Hi guys,
I have written an article for Dive New Zealand magazine on this. I will paste the content below - but that is why it sounds like an article. There is a bit of CO2 physiology background first.
CO2 is produced in the tissues during the utilisation of oxygen. It is eliminated from the body by breathing, and the more we breathe the more CO2 is eliminated. Breathing to eliminate CO2 is usually precisely controlled by the brain to keep CO2 in the body at a stable level. If CO2 levels rise, the brain will ‘drive’ more breathing to bring CO2 back to normal and vice versa. This is a completely automatic function which takes place without us thinking about it.
This normal process of CO2 control can be disturbed in diving because of an increase in the work required to breathe. The work of breathing increases because we are respiring a denser gas through a regulator or rebreather. In some people more than others, when the work of breathing rises the brain seems less sensitive to rising levels of CO2 and will avoid driving the extra breathing work required to keep CO2 levels normal. Thus, when underwater, divers are prone to having body CO2 levels rise, particularly when exercising and when the work of breathing is high, simply because they don’t breathe enough to eliminate the CO2 being produced. This phenomenon can occur during use of any underwater breathing apparatus (including open circuit scuba and rebreathers) and is referred to as “CO2 retention”.
Another cause of increased CO2 levels during diving is inhalation of CO2 during breathing. CO2 inhalation is a specific hazard of rebreather diving because these devices recycle the exhaled gas which is then “rebreathed”. The CO2 must be removed from the exhaled gas and this is achieved by a CO2 ”scrubber” canister which contains a granular chemical compound called “soda lime” which absorbs CO2. Soda lime has a limited absorption capacity and must be replaced regularly. If the scrubber fails for some reason (such as the soda lime becoming exhausted, incorrect installation of the canister, or failure to install it at all!) the diver will inhale CO2 and breathing becomes much less efficient at removing CO2 from the lungs. CO2 levels in the body can increase even if the diver breathes heavily. The more CO2 is inhaled, the worse this problem is likely to be.
This double hazard of CO2 in rebreather diving understandably raises questions about whether CO2 can be monitored when diving rebreathers. The answer is yes, but this is where things can get a little confusing. The first thing to clearly understand is that all of the monitoring systems currently available in rebreathers are focussed on the prediction or detection of inhaled CO2, that is CO2 breaking through the scrubber. I will focus on these first, before moving on to the slightly more complicated topic of systems capable of measuring CO2 levels in the diver.
Inhaled CO2 detection
Warning systems for inhaled CO2 take two forms: temperature sticks and CO2 monitors.
Temperature sticks are designed to indicate when a scrubber has reached the end of its capacity to absorb CO2, and therefore the point at which CO2 could begin to break through and be re-inhaled. They do this by monitoring the temperature change through the scrubber as the soda lime gradually becomes exhausted, and this segues into the key point about these devices: they don’t actually detect CO2 when it breaks through the scrubber; they simply predict when it is likely to occur. We have just finished a fairly comprehensive investigation of how accurately they achieve this in both the Inspiration and Revo rebreathers. The data are about to be submitted for publication, and I cannot reproduce them here. However, it is a fair summary to say that temp sticks work well. I will describe those data in detail once they are published. Returning to the point that temp sticks don’t actually detect CO2, an important corollary is that they can’t detect or predict CO2 breakthrough because (for example) of a fault with scrubber installation. Only a CO2 monitor can do that.
CO2 monitors are effectively gas analysers which use infra-red light absorbance technology to detect CO2 in gas. Thus, the gas downstream of the scrubber is analysed for CO2 (if the scrubber is working well there should be none) before it is inhaled by the diver. If inhaled CO2 levels exceed a threshold (usually set around 5 millibars) then an alarm state is activated. Only a few rebreathers offer these devices, and their uptake and review by users has been mixed. The principal challenge with them is that the infra-red absorbance system does not like moisture, and in the 100% humidity environment of a rebreather loop they are prone to giving false positive alarms. We tested the Inspiration version of this device and found it to be reliable provided the user is fastidious in following moisture protection guidelines.
If you cast your mind back to the introduction where we talked about the mechanisms of CO2 problems, the obvious vulnerability of inhaled CO2 detection systems is that while they can tell us if we are inhaling CO2, they cannot detect CO2 retention by the diver. Remember, a diver may fail to breathe enough to eliminate all CO2 produced and retain CO2, even if there is no CO2 inhalation.
Measuring CO2 levels in the diver
The only way to detect high levels of CO2 in the diver, whether the cause is CO2 rebreathing or retention due to inadequate breathing, is to measure CO2 in the exhaled breath. The principle underpinning this strategy is that the composition of the gas in the alveoli (small air sacks) of the lungs is in approximate equilibrium with the gas pressures in the arterial blood. Therefore, if we can measure the PCO2 in the alveolar gas, then we have a good estimation of the CO2 levels in the diver’s blood. To ensure that we are measuring alveolar gas we wait until the very end of the exhalation because at that point all the gas coming out of the divers mouth should be coming from the alveoli. That is why these measurements are referred to as ‘end tidal CO2’ measurements.
This strategy is used in operating theatres all over the world every day to monitor CO2 levels in patients undergoing anaesthesia. Unfortunately it is not as simple as it sounds. One either has to have one of the infrared sensors described above positioned right at the mouth, or gas has to be sampled from the mouthpiece and passed to an analyser nearby (the latter is the most common method in an operating room). Neither strategy would be easy in a rebreather. Putting an infrared sensor in the mouthpiece is tricky both because of space and humidity, and sampling to a remote sensor would require a lot of power to drive the sampling pump. In addition, whereas detection of CO2 in inhaled gas does not need to be particularly accurate (its presence or absence is the main issue), if we are going to base dive management decisions on end tidal CO2, the measurements have to be very accurate because it is a tightly controlled physiological parameter.
Accurate and reliable end tidal CO2 measurements in a rebreather would be a very useful feature, but no device has currently achieved this. One manufacturer has promised end tidal CO2 for over a decade but it has never materialised. Indeed, the original proposal to measure it was flawed and it is unclear whether subsequent modifications to the system will work. Hopefully it is something to look forward to in the future.
Simon M
