Six circuits built from the same five parts. Where the fresh gas inlet and the APL valve sit decides everything — including the reversal between A and D.
What they are
Semi-open systems with no CO₂ absorber and no unidirectional valves. Rebreathing is prevented purely by fresh gas flow — which is why the required flow differs so much between them.
The five components
Fresh gas inflow · reservoir bag · corrugated tubing · APL (expiratory/pop-off) valve · patient connection.
Every Mapleson system contains the same five parts. The classification depends entirely on where the FGF inlet and the APL valve sit relative to the patient.
The six systems
Name
FGF inlet
APL valve
FGF: spontaneous
FGF: controlled
A
Magill
Machine end
Patient end
0.7–1 × MV most efficient
3 × MV — very poor
B
—
Patient end
Patient end
2 × MV
2–2.5 × MV
C
Waters'
Patient end
Patient end
2 × MV
2–2.5 × MV
D
Bain (coaxial)
Patient end
Machine end
2–3 × MV
1–2 × MV most efficient
E
Ayre's T-piece
Patient end
None
2–3 × MV
3 × MV
F
Jackson-Rees
Patient end
Open-tailed bag
2–3 × MV
2–3 × MV
MV = minute volume.
Efficiency ranking — the single most examined fact
Spontaneous ventilation: A > DFE > CB
Controlled ventilation: DFE > BC > A
The reversal IS the point of the classification. Mapleson A is the best circuit for spontaneous breathing and among the worst for controlled ventilation. Mapleson D is the exact opposite.
Why A is efficient — and why it collapses
Spontaneous breathing
During expiration, dead-space gas (CO₂-free) returns first and fills the tubing.
Continuing fresh gas flow then pushes the later, alveolar CO₂-rich gas out through the patient-end APL valve.
At end-expiration the tubing therefore contains fresh gas plus dead-space gas — both CO₂-free.
So FGF need only match alveolar minute ventilation, roughly 70 mL/kg/min.
Controlled ventilation
The APL valve must be partly closed to generate inspiratory pressure. Alveolar gas is therefore retained rather than vented, and flows of 3 × MV become necessary.
Mapleson A should not be used for controlled ventilation.
The Lack circuit
The coaxial modification of A — expired gas travels down the inner tube to a machine-end APL valve, which makes scavenging convenient.
It remains a spontaneous-ventilation circuit, with the same limitations as the Magill.
Mapleson D and the Bain circuit
FGF enters at the patient end, so during the expiratory pause fresh gas flushes alveolar gas down the corrugated tube toward the machine-end APL valve. Controlled ventilation actively assists this washout — hence the efficiency.
Bain is the coaxial version: the inner tube delivers fresh gas to the patient, the outer corrugated tube carries expired gas back.
Practical flows: 70–100 mL/kg/min for controlled ventilation to achieve normocapnia, and 150–200 mL/kg/min for spontaneous breathing.
The inner tube warms incoming fresh gas by countercurrent exchange with expired gas — a modest heat-conservation benefit.
The Bain-specific hazard and the Pethick test
Disconnection or kinking of the inner tube is invisible on external inspection and converts the whole circuit into enormous dead space.
Pethick test: occlude the patient end, fill the circuit using the oxygen flush, then release the occlusion while still flushing.
If the inner tube is intact, the jet of gas at the patient end creates a Venturi effect that empties the reservoir bag.
If the bag stays inflated, the inner tube is faulty.
E and F in paediatrics
Ayre's T-piece (E) has no valves and no bag — minimal resistance and negligible dead space, which is why it was designed for neonates and infants.
The expiratory limb should hold at least one tidal volume, to prevent entrainment of room air — but not so large as to permit rebreathing.
Jackson-Rees (F) adds an open-ended bag to the expiratory limb. This allows respiratory movement to be observed, ventilation to be assisted or controlled by occluding the tail, and scavenging to be attached.
F became the standard paediatric circuit below about 20 kg, though modern low-resistance circle systems with small-diameter tubing have largely displaced it.
Practical notes
Mapleson B is essentially obsolete. C survives as the short "Waters" bag-and-mask assembly used in resuscitation and recovery areas — valued for portability, not efficiency.
All Mapleson systems consume far more fresh gas than a circle system with soda lime. That is the main argument against routine use: cost, volatile agent consumption, atmospheric pollution, and loss of heat and humidity.
The "× MV" figures are guides, not guarantees. Actual CO₂ elimination depends on tidal volume, respiratory rate, the length of the expiratory pause and the patient's own CO₂ production — so capnography, not a flow formula, confirms adequacy.