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Mapleson Circuits

Introduction

Mapleson circuits are semi-open, non-rebreathing anaesthesia breathing systems used to deliver fresh gas, volatile anaesthetic agents, and oxygen while eliminating carbon dioxide. In 1954, W. W. Mapleson described five arrangements, A to E; Mapleson F was later added as the Jackson-Rees modification.

They do not contain unidirectional valves or a carbon dioxide absorber. Therefore, prevention of carbon dioxide rebreathing depends primarily on:

Adequate fresh gas flow (FGF)
Correct arrangement of circuit components
Effective venting of exhaled gas through the APL valve or open expiratory limb
Mode of ventilation, spontaneous or controlled

Note: I do not have the full Dorsch and Dorsch text in the available library. This six-page exam-style note follows the standard descriptions consistent with Dorsch and Dorsch, cross-checked with major anaesthesia texts.

1. Components of a Mapleson Circuit

The basic components are:

Fresh gas inlet (FGI): Delivers oxygen and anaesthetic gases from the anaesthesia machine.
Patient connection: Connects the circuit to a face mask, supraglottic airway, or endotracheal tube.
Reservoir tube: Acts as a gas reservoir and provides a low-resistance pathway.
Reservoir bag: Stores gas, permits manual ventilation, and gives a visual and tactile indication of ventilation.
APL valve: Adjustable pressure-limiting valve, also called a pop-off valve. It allows release of excess gas and limits circuit pressure.

The different Mapleson systems are formed only by changing the relative positions of the fresh gas inlet, reservoir bag, APL valve, and patient end. This change markedly alters their efficiency in spontaneous and controlled ventilation.

Mapleson circuits are lightweight, simple, inexpensive, and have low resistance. Their major limitation is their requirement for high FGF to avoid rebreathing. - Morgan and Mikhail’s Clinical Anesthesiology, 7e, p. 83

2. Principle of Carbon Dioxide Elimination

During expiration, the patient initially exhales gas from anatomical dead space, followed by alveolar gas containing carbon dioxide. In a Mapleson circuit, the continuing FGF should wash carbon dioxide-rich alveolar gas away from the patient before the next inspiration.

If FGF is inadequate:

Alveolar gas remains in the circuit.
The patient rebreathes carbon dioxide.
Inspired carbon dioxide and end-tidal carbon dioxide rise.
Hypercapnia may occur.

The amount of rebreathing is influenced not only by FGF, but also by minute ventilation, tidal volume, respiratory rate, I:E ratio, expiratory pause, peak inspiratory flow, reservoir tube volume, use of mask versus endotracheal tube, and sampling location for carbon dioxide measurement. - Barash, Cullen, and Stoelting’s Clinical Anesthesia, 9e, pp. 2010-2011

General rule

Spontaneous ventilation: the circuit should preferentially eliminate expired alveolar gas during expiration.
Controlled ventilation: the circuit should direct fresh gas toward the patient and push alveolar gas away from the patient.

3. Classification of Mapleson Circuits

Circuit	Common name	Main feature
A	Magill circuit	FGI far from patient; APL valve near patient
B	Mapleson B	FGI and APL valve near patient; bag at far end
C	Waters to-and-fro	Short version of B, without corrugated tubing
D	Mapleson D	FGI near patient; APL valve and bag at far end
E	Ayre’s T-piece	Mapleson D without reservoir bag or APL valve
F	Jackson-Rees circuit	Mapleson E with open-ended reservoir bag

The circuits can be functionally grouped as:

Group A: Mapleson A
Group B/C: Mapleson B and C
T-piece group: Mapleson D, E, and F
Barash, Cullen, and Stoelting’s Clinical Anesthesia, 9e, p. 2010

4. Mapleson A Circuit

Arrangement

Patient - APL valve - corrugated tubing - reservoir bag - FGI

Fresh gas inlet is at the end farthest from the patient.
APL valve is close to the patient.
Also known as the Magill circuit.

Function during spontaneous ventilation

Mapleson A is the most efficient circuit for spontaneous ventilation.

Mechanism

At the start of expiration, dead-space gas enters the circuit.
Alveolar gas then reaches the APL valve near the patient.
With the APL valve open, carbon dioxide-rich alveolar gas is vented.
During the expiratory pause, continuing FGF fills the circuit with fresh gas and flushes remaining alveolar gas through the APL valve.
On the next inspiration, the patient receives mainly fresh gas plus dead-space gas.

A fresh gas flow approximately equal to the patient's minute ventilation usually prevents significant rebreathing.

Function during controlled ventilation

Mapleson A is the least efficient circuit for controlled ventilation.

During positive-pressure ventilation, the APL valve must be partially closed to generate pressure. Consequently, exhaled alveolar gas is not efficiently expelled during expiration. Very high FGF is required to avoid rebreathing, often more than 3 times minute ventilation, and some sources cite flows as high as 20 L/min in adults. - Barash, Cullen, and Stoelting’s Clinical Anesthesia, 9e, p. 2011

Advantages

Most economical Mapleson system for spontaneous breathing.
Low resistance.
Simple and lightweight.
Suitable for spontaneous ventilation in adults.

Disadvantages

Very inefficient for controlled ventilation.
High FGF requirement causes anaesthetic gas wastage.
Operating room pollution may occur.
APL valve close to the patient may interfere with surgery near the head and neck.

Use

Spontaneously breathing adult patient.
Historically used during inhalational anaesthesia.
Less commonly used in modern practice because circle systems are more economical.

5. Mapleson B and C Circuits

Mapleson B arrangement

Patient - FGI and APL valve - corrugated tubing - reservoir bag

In Mapleson B, both the FGI and APL valve are near the patient. The reservoir bag lies at the other end.

Mapleson C arrangement

Patient - FGI, APL valve, and reservoir bag close together

Mapleson C is a short version of Mapleson B. It has little or no corrugated tubing and is called the Waters to-and-fro circuit.

Functional characteristics

Both B and C are relatively inefficient during both spontaneous and controlled ventilation. Their FGF requirements are approximately similar in each mode and are usually around twice the minute ventilation, sometimes higher depending on ventilation pattern.

The reason is that fresh gas and exhaled gas mix near the patient, increasing the tendency toward rebreathing unless a high FGF is used.

Advantages

Compact, especially Mapleson C.
Low resistance.
Mapleson C can be useful for short periods of manual ventilation.
Convenient during resuscitation or transport in selected settings.

Disadvantages

High FGF is required in both spontaneous and controlled ventilation.
Inefficient compared with Mapleson A for spontaneous breathing.
Inferior to D, E, and F circuits for controlled ventilation.
Limited modern use.

Use

Mapleson C may be used for emergency ventilation, resuscitation, and transport.
Mapleson B is rarely used today.

6. Mapleson D Circuit

Arrangement

Patient - FGI - corrugated tubing - reservoir bag and APL valve

In this circuit, the FGI is close to the patient, while the APL valve and reservoir bag are at the distal end.

This is essentially the reverse arrangement of Mapleson A. Simply interchanging the FGI and APL valve changes a Mapleson A into a Mapleson D circuit. - Morgan and Mikhail’s Clinical Anesthesiology, 7e, p. 85

Function during controlled ventilation

Mapleson D is efficient for controlled ventilation.

Mechanism

During inspiration, fresh gas enters near the patient.
It is delivered directly toward the patient.
During expiration, exhaled alveolar gas travels away from the patient toward the reservoir bag and APL valve.
The continuing FGF helps push carbon dioxide-rich gas toward the APL valve for elimination.

Thus, fresh gas flow opposes the return of exhaled gas to the patient.

Fresh gas flow

For controlled ventilation, a practical FGF is approximately 1.5 to 2 times minute ventilation. For spontaneous ventilation, a higher flow is required, commonly 2 to 3 times minute ventilation.

Advantages

Efficient during controlled ventilation.
Low resistance.
Can be used for both spontaneous and controlled ventilation.
Useful when the anaesthetist is some distance from the patient.
Reservoir bag and APL valve are away from the surgical field.

Disadvantages

Higher FGF requirement than a circle system.
Increased operating room pollution if scavenging is inadequate.
Heat and humidity are poorly conserved.
Rebreathing occurs if FGF is inadequate.

Bain Circuit: Coaxial Mapleson D

The Bain circuit is a coaxial modification of Mapleson D.

Design

A narrow inner tube carries fresh gas to the patient end.
Exhaled gas travels back through the outer corrugated tube.
The reservoir bag and APL valve are at the machine end.

Advantages

Lightweight and less bulky.
Low resistance.
Convenient for head and neck procedures and remote locations.
Exhaled warm gas in the outer tube partially warms inspired fresh gas by countercurrent heat exchange.
The transparent outer tube allows inspection of the inner tube.

Hazards

The major danger is disconnection, kinking, or damage to the inner fresh-gas tube. This can cause:

Rebreathing
Hypercapnia
Hypoxaemia
Failure to deliver the intended anaesthetic gas mixture

The inner tube must therefore be checked before use and inspected throughout anaesthesia. - Miller’s Anesthesia, 10e, pp. 2353-2354

7. Mapleson E Circuit: Ayre’s T-Piece

Arrangement

FGI near patient - T-piece - open expiratory limb

The Ayre’s T-piece has:

Fresh gas inlet near the patient.
Open-ended expiratory limb.
No APL valve.
No reservoir bag.

It is a simple circuit with very low resistance.

Advantages

Very low resistance to breathing.
Lightweight.
Minimal drag on the endotracheal tube.
Suitable for neonates and small children.
Easy to assemble.
Useful when a low-resistance circuit is required.

Disadvantages

High FGF required.
No reservoir bag, so the anaesthetist cannot observe bag movement or feel lung compliance.
Manual controlled ventilation is difficult and carries a risk of excessive airway pressure.
No effective scavenging of waste gas.
Poor heat and humidity conservation.

Mapleson E is used primarily for spontaneous ventilation. In modern practice it may be used for spontaneous breathing trials or liberation from mechanical ventilation in selected ICU patients. - Miller’s Anesthesia, 10e, pp. 2352-2353

8. Mapleson F Circuit: Jackson-Rees Modification

Arrangement

Mapleson F is a modification of the Ayre’s T-piece:

FGI near patient - T-piece - expiratory limb ending in an open-tailed reservoir bag

It consists of a Mapleson E circuit with a reservoir bag attached to the expiratory limb. The tail of the bag is open and may be partially occluded to permit manual ventilation.

Advantages

Very low resistance, making it useful in neonates and children.
Reservoir bag allows observation of respiration.
Allows manual ventilation.
Gives tactile feedback about lung compliance.
More useful than Mapleson E for assisted or controlled ventilation.
Lightweight and compact.

Disadvantages

Requires high FGF.
Waste anaesthetic gas pollution can be significant.
The open tail makes scavenging difficult.
Excessive occlusion of the tail can cause high airway pressure and barotrauma.
Inadequate FGF may cause rebreathing.

Fresh gas flow

Approximate flows needed to prevent rebreathing:

Spontaneous ventilation: 2.5 to 3 times minute ventilation
Controlled ventilation: 1.5 to 2 times minute ventilation
Miller’s Anesthesia, 10e, p. 2353

Use

Commonly used in paediatric anaesthesia.
Suitable for transport and manual ventilation in infants and small children.
Useful when low circuit resistance is particularly important.

9. Relative Efficiency of Mapleson Circuits

During spontaneous ventilation

Most efficient to least efficient:

A > D/E/F > B/C

Mapleson A requires the lowest FGF, approximately equal to minute ventilation.
D, E, and F require higher flows.
B and C are relatively inefficient.

During controlled ventilation

Most efficient to least efficient:

D/E/F > B/C > A

D, E, and F are preferred for controlled ventilation.
B and C are intermediate.
Mapleson A is very inefficient.

Circuit	Spontaneous ventilation	Controlled ventilation
A	Best, FGF about 1 × minute ventilation	Worst, needs very high FGF
B	Inefficient	Inefficient
C	Inefficient	Inefficient
D	Moderately efficient	Efficient
E	Moderately efficient	Efficient but limited by absent bag
F	Moderately efficient	Efficient, especially in paediatrics

FGF values are practical approximations. The patient’s capnogram, inspired carbon dioxide, chest movement, reservoir-bag behaviour, airway pressure, and clinical condition must be monitored rather than relying on a formula alone.

10. General Advantages and Disadvantages

Advantages of Mapleson circuits

Simple construction.
Lightweight and portable.
Low resistance to breathing.
Inexpensive.
Rapid alteration of inspired gas composition when FGF is changed.
No carbon dioxide absorbent is required.
No risk of absorbent-related degradation of volatile agents.
Particularly useful in paediatric practice, transport, and selected emergency situations.

Disadvantages

Require high FGF to avoid carbon dioxide rebreathing.
High consumption of oxygen, carrier gas, and volatile anaesthetic.
Increased environmental contamination by waste anaesthetic gases.
Poor conservation of heat and humidity.
Scavenging is often difficult.
Rebreathing can occur rapidly when FGF is inadequate or the circuit is incorrectly assembled.
Less economical than a circle system.

Mapleson systems have few components and low resistance, but their need for high gas flows makes them less economical than circle systems; heat and humidity conservation and waste-gas scavenging are also less efficient. - Miller’s Anesthesia, 10e, pp. 2352-2353

11. Conclusion

Mapleson circuits are semi-open breathing systems in which carbon dioxide elimination depends on FGF washout, not on a carbon dioxide absorber. Their efficiency is determined by the arrangement of the FGI, APL valve, reservoir bag, and patient connection.

Mapleson A is best for spontaneous ventilation.
Mapleson D and Bain circuit are preferred for controlled ventilation.
Mapleson E and F are low-resistance T-piece systems useful especially in paediatric practice.
Mapleson B and C are now rarely used because of poor efficiency.

The key safety principles are adequate FGF, continuous capnography, careful circuit assembly, and checking for rebreathing or obstruction.

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Mapleson Breathing Systems

Source: Dorsch & Dorsch, Understanding Anesthesia Equipment, Chapter 8, pp. 209-220.

Definition and principle

Mapleson systems are carbon dioxide washout or flow-controlled breathing systems. They lack:

Carbon dioxide absorber
Unidirectional valves
Separate inspiratory and expiratory limbs

Therefore, carbon dioxide elimination depends on adequate fresh gas flow (FGF) washing expired gas out of the circuit. Rebreathing occurs when inspiratory flow exceeds FGF. The best practical guide for adjusting FGF is end-tidal CO₂ monitoring.

The basic components are:

Patient connection
Fresh gas inlet
Reservoir bag
Corrugated tubing
Adjustable pressure-limiting (APL) valve

Six systems are recognized: A, B, C, D, E, and F.

Mapleson A System

Configuration

Also called the Magill system.

Arrangement:
Fresh gas inlet and reservoir bag - corrugated tubing - APL valve - patient

Its distinguishing feature is that fresh gas enters at the end away from the patient, near the reservoir bag. The APL valve lies near the patient.

Lack modification

The Lack system is a modified Mapleson A with an added expiratory limb from the patient connection to an APL valve at the machine end.

Advantages:

APL valve is easier to reach.
Waste-gas scavenging is easier.

Disadvantage: Slightly increased work of breathing.

Use and functional analysis

Spontaneous ventilation

Keep the APL valve fully open.
During expiration, dead-space gas and then alveolar gas enter the tubing.
Once the bag fills, alveolar gas escapes through the APL valve.
Continuing FGF flushes expired gas from the circuit before the following inspiration.
It is the most efficient Mapleson system during spontaneous breathing.

Rebreathing starts at an FGF of approximately 56-82 mL/kg/min, and recommended FGF to avoid rebreathing is 51-85 mL/kg/min.

Controlled or assisted ventilation

Partially close the APL valve and compress the reservoir bag.
During expiration, expired gases remain in the tubing, with carbon dioxide-rich alveolar gas nearest the patient.
At the start of the next inspiration, alveolar gas is inhaled first.
Therefore, there is substantial rebreathing and wastage of fresh gas.

Hazards

A ventilator that vents excess gas should not be used because the entire circuit can become dead space.
Standard anaesthesia machine ventilators are unsuitable for classic Mapleson A use.
Incorrectly assembled Lack systems, with FGF near the APL valve rather than the reservoir bag, produce excessive dead space.

Mapleson B System

Configuration

The FGF inlet and APL valve are near the patient. The reservoir bag is at the far end of the corrugated tubing.

Use

For spontaneous ventilation, keep APL fully open.
For assisted or controlled ventilation, partly close the APL valve to inflate the lungs.

Functional analysis

Spontaneous ventilation

Near the end of expiration, tubing close to the patient contains fresh gas mixed with some alveolar gas.
To prevent rebreathing, FGF should equal peak inspiratory flow, usually 20-25 L/min.
A flow greater than twice minute volume has been recommended, although lower flows of 0.8-1.2 times minute volume may sometimes suffice.

Controlled ventilation

Slightly more efficient than Mapleson A because fresh gas accumulates near the patient during the expiratory pause.
Performance is variable because inspired gas composition depends greatly on the respiratory pattern.
Recommended FGF: 2-2.5 times minute volume.

Mapleson C System

Configuration

Mapleson C is essentially a Mapleson B without corrugated tubing.

Use and function

Its use is similar to Mapleson B.

During spontaneous breathing, it is almost as efficient as Mapleson A if the expiratory pause is minimal.
Its efficiency decreases as expiratory pause lengthens.
Recommended FGF:
- Spontaneous ventilation: 2 times minute volume
- Controlled ventilation: 2-2.5 times minute volume

Mapleson D System

Configuration

The Mapleson D, E, and F systems have a T-piece near the patient.

Arrangement:
Patient - T-piece with FGF inlet - corrugated tubing - reservoir bag and APL valve

The Mapleson D is the most efficient Mapleson system for controlled ventilation. Scavenging of waste gas is comparatively easy because the APL valve is distant from the patient.

Bain modification

The Bain circuit is a coaxial Mapleson D system:

Fresh gas travels through a narrow inner tube.
Exhaled gas travels in the outer corrugated tube.
The clear outer tube permits inspection of the inner tube.
A long Bain circuit can be used for remote anaesthesia, for example in MRI.

Use

Spontaneous ventilation

Leave the APL valve open.
Excess gas vents through the APL valve during expiration.

Assisted/manual ventilation

Partly close APL valve and squeeze reservoir bag.
Excess gas vents during inspiration.

Mechanical ventilation

Replace reservoir bag with ventilator hose and close the APL valve.
Excess gas vents through the ventilator spill valve.

Functional analysis

During expiration, expired gas mixes with fresh gas and moves away from the patient. During the expiratory pause, continuing FGF flushes expired gas down the tube. A long expiratory pause reduces rebreathing.

Spontaneous breathing

Recommended FGF: usually 1.5-3 times minute volume.
Reported weight-based values: 100-300 mL/kg/min.

Controlled ventilation

A low FGF permits inspired carbon dioxide rebreathing.
Prolonged inspiratory time, high respiratory rate, and an inspiratory plateau increase rebreathing.
A long expiratory pause decreases rebreathing.
With high FGF, the system behaves more like a non-rebreathing circuit, but at the cost of higher gas consumption and loss of heat and humidity.

Bain hazards

If the inner fresh-gas tube becomes disconnected, leaks, kinks, twists, is omitted, or is incorrectly assembled, fresh and expired gas may mix. The entire circuit then behaves as dead space, causing significant rebreathing.

Preuse checks for Bain circuit

Perform the usual leak test.
Occlude the inner tube at the patient end with a low oxygen flow running. The flowmeter indicator should fall if the inner tube is intact and connected.
With oxygen flush, a Venturi effect should cause the reservoir bag to deflate if the inner tube is patent.

Mapleson E System

Configuration

Also called the Ayre’s T-piece.

It is a T-piece with a fresh gas inlet near the patient and an open expiratory limb. It has no reservoir bag and no APL valve.

Use

Commonly used to supply oxygen or humidified gas to spontaneously breathing patients.
Use for anaesthesia has decreased because waste-gas scavenging is difficult.
For spontaneous ventilation, leave the expiratory limb open.
Controlled ventilation can be performed by intermittently occluding the expiratory limb, but assisted ventilation is difficult.

Functional features

Without an expiratory limb, spontaneous breathing causes no rebreathing.
If an expiratory limb is added, the FGF required is similar to Mapleson D.
During controlled ventilation, only fresh gas inflates the lungs, so rebreathing does not occur.

Air dilution

During spontaneous breathing, air dilution may occur if reservoir tubing volume is less than tidal volume. It can be prevented by:

FGF exceeding peak inspiratory flow, usually 3-5 times minute volume, or
FGF of 2 times minute volume with reservoir tubing volume at least one-third of tidal volume.

Hazard

Intermittently occluding the expiratory limb can cause overinflation and barotrauma because there is no reservoir bag for tactile feedback and no APL valve to limit pressure.

Mapleson F System

Configuration

Also known as the Jackson-Rees or Rees modification of Ayre’s T-piece.

It is a Mapleson E system with a reservoir bag that has an open tail or relief opening for venting excess gases.

Use

For spontaneous breathing, leave the relief opening fully open.
For controlled or assisted ventilation, partially occlude the tail to distend the bag and squeeze the bag.
A ventilator hose may replace the bag for mechanical ventilation.

Functional analysis

Mapleson F functions like Mapleson D.

FGF requirements for preventing rebreathing during spontaneous and controlled ventilation are the same as for Mapleson D.
It offers lower work of breathing than a paediatric circle system, though the difference may only matter in very sick spontaneously breathing patients.

Hazards

Similar to Mapleson E:

Excessive pressure and barotrauma may occur during manual ventilation.
A reservoir bag makes excessive pressure less likely than with Mapleson E.
Addition of an HME may increase resistance and delay inhalational induction.

Respiratory Gas Monitoring

Except for Mapleson A, FGF enters near the patient, making accurate expired-gas sampling difficult.

For accurate expiratory gas measurement, sampling should be close to the patient:

At the tracheal tube connector, or
About 2 cm distal in the elbow connector.

Sampling at the junction of the breathing system and elbow connector can produce inaccurate values even at low FGF.

Advantages

Simple, inexpensive, rugged equipment.
Few moving parts and easily disinfected or sterilized.
Useful for emergency positive-pressure ventilation.
Usually low resistance to breathing.
Lightweight with little drag on face mask or tracheal tube.
Easy to position and useful in remote locations.
Lower compression and compliance volume losses than circle systems.
Rapid change in inspired gas concentration after changing FGF.
Coaxial circuits, Lack and Bain, conserve some heat through warming of inspiratory gas by expired gas.
No carbon dioxide absorbent, hence no absorbent-related carbon monoxide or compound A formation.

Disadvantages

High FGF requirement causes increased cost and atmospheric pollution.
High FGF reduces inspired heat and humidity unless humidification is added.
Optimal FGF can be difficult to establish and must be changed when ventilation mode changes.
Reduced FGF can rapidly produce rebreathing.
In A, B, and C systems, the APL valve lies close to the patient and may be inaccessible; scavenging is awkward.
E and F systems are difficult to scavenge.
Mapleson E may cause air dilution.
Not suitable for malignant hyperthermia because FGF may not be increased enough to eliminate the markedly increased carbon dioxide load.

High-yield summary

System	Best setting	Key point
A - Magill	Spontaneous ventilation	Most efficient during spontaneous breathing
B	Rarely used	Requires high FGF in both modes
C	Short-term/manual use	Mapleson B without corrugated tubing
D	Controlled ventilation	Most efficient Mapleson circuit for controlled ventilation
Bain	Controlled ventilation	Coaxial Mapleson D; check inner tube
E - Ayre’s T-piece	Small patients, spontaneous breathing	No bag or APL valve; risk of barotrauma during manual ventilation
F - Jackson-Rees	Paediatric manual ventilation	T-piece with open-tailed bag; functions like Mapleson D

Source: Dorsch & Dorsch, Understanding Anesthesia Equipment, Chapter 8, pp. 209-220.

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Mapleson Circuits: One-Page Comparison Table

Circuit	Alternative name / configuration	FGF: spontaneous ventilation	FGF: controlled ventilation	Main use / relative efficiency	Major advantages	Important limitations / hazards
A	Magill. FGF and reservoir bag at machine end; APL valve near patient.	About 51-85 mL/kg/min	Inefficient, substantial rebreathing	Most efficient for spontaneous ventilation	Low FGF requirement during spontaneous breathing	Unsuitable with conventional anaesthesia-machine ventilators. APL valve is near patient and scavenging is awkward.
A modification	Lack circuit. Mapleson A with an expiratory limb leading to APL valve at machine end.	Similar principle to A	Not preferred	Spontaneous breathing	Easier APL adjustment and scavenging	Slightly increases work of breathing. Incorrect assembly can create large dead space.
B	FGF inlet and APL valve near patient; reservoir bag at far end of corrugated tubing.	Usually high: up to peak inspiratory flow, about 20-25 L/min; reported range 0.8-1.2 × MV may suffice	2-2.5 × MV	Rarely used; variable efficiency	Simple circuit	High FGF requirement; variable performance, especially during controlled ventilation.
C	Waters to-and-fro. Mapleson B without corrugated tubing.	2 × MV	2-2.5 × MV	Short, compact form; can be nearly as efficient as A if expiratory pause is minimal	Compact and simple	Efficiency declines as expiratory pause increases; high FGF required.
D	T-piece near patient with FGF inlet; corrugated tube leading to reservoir bag and APL valve at machine end.	Usually 1.5-3 × MV or 100-300 mL/kg/min	Most efficient Mapleson circuit for controlled ventilation; adequate FGF and a long expiratory pause reduce rebreathing	Best Mapleson circuit for controlled ventilation	APL valve away from patient; excess-gas scavenging is relatively easy; useful for manual or mechanical ventilation	High FGF, gas wastage, loss of heat/humidity. Rebreathing increases with short expiratory pause, long inspiratory time, high rate, or inspiratory plateau.
D modification	Bain circuit. Coaxial D: inner tube carries FGF to patient; outer tube carries expired gas.	Same principle as D	Same principle as D	Controlled ventilation; remote sites such as MRI	Lightweight, less bulky; inner tube visible through clear outer tube; some warming of inspired gas	Inner-tube disconnection, leak, kink, twist, blockage, or faulty assembly can turn the circuit into dead space and cause severe rebreathing. Perform a preuse inner-tube integrity test.
E	Ayre’s T-piece. T-piece with FGF near patient and open expiratory limb; no bag and no APL valve.	No rebreathing if there is no expiratory limb. With limb, same FGF requirement as D.	No rebreathing because fresh gas inflates lungs	Spontaneous oxygen/humidified-gas delivery, particularly small patients	Very simple and low resistance	Difficult scavenging. During spontaneous breathing, air dilution may occur if reservoir tubing is too short. Manual occlusion for controlled ventilation risks overinflation/barotrauma because there is no bag or APL valve.
F	Jackson-Rees / Rees modification of Ayre’s T-piece. E system plus reservoir bag with open tail or relief opening.	Same as D	Same as D	Paediatric spontaneous, assisted, or manual ventilation	Reservoir bag gives visual and tactile feedback; lower work of breathing than a paediatric circle system	Same general risks as E, though excessive pressure is less likely because a bag is present. Scavenging can be difficult.

Key exam points

All Mapleson systems lack CO₂ absorber, unidirectional valves, and separate inspiratory/expiratory limbs. CO₂ elimination is by fresh-gas washout.
End-tidal CO₂ monitoring is the best guide to adequate FGF.
Best for spontaneous ventilation: Mapleson A.
Best for controlled ventilation: Mapleson D, including the Bain modification.
E and F: T-piece systems. F adds a reservoir bag.
A low FGF in any system can cause rebreathing, reflected by raised inspired and/or end-tidal CO₂.

Source: Dorsch & Dorsch, Understanding Anesthesia Equipment, Chapter 8, pp. 209-220. MV = minute volume; FGF = fresh gas flow; APL = adjustable pressure-limiting valve.

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