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

Fresh Gas Inlet

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).

Unidirectional Valves

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.

The unidirectional valves are found at the start of the inspiratory and end of the expiratory limbs (Figure 7-3). The inspiratory valve allows flow toward the patient only with no flow back toward the machine. The expiratory valve allows flow away from the patient only with no flow back toward the patient. The valves are somewhat mirror images of the other. We will discuss how the inspiratory valve works first.

The valve consists of a flat plastic disk about the size of a 50 cent piece but thinner. On almost all machines, it is seated horizontally on a round, thin-walled valve seat. It is held in place by a spider-like looking cage over the top of the disk, and the whole assembly is under a clear plastic dome that is visible from the operator’s position. Flow going through the valve lifts the disk off its seat, allowing flow traveling in one direction. The instant that the flow is reversed (e.g., exhalation), the disk is pushed back on its round valve seat, thereby blocking flow.

The expiratory unidirectional valve works similarly. Expiratory flow is directed under the disk to lift it off its valve seat. The instant that reverse flow is detected, the disk is pushed back against its valve seat.

The valves are situated on the machine so that they can be clearly seen and inspected for proper function. They are also labeled either by name or by an arrow indicating flow travel. On some machines, such as the Datex-Ohmeda ADU, the unidirectional valves are mounted vertically and not horizontally. Their function is the same. Things can go wrong with unidirectional valves, and we will discuss that later in the chapter.

Inspiratory and Expiratory Ports

These are where the circle tubing connects to the machine. The ports are found immediately downstream of their respective unidirectional valves. The size of the ports is 22 mm. The ports stick out about 1.5 inches or so perpendicularly from their mounting to allow the circle limbs to slip on tightly. The ports are usually made out of metal. But sometimes they are made out of plastic (like the Draeger Apollo), are angled, and have a collar around them that can be loosened to direct the angled port in a different way. (These collars can be the site for leaks; more on this later.)

Circuit Tubing

Circuit tubing is made of plastic and is corrugated. The corrugations are there to decrease kinking and to allow the plastic tubing to bend easily. The tubing comes in adult and pediatric sizes. It is white in color compared with the blue color of stiffer critical care ventilator circuits. Because anesthesia circuit tubing is softer and more pliable than critical care ventilators, the actual tidal volume (TV) may not be what you think you dialed in initially. Gas is compressible, and the circuit tubing is expandable—you can often see it “wiggle” during a ventilatory cycle. This is more visible if the patient’s peak airway pressure is high.

On older machines, this would often change the delivered TV to a significant degree. However, modern anesthesia ventilators have more sophisticated volume sensors in a feedback loop to deal with this wasted ventilation.

Y Piece

The Y piece is where the inspiratory and expiratory limbs of the circuit meet, and form a common pathway to the circuit–patient interface (e.g., facemask, endotracheal tube, or supraglottic device). It is the only part of the circle circuit where there is dead space, meaning there is rebreathing of gas containing carbon dioxide.

Of course, we say the circle circuit is all about rebreathing, but the carbon dioxide is eliminated in the course of rebreathing. With the Y piece, there is a small amount of gas that has carbon dioxide in it that is rebreathed because the gas at the end of expiration doesn’t make it all the way out past the Y piece and down the expiratory limb. That small volume of carbon dioxide containing gas is still in the Y piece when inspiration begins. This is true for controlled or spontaneous ventilation.

Think about the volume of a Y piece. It is not very much when compared with an adult TV, but the dead space volume from the Y piece can be significant for neonates and can lead to hypercarbia in neonatal anesthesia. Y piece volumes can differ slightly depending on the shape and configuration of the circuits from different manufacturers, and there is a difference in volumes between adult and pediatric circuits.

The Y piece can also have adapters built into it to attach gas sampling lines and even temperature sensors for heated humidifiers.

Along with the Y piece, there is the “elbow,” a small right angle connector whose main function is to redirect the circuit at its takeoff from the facemask, endotracheal tube, or supraglottic device. It is made to accept a 22-mm female adapter (facemask) or 15-mm male adapter (endotracheal tube, supraglottic device). The Y piece itself has the same connector size ability. Y pieces and elbows are prime candidates for disconnection locations.

Adjustable Pressure-Limiting Valve

This valve, more commonly known as the adjustable pressure-limiting (APL) or “pop-off” valve, is located downstream of the expiratory unidirectional valve. Its function may be familiar to most clinicians, but how it works is not well understood by most of us. Exactly what is happening when we turn the APL knob?

The APL valve is the interface between the circuit and the scavenger system (Figure 7-4). It controls the pressure in the circuit during manual or spontaneous ventilation with a circle circuit. When the APL is completely open, excess volume from the circuit is vented into the scavenger. (Remember that excess volume is related to whatever your fresh gas flow (FGF) is; what goes in must come out of the machine, either through the scavenger or through leaks somewhere else in the circuit such as a poorly sealed facemask.)

As you turn the APL knob clockwise, the APL valve opening gets smaller, so less of your excess volume goes into the scavenger. That is why it helps to turn your APL knob closed whenever you have a difficult mask airway—more volume is available to you to try to squeeze into the patient’s lungs. Turning the knob counterclockwise opens the valve. Some valves have knobs that click as they are adjusted, and some have numbers corresponding to the airway pressure at a specific setting.

There is a spring-loaded valve inside the APL housing. It may be a disk type valve, or it may be a type that is more like a water faucet, opening a valve seat as you turn the stem. There will also be a unidirectional disc valve similar to the inspiratory and expiratory valves that will prevent backpressure and subsequent barotrauma from the scavenger.

The APL valve is not part of the in-line path of the circle circuit but is more like a branch or sideline device that allows us to “pop off” excess volume (and therefore pressure) when we do not need it based on our clinical experience. It is bypassed totally when the ventilator is turned on. Excess gas from the ventilator is vented into the scavenger by another pathway.

As mentioned, the APL valve is located downstream of the expiratory unidirectional valve and is close to the reservoir bag. It could be placed in several different positions, but in this position, the excess gas is vented into the scavenger before it goes through the carbon dioxide absorber, saving absorbent because we aren’t wasting our absorbing capacity on gas we are going to discard.

The location on the machine itself of the APL valve knob varies not only from manufacturer to manufacturer but also from model to model. In general, the knob is close to the reservoir bag because it ergonomically makes sense to have knob and bag close because the right hand is used for both. Some knobs are mounted horizontally, and some are mounted vertically.

Reservoir Bag

This is one of the most familiar components of an anesthesia machine, and even laypeople have seen a reservoir bag moving in and out on TV shows. It seems so simple; what else do we need to know about it except it is made of rubber and we squeeze it?

The bag allows us to manually ventilate the patient. It also allows us to monitor the patient’s spontaneous ventilations. As the name suggests, the bag is also a reservoir for gas for the circuit. In addition, for some types of anesthesia ventilators, such as the Draeger piston drive machines, the reservoir bag is an integral part. FGF is diverted (decoupled) into the reservoir bag during inspiration. If you disconnected the bag on such a machine while using the ventilator, you would cause a big leak. For bellows-type ventilators, the reservoir bag plays no role; disconnecting the bag causes no leak or malfunction. We discuss this in more depth in Chapter 8 on anesthesia ventilators.

The bag connects to the machine by a swivel arm, either rigid or flexible like a gooseneck lamp. The bag connection is 22 mm female. This allows the bag to be attached to the anesthesia circuit to act like a lung to check the ventilator (but remember, taking off the bag will cause a leak on ventilators such as the Draeger Apollo).

The location of the bag in the circle is usually between the expiratory unidirectional valve and the carbon dioxide absorber. The bag’s function is closely associated with that of the APL valve, as discussed previously. However, on some machines, the reservoir bag is found in different sites along the circle.

Bags are made from both latex and nonlatex material and come in adult (3-L volume), pediatric (1-L volume), and in-between (2-L volume) sizes.

Selector Switch

This switch merely acts like a stopcock to change flow from the bag and APL to the ventilator. It is important to remember that when switched from bag to ventilator, the APL valve is bypassed. The switch can be mechanical or electronic; going from bag to ventilator automatically turns on the ventilator and vice versa. Remember that with more sophisticated anesthesia ventilators that have pressure support mode along with volume control and pressure control, the reservoir bag is not part of the pressure support circuit even though the patient may be breathing spontaneously. The authors have seen episodes when a spontaneously breathing patient emerges from anesthesia while still on pressure support and is extubated, then goes into laryngospasm or obstructs, and no positive-pressure ventilation is produced from squeezing the reservoir bag because the selector switch is still on ventilator/pressure support mode (Figure 7-5).

Related posts:

Electrical Systems Medical Gas Supply Systems Malignant Hyperthermia and the Anesthesia Machine Capnography and Gas Monitoring Fail-Safe Systems Bag Valve Mask and Mapleson Circuits

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