Two families do almost all the work: the circle system, which absorbs CO₂, and the Mapleson systems, which wash it out with fresh gas. A widely shared poster blends the two into categories that don't exist.
The poster presents four circuits: circle, semi-closed, non-rebreathing Bain, and a "Mapleson (modified Bain)" with a CO₂ absorber. The first three labels describe how a circuit is run, not different hardware, and the fourth circuit doesn't exist. Its clinical pearls about valves and the APL are sound; its circuit descriptions are not.
Semi-closed and closed are the same circle system at different fresh gas flows. Turn the flow down and a semi-closed circle becomes a low-flow, then a closed, circle.
Bain: non-rebreathing circuit.The Bain is a coaxial Mapleson D. Whether it rebreathes depends entirely on fresh gas flow: at too low a flow, exhaled gas is rebreathed and CO₂ rises. It has no valves that make it non-rebreathing.
Mapleson (modified Bain): adds a CO₂ absorber.Mapleson systems by definition have no absorber. They clear CO₂ only by fresh gas flow. A circuit with an absorber and one-way valves is a circle system.
Semi-closed circuit as a separate circuit type.Semi-closed describes a circle system run with fresh gas flow above the patient's uptake, with excess vented through the APL valve. The poster's semi-closed diagram is a circle system.
Circle system (closed system): requires closed APL valve.Only a truly closed circle has the APL valve shut. In everyday practice the circle runs semi-closed, with the APL open to scavenging.
Mapleson flows of 3–6 L/min; Bain 6–10 L/min.Mapleson flow requirements depend on the system and on whether ventilation is spontaneous or controlled, and scale with body size. A single flow range per circuit is misleading (see section 02).
Check one-way valves; APL valve controls pressure and directs gas to scavenging.Correct. An incompetent circle valve causes rebreathing and rising inspired CO₂.
Fresh gas flow (FGF) needed to prevent rebreathing, as multiples of minute ventilation (MV). Approximate; adult values.
| System | Common name | Spontaneous | Controlled | Use |
|---|---|---|---|---|
| A | Magill, Lack (coaxial) | ≈ 1 × alveolar MV (~70 mL/kg/min) | 2–3 × MV, inefficient | Best for spontaneous breathing |
| B, C | C = "Waters" bagging circuit | 1.5–2 × MV | 1.5–2 × MV | Resuscitation and transport bagging (C) |
| D | Bain (coaxial D) | 2–3 × MV (150–200 mL/kg/min) | 1–2 × MV (70–100 mL/kg/min) | Best for controlled ventilation |
| E | Ayre's T-piece | 2.5–3 × MV | Intermittent occlusion | Historical pediatric use |
| F | Jackson-Rees | 2.5–3 × MV | ~2 × MV | Pediatrics, transport |
Efficiency, the classic recall. Spontaneous: A > D, F, E > C, B. Controlled: D, F, E > B, C > A. Mapleson A wins when the patient breathes; D wins when you breathe for them.
Bain-specific hazard. If the inner fresh-gas tube disconnects or kinks, the whole outer limb becomes dead space and the patient rebreathes. The Pethick test (occlude, fill the bag, then release with high oxygen flush) checks inner-tube integrity before use.
Two one-way valves, an inspiratory and expiratory limb, a CO₂ absorber, a reservoir bag, and an APL valve. Because CO₂ is absorbed chemically, fresh gas flow can be cut far below minute ventilation. That conserves volatile agent and preserves heat and humidity.
| Mode | Typical FGF | Practical point |
|---|---|---|
| High flow | > 2 L/min | Fast changes in delivered concentration; wasteful |
| Low flow | ≈ 0.5–1 L/min | Economical; inspired concentration lags the dial, so monitor end-tidal agent and FiO₂ |
| Closed | = uptake (≈ 3–4 mL/kg/min O₂) | APL closed; needs a leak-free circuit and continuous gas analysis |
Exhausted absorbent shows as a rising inspired CO₂ on the capnogram. Desiccated absorbent can degrade volatiles; keep it moist and change it on schedule.