A reader of the dekoblog wanted to know the background of the air breaks that are instructed in the Trimix course. Good question!
The narrative behind air breaks is that they are used to control oxygen toxicity during decompression with pure oxygen. The diver temporarily switches from pure oxygen to air or nitrox during decompression to reset the CNS-"clock". For example, during a 21-minute decompression stop at a depth of six meters, after 10 minutes of pure oxygen, the diver could switch to air for five minutes (air break), followed by a further 11 minutes of pure oxygen until the decompression obligation is fulfilled. The air break itself is not counted as part of the decompression time.
The CNS toxicity of oxygen poses a significant risk to recreational and technical divers. The question is whether air breaks can actually manage oxygen toxicity. An example illustrates the problem:
According to the US government agency NOAA, which sets the global SCUBA standards for nitrox diving and decompression with pure oxygen, pure oxygen can be breathed at a depth of 6 meters (O2 partial pressure of 1.6 bar) for a maximum of 45 minutes without risk of CNS toxicity. In the above example, the 21 minutes would be on the safe side. However, it is completely irrelevant for the calculation of CNS toxicity whether these 21 minutes are completed in one go or in sections. In any case, 21 minutes of pure oxygen breathing utilizes 46% of the CNS "clock" (21 minutes divided by the max. permitted 45 minutes). The 5 minutes of air break do not change this at all (and have hardly any negative effect on the CNS clock due to the very low partial pressure of O2).
Air Breaks reduce the risk of CNS toxicity - during hyperbaric chamber treatments.
But why do various diving organizations train air breaks? The origin probably lies in the practice of pressure chamber treatments, in which patients are exposed to an O2 partial pressure of 2.8 bar in order to achieve maximum effects. During such treatments, air breaks are interspersed every 20 minutes to avoid epileptic seizures (e.g. Navy Table 6). This practice is based on sound science. However, we are talking here about an O2 partial pressure that is far above the maximum permitted value (1.6 bar) for SCUBA diving!
The US Navy diving manual also prescribes air breaks, but for a decompression stage at 30 feet water depth after 30 minutes of oxygen breathing (O2 partial pressure 1.9 bar).
It is important to understand that the method is used at much higher oxygen partial pressures and in different settings than SCUBA diving.
Can the benefits of air breaks be directly transferred to diving?
It is beyond my knowledge who came up with the idea of adopting air breaks tel quel for decompression stops during a scuba dive. The (cautious) NOAA guidelines do not mention air breaks at all. From this one could conclude that they are negligible.
Is there an argument in defense of air breaks? There are considerations related to the optimization of desaturation. Oxygen is a vasoconstrictor that constricts blood vessels and can negatively affect tissue perfusion. Theoretically, air breaks could dilate the vessels by briefly lowering the partial pressure of O2, improve blood flow and thus promote the elimination of inert gases. However, a significant benefit for desaturation remains questionable.
Mixed conclusion
Conclusion: Air breaks are established for pressure chamber treatments and decompression with O2 partial pressures that are significantly higher than the maximum permitted values for scuba diving. Their benefit for scuba diving is unproven.
This raises one final question: Can air breaks do any harm? I don't think so. There is nothing wrong with taking advantage of the potential benefits of air breaks during extended decompression stops, provided the dive is carefully planned within the recommended limits and the management of CNS oxygen toxicity is based on NOAA guidelines.
Air breaks are not an alternative to adhering to the upper limit for the O2 partial pressure (1.6 bar).
Decompression sessions with O2 partial pressure values above 1.6 bar must be avoided and it would be a dangerous misconception that air breaks allow decompression with oxygen partial pressure values above 1.6 bar.
More on oxygen toxicity during diving and the CNS-"clock" in another article soon.
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