solution VERY dangerous. would for all manufacturers to quote an Arguably the most important secondary 5. I can store a part used scrubber endurance based on a worst case scenario. finding of our study is that partial scrubber indefinitely and use again and obtain This would then create a ‘level playing failure in a rebreather is a particularly the same performance. Author’s note: field’ from a user information/safety insidious fault if divers rely on a pre- The variabilities of container integrity standpoint. This is exactly what the CE test breathe to detect it. By modestly increasing and water content loss (evaporation) standard for rebreathers (EN14143) tries to ventilation, subjects typically maintain normocapnia during a surface pre-breathe rebreather design, increased ventilation thermal front movement can be detected in this condition, resulting in a false negative rate. In addition, the increased ventilation using a series of temperature sensors (a that is dangerous because normocapnia is can produce an increase in work of thermal array) arranged linearly within much less likely to be maintained during the breathing and the associated CO2 the scrubber. As each sensor peaks, then dive itself. retention, especially at depth. a percentage remaining can be estimated The observations from this trial point to Unmanned trials do however indicate that and compared to a tested endurance to the fact that the only reliable test of a in the first 2/3rd of the scrubber life, even predict time remaining. scrubber systems integrity should utilize a with increased ventilation/work-rate that the While thermal array monitors are a gaseous CO2 sensor. scrubber can ‘cope’. However, it is clear that good steady state system, accuracy is this is not always the case in the final 1/3rd. All absorbents are the same challenged in shallow and warm water and it cannot predict bypass as a result of high As previously mentioned, there are CO2/endurance sensing workload. several manufacturers and types of Gaseous CO2 sensors are not new and commercially available absorbent. A have been used in surface applications for common misconception is that ‘absorbent years, and are a way of instantly measuring is absorbent’ and one will last just as long CO2 in a breathing loop. as the other. More recently a form of CO2 sensor has Again, in a New Zealand trial⁶, two been used for diving with rebreathers. common diving absorbents were tested. It is known as a non-dispersive infra-red Author’s note: These tests were completed (NDIR) sensor. Put simply, using infra-red at surface pressure and testing at depth light it measures the spectral response will change the results although the of CO2 when the light is passed over relationships will remain broadly the same. it. Unfortunately, the spectral response of water is similar to CO2 and can also Sofnoline 797⁷ (maximum granule size Figure 4 - Thermal front monitor – AP Diving be detected. This can create a ‘false 2.5mm) and Spherasorb⁸ (maximum positive’ (high reading) and a shorter dive In short, all types of sensor have one issue granule size (5mm) were tested with the endurance, which in itself can be seen as or another that can also relate to how it is following results. a ‘fail safe’ scenario. Some manufactures deployed within the rebreather. The only The mean operating duration to CO2 have successfully integrated NDIR sensors way to verify suitability is to review test breakthrough was 138 +/-4 (SD) minutes in rebreathers with additional moisture results. for Spherasorb and 202 +/- 4 minutes filtration, sometimes at the expense of In summary, gaseous CO2 sensors help for 2.64kg Sofnoline. Author’s note: sensor response time and long duration with: Spherasorb weight not referenced, likely use (due to filter degradation) is still n Bad CO2 seal to be less due to lower bulk density of potentially an issue. Gas density can also n Bad absorbent Spherasorb. The difference between the peak affect these sensors but can in most cases n Absorbent abuse (used too long, respiratory pressures was 10% less during be compensated for. repacked) use of Spherasorb, suggesting lower work of n Breakthrough due to high work breathing. Gaseous CO2 sensors do not help with: Under conditions simulating work at 6MET n Detecting CO2 retention during use of a rebreather, a canister n Duration monitoring packed with Spherasorb reached CO2 breakthrough 32% earlier with 10% less Thermal arrays help with: mass than Sofnolime packed to similar n Endurance prediction volume. Divers cannot alternate between Thermal arrays do not help with: these two preparations and expect the n CO2 bypass as a result of high same endurance. workload/seal failure The scrubber ‘curve’ Choosing an absorbent When tested at a steady depth, Figure 3 - Experimental CO2 sensor The correct choice of a CO2 absorbent temperature and CO2 generation rate, the CO2 ‘curve’ is mostly a flat line (0mb Another, less common sensor used uses a product may not be as simple as it first CO2) for at least 2/3rd of the endurance chemical gel patch which changes colour seems. For a lot of divers, cost is an issue. time. In the last 1/3rd it starts to rise until as more or less CO2 passes over it. This For others, especially deep divers, it might it passes the 5mb and then 10mb safety change is then measured and directly be endurance or work of breathing. alarm limits⁹. The ‘lift’ towards 5mb often correlated to a CO2 level. These are less The key point here is you need to decide only occurring in the last 10-15% of the prone to moisture interference and are gas what your objectives are and above all you endurance time and the rise to 10mb in density independent. should prioritize safety as the driver. the last 10-20 minutes or so. At first glance Gaseous sensors aside, a scrubber’s it would appear that we can safely exercise As an example, if you are only conducting endurance prediction is the next right up to nearly the end of the scrubber short, shallow dives then a product that challenge. Current systems rely on the life without a bypass taking place, even at is more cost effective at the expense of fact that as absorbent is used within a the extreme CE test rate and/or having ill endurance may be applicable. scrubber housing, a thermal ‘front’ slowly effects due to hypercapnia. moves through the scrubber tracking the If all your diving is in warm water, then The reality is not so simple and can active section of the absorbent. As that again a less efficient product may be depend on several elements such as section is used, the front moves on. This suitable. Conversely, if you diving is deep, long 8. Do not store part used absorbent for and cold then ideally would want the later use. best performing product possible from 9. Do not rely on the 5-minute pre- an endurance perspective. That should be breathe as the only CO2 system test: balanced against any work of breathing issues of a potentially more densely a. Check mouthpiece mushroom valves on every dive packed (smaller granules) absorbent if b. Check CO2 seals that is the choice, much of the latter being c. Properly pack the scrubber mitigated by using the correct diluent at depth. 10. Do not dive with a leaking rebreather, this will: a. Increase WOB b. Decrease absorbent efficiency c. May generate a caustic solution that could be swallowed SUMMARY The current ‘state of the nation’ is that there are several types of absorbents Figure 5 - Absorbent friability should be available for use. Only a small number of considered these absorbents have been tested and qualified in the now available larger range Consideration on long dives should also of recreational diving rebreathers. As a be given to the product’s ‘friability’, its result, there is some confusion with regard tendency to break into smaller particles, to the safe limits of use. bypass any scrubber filtration and mix with condensed water to generate a Military groups do qualify absorbent caustic cocktail. Friability is tested for by endurance through testing. However this most manufacturers and can normally be only provides a worst case (extreme work simply evaluated by comparing how much rate) scenario. dust there is at the bottom of a used barrel In addition, the complete picture of of product. issues related to CO2 is a complex event The bottom line with absorbent use is, you and while elements of it are covered in have choices, make them wisely. isolation in most training documentation, the more ‘joined up picture’ is seldom Safety Top 10 discussed in detail. 1. Find out and understand the CO2 As a user group and as technology evolves, endurance limits of your chosen rebreather design. it is vital that we become as educated as possible with regard to the training and 2. Hypercapnia is insidious and management of CO2. Whether choosing potentially lethal – do not assume you a new rebreather, a training program, a can detect or ‘handle’ it. specific manufacturer or an absorbent 3. Use absorbent that has been supplier, it is important that we confirm qualified for your rebreather and is the detail and validity of the information in date. Be fully conversant with the we receive. manufacturer’s test data. You have a right to ask, use it. ¹ Warkander and Shykoff. NEDU dated July/Aug 2012 4. Do not assume all absorbents give the ² Courtesy Gavin Anthony same duration. ³ Courtesy Gavin Anthony 5. Understand the scrubber curve, avoid ⁴ Diving and Hyperbaric medicine. Volume 48. 2nd June high work rates towards the end. 2018. Pollack, Gant, Harvey, Mesley, Hart, Mitchell. ⁵ Diving and Hyperbaric Medicine Volume 45 No. 1 March 6. Use the right gas at the right depth to 2015. Deng, Pollock, Gant, Hannam, Dooley, Mesley and minimize WOB and CO2 retention. Gas Mitchell. density should not exceed 6.2 g/L. ⁶ The duration of two carbon dioxide absorbents in a closed circuit rebreathing system. Harvey, Pollack, Gant, 7. CO2 sensing is a major safety feature Hart, Mesley and Mitchell with extended use and changing ⁷ Trademark Molecular Products Ltd workload. If at all possible, use a CO2 ⁸ Trademark Intersurgical Ltd sensor. ⁹ Reference CE EN14143