Due to their efficient gas consumption, closed-circuit rebreathers (CCRs) are being used more and more frequently in demanding non-comercial, non-military diving. CCRs recycle the exhaled gas by eliminating CO2 in a container (CO2 scrubber) filled with absorbent material, usually soda lime. The complexity of CCR is higher compared to conventional SCUBA and comes with potential sources of error associated with the CO2 scrubber. Failure to replace used breathing lime or improper packing of the scrubber can result in hypercapnia (CO2 poisoning) with the risk of serious incidents.
To mitigate this risk, divers are taught during CCR training to perform a prebreathe phase of several minutes before the dive and to breathe through the rebreather before descending to check for early symptoms of hypercapnia, such as shortness of breath or headaches. It is generally assumed that this is sufficient to detect a problem with the CO2 scrubber. However, the validity of this practice has rarely been formally evaluated. One study investigated this a few years ago.
The study
The study involved 30 trained divers who were randomly exposed to two of three conditions: normal breathing chalk, partially failing breathing chalk or no breathing chalk at all. Each diver completed a five-minute prebreath test under two of these conditions with a 20-minute break between tests. An Inspiration Evolution Plus system was used as the CCR. Physiological parameters such as inhaled CO2, end-tidal CO2 (CO2 at the end of exhalation), ventilation (respiratory minute volume) and heart rate were continuously monitored. The primary outcome was the proportion of divers who stopped prebreathing due to symptoms of hypercapnia.
Results
The prebreathing showed varying degrees of sensitivity in detecting failures of the CO2 scrubber. None of the divers aborted the test with a normally functioning scrubber, only 2 out of 20 aborted the test with partially failing scrubber and 15 out of 20 aborted the test with no scrubber. This results in a sensitivity of 10 % for partially functioning breathing scrubber and 75 % for missing lime with a specificity of 100 %. The main symptoms reported by subjects who discontinued the test were shortness of breath and a feeling of harder breathing. The physiological data showed no significant changes with normal function of the scrubber, whereas with partially failing scrubber the increase in inhaled CO2 was compensated by a slight increase in ventilation, preventing significant hypercapnia. In the absence of the absorber, CO2 levels rose steadily, with divers responding differently, with a wide range of ventilation.
The results are sobering. 90% of the test subjects did not notice that the scrubber was not fully functional. 25% did not even stop the test when no scrubber was present! The prebreathe test therefore appears to be obsolete for detecting a malfunction of the scrubber.
Most divers don't notice when the scrubber is not working properly, one in four doesn't even notice when there's no absorbent at all!
Physiologic compensation
The study suggests that the low sensitivity of prebreathing is due to the body's ability to compensate for small increases in CO2 during the test, so that divers do not experience symptoms that would cause them to stop prebreathing. This compensation is unlikely to be as effective during an actual dive, which makes a supposedly well-functioning scrubber particularly dangerous.
Prebreathing: Important, but not für CO2.
The authors come to the same conclusion: "While prebreathes are useful to evaluate other primary functions, the five-minute prebreathe is insensitive for CO2 scrubber faults in a rebreather. Partly-failed conditions are dangerous because most will not be detected at the surface, even though they may become very important at depth." especially because "partial scrubber failure in a rebreather is a particularly insidious fault if divers rely on a prebreathe to detect it. By modestly increasing ventilation, subjects typically maintain normocapnia during a surface prebreathe in this condition, resulting in a false negative that is dangerous because normocapnia is much less likely to be maintained during the dive itself."
Use fresh lime generously!
CO2 sensors urgently wanted
Once again it becomes clear that it is almost impossible to subjectively perceive a malfunction of the rebreather and react appropriately. The same problem arises for potential hypoxia. However, if the oxygen partial pressure can be reliably measured by sensors, the issue of CO2 is more critical. There are still no commercially available CO2 sensors for CCR. It is therefore essential to use a sufficient quantity of correctly packed soda lime.
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