The Circle System

The Circle System

Is most popular anesthetic breathing circuit. The circle system essential components are arranged in a circular manner.

Components of the Circle System

It consists of (1) a fresh gas inlet, (2) inspiratory and expiratory unidirectional check valves, (3) inspiratory and expiratory corrugated tubing, (4) a Y-piece connector, (5) an adjustable pressure-limiting (APL) valve, also referred to as an overflow or pop-off valve, (6) a reservoir bag, (7) a canister containing carbon dioxide absorbent, (8) a bag/vent selector switch, and (9) a mechanical anesthesia ventilator. (Figure 3.4)

The inspiratory and expiratory valves are placed in the system to ensure gas flow through the corrugated hoses remains unidirectional. The fresh gas inflow enters the circle by a connection from the common gas outlet of the anesthesia machine. The circle system prevents rebreathing of carbon dioxide by chemical neutralization of carbon dioxide with carbon dioxide absorbents.

A circle system can be classified as semiopen, semiclosed, or closed, depending on the amount of fresh gas inflow (Table 3.1). In a semiopen system, very high fresh gas flow is used to eliminate rebreathing of gases. A semiclosed system is associated with rebreathing of gases and is the most commonly used breathing system. In a closed system, the inflow gas exactly matches that being consumed by the patient.

Rebreathing of exhaled gases in the semiclosed and closed circle systems results in:

  • Some conservation of airway moisture and body heat and
  • Decreased pollution of the surrounding atmosphere with anesthetic gases when the fresh gas inflow rate is set at less than the patient's minute ventilation.

Figure 3.4 Schematic diagram of the components of a circle absorption anesthetic breathing system

Rotation of the bag/vent selector switch permits substitution of an anesthesia machine ventilator (V) for the reservoir bag (B). The volume of the reservoir bag is determined by the fresh gas inflow and adjustment of the adjustable pressure-limiting (APL) valve.

Disadvantages of the circle system include:

  • Increased resistance to breathing because of the presence of unidirectional valves and carbon dioxide absorbent.
  • Bulkiness with loss of portability.
  • Enhanced opportunity for malfunction because of the complexity of the apparatus.

Fresh Gas Inlet and Unidirectional Valves

Fresh gas enters the circle system through a connection from the common gas outlet of the anesthesia machine. Two unidirectional valves are situated in different limbs of the corrugated tubing such that one functions for inhalation and the other for exhalation. These valves (1) permit positive-pressure breathing and (2) prevent the rebreathing of exhaled gases until they have passed through the carbon dioxide absorbent canister and have had their oxygen content replenished. If the unidirectional valves are functioning properly, the only dead space in the circle system is between the Y-piece and the patient.

Corrugated Tubing

The inspiratory and expiratory corrugated tubes serve as conduits for delivery of gases to and from the patient. Their large bore provides minimal resistance, and the corrugations provide flexibility, resist kinking

Y-Piece Connector

A Y-piece connector at the patient end of the circuit has (1) a curved elbow, (2) an outer diameter of 22 mm to fit inside a facemask, and (3) an inner diameter of 15 mm to fit in to an endotracheal tube connector.

Adjustable Pressure-Limiting Valve

When the bag/vent selector switch is set to bag, the APL (overflow or pop-off) valve (1) allows venting of excess gas from the breathing system into the waste gas scavenging system and (2) can be adjusted to allow the anesthetist to provide assisted or controlled ventilation of the patient's lungs by manual compression of the gas reservoir bag. The APL valve should be fully open during spontaneous ventilation so that circuit pressure remains negligible throughout inspiration and expiration.


Manometer measures the amount of pressure in the breathing circuit. The manometer is usually located on top of the absorber. Over pressurization of the system can cause damage to the lungs of the patient.

Reservoir Bag

When the bag/vent selector switch is set to bag, the gas reservoir bag maintains an available reserve volume of gas to satisfy the patient's spontaneous inspiratory flow rate from the anesthesia machine. The bag also serves as a safety device because its distensibility limits pressure in the breathing circuit to less than 60 cm H2O, even when the APL valve is closed.

Anesthesia Machine Ventilators

When the bag/vent selector switch is set to vent, the gas reservoir bag and APL valve are eliminated from the circle anesthetic system and the patient's ventilation is delivered from the mechanical anesthesia ventilator. Anesthesia ventilators are powered by compressed gas, electricity, or both. Most conventional anesthesia machine ventilators are pneumatically driven by oxygen or air that is pressurized and, during the inspiratory phase, routed to the space inside the ventilator casing between the compressible bellows and the rigid casing.


Humidification is a form of vaporization in which water vapor (moisture) is added to the gases delivered by the anesthetic breathing system to minimize water and heat loss. The water formed and the heat generated by chemical neutralization of carbon dioxide help humidify and heat the gases in the breathing circuit.

Last modified: Tuesday, 15 November 2016, 5:01 PM