do you have a link to the source? I'd like to read it, thanks!
Hello ginti,
Delving into this area is slightly tangential to the topic of the thread, but I have posted a fully labelled version of the graph below.
This comes from a paper written by David Doolette and I for Comprehensive Physiology [1]. For those who are physiologically minded / trained I have also uploaded the paper (pdf available below), but it might be heavy going if you are not already trained in the field.
Basically, the graph shows how the maximum voluntary ventilation (MVV) (the amount of gas you can move in and out of your lungs per minute if you breathe as hard as you can) is affected as you breathe air at increasing depth. The mechanism for this effect is complicated (see the paper) but it is primarily mediated by increasing gas density. On the face of it, the effect is fairly dramatic. For example, if you descend to 30m (~100') your MVV falls from 200 L/min to 100 L/min, i.e., halved! These measurements were made using low resistance lab equipment in a dry chamber, so the effect is almost certainly greater in real diving when immersed and breathing through a regulator or rebreather.
I said "on the face of it" deliberately because, for example, even if your MVV is limited to 100L/min at say 100', that should still be enough ventilation capacity to keep your CO2 normal even during moderate exercise. Thus, in most settings it is unlikely that limited ability to move gas in and out of the lungs is truly responsible for CO2 retention in diving (remember that CO2 elimination is directly proportional to breathing - breathe more and you get rid of more CO2, breathe less and the opposite applies). There are almost certainly exceptions to this though - if you combine very dense gas with poor equipment choices there may truly be situations where you simply can't breathe enough to eliminate the CO2 you are producing. That is a very dangerous and likely fatal scenario - one famous example has previously been published [2]. Although such events are probably rare, I still believe instructors like Miyaru are absolutely correct to use the dramatic effect of gas density on MVV to reinforce to students how important gas density is in a tangible way. The biggest problem with gas density (see next paragraph) is harder to explain and illustrate.
Another more common cause of CO2 retention in diving is the effect that harder work of breathing has on breathing control. We are programmed to increase our breathing if CO2 levels begin to rise, thus eliminating the excess CO2 from the body. That process is operating in everyone as they read this post - just not in the part of the brain that you are conscious of. However, in some people (sometimes referred to as "CO2 retainers") the response is suppressed if the work involved in increasing breathing is abnormally high. Work of breathing is increased in multiple ways during diving (eg equipment resistance, dense gas) and so in "CO2 retainers", if body CO2 levels rise during diving, they may not respond as efficiently with increased breathing to get rid of it, and the deeper you are and the denser your gas, the more true this is likely to be.
Thus, to summarise, CO2 levels may rise dangerously in diving for two reasons: because we literally can't breathe enough to eliminate the CO2 we are producing (probably very rare), or because increased work of breathing has a suppressing effect on the brain's normal response to rising CO2 and we subconsciously choose not to increase breathing. Both of these mechanisms can happen on open circuit or rebreathers. A third mechanism that can lead to CO2 buildup relevant only to rebreather diving is failure of a CO2 scrubber so that the diver is actually inhaling CO2.
Simon M
1. DOOLETTE DJ, MITCHELL SJ. Hyperbaric conditions.
Comprehensive Physiol 1, 163-201, 2011
2. MITCHELL SJ, CRONJE F, MEINTJES WAJ, BRITZ HC. Fatal respiratory failure during a technical rebreather dive at extreme pressure.
Aviat Space Environ Med 78, 81-86, 2007
