Traditional Circle Breathing 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. A newer version of the traditional circle system, referred to as a universal F or single-limb circuit, is growing in popularity. Although these systems appear different externally, they have the same overall functional layout as the traditional circle system and the following discussion is also applicable to them.

The circle system prevents rebreathing of carbon dioxide by use of carbon dioxide absorbents 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.^[105] 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 consumed by the patient. In a closed system, there is complete rebreathing of exhaled gases after absorption of carbon dioxide, and the overflow (pop-off) valve or relief valve of the ventilator remain closed.

The circle system ( Fig. 9-22 ) 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 carbon dioxide 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.

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 carbon dioxide absorber, and the site of fresh gas entry. However, to prevent rebreathing of carbon dioxide in a traditional circle system, three rules must be followed:

1. Unidirectional valves must be located between the patient and the reservoir bag on the inspiratory and expiratory limbs of the circuit.
2. The fresh gas inflow cannot enter the circuit between the expiratory valve and the patient.
3. The overflow (pop-off) valve cannot be located between the patient and the inspiratory valve.

Figure 9-22 Components of the circle system. APL, adjustable pressure limiting; B, reservoir bag; V, ventilator.

The most efficient arrangement of the circle system 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, the one used on most conventional anesthesia machines (see Fig. 9-22 ), is somewhat less efficient because it allows alveolar and dead space gas to mix before venting. ^{[106] [107]}

The advantages of the circle system include a relative stability of inspired gas concentrations, conservation of respiratory moisture and heat, and prevention of operating room pollution. It also 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 different connections. Multiple connection sites set the stage for misconnections, disconnections, obstructions, and leaks. In a closed claim analysis of adverse anesthetic outcomes arising from gas delivery equipment, one third (25 of 72) of malpractice claims resulted from misconnections or disconnections of the breathing circuit.^[2] Malfunctioning valves can cause serious problems. Rebreathing can occur if the valves stick in the open position, and total occlusion of the circuit can occur if they are stuck closed. If the expiratory valve is stuck shut, breath stacking and tension pneumothorax can occur. Obstructed filters located in the expiratory limb of the circle breathing system have caused increased airway pressures, hemodynamic collapse, and bilateral tension pneumothorax. Obstruction of the expiratory filter may occur because of defective filters, patients' secretions, or albuterol nebulization. ^{[108] [109] [110]}

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