Section 2: Scientific Principles
Part A: Inhaled Anesthetics
Chapter 7: Inhaled Anesthetic Delivery Systems

Circle System

The circle system is the most popular breathing system in the United States. It is so named because its components are arranged in a circular manner. This system prevents rebreathing of CO2 by soda lime absorption but allows partial rebreathing of other exhaled gases. The extent of rebreathing of the other exhaled gases depends on component arrangement and the fresh gas flow rate.

A circle system can be semiopen, semiclosed, or closed, depending on the amount of fresh gas inflow. 85  A semiopen system has no rebreathing and requires a very high flow of fresh gas. A semiclosed system is associated with rebreathing of gases and is the most commonly used system in the United States. A closed system is one in which the inflow gas exactly matches that being taken up, or consumed, by the patient. There is complete rebreathing of exhaled gases after absorption of CO2, and the overflow (pop-off) valve is closed.

The circle system (Fig. 7–21) consists of seven components: (1) a fresh gas inflow source; (2) inspiratory and expiratory unidirectional valves; (3) inspiratory and expiratory corrugated tubes; (4) a Y-piece connector; (5) an overflow or pop-off valve, referred to as the APL valve; (6) a reservoir bag; and (7) a canister containing a CO2 absorbent. The unidirectional valves are placed in the system to ensure unidirectional flow through the corrugated hoses. The fresh gas inflow enters the circle by a connection from the common gas outlet of the anesthesia machine.

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FIGURE 7–21 Components of the circle system. B, reservoir bag; V, ventilator; APL, adjustable pressure limiting.

Numerous variations of the circle arrangement are possible, depending on the relative positions of the unidirectional valves, the pop-off valve, the reservoir bag, the CO2 absorber, and the site of fresh gas entry. However, to prevent rebreathing of CO2, three rules must be followed: (1) a unidirectional valve must be located between the patient and the reservoir bag on both the inspiratory and the expiratory limbs of the circuit, (2) the fresh gas inflow cannot enter the circuit between the expiratory valve and the patient, and (3) the overflow (pop-off) valve cannot be located between the patient and the inspiratory valve. If these rules are followed, any arrangement of the other components prevents rebreathing of CO2 . 86 

The most efficient circle system arrangement that allows the highest conservation of fresh gases is one with the unidirectional valves near the patient and the pop-off valve just downstream from the expiratory valve. This arrangement conserves dead space gas and preferentially eliminates alveolar gas. A more practical arrangement, used on all contemporary anesthesia machines (see Fig. 7–21 ), is less efficient because it allows alveolar and dead space gas to mix before venting. 86, 87 

The advantages of the circle system include relative stability of inspired concentration, conservation of respiratory moisture and heat, and prevention of operating room pollution. Additionally, it can be used for closed-system anesthesia or with low oxygen flows. The major disadvantage of the circle system stems from its complex design. The circle system has approximately 10 connections, and the multiple connection sites set the stage for misconnections, disconnections, obstructions, and leaks. In a recent closed claim analysis of adverse anesthetic outcomes arising from gas delivery equipment, a third (25/72) of malpractice claims resulted from breathing circuit misconnections or disconnections. 2  Malfunctioning valves can cause serious problems. Rebreathing can occur if the valves stick in the open position. Total occlusion of the circuit can occur if they are stuck closed. Finally, the bulk of the circle offers less convenience and portability than the Mapleson systems.