Circle System Fresh Gas Flow — FRCA Final MCQ

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ModerateAnaesthetic Equipment & SafetyCircle System Fresh Gas FlowFRCA Final

During maintenance of anaesthesia, a patient is ventilated through a correctly assembled circle breathing system with competent unidirectional valves and a functioning carbon dioxide absorber. Which statement best describes the minimum total fresh gas flow required specifically to prevent an inspired carbon dioxide concentration caused by rebreathing?

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Correct answer: B — No fixed flow; carbon dioxide is removed by the absorber

Option B is correct. A circle system intentionally permits rebreathing of exhaled gas, but a functioning absorber removes its carbon dioxide. Therefore, unlike a Mapleson system, there is no minimum fresh gas flow determined specifically by the need to prevent inspired CO2; flows can approach closed-circuit conditions. Fresh gas flow is not irrelevant overall: it must replace patient uptake, maintain circuit volume and an adequate inspired oxygen concentration, and deliver the required volatile agent. Thus 0.5 L/min is a commonly used minimal-flow setting, not a universal threshold for CO2 elimination. Flows equal to or twice minute ventilation and 3 L/min would reduce the proportion of gas rebreathed but are unnecessary when the absorber and unidirectional valves function. Inspired CO2 suggests absorber exhaustion, channeling, valve failure or circuit misassembly.

Reference: Feldman JM, Hendrickx J, Kennedy RR. Carbon Dioxide Absorption During Inhalation Anesthesia: A Modern Practice. Anesthesia & Analgesia. 2021;132:993–1002. https://pubmed.ncbi.nlm.nih.gov/34257206/