exhaled CO2 is variable based on workload and O2 consumption.
Hi rjack,
This is true in one sense, but maybe not in the way you mean.
If we exercise harder, we consume more oxygen and produce more CO2. That CO2 is carried to the lungs in the blood, and breathed out. So, it is technically correct to say that more CO2 will be being eliminated by exhalation during higher workload, and less at rest.
BUT:
The amount of CO2 in each breath will (or more correctly should) remain the same.... we just breathe more frequently to eliminate that extra CO2. Monitoring the PCO2 in the exhaled gas therefore remains a valid way of appraising body CO2 levels even during exercise. I will explain in a little more detail.
First a bit of respiratory physiology. It is really important to understand that it is the amount of breathing that determines the amount of CO2 removed from the body. If you breathe more, then more CO2 is eliminated, and if you breathe less, then less CO2 is eliminated. Not surprisingly, breathing is regulated by a region of the brain to keep CO2 levels normal, without us having to think about it. Thus, in your exercise scenario, more CO2 is produced and so the brain drives more breathing to keep the body CO2 levels normal. This means more breaths, but the amount of CO2 in each breath will stay approximately the same. Indeed, during exercise on land the brain tends to over-compensate with excess breathing and body CO2 levels often fall a little. This provides an interesting contrast with diving which I will come back to later.
Now to monitoring body CO2 levels. The most common form of body CO2 level monitoring is the use of "end tidal CO2". This means measuring the partial pressure of CO2 (PCO2) in the exhaled breath at the very end of each exhalation as doctormike has explained. The reason this works is that the gas coming out of the mouth at the end of exhalation has come from the deepest part of the lungs - the alveoli. The alveoli are in intimate contact with the blood, and blood and alveolar gas pressures equilibrate during each inhalation and exhalation. Thus, by measuring the PCO2 in the exhaled alveolar gas, we are effectively measuring the PCO2 in the blood. There are a few assumptions and potential sources of inaccuracy, but it mainly works well. I (and every anesthesiologist in the world) use this technology every day wen mechanically ventilating patients (who are not breathing for themselves) in order to ensure ventilation is adequate to keep the blood CO2 levels normal.
The big advantage of measuring end tidal CO2 is that it detects an increase in body CO2 no matter what is causing it. The focus in this thread has been on the possibility that CO2 might break through the scrubber canister in a rebreather. That is certainly one potential cause for failure to adequately eliminate CO2 and a rise in body CO2. But it is often under-appreciated that the most common cause of high body CO2 levels in diving has nothing to do with CO2 breaking through a scrubber; it is simply a failure to breathe enough to eliminate the CO2 we are producing. This is often referred to as "CO2 retention". Put simply (because it is a complex phenomenon) when the work of breathing is increased, the brain becomes less sensitive to rising body CO2 levels, and is less likely to drive enough breathing to keep CO2 levels normal. Thus, CO2 level rise simply because we don't breathe enough to eliminate it. Since the work of breathing is invariably increased in diving (by things like dense gas and equipment resistance) CO2 retention is commonly seen. Exercise makes it worse, and the worst combination is heavy exercise with a significantly increased work of breathing. In addition, some people are more prone to CO2 retention than others.
The point is that end tidal CO2 will detect problems due to inhaling CO2 or due to CO2 retention whereas a CO2 monitor on the inhale side of a rebreather circuit will only tell you if you are inhaling CO2. Put another way, a diver could have dangerously high body CO2 levels due to CO2 retention, but the CO2 monitor on the inhale side would be showing zero. That is not to say that inhale CO2 monitors have no value, and it is possible to have both. Indeed, if you
look at the capnography trace provided by doctormike you can see that it can show you: first, that the inhaled PCO2 falls to zero during inhalation (labelled phases 0 and 1), that is, there is no breakthrough; and second, what the end tidal PCO2 is at the end of exhalation. So you get the best of both worlds. However, this requires that the monitor itself is in the mouthpiece, or that gas is continuously sampled from the mouthpiece.
No one has managed to achieve this in a rebreather yet. As others have suggested, this relates in part to the challenges of putting an infra-red CO2 sensor in a 100% humid environment. The Apocalypse iCCR was designed to monitor end tidal CO2, but the original description of their methodology (almost 10 years ago now) was flawed, and a description of subsequent modifications is difficult to interpret and yet to be verified. They are still saying it is coming but who would know.
Simon M
