Ever since I watched the movie “Diving into the Unknown” about the Plura cave disaster, one question has been on my mind: How can it happen that a rebreather diver suddenly loses consciousness due to stress and drowns? The answer could lie in the special functioning of a rebreather's scrubber canister.
The Plura Cave disaster
In 2014, five experienced Finnish cave divers undertook a challenging dive in the Norwegian Plura cave - with a tragic outcome. Two of them lost their lives.
The group was divided into two and three divers. At the greatest depth of the dive, around 130 meters below the surface, one diver got stuck in a narrow passage. He panicked and presumably died of hypercapnia (CO₂ poisoning) within a very short time.
The second group of three divers followed hours later. They came across the dead man and the blocked section - and the situation escalated again. One of the divers presumably panicked and died by drowning within a few minutes. The other three were able to save themselves despite suffering from severe decompression sickness. Months later, they recovered the bodies of their friends in another risky dive after official rescue attempts had failed.
Death by panic?
I do not claim to make a definitive judgment on the accident. But one crucial question remains: Why did the second diver who died suffer an uncontrollable breathing problem so unexpectedly, which led to his death?
There is little doubt that hypercapnia also led to his unconsciousness. What is striking, however, is that there is no evidence of a malfunction of his device or excessive physical exertion - two major factors that can normally contribute to CO₂ poisoning. So what was the mechanistic trigger for this fatal outcome?
One possible explanation could lie in an often overlooked feature of the rebreather: the interaction between CO₂ and the scrubber's lime.
The importance of the CO₂ resident time
For the CO₂ to be effectively absorbed in the rebreather, it must remain in the scrubber for a sufficiently long time. This resident time depends on several factors:
The further the CO₂-path through the scrubber and the more densely the lime is packed, the longer the CO₂ remains in contact with the absorbent.
Less lime or a looser packing of the scrubber shortens the contact time and reduces CO₂ absorption.
However, another decisive factor is often overlooked: the breathing minute volume.
At first glance, one might assume that a higher breathing rate improves CO₂-elimination because the gas flows through the lime more frequently. Paradoxically, however, the opposite is the case. The higher the breathing minute volume, the shorter a single CO₂-molecule remains in the scrubber - and the worse it is absorbed.
The former head of research at the Navy Experimental Diving Unit (NEDU), John Clarke, describes this problem in his book “Breakthrough - Revealing the Secrets of Rebreather Scrubber Canisters”. His simulations show that CO₂-absorption decreases as soon as the breathing minute volume rises above a certain level.
The deadly vicious circle of panic
It is precisely this mechanism that can be fatal for a rebreather diver in panic. Panic leads to hyperventilation - the breathing rate increases dramatically. As a result, the exhaled gas flows faster through the scrubber, the CO₂ resident time decreases and less CO₂ is captured. A dangerous vicious circle sets in: increasing CO₂ leads to a further increase in the breathing rate, the CO₂ resident time decreases further, which leads to a further increase in CO₂ and so on. Finally, the respiratory physiology is overwhelmed by further factors such as the high breathing resistance due to the high breathing gas density. The system collapses and ends in severe hypercapnia with unconsciousness. It is precisely this mechanism that could explain the second death in the Plura Cave disaster - even if the diver did not initially have a CO₂-problem.
Why it is vital to have controlled breathing
Unfortunately, CO₂ contact time in the scrubber and CO₂ concentration in the blood are not independent variables, but are related. This is because the higher the CO₂ concentration in the blood, the higher the breathing rate and the shorter the CO₂ contact time in the scrubber. In the event of a high CO₂ build-up, this means a decisive disadvantage: just when as much CO₂ as possible should be absorbed, the CO₂ contact time falls, which hampers CO₂ absorption in the scrubber.
This finding illustrates how essential it is to avoid a high breathing rate during a rebreather dive. Even emergency situations that were not initially caused by increased CO₂ can quickly get out of control due to hyperventilation. If other factors are added, such as high breathing resistance at depth or heavy physical exertion, the result can be life-threatening hypercapnia with unconsciousness and drowning within a very short time.
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