The circle system is the most popular breathing system in the US. The circular design includes the following components: (1) a fresh gas inlet; (2) an inspiratory unidirectional valve and inspiratory breathing tube; (3) a Y-connector; (4) an expiratory unidirectional valve and expiratory breathing tube; (5) an APL valve; (6) a reservoir; and (7) a CO2 absorber. The fresh gas inlet is placed between the absorber and the inspiratory valve in the circle system to prevent fresh gas from bypassing the patient during exhalation.
In a circular system, the patient will rebreathe alveolar gas. Rebreathing alveolar gas conserves heat, humidity and anesthetic. However, the patient will also rebreathe CO2 in exhaled gas, which must be removed to prevent hypercapnia.
CO2 chemically combines with water to form carbonic acid. CO2 absorbents such as soda lime (or calcium hydroxide lime) contain hydroxide salts that are capable of neutralizing carbonic acid. Soda lime consists primarily of calcium hydroxide (80% Ca(OH)₂ along with NaOH, H₂O and KOH) and is the most common absorbent. It is capable of absorbing up to 23 L of CO2 per 100 g of absorbent. With an absorber, the circle system prevents rebreathing of CO2 at reduced fresh gas flows (≤1 L) or even fresh gas flows equal to the uptake of anesthetic gases and oxygen by the patient and the circuit itself. At fresh gas flows greater than 5 L/min, rebreathing is so minimal that a CO2 absorber is usually unnecessary.
Exhaustion of the CO2 absorbed or failure of either the expiratory and inspiratory valves can cause the patient to rebreathe CO2 and may result in hypercapnia.
Updated definition 2020:
Circle systems are the most common type of anesthetic circuit used. Circle systems include some variation of the following components: 1) an inspiratory and expiratory leg of tubing, 2) unidirectional valves at each of the above legs, 3) a y-piece connector at the junction of the above legs, 4) a fresh gas flow inlet, 5) an adjustable pressure limiting (APL) valve, 6) a reservoir bag for reserve flow capacity, 7) a CO2 absorbent cannister. A mechanical ventilator and switch to swap between bag and vent is generally present but not essential to the concept of the circle system itself.
Circle systems can be classified as semiopen, semiclosed or closed. A given anesthesia breathing system can act as anyone of these classifications depending on the amount of fresh gas flow used. A system with a very high amount of fresh gas flow will act as a semiopen system with very little recirculation of expired gas. In a semiopen system a large amount of expired gas will go to scavenge/waste (see image below). In a system with very little fresh gas flow, where the inflow is equal to gas consumed by the patient, the system will act as semiclosed. In a semiclosed system very little expired gas will go to scavenge/waste. A truly closed system is generally not seen in clinical practice, but is often used for illustration purposes of teaching circuit dynamics. This is a system in which no fresh gas flow is introduced to the circuit.
Rebreathing of expired gas in a circle system has two primary advantages: 1) some conservation of heat and moisture taken off the patient by expiration, 2) reduced use of anesthetic gas from the vaporizer with a reduction of wasted gas going to the environment.
Rebreathing of expired gas however does have some disadvantages: 1) the need for unidirectional valves to ensure proper flow direction of inspired and expired gas, 2) the need for a carbon dioxide absorbent, 3) loss of simplicity and portability due to added components, 4) the added components in the circuit increase the resistance to breathing and add potential for malfunction.
In a circle system rebreathing of expired CO2 by the patient is largely prevented by using a CO2 absorbent cannister. Rebreathing is also prevented by the unidirectional valves which ensure expired gas is directed away from the patient during exhalation, and fresh gas is directed towards the patient during inhalation. As mentioned above, the increased complexity of the circle system provides the potential for malfunction. A faulty unidirectional valve can cause expired CO2 to be inhaled prior to passing through the CO2 absorber. Similarly, exhaustion or depletion of the CO2 cannister can result in rising CO2 levels as the patients expired CO2 is circled back and inhaled again. Carbon dioxide absorbents eliminate CO2 through a series of chemical reactions supported by the neutralization of CO2 with water and subsequent products by the granules in the cannister. As granules are consumed and no longer able to support the chain of chemical reactions, CO2 levels can rise. This rise can be seen on capnography, often with a distinctive pattern of rise.