Performance Characteristics of the Circle System
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 (closed-system anesthesia). At fresh gas flows greater than 5 L/min, rebreathing is so mini-mal that a CO 2 absorber is usually unnecessary.
With low fresh gas flows, concentrations of oxygen and inhalation anesthetics can vary mark-edly between fresh gas (ie, gas in the fresh gas inlet) and inspired gas (ie, gas in the inspiratory limb of the breathing tubes). The latter is a mixture of fresh gas and exhaled gas that has passed through the absorber. The greater the fresh gas flow rate, the less time it will take for a change in fresh gas anesthetic concentration to be reflected in a change in inspired gas anesthetic concentration. Higher flows speed induction and recovery, compensate for leaks in the circuit, and decrease the risks of unanticipated gas mixtures.
Th at part of a tidal volume that does not undergo alveolar ventilation is referred to as dead space. Thus, any increase in dead space must be accompa-nied by a corresponding increase in tidal volume, if alveolar ventilation is to remain unchanged.Because of the unidirectional valves, appara-tus dead space in a circle system is limited tothe area distal to the point of inspiratory and expira-tory gas mixing at the Y-piece. Unlike Mapleson circuits, the circle system tube length does not affect dead space. Like Mapleson circuits, length does affect circuit compliance and thus the amount of tidal volume lost to the circuit during positive-pressure ventilation. Pediatric circle systems may have both a septum dividing the inspiratory and expiratory gas in the Y-piece and low-compliancebreathing tubes to further reduce dead space, and are lighter in weight.
The unidirectional valves and absorber increase cir-cle system resistance, especially at high respiratory rates and large tidal volumes. Nonetheless, even pre-mature neonates can be successfully ventilated using a circle system.
Medical gas delivery systems supply dehumidified gases to the anesthesia circuit at room temperature. Exhaled gas, on the other hand, is saturated with water at body temperature. Therefore, the heat and humidity of inspired gas depend on the relative pro-portion of rebreathed gas to fresh gas. High flows are accompanied by low relative humidity, whereas low flows allow greater water saturation. Absorbent granules provide a significant source of heat and moisture in the circle system.
The minimal risk of microorganism retention in circle system components could theoretically lead to respiratory infections in subsequent patients. For this reason, bacterial filters are sometimes incorpo-rated into the inspiratory or expiratory breathing tubes or at the Y-piece.