Inhalation anaesthesia has been considered the most common anaesthetic modality nowadays. Although it is not the focus of this post, it had spread out because of the autonomy in anaesthetic time and provides both fast changes of anaesthetic plans and recovery. But it is only possible if we have good anaesthetic machines for vaporising and suppling the anaesthetic to the patient. Another post about anaesthetic machine components can be check out in this link, ok?
Inhalation anaesthesia is only safe to use if is able to (a) provide anaesthetic to the patient, (b) O2 in suitable concentrations and (c) avoid CO2 rebreathing. To make it possible, rebreathing circuits (circle circuit system) have been used, which prevent CO2 rebreathing by soda lime and one-way valves. There is no doubt it is the best option, once we can save anaesthetic, O2 and also reduce air pollution, but it is not feasible for all animals. It is because these patients need to exert enough expiratory pressure to overcome the circuit mechanical resistance. By convention, the semiclosed circle circuit is safe to use for patients over 5-7 kg.
So, how can we anesthetise patients under 5kg, like quite small dogs, cats, birds and reptiles by inhalation anaesthesia? Weel, in these situations, we must use non-rebreathing circuits, which are generally lightweight and have low mechanical resistance.
Mapleson A, B, C….
The first non-rebreathing circuits became from the early 20th century. A dozen circuits were designed, between the 1920s and 1980s, with different designs, but all of them trying to “deliver the best cost-benefit ratio”. In general, everyone follows the idea of providing anaesthetic, O2 and avoiding CO2 rebreathing. Here comes the question... But, are they all the same or not?
This question was already asked back in the 1950s, by Dr William Mapleson. Even though people believe Dr Mapleson's expertise was in anaesthetic equipment development, all of this happened by chance. Actually, his studies focused on neuromuscular blockers, but he came across to non-rebreathing circuits by Dr William Mushin, a friend of him. Years later, he published a paper, which assesses a bunch of non-rebreathing circuits, classifying them by letters (A to E) (Figure 1). Funny situation... from an unpretentious study, Dr Mapleson became known worldwide. He used to say he became famous only "because he knew the letters of the alphabet".
Arranging non-rebreathing circuits
There are many possibilities for arranging the non-rebreathing circuits. The common characteristics of them have already been addressed in the video lesson Anaesthetic Circuits , from the web series “Anaesthesia Unravelled”. Now here we will talk about the most well know non-rebreathing circuits have been used in Veterinary Medicine, grouping them by “structural affinity”, without any intention to compare with Mapleson's classification. I just did it like this because I confess "Alphabet classification" make me confused...
Magill, Lack and Bain Circuits
The Magill Circuit (Ivan Magill, 1928) consists of a corrugated hose which has connected to a reservoir bag, at the end part, and an expiratory valve (pop-off valve), close to the patient. The fresh gas flow (FGF) comes far from the patient, next to the reservoir bag (Figure 2). We can see the CO2 scavenging system is done by the FGF, in the opposite direction of the expired gas, forcing it to go away through the pop-off valve. To prevent rebreathing, the FGF must be at least 1 x VM and the corrugated hose volume must exceed the VT of the patient.
The main advantage of the Magill circuit is that the expired gas is easily removed by the FGF, not allowing CO2 to remain in the hose and/or reservoir bag. But in controlled ventilation CO2 rebreathing occurs, being a disadvantage of it. Two more issues: the first one is the mechanical resistance at the expiratory moment, by the opposite direction of the FGF and the other, the tricky position of the pop-off valve.
The Lack Circuit (L.A. Lack, 1976) is basically the same as Magill's, but a design that makes easy to adapt a scavenging hose. The FGF continues to come from the end part of it, but the expired gas goes through a thin inner hose, which takes the gas away by the pop-off valve, also located far from the patient (Figure 3). Here we have the advantages and disadvantages seen on Magill circuit.
The Bain Circuit (Bain & Spoerel, 1972) was designed to offer more advantages than the previous ones. Contrary to the Lack circuit, the FGF comes from an inner tube to the patient and the expired gas, by an outer hose (Figure 4). This design allows the inspired gas to be humidified and warmed by the expired gas.
In theory, this idea would be excellent, but it is a bit contested, mainly because the inner tube is considered too narrow for providing sufficient FGF to the patient. So, some amount of the expired gas might be rebreathed. To solve this issue, a FGF more than 2 x VM must be delivered, which increases the costs the procedure, air pollution and mechanical resistance as well. Maybe the humidification and warming should not be effective in high FGF situations.
Ayre's T-piece and Jackson Rees Circuits
Both circuits have an interesting design, and they do not promote mechanical resistance on expiration, because the FGF does not come from the opposite direction to the expired gas, like the others described.
The Ayre's T-piece (Thomas Ayre, 1937) is a quite simple circuit. It is made by a T-piece and a small hose, with no reservoir bag or pop-off valve. The FGF is delivered close to the patient, but in a side direction, exerting low mechanical resistance, and being useful for tiny animals (up to 2 kg) (Figure 5).
We can see this circuit has no reservoir bag. Some patients can breathe ambient air from the end of the hose, diluting the anaesthetic and O2 as well. To avoid this, the FGF must exceed 3 x VM of the patient. It is noteworthy that it is not very efficient to apply controlled ventilation.
A kind of Ayre's T-piece evolution is the Jackson Rees Circuit (Gordon Jackson Rees, 1950), which has a reservoir bag at the end of the hose (Figure 6). Firstly it was designed with an open-ended reservoir bag, but the current models have the pop-off valve between the hose and the bag, far from the patient. This format reduced any possible “contamination” of the FGF by the ambient air and made it possible controlled ventilation manoeuvres. The FGF must be high to scavenge the expired CO2 from the circuit.
Anis Baraka (1969) added another T-piece on the Jackson Rees circuit (Figure 7). Here in Brazil this circuit is known as Baraka Circuit, but in other countries it is well known as Ayre's double T-piece or Jackson Rees II. It is a hybrid because it can works like Magill circuit, when the FGF is provided from the distal T-piece (Figure 7a), or like Jackson Rees, when the FGF comes from the proximal T-piece (Figure 7b). It has no pop-off valve and, in both situations, the expired gas goes away through the other T-piece. This circuit has the advantages and disadvantages already highlighted for those similar circuits, according to the FGF location.
F Circuit (Mera circuit)
The F circuit (Atsuo Fukunaga, 1978) was designed to be a hybrid, being used as a non-rebreathing or rebreathing circuit, according to the convenience. It is basically a Bain circuit, with a flexible hose at the end of it, which could be expanded or contracted. Thus, this hose woks like scavenger (non-rebreathing design) or like an expiratory limb, connected to the expiratory valve (Figure 8).
Besides the advantage it is a hybrid, it is also lightweight and easy to hand, compared to the circle circuit system, once it is a coaxial system. As a disadvantage, the FGF must be higher than for the circle circuit, to avoid CO2 rebreathing.
So many information about them can make us crazy, can't we? I understand you... don't worry about. In children, for instance, it is a bit easier to apply these concepts because of their weight difference, and respiratory strength as well, is not so range than the vet patients.
Basically, we must avoid CO2 rebreathing. So, we must take account of the hose size, prioritizing those with volume higher than the patient's VT, and the FGF as well. In spontaneous ventilation, non-rebreathing circuits with FGF comes far from the patient (Magill and Lack) are good options, as they are more effective to scavenge expired CO2. In controlled ventilation, those which deliver FGF close to the patient (Jackson Rees and variants) should be used.
Two other important things to avoid rebreathing are (a) trying to prevent tachypnea, because the respiratory pause is essential to the CO2 scavenge, and the accurate monitoring of CO2, by a good capnograph. The best one now? it's up to you!
Bain JA, Spoerel WE. A Streamlined anaesthetic system. Can Anaesth Soc J. 19: 426-435, 1972.
Baraka A et al. Rebreathing in a double T-piece system. Brit J Anaesth. 41:47-53, 1969.
Clutton E. The right anaesthetic breathing system for you? In Practice. 17:229-231, 1995.
Fukunaga A. The F breathing circuit, a universal single-limb breathing circuit. J Anesth. 33:317-320, 2019.
Mapleson WW. The elimination of rebreathing in various semi-closed anaesthetic systems. Brit J Anaesth. 26:323-332, 1954.
McIntyre JWR. Anaesthesia breathing circuits. Can Anaesth Soc. 33:98-105, 1986.
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