Breathing Circuit Classification
Breathing systems are classified into two broad categories: Rebreathing circuits and Non-rebreathing (NRB) circuits.
In many references the size of the patient often dictates which system is used for a particular patient with 7 kg being this magical weight that determines the cut off. Patients over 7 kg use a rebreathing circuit and patients under 7 kg use a NRB circuit.
Interestingly, but not surprisingly, there is NO scientific evidence to support this weight cut off. I have spoken to several older generation anesthesiologists, and they too cannot pinpoint where this number came from. I had one anesthesiologist tell me they believe the 7 kg was just randomly selected because people needed a number when it became popular to use both types of breathing circuits in veterinary medicine.
The reality is that body weight alone is NOT the main criteria for breathing circuit selection. Instead, the advantages and disadvantages should be taken into consideration when deciding the best breathing circuit to use for an individual patient undergoing an anesthetic event for a particular procedure.
Here are the advantages and disadvantages of each circuit:
Non-rebreathing Circuits (Bain Coaxial and Jackson Rees)
Advantages:
They are lightweight and simple to use.
They are inexpensive to purchase and maintain. They do not require any long-term maintenance or changing of parts. However, they are not meant to last forever. The inner tube of the Bain coaxial must be intact to properly operate so close inspection of the inner tube prior to every use is imperative. The slide valve on the Jackson Rees circuit acts as the APL valve (pop-off valve). This valve can stop working and therefore should always be tested prior to use to ensure it can properly release the pressure in the circuit.
Resistance to breathing is minimal which means that it is easier for small patients to spontaneously breathe. This is the main reason why NRB circuits became so popular for small patients. If the patient is manually ventilated or placed on a ventilator then this is a moot point even in the smallest 1kg patient.
Allows for a rapid change in anesthetic depth when the vaporizer dial is changed due to the high fresh gas flow rate required for proper function combined with the overall low circuit volume.
Disadvantages:
There is no mechanism to extract CO2 from the exhaled gases and therefore requires higher oxygen flow rates to flush the exhaled CO2 from the circuit before the patient takes another breath. Utilizing too low of an oxygen flow rate will result in excessive rebreathing of CO2.
The high oxygen flow rate is going to use more oxygen and inhalant so while the circuit is inexpensive to purchase and maintain it becomes very expensive to use this circuit.
The high fresh gas flow rates mean there is minimal conservation of heat and moisture, which will contribute to hypothermia and drying of the respiratory tract.
Because the circuit is using more inhalant that will ultimately contribute to more waste anesthetic gas (WAG) being released into the atmosphere which is not good from an environmental standpoint. It could also mean that there is increased operating room pollution if the scavenging system is ineffective.
The low volume of the circuit and inability to expand combined with the high O2 flow rates means that a patient is more at risk for barotrauma should the APL valve accidentally be left closed.
Rebreathing Circuits (Dual Wye and Universal F)
Advantages:
The parts of a rebreathing circuit are designed so that gases only move in one direction allowing it to be very efficient.
These circuits utilize lower oxygen flow rates to properly operate. There are several different oxygen flow rates used for these circuits. The lower the oxygen flow rate the more the gases (oxygen and inhalant) are recirculated.
The circuit utilizes CO2 absorbent which extracts CO2 from the exhaled gases. This is what allows the lower oxygen flow rates to be utilized. The lower the oxygen flow the more the circuit depends on the CO2 absorbent to extract the CO2 before the gases are rebreathed by the patient.
The lower oxygen flow rates mean that the circuit conserves heat, moisture oxygen and inhalant making it very economical to operate. It promotes conservation of heat within the circuit helping to minimize heat loss.
They are more environmentally friendly because they produce less WAG.
The larger circuit volume means that there is a buffer against barotrauma should the APL valve be left closed for a few seconds.
Disadvantages:
The CO2 absorbent and one-way valves add resistance to breathing which is a concern for small patients when spontaneously breathing, especially during long anesthetic episodes. If manual or mechanical ventilation is used, then resistance to breathing becomes a moot point.
The larger circuit volume means that it takes longer to establish equilibrium within the circuit and therefore the patient’s brain once the vaporizer is turned on. Any time the circuit becomes disconnected (e.g. moving patient) or a change is made on the vaporizer dial setting then equilibrium needs to be re-established. This is why oxygen flow rates are higher immediately after induction and turning on the vaporizer. Once equilibrium has been established, the oxygen flow rate can be turned down to the maintenance rates.
The CO2 absorbent must be changed regularly as it becomes exhausted so there is an added cost to maintain and operate these circuits.
The added parts of the circuit means that there are increased places for leaks to develop such as the seals around the CO2 absorbent canister and one-way valves and the connections for the breathing hoses.
Limitations to consider for a rebreathing circuit:
A 2.3 kg cat can be placed on a pediatric dual wye rebreathing circuit but there are some considerations for proper use. First, most vaporizers require a minimum oxygen flow rate of 250-500 mL/min to ensure proper and accurate inhalant output. If I calculate this patient’s maintenance oxygen flow rate at 44 mL/kg/min that equals 101 mL/min or 0.1 L/min. If I turn the flow meter down to 0.1 L/min then the concentration of inhalant going into the circuit may or may not be the percentage that is indicated on the vaporizer dial. The only way to know for sure is to use a gas analyzer and measure the inspired and expired inhalant concentration. This is an expensive piece of equipment that is not commonly found in most general practices. Therefore, it is better to not go below the minimum recommended oxygen flow rate. I use 500 mL/min (0.5L/min) as the low end but some of the newer vaporizers can go as low as 250 mL/min (0.25L/min). You must read your vaporizer manual to figure out the low end of the calibration range for oxygen flow rate. Regardless of what is calculated, it is important to not go below this minimum value.
The second consideration for using a rebreathing circuit on a small patient (e.g., 2.3 kg cat) is their breathing. Because of the added resistance, they can get fatigued easily if spontaneously breathing. Using a balanced anesthesia drug protocol where the inhalant is kept low will help the patient be able to properly maintain ventilation parameters. However, I would not feel comfortable using a rebreathing circuit on a small patient without the ability to monitor ETCO2. I cannot tell you when a patient will be able to properly ventilate and when they will not. You need to monitor ETCO2 so that you know when to step in and support ventilation.
As an aside, a small patient on a NRB circuit can also not properly ventilate on their own especially if they are too deep. The circuit alone is not enough to ensure proper ventilation. Monitoring ETCO2 is imperative for a small patient on a NRB circuit as much as it is with a rebreathing circuit!
The third consideration is additional mechanical dead space. All the breathing circuits (e.g. rebreathing and NRB) will contribute to mechanical dead space where the circuit attaches to the endotracheal tube. The pediatric universal F circuit contributes more mechanical dead space than an adult universal F circuit so there is no reason to use a pediatric universal F circuit. I prefer to use a pediatric dual wye on smaller patients.
Limitations to consider for a non-rebreathing circuit:
Can a 20 kg dog be placed on a Jackson Rees non-rebreathing circuit?
It depends!
A Jackson Rees NRB circuit requires a minimum oxygen flow rate of 300 mL/kg/min. For this 20 kg patient that means they require 6000 mL/min or 6 L/min for the oxygen flow rate.
Most small animal flow meters only go up to 4 L/min, but the large animal flow meters go to 10 L/min. If you only had a flow meter that went to 4 L/min then you could not properly utilize the Jackson Rees NRB circuit on this 20 kg patient. Using too low of O2 flow rate will greatly contribute to rebreathing of CO2. You would not know there was rebreathing of CO2 unless you were monitoring it with a capnograph!
If you had a flow meter that went to 10 L/min then you could utilize the Jackson Rees circuit on this 20 kg patient. But then the question remains.....why would you do this in lieu of a rebreathing circuit? Having the oxygen flow rate at 6 L/min is going to be extremely wasteful for both oxygen and inhalant so not only is it more expensive to use but you are also contributing more to WAG which is not good for the environment.
There is no advantage to using a NRB circuit on a larger patient unless semantics is playing a role. I used to work at a facility where our only option for a breathing circuit in the MRI was a Bain Coaxial NRB circuit. In this instance, we had a large animal flow meter mounted to our anesthesia machine. The oxygen flow rate for a Bain Coaxial NRB circuit is 200 mL/kg/min which meant the largest size patient we could safely anesthetize in the MRI was 50 kg if we were going to use the small animal anesthesia machine.
Bottom line.....weight is NOT the sole variable that should be used to determine the breathing circuit! It does not matter which circuit you use on a patient if you understand the advantages and disadvantages of the circuit you are using and follow the requirements for proper operation.
