Most veterinary practices have a policy in place to decide which patients use a rebreathing circuit and which use a non-rebreathing circuit during gas anesthesia. In general, smaller patients use a non-rebreathing circuit and larger patients use a rebreathing circuit. Your hospital designates the actual cut-off from rebreathing and non-rebreathing, based on the weight of the animal. But there’s more than the weight of the animal at play here, so let’s take a closer look at the decision to use a non-rebreathing circuit over a rebreathing circuit.
Anesthesia breathing circuits fall into two main categories, based on how they manage the patient’s expired CO2. Rebreathing circuits use a carbon dioxide absorber to trap and remove CO2 so the patient can breathe gases that have been recycled through the machine. Non-rebreathing circuits use high gas flows to washout expired CO2 from the circuit before the patient takes its next breath.
Regardless of the shape or configuration of rebreathing circuits, they all share five essential components:
- Hoses
- Rebreathing bag
- Unidirectional flow valves
- CO2 absorbent
- Pop-off valve
By contrast, non-rebreathing circuits are comprised of only hoses. A rebreathing bag and pop-off valve are often part of a non-rebreathing circuit, but they are not essential to its function
The advantages of a rebreathing circuit make it the first choice for anesthesia. Rebreathing circuits require lower gas flows which saves money for the practice as well as reducing pollution into the atmosphere by waste anesthetic gas. But the more significant advantages are to the patient. Rebreathing circuits help keep patients warmer and help to retain moisture. Non-rebreathing circuits require high gas flows and steal heat and moisture from the patient. And the loss of body heat and moisture are key complications of anesthesia that we face with every patient, especially smaller patients.
So why not put every patient on a rebreathing circuit?
That would be the ideal. Unfortunately the one significant disadvantage of rebreathing circuits is that the combined five essential components of the circuit increase the resistance to breathing. When you decide that your patient should be on a non-rebreathing circuit, you are actually deciding that you think your patient cannot overcome the resistance of a rebreathing circuit. Not surprisingly, the one significant advantage of a non-rebreathing circuit is the minimal resistance to breathing. 
Imagine the difference in resistance between taking a drink from a soda straw, and taking a drink from a garden hose. Although it takes only a small amount of energy to draw water up a soda straw, it’s still significantly more energy than it takes to get a drink of water flowing from a garden hose. Similarly, it requires far less effort to take a breath from a non-rebreathing circuit than from a rebreathing circuit. In order to draw a breath from a rebreathing circuit, the patient must be able to generate enough negative pressure to overcome the resistance of the hoses, lift the unidirectional flow valves and draw it through the soda lime.
Test the limits
Maybe it’s time to examine your hospital’s policy toward non-rebreathing circuits. I’ve seen target weights set at 20 pounds, at 5 pounds, and at every weight in between. And I’ve seen practices that don’t use non-rebreathing circuits at all. They can’t all be right, can they?
Non-rebreathing circuits are expensive to use, flowing nearly ten times more oxygen and anesthetic gas into the scavenge system as rebreathing circuits, and they syphon off body heat and moisture from patients. One way to extend the weight range for the use of non-rebreathing circuits into lower numbers is to challenge the efficiency of your current rebreathing circuit.
A simple first step
The unidirectional flow valves and the CO2 absorber are significant sources of resistance in your circle rebreathing system, and that can be reduced considerably by evaluating your current system and replacing outdated parts of the gas machine.
However, a more accountant-friendly first step is to examine the tubing you use in your rebreathing circuit. Hoses account for up to 50% of the resistance the patient has to overcome in order to take a breath. And replacing the breathing hoses of your rebreathing circuit is not expensive at all.
Resistance test results reported in a recent study show that the corrugations in standard breathing hoses and in the popular Universal “F” circuits used in many small animal practices today are sources of resistance to breathing. The same test results show that removing the corrugations from the inside of the hose reduces resistance remarkably. Removing the corrugations also allows the size of the tubing to be reduced, further improving the efficiency of the system. Smooth wall circuits are even less resistant to flow than smaller corrugated pediatric circuits.
Darvall has pioneered smooth wall circuits and has them available as heated and non-heated, in two sizes: 12mm for patients under 45 pounds and 16mm for patients over 45 pounds. Both hold less volume than their standard 22mm corrugated counterparts which make them more efficient and makes your gas machine more responsive. The smaller 12mm hose is so efficient that with some CO2 absorbers, you can use it on patients as small as 2 Kg. And as a special bonus for those of us who wash breathing circuits, without corrugations nothing traps water inside the hoses. Smooth wall circuits dry almost immediately. That means no more swinging circuits over our heads to get them dry enough to use.
Non-rebreathing circuits cost your practice money and cost your smaller patients body temperature. Examine your hospital policy toward non-breathing circuits and test the limits of your weight range. A capnograph will indicate when a patient is inspiring CO2, and can be used to determine if a non-rebreathing circuit is necessary. Using the most efficient hoses for your rebreathing circuits will go a long way toward lowering your target weights, and allow you to put smaller patients on rebreathing circuits. Darvall offers smooth wall circuits in heated and non-heated. Changing to smooth wall circuits is a great first step to improving the efficiency of your gas machine and allows you to use a circle rebreathing circuit on smaller patients than you normally would.
Follow these links for more information
Darvall Smooth Wall Tubing (SWT): Low Resistance & Volume
Darvall Smooth-Wall Circuits – Efficient & Responsive Anesthesia Breathing Circuits
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
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