Estimated reading time: 8 minutes
So this is one of the very basic topics which each one of you would get either in a viva or in the basic science of anesthesiology as a long note or to describe any of the circuits which I’m going to discuss today and on the 16th for a short note as well. In viva you can be asked about the functional analysis of every circuit that you are reading and the functions of each part or what are the specifications for each part of these anesthesia circuits can be asked in any viva question which you are going to face and it will be a lifelong lesson for every one of you to know about these anesthesia circuits.
So basically what is the anesthesia circuit it is an assembly of components which connects the patient’s airway to the anesthesia machine and it is from this the artificial atmosphere and into which the patient will breathe throughout the anesthesia that you are administering. So this breathing circuits you have to know the history of how these breathing circuits evolved before we jump into the actual breathing systems.
So somewhere it will be given as 1917 when Sir Ivan Magill devised the Magill circuit which is now present as Mapleson A circuit and somewhere in anesthesia museum, it will be given as in 1928 Sir Ivan Magill devised the Mapleson A circuit.
And in 1927 the Ralph M Walters, and Ralph Milton Walters devised this to-and-fro system of carbon dioxide canister. This has become very popular now because this canister can be autoclaved and it has a carbon dioxide absorber as well. So those patients who have got a respiratory infection such as tuberculosis they were using this machine in particular to autoclave it and then use it so that they can prevent the infection which can occur from patient to patient if they are using the anesthesia circuits.
And in 1937 Philips Ayer was the one who devised these specifications for pediatric systems as the ISTPs. Before then there were no gross circuits which were available for pediatric population. There was Mapleson A, B and C but there was no pediatric population could be served through the circuits because it would drag the endotracheal tube and lots of effort for spontaneous ventilation as well as controlled ventilation was there with all the Mapleson systems which were available back then.
And if you have to read about Ayre he is a very peculiar person who has got a cleft lip cleft palate himself and I think he got operated also. He is a very great person and he has suffered so much of personal losses yet he beautifully made these ISTPs to help people to help the pediatric population who would get anesthetized. And the revolutionary discovery was by Brain Sword with whom he devised a closed system closed circuit and this closed circuit is being modified and used nowadays with the carbon dioxide observer and this person is the person behind it.
One should not forget about the Mapleson system which was devised in 1954 William Mapleson gave actually five systems A, B, C, D and E. E’s modification became Jacksonry’s modified circuit E to make it as Jacksonry’s circuit which is F circuit. This was a milestone article which was published in the British Journal of Anesthesia. If anyone would be interested to know about how Mapleson classified all these systems we can go through this landmark article in BJA and this is another landmark article which was given by Dr. Bain and Sporell who they both devised the Bain circuit which we are using now which is again a modification of Mapleson D circuit and this circuit was long enough and this enabled the scavenging system for actively scavenging the anesthesia gases which are there in the which we are using day in and day out and William Sporell as well as Bain showed that as you can see in this picture they showed that scavenging also is possible through the suction system and they also anesthetized patients with cleft lip and cleft palate with Bain circuit and proved that without effective rebreeding patients can be anesthetized using Bain circuit which is again a modification of Mapleson D. And based on these anesthesia circuits these anesthesia circuits have a requirement which they should meet before using it up to a particular patient.
There are essential requirements and desirable requirements. The essential requirement is the one which has to be met for anaesthetizing the patient. So the primary requirement will be it should deliver the anesthetic gases as well as the fresh gas flow at the same concentration in the shortest time possible to the patient and these circuits should eliminate the carbon dioxide that the patient is producing as and when possible without a possibility of inhaling the carbon dioxide which the patient is expiring and the apparatus should have a minimal dead space apparatus dead space as possible and these systems should have a very low resistance.
So these are the essential requirements for the anesthesia breathing circuits and the desirable requirements are it should actually consume less fresh gas flow it should have conservation of heat and humidification of inspired air is one desired quality of breathing circuit and should be lightweight and inexpensive and it should be convenient for your usage like you should not hang hold the mask have a tight-fitting mask and the circuit should not drag you down or drag the endotracheal duct down because of its weight.
So it should be convenient during usage whether it is spontaneous or controlled it should be efficient for both and it can be it should be useful for both adults and paediatrics there should be an effective scavenging system like as you can attach through veins a suction cannula or a suction apparatus at the APL well to vent the anesthetic gases so that will minimize the data pollution and you should prevent the patient from barotrauma and as I said it should be inexpensive and these breathing circuits are classified as two ways one is drips and another is the convoy and the convoy is a modified convoy so drips modified I mean devised these breathing systems based on the volume of the reservoir and the amount of rebreathing that is allowed within the circuit. So this system is not used nowadays because of the obvious disadvantages where he classified the systems into open semi-open, semi-closed and closed so in the open system there is no reservoir and no rebreathing in a semi-open there is a good reservoir but there is no rebreathing and in an an semi-closed there is a good reservoir and a partial rebreathing and in closed there is a reservoir and there is a complete rebreathing.
So what are the examples of these are drips gave it as the open method is open circuit is the one which is the ether by ether drop method which is a Schimelbusch method but do we really see a circuit in Schimmelbusch mask no it is just the mask which is held and a cloth which is put and then the ether is given as an open drop method there is no circuit which is involved in a Schimelbusch mask so this is again the first thing itself became an absolute one. He classified say Mapleson A to E as semi-open but some of them came with the notion that how can it be a open system when the APL valve is partially closed and you are allowing a partial rebreathing so and that again led to a controversy and there is another system called as a semi-closed system so what drips did was he said partial open APL valve in a closed circuit or in a complete circle system is a semi-open semi-closed circuit and a completely closed APL valve belongs to a closed circuit is what drips told so the obvious disadvantages are the Mapleson system itself can act as a semi-closed one as well as semi-open one and there is no clear cut data which says this system belongs to this and there is no circuit which is involved in the open drop method so then came the modified convoy there are so many methods of classifying so these two will be enough for us to know and modified convoy is the one where he classified the breathing systems into two without carbon dioxide absorption and with carbon dioxide absorption so based on the flow if there are any flow control valves or unidirectional flow is aided by a valve whether it be a Ruben valve or an unidirectional flow valve so if there is an unidirectional flow valve then it is an unidirectional system if there are no valves and the fresh gas flow as well as is flowing bidirectionally then it becomes a bidirectional flow so with carbon dioxide absorption the unidirectional system is the circle system and the bidirectional flow system is the waters to and fro canister and in breathing systems without carbon dioxide absorption the unidirectional flow is the non-repeating system and in bidirectional flow we have got the maplesons a b c lag system and the miller circuit and the humphreys ade system so this is the bidirectional flow.
To know more about it: Subscribe to the Conceptual Anesthesia
