The circle circuit is not a true circle in shape, of course. It is a circle in that it is a continuous loop that recycles gas and anesthetic agent from the patient. It is the end point of gas delivery to the patient. It all seems simple, but on second look, there are a lot of things that make the circle circuit possible. Some of them you may know about, but some you may not know about.
In this chapter, we will go from one end of the circle and back again, going all around the circuit to understand it. One part of the circle system, the carbon dioxide absorber, is discussed in its own chapter.
There are two important points that allow the circle circuit to work: one is that there is flow going only one way through it, and the other is that carbon dioxide is removed from the exhaled breath. The circle circuit has many parts, so we will discuss each one. We will start inside the machine and work our way around.
Parts of the Circle Circuit
We will begin the journey at the fresh gas outlet (Figure 7-1). The fresh gas inlet is where the gas from the pneumatic part of the machine enters the circuit. It is really an extension of the common gas outlet that we discussed in the chapter on the low-pressure pneumatic; kind of like a road that changes its name at an intersection, it is called the common gas outlet on one end and the fresh gas outlet on the other end. The fresh gas inlet can enter the circle at any place theoretically but almost always comes in upstream of the inspiratory unidirectional valve and downstream of the absorber (Figure 7-2).
Major parts of the circle circuit. APL, adjustable pressure-limiting. (Reproduced with permission from Morgan GE, Mikhail MS, Murray MJ. Clinical Anesthesiology. 4th ed. New York, NY: McGraw-Hill; 2006. Figure 3-1.)
Schematic of the common gas outlet at the end of pneumatic section of machine, becoming the fresh gas inlet at the start of the ventilatory section of the machine.
Remember we said there is one-way flow through the circle. It is the unidirectional valves that make this so. The unidirectional valves are another part of the anesthesia machine that were designed simplistically and not overengineered to do their job. They require no electricity either.