Your anesthetized patient exhales carbon dioxide. But you already know that. And you know that you are exhaling carbon dioxide as you’re reading this. You may not have thought about it in awhile, but you also know how carbon dioxide gets in your lungs so you can exhale it. Carbon dioxide is the waste gas that is produced as part of the body’s energy-making processes. The lungs and respiratory system facilitate oxygen absorption into the body, and enables the body to get rid of carbon dioxide in the air breathed out. To extend our glance at this system of gas exchange to a global view, plants absorb carbon dioxide that we exhale and they exhale oxygen. It’s a pretty efficient relationship.
Exhaled carbon dioxide is handled differently in the two different types of anesthetic breathing systems. With a non-rebreathing system (Bain, Mapleson, etc), the exhaled carbon dioxide is managed with high oxygen flow rates. The oxygen flow rate must be high enough to move the exhaled carbon dioxide out of the circuit and refill the circuit with fresh gas before the patient takes its next breath.
Exhaled carbon dioxide is managed differently in a rebreathing circuit. By adding a carbon dioxide absorber into the circuit – thereby taking the exhaled carbon dioxide out of the breathing circuit – we are able to use lower oxygen flow rates which saves oxygen, anesthesia gas, patient body temperature, and money. The carbon dioxide absorber most commonly used is a soda lime canister.
Soda lime is a granular mixture, consisting predominantly of calcium hydroxide, together with small amounts of potassium hydroxide and sodium hydroxide. A color change indicator is also usually in the mixture which lets you know when the soda lime is spent and no longer absorbing carbon dioxide.
Since soda lime absorbs carbon dioxide, it stands to reason that it will reach its capacity at some point, and will need to be changed. How do we know when that is?
There is a color change indicator in soda lime that many rely on as a signal to change the soda lime. And it’s not to be ignored. However, it can’t be the definitive indicator of saturated soda lime because the color changes back to white after prolonged disuse. So, if the color changed on yesterday’s late shift but the soda lime wasn’t changed after that shift, when you come in the next morning, the color will have turned back to white and you wouldn’t know it needs attention.
The manufacturers of soda lime set the specific recommendations for changing the soda lime – and again, these recommendations are not to be ignored. They recommend that soda lime be changed after 6 – 10 hours of use. Although that’s a wide range of time, and it doesn’t take into account the size of your absorber relative to the size of the patients (larger animals on smaller-capacity absorbers will saturate the soda lime more quickly), there is a take-home message here: record the time of use somewhere on the gas machine. Follow this link to download a chart that you can print and attach to your machine each time you change the soda lime. It’ll help a lot.
Watching for color change and using a chart to record the time of use are great guidelines. The final test is in your hands – literally. Feel the soda lime granules. Fresh soda lime crumbles easily when you crush it between your fingers. Exhausted soda lime feels hard like tiny gravel between your fingers.
Put your gas machines on a regular maintenance program. Set a time interval that you and your crew will break down the machines, clean them, and test them to be sure everything is working the way it should. The interval will depend on how often you use the machines. If they are used everyday, consider a weekly interval. If they are used only occasionally during the week, the interval could be stretched out to monthly. The important thing is that the maintenance day for the machines is set and regular. Between maintenance days, watch for the soda lime color to change and tick off the time of use on the soda lime chart. But during each regular cleaning, be sure to test the soda lime with your fingers, and let that be the definitive indicator to change the soda lime.
For more information about carbon dioxide absorbers, click on the links below.
Ken Crump AAS, AHT is a writer and animal anesthetist, and writes Making Anesthesia Easier for DarvallVet, a division of Advanced Anesthesia Specialists. He makes dozens of Continuing Education presentations on veterinary anesthesia and oncology across the United States and in Canada. Ken retired from the Veterinary Teaching Hospital at Colorado State University in 2008.