1 What are the different types of anesthesia breathing circuits?

Breathing circuits are usually classified as open, semiopen, semiclosed, or closed. They include various components configured to allow the patient to breathe (or be ventilated) with a gas mixture that differs from room air.

2 Give an example of an open circuit

An open circuit is the method by which the first true anesthetics were given 160 years ago. A bit of cloth saturated with ether or chloroform was held over the patient’s face. The patient breathed the vapors and became anesthetized. The depth of anesthesia was controlled by the amount of liquid anesthetic on the cloth; thus it took a great deal of trial and error to become good at the technique.

3 Give an example of a semiopen circuit

The various semiopen circuits were fully described by Mapleson and are commonly known as the Mapleson A, B, C, D, E, and F circuits (Figure 19-1). All have in common a source of fresh gas, corrugated tubing (more resistant to kinking), and a pop-off or adjustable pressure-limiting valve. Differences among the circuits include the locations of the pop-off valve, fresh gas input, and whether or not a gas reservoir bag is present. Advantages of the Mapleson series are simplicity of design, ability to change the depth of anesthesia rapidly, portability, and lack of rebreathing of exhaled gases (provided the fresh gas flow is adequate). Disadvantages include lack of conservation of heat and moisture, limited ability to scavenge waste gases, and high requirements for fresh gas flow. These are rarely used today except for patient transport.

Figure 19-1 Mapleson A, B, C, D, E, and F circuits. FGF, Fresh gas flow.

(From Willis BA, Pender JW, Mapleson WW: Rebreathing in a T-piece: Volunteer and theoretical studies of the Jackson-Rees modification of Ayre’s T-piece during spontaneous respiration, Br J Anaesth 47:1239–1246, 1975, with permission.)

4 Give an example of a semiclosed circuit

The prototypical semiclosed circuit is the circle system, which is found in most operating rooms in the United States (Figure 19-2). Every semiclosed system contains an inspiratory limb, expiratory limb, unidirectional valves, carbon dioxide (CO₂) absorber, gas reservoir bag, and a pop-off valve on the expiratory limb. Advantages of a circle system include conservation of heat and moisture, the ability to use low flows of fresh gas (thereby conserving volatile anesthetic and the ozone layer), and the ability to scavenge waste gases. A disadvantage is its complex design; it has approximately 10 connections, each of which has the potential for disconnection.

Figure 19-2 Circle system.

(From Andrews JJ: Inhaled anesthetic delivery system. In Miller RD, editor: Anesthesia, ed 4, New York, 1994, Churchill Livingstone, pp 185–228.)

5 Give an example of a closed circuit

Like the semiclosed circuit, the closed circuit is a circle system adjusted so the inflow of fresh gas just matches the patient’s oxygen (O₂) consumption and anesthetic agent uptake. The CO₂ is eliminated by the absorber.

6 Rank the Mapleson circuits in order of efficiency for controlled and spontaneous ventilation

Controlled: D > B > C > A (Pneumonic: Dog Bites Can Ache)

Spontaneous: A > D > C > B (Pneumonic: All Dogs Can Bite)

7 What circuit is most commonly used in anesthesia delivery systems today?

Almost every anesthesia manufacturer supplies a circle system with its equipment. When compared with other available circuits, the circle system provides the most advantages.

8 How is a breathing circuit disconnection detected during delivery of an anesthetic?

Breath sounds are no longer detected with an esophageal or precordial stethoscope, and, if the parameters are properly set, the airway pressure monitor and tidal volume–minute volume monitor alarm will sound. The capnograph no longer detects CO₂, and soon thereafter the O₂ saturation begins to decline. Exhaled CO₂ is probably the best monitor to detect disconnections; a decrease or absence of CO₂ is sensitive although not specific for disconnection.