CO2 handling is crucial in closed circuit rebreather (CCR) diving because excessive CO2 buildup, known as hypercapnia, can be life-threatening. Rebreathers recycle exhaled air by removing CO2 using a scrubber system. However, if the scrubber fails or becomes saturated, CO2 can be inhaled again, leading to dangerous CO2 levels in the diver’s body. Early detection through CO2 monitoring would allow divers to address these risks in real time, preventing accidents caused by CO2 toxicity or scrubber malfunctions, significantly improving dive safety. Why are CO2-sensors still not routinely implemented in modern rebreathers in contrast to oxygen-sensors? This and the next blog post will deal with scrubber monitoring systems and CO2-sensors.
CO2 monitoring refers to measuring the carbon dioxide (CO2) concentration in the gas exhaled by a diver. It is widely used in medical fields like anesthesiology and intensive care medicine, where it helps monitor ventilation and detect early signs of respiratory issues. Bringing this technology into diving is seen as a major safety advancement, as it could help detect two key risks: CO2 buildup in the body (hypercapnia) and CO2 rebreathing due to scrubber failure in CCRs. Both scenarios can lead to life-threatening situations underwater. However, these are not yet routinely availabe. The manufacturers therefore resort to systems that monitor the scrubber function.
Scrubber monitoring
Scrubber monitoring systems are designed to track the performance and status of CO2 scrubbers in CCRs, which are responsible for removing exhaled carbon dioxide from the breathing loop. Some CCR manufacturers use "temp sticks" to monitor CO2 scrubbers. These devices don’t measure CO2 directly but predict scrubber depletion and remaining capacity by measuring the heat generated during the reaction between CO2 and soda lime, helping to detect when the scrubber is nearing exhaustion.
However, they do not give information about the real CO2-concentration in the breathing gas. It can be assumed that these systems allow for a large safety margin for reasons of liability, which results in an “excessive” lime replacement indication. This could be shown to a limited degree in a study, which tested «temp sticks» in two rebreathers: Inspiration and rEvo. Tests simulated dives at surface pressure and shallow depths. At depth, temperature sticks provided timely warnings of CO2 scrubber depletion in most, but not all cases. However, they were less accurate during simulated exercise at surface pressure. rEvo tended to give early warnings (approximately 60 min. prior), while Inspiration warnings were closer to or slightly after the CO2 breakthrough.
Sensitivity and specificity for divers
Here you can see the fundamental problem: on the one hand, you want the warning mechanism to report a breakthrough in CO2 absorption with maximum reliability; on the other hand, you don't want it to be reported too early, so that you can't dive, have to abort the dive early or have to change the lime too often. The problem is known as “sensitivity” and “specificity” and applies to alle medical tests. Sensitivity indicates the reliability with which a test predicts an unfavorable result. The specificity indicates the certainty with which the test does not falsely report a normal result as unfavorable.
In the case of the study mentioned, the “temp stick” of the inspiration warned early enough in 6 out of 8 cases and too late in 2. This results in a sensitivity of 6/8, i.e. 75%. In 25% of cases, therefore, it was missed that the scrubber no longer performed well enough. The situation was different with the rEvo: in all 5 tests it warned about 60 minutes before a CO2 breakthrough. One could therefore say that it falsely delivered unfavorable results (of course, it is debatable how early a warning should come before exhaustion oft he lime). The specificity here was therefore rather low.
Ideally, we would like to have a scrubber-test that is 100% sensitive and 100% specific. This doesn't exist in real life.
What if the “temp stick” in Inspiration were simply made more sensitive? Then the sensitivity would increase, but the specificity would drop.
Let's assume that the above-mentioned “temp stick” now detects an expired lime in 95% of cases instead of just 75%. It can be reasonably assumed that false alarms would occur in this case. If we assume that the price for the increase in sensitivity is that the specificity drops from 100% to 80%, this means in practical terms that a CO2 breakthrough would be missed in 5 out of 100 dives, but 20 out of 100 dives could not be carried out unnecessarily or would have to be aborted prematurely due to a false alarm. This essentially translates into a problem with the measurement on every 4th dive. This is unacceptable in practice.
Know your lime!
Therefore, it is still best to know the performance of the lime used. Sofnolime 797 is one of the most frequently used products. Its effectiveness was compared to Spherasorb in a CCR (Inspiration). In the study, the metabolic rate during the simulation was set at 6 METs (metabolic equivalents), Under these conditions, the absorbents lasted as follows:
Spherasorb: 138 minutes before CO2 breakthrough.
Sofnolime 797: 202 minutes before CO2 breakthrough.
6 METs correspond to at least a moderate physical effort. In other words, the simulation was not an easy dive. This shows the good performance of Sofnolime even during physically demanding dives and underlines the fact that there are differences between specific products.
The methods mentioned only provide indirect information about the CO2 in the breathing loop. The holy grail of rebreather diving would be direct knowledge of the CO2 concentration in the inhaled gas. The next blog post will deal with the challenges involved here.
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