VENTILATIECIRCUIT – HOWEQUIPMENTWORKS
A “breathing system” is an arrangement of tubes and other components that transports
gases between the anaesthetic machine and the patient. A very common breathing system
used in anaesthesia is the “circle breathing system” and I will introduce its functioning to
you. This breathing system has many advantages which we will discuss later. To make
things easier, I will from now on, shorten “the circle breathing system” to “circle system“.
I think a good way to understand how the circle system works is to “construct” one, step
by step. Let us begin “making” our circle system by drawing a circular tube. The gases inside
the circle system go around in a circular way.
Let us start adding parts to our circle, in a step by step way. The first part I would like to
add to the circle is a patient! However, though we have connected the patient to the circle,
he will unfortunately not be able to breath in or out from it. This is because the circular
tube is made of a non- stretchable material and therefore it cannot expand to accept the
patient’s expiration, and nor can it contract when the patient tries to inspire from it.
To allow the patient to breath in and out, we attach a flexible bag (called reservoir bag) to
the circle system. Now the patient can breath, through the tubes, into and out of the
flexible reservoir bag.
,However, if we leave our patient like this, he will not survive, since we are forgetting to
give him something vital for life. We need to urgently give our patient oxygen! The oxygen
(and other gases) come out of the flow meters of your anaesthetic machine. The flow
meters allow you to control the flow of the various gases that you supply to your patient.
The total flow of gases coming out of the flow meters is called ”total fresh gas flow” or
more commonly , simply referred to as, “fresh gas flow”.
So, to keep our patient alive, we supply fresh gas flow (containing oxygen, shown as blue
dots) from the flow meters into the circle system.
Unfortunately there still is a problem. Though we are giving oxygen into the circle, it is not
reaching the patient! The reason for this is that he is breathing in his own expired air
(shown in grey) which of course does not have much oxygen. This breathing of his own
oxygen depleted air will make him hypoxic!
As I will explain to you, the solution is to “force” the patient to inspire from one section of
the circle and to expire into a different section of the circle. I will explain later how we
“force” the patient to do this. In the diagram, we ”force” the patient to inspire from the
circle system tubing labelled as ”i“.
2
,We then “force” our patient to expire to a different part of the circle system (the part
labelled “e“). I.e. the inspiratory pathway and the expiratory pathway are separate.
In this way, the patient will inspire oxygen rich fresh gas rather than the gases he just
expired. Nextly, I will explain how we will “force” the patient to breath the way we just
discussed.
We ”force” the patient to inspire from one part of the circle, and expire into the other part
of the circle, using what are called “one way valves“. As their name suggests, these valves
allow gas to pass one way, and not the other way. The valve has a disc that opens only in
one direction, allowing gases to only go in that direction. In the example, the one way valve
is designed to allow flow in the direction of the green arrow and not allow flow to go in
the opposite direction. The top of the valve enclosure is usually transparent so that you
can observe the disc moving and confirm that it is working properly. The valve will usually
have a “arrow ” marking, showing the direction of flow it allows.
3
, We add two one way valves into the circle system as shown. One allows flow only towards
the patient and the other allows flow only away from the patient.
During inspiration, the valve labelled ”expiratory one way valve” closes, preventing the
patient from inspiring the gases he just breathed out. On the other side, the valve labelled
”inspiratory one way valve” opens, letting the patient inspire gases rich with oxygen. The
tubing from the inspiratory one way valve to the patient carries only inspiratory gases, and
we can therefore call it the “inspiratory tubing“.
During expiration, the reverse happens. The inspiratory one way valve closes, preventing
the expired gases going into the inspiratory tubing. Instead, the valve labelled “expiratory
one way valve” opens, letting expired gases go via the tubing between it and the patient.
The tubing between the patient and the expiratory one way valve carries only expired
gases, so we can therefore call it the “expiratory tubing“.
4
A “breathing system” is an arrangement of tubes and other components that transports
gases between the anaesthetic machine and the patient. A very common breathing system
used in anaesthesia is the “circle breathing system” and I will introduce its functioning to
you. This breathing system has many advantages which we will discuss later. To make
things easier, I will from now on, shorten “the circle breathing system” to “circle system“.
I think a good way to understand how the circle system works is to “construct” one, step
by step. Let us begin “making” our circle system by drawing a circular tube. The gases inside
the circle system go around in a circular way.
Let us start adding parts to our circle, in a step by step way. The first part I would like to
add to the circle is a patient! However, though we have connected the patient to the circle,
he will unfortunately not be able to breath in or out from it. This is because the circular
tube is made of a non- stretchable material and therefore it cannot expand to accept the
patient’s expiration, and nor can it contract when the patient tries to inspire from it.
To allow the patient to breath in and out, we attach a flexible bag (called reservoir bag) to
the circle system. Now the patient can breath, through the tubes, into and out of the
flexible reservoir bag.
,However, if we leave our patient like this, he will not survive, since we are forgetting to
give him something vital for life. We need to urgently give our patient oxygen! The oxygen
(and other gases) come out of the flow meters of your anaesthetic machine. The flow
meters allow you to control the flow of the various gases that you supply to your patient.
The total flow of gases coming out of the flow meters is called ”total fresh gas flow” or
more commonly , simply referred to as, “fresh gas flow”.
So, to keep our patient alive, we supply fresh gas flow (containing oxygen, shown as blue
dots) from the flow meters into the circle system.
Unfortunately there still is a problem. Though we are giving oxygen into the circle, it is not
reaching the patient! The reason for this is that he is breathing in his own expired air
(shown in grey) which of course does not have much oxygen. This breathing of his own
oxygen depleted air will make him hypoxic!
As I will explain to you, the solution is to “force” the patient to inspire from one section of
the circle and to expire into a different section of the circle. I will explain later how we
“force” the patient to do this. In the diagram, we ”force” the patient to inspire from the
circle system tubing labelled as ”i“.
2
,We then “force” our patient to expire to a different part of the circle system (the part
labelled “e“). I.e. the inspiratory pathway and the expiratory pathway are separate.
In this way, the patient will inspire oxygen rich fresh gas rather than the gases he just
expired. Nextly, I will explain how we will “force” the patient to breath the way we just
discussed.
We ”force” the patient to inspire from one part of the circle, and expire into the other part
of the circle, using what are called “one way valves“. As their name suggests, these valves
allow gas to pass one way, and not the other way. The valve has a disc that opens only in
one direction, allowing gases to only go in that direction. In the example, the one way valve
is designed to allow flow in the direction of the green arrow and not allow flow to go in
the opposite direction. The top of the valve enclosure is usually transparent so that you
can observe the disc moving and confirm that it is working properly. The valve will usually
have a “arrow ” marking, showing the direction of flow it allows.
3
, We add two one way valves into the circle system as shown. One allows flow only towards
the patient and the other allows flow only away from the patient.
During inspiration, the valve labelled ”expiratory one way valve” closes, preventing the
patient from inspiring the gases he just breathed out. On the other side, the valve labelled
”inspiratory one way valve” opens, letting the patient inspire gases rich with oxygen. The
tubing from the inspiratory one way valve to the patient carries only inspiratory gases, and
we can therefore call it the “inspiratory tubing“.
During expiration, the reverse happens. The inspiratory one way valve closes, preventing
the expired gases going into the inspiratory tubing. Instead, the valve labelled “expiratory
one way valve” opens, letting expired gases go via the tubing between it and the patient.
The tubing between the patient and the expiratory one way valve carries only expired
gases, so we can therefore call it the “expiratory tubing“.
4
