Anaesthesia equipment -Breathing system,airway management

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anaesthesia breathing circuit circle system components

A clinical photograph depicting the assembly of an anesthesia circle system for medication delivery. The central focus is a Y-piece connector facilitating the administration of inhaled salbutamol during general anesthesia. On one end, a transparent corrugated breathing tube is attached via a gray connector. On the opposite side, a metered-dose inhaler (MDI) canister is inserted into a specialized delivery port on a white rigid segment. A distinctive teal-colored, flexible, ribbed adaptor is positioned between the gray cylindrical connector and the delivery port. This setup demonstrates a method for integrating a salbutamol aerosol canister into a closed or semi-closed anesthesia breathing circuit to treat bronchospasm or improve ventilation-perfusion matching during surgical procedures. The photograph illustrates the physical components and mechanical interface required for aerosolized drug delivery in an intraoperative setting.

This composite image consists of a technical illustration (left) and a corresponding clinical photograph of a medical manikin (right) demonstrating the Kelley Circuit, a specialized respiratory assembly for tracheostomized patients. The assembly is secured to the neck via a standard tracheostomy tube and neck strap. Key components include a ProTrach XtraCare heat and moisture exchanger (HME) featuring an integrated electrostatic filter, which is attached to the lateral ventilator hub. Connected directly to the ISO 15 hub of the tracheostomy tube is a closed-circuit suction system, visible as a clear, flexible tube partially enclosed in a transparent protective plastic sleeve with green mechanical valves and connectors. This configuration is designed to provide viral and bacterial filtration while allowing for airway clearance without disconnecting the patient from the circuit, thereby reducing the risk of aerosolization and tube drag. It is clinically relevant for critical care and respiratory therapy education, specifically regarding the management of spontaneously breathing patients requiring humidification and frequent suctioning.

This clinical photograph serves as a schematic diagram illustrating the assembly of a non-invasive high-frequency oscillatory ventilation (NIHFOV) circuit. The image displays three primary medical device components arranged horizontally on a neutral background, with double-ended black arrows indicating the connection sequence. From left to right, the components include: 1) A clear, teardrop-shaped silicone non-invasive ventilation (NIV) mask featuring a blue four-pronged headgear attachment ring and a distal sampling port. 2) A clear, cylindrical disposable leak valve designed to facilitate the venting of exhaled CO2. 3) A proximal segment of a flexible, transparent ventilator breathing circuit showing multi-lumen tubing. The setup demonstrates the critical interface between the ventilator tubing, the mandatory leak valve for safety and gas exchange, and the patient interface (mask). This visual is intended for clinical training and educational instruction on the proper configuration of NIHFOV hardware for respiratory support.

This medical anatomical diagram illustrates the Papez circuit, a fundamental neural pathway of the limbic system involved in memory and emotion. The illustration uses a sagittal view of the human brain to map the connectivity between key structures. The circuit is depicted as a closed loop with directional blue arrows indicating the flow of information. Key components labeled include: (A) Hippocampal Formation, (B) Fornix, (C) Mammillary Body, (D) Anterior Thalamic Nucleus, (E) Cingulate Gyrus, (F) Cingulum, and (I) Parahippocampal Gyrus/Entorhinal Cortex. The pathway follows the sequence: Hippocampus → Fornix → Mammillary bodies → Mammillothalamic tract → Anterior thalamic nucleus → Cingulate gyrus → Cingulum → Entorhinal cortex → Hippocampus. Additionally, adjacent limbic structures are identified, including (H) Hypothalamus, (J) Amygdala, and (S) Septal Area. This educational diagram is designed to demonstrate the functional neuroanatomy and connectivity essential for long-term memory processing and limbic system integration.

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Mapleson circuit classification breathing system

This clinical photograph illustrates a pediatric patient under procedural sedation or general anesthesia, managed with a specialized respiratory circuit. A clear, contoured anesthesia face mask is positioned over the child's nose and mouth, secured firmly by a multi-holed black rubber head strap (Spider strap) to maintain an airtight seal for Continuous Positive Airway Pressure (CPAP). The mask is connected to a Jackson-Rees circuit, a modification of the Mapleson F system, characterized by the visible reservoir bag and fresh gas flow tubing. Key components include an elbow connector at the mask apex, a wide-bore corrugated tube leading to the reservoir bag, and a thin pressure-monitoring line or supplemental oxygen tubing. White gauze padding is placed under the straps near the neck and cheeks to prevent pressure-related skin injury. In the immediate vicinity, a syringe and a medical sensor (likely a pulse oximetry or CO2 monitoring cable) are visible on the sterile field. This setup is clinically used to provide airway support and maintain functional residual capacity (FRC) during radiological procedures or surgery.

A comparison of a normal (a, c) and a defective (b, d) heated, humidified breathing circuit. The images show a corrugated, translucent blue plastic tube used in anesthesia and intensive care. Inside the circuit, an internal assembly containing a heating wire, temperature sensor, and fluid tubing is wrapped in a white cotton-like insulating material. In the normal circuit (a, c), the cotton wrapping is thin and uniform, preserving a clear patent lumen for airflow. In the obstructed circuit (b, d), which represents a manufacturing error, the white cotton material is irregularly thick and bulky, almost completely occluding the internal diameter of the tube. This visual demonstration highlights a critical equipment-related cause of difficult ventilation, where high circuit resistance due to internal lumen obstruction can lead to nearly undetectable end-tidal CO2 and low tidal volumes despite high pressure settings.

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endotracheal tube intubation laryngoscopy airway

This sequence of clinical images demonstrates the four steps of endotracheal intubation (ETI) using a polycarbonate barrier box on an airway trainer manikin. The setup is designed to minimize aerosol exposure during airway management. Step 1: An operator wearing personal protective equipment (PPE), including blue gloves, inserts hands through the rear access ports of the transparent box to begin laryngoscopy. Step 2: The operator uses a Macintosh blade laryngoscope to visualize the glottis while an assistant provides a 7.0 mm cuffed endotracheal tube through a side port. Step 3: The operator inserts the tracheal tube into the trachea. Step 4: The tube is secured, the cuff is inflated, and an Ambu-Bag (manual resuscitator) is connected to begin positive-pressure ventilation. The images highlight the ergonomic constraints and procedural adjustments required when performing direct laryngoscopy within a confined barrier enclosure, commonly utilized in protocols for highly infectious respiratory diseases such as COVID-19.

This composite image illustrates the design and clinical application of the Airway Scope® (AWS), a portable, battery-operated video-laryngoscope used for endotracheal intubation. Figure A shows the device's physical components: an orange handle with an integrated 6.1 cm LCD monitor and a single-use 'Intlock' blade. An endotracheal tube (ETT) with a blue radiopaque stripe is preloaded into the blade’s dedicated side-channel guide, which facilitates direct advancement into the trachea. Figure B demonstrates the device's functional output during indirect laryngoscopy. The LCD screen displays a real-time view of the upper airway, specifically focusing on the glottic opening. To assist the clinician, a green crosshair target is superimposed on the digital image; alignment of this target with the glottis indicates the correct path for ETT insertion. This visual aid is designed to simplify intubation by providing a wide viewing angle (180°) and clear anatomical visualization of the vocal cords, thereby potentially increasing first-pass success rates in both novice and expert practitioners.

A high-angle clinical photograph displays standard and advanced airway management equipment for endotracheal intubation, positioned alongside a Laerdal Airway Management Trainer mannequin. The equipment includes three types of laryngoscopes: a conventional Macintosh-style laryngoscope with a metal handle and curved blade, and two video laryngoscopes featuring integrated digital screens and specialized curved blades for enhanced glottic visualization. Also present is a Bag Valve Mask (BVM) for manual ventilation. Centrally located is a clear cuffed endotracheal tube (ETT) with an internal stylet bent into a 'hockey-stick' configuration to facilitate insertion; the ETT includes a visible pilot balloon, connector, and depth markings. Ancillary procedural tools include a ruler for measurement and a digital timer for monitoring intubation time. This setup is typical for anesthesia training, emergency medicine simulation, or comparative studies between direct and video laryngoscopy techniques.

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Anaesthesia Equipment: Breathing Systems & Airway Management

Sources: Morgan & Mikhail's Clinical Anesthesiology 7e, Bailey & Love's Surgery 28e, Miller's Anesthesia 10e

PART 1: BREATHING SYSTEMS

Overview

A breathing system connects the anaesthesia machine's fresh gas outlet to the patient's airway. It must deliver oxygen and anaesthetic agent, eliminate CO₂, and conserve heat and humidity. The key systems are:

Insufflation / Open drop (historical)
Mapleson circuits (A-F) - semi-open/semi-closed
Circle system - closed or semi-closed
Resuscitation bags (AMBU)

1. Mapleson Circuits

Mapleson classified semi-open breathing systems into types A-F based on the arrangement of the fresh gas inlet, adjustable pressure-limiting (APL) valve, reservoir bag, and corrugated tubing.

Bain circuit (Mapleson D coaxial design) - APL valve, fresh gas tubing, breathing tube, breathing bag and mask

FIGURE: Bain circuit - a Mapleson D coaxial design with fresh gas tubing inside the corrugated breathing tube (Morgan & Mikhail 7e)

Circuit	Description	Best For	FGF Required
A (Magill)	APL valve near patient, FGI near bag	Spontaneous ventilation	= Alveolar MV
B	FGI near APL, both near patient	Rarely used	High
C	FGI and APL close together	Short procedures	High
D (Bain)	FGI near patient, APL near bag	Controlled ventilation	2-3x MV
E (Ayre's T-piece)	No bag, no valve - T-piece	Paediatrics	High
F (Jackson-Rees)	Open-tailed bag added to E	Paediatrics	High

Key principle: The position of the FGI relative to the APL valve determines efficiency.

Mapleson A is most efficient for spontaneous ventilation (FGF = minute ventilation).
Mapleson D/Bain is most efficient for controlled ventilation (FGF = 2-3x minute ventilation).

Bain Circuit: A coaxial Mapleson D where the fresh gas inlet tube runs inside the corrugated outer tube. Advantages: less bulk, partial heat and humidity conservation by counter-current exchange. Disadvantage: kinking or disconnection of inner tube can cause unrecognised rebreathing.

2. The Circle System

The circle system is the most widely used breathing system in adults. It adds CO₂ absorber + unidirectional valves to overcome the high FGF requirements of Mapleson circuits.

APL valve diagram showing spring-loaded valve disc, reservoir bag, and connections to expiratory limb, scavenger, and CO₂ absorber

FIGURE: APL (Adjustable Pressure-Limiting) valve with reservoir bag and connections in circle system (Morgan & Mikhail 7e)

Components of the Circle System

#	Component	Function
1	Fresh gas inlet	Delivers O₂ + anaesthetic from machine
2	Inspiratory unidirectional valve	Ensures one-way flow to patient
3	Corrugated breathing tubes	Connect Y-piece to valves
4	Y-piece	Patient connection point
5	Expiratory unidirectional valve	Ensures one-way flow from patient
6	APL (pop-off) valve	Vents excess gas to scavenger; adjustable
7	Reservoir bag	Accommodates tidal volume, allows manual ventilation
8	CO₂ absorber (soda lime)	Chemically absorbs exhaled CO₂

CO₂ Absorber Chemistry (Soda Lime)

Soda lime is composed primarily of calcium hydroxide (80%), sodium hydroxide, water, and potassium hydroxide. It absorbs up to 23 L CO₂ per 100 g of absorbent.

Reaction:

CO₂ + H₂O → H₂CO₃
H₂CO₃ + 2NaOH → Na₂CO₃ + 2H₂O + Heat
Na₂CO₃ + Ca(OH)₂ → CaCO₃ + 2NaOH

A pH indicator dye (ethyl violet) changes from white → purple as the absorbent is exhausted.
Granule size is 4-8 mesh (balance between surface area and airflow resistance).
Dry soda lime degrades volatile agents - desflurane produces the most carbon monoxide; sevoflurane can produce Compound A.

Optimal Component Arrangement

Unidirectional valves near patient (prevent backflow; not at Y-piece).
Fresh gas inlet between absorber and inspiratory valve.
APL valve between absorber and expiratory valve, near reservoir bag.
Reservoir bag in expiratory limb (decreases exhalation resistance).

Performance Characteristics

Feature	Details
FGF requirement	≤1 L/min with absorber; closed-system possible
Dead space	Limited to Y-piece (unidirectional valves eliminate tube dead space)
Resistance	Increased by valves and absorber; still safe for neonates
Heat/humidity	Conserved at low flows; CO₂ granules generate heat
Bacterial contamination	Bacterial filters recommended at Y-piece or in limbs

Disadvantages of Circle System

Larger, less portable
Greater complexity - higher risk of disconnection/malfunction
Absorbent-related complications (CO, Compound A)
Unpredictable inspired concentrations at low FGF

Monitoring Components in the Circle System

Oxygen analyzers (Clark electrode / galvanic / paramagnetic): must be present during all GA; sensor placed in inspiratory or expiratory limb; paramagnetic type is self-calibrating.
Spirometers (Wright respirometer): measure exhaled tidal volume; placed near expiratory valve; under-read at low flows, over-read at high flows.
Capnography: measures end-tidal CO₂; gold standard for confirming circuit integrity and tracheal tube position.

3. Resuscitation Bags (AMBU / Bag-Mask Unit)

Used for emergency ventilation. Features a non-rebreathing valve (unlike Mapleson or circle systems). Can deliver ~100% oxygen. Portable and simple. Bag-mask ventilation maintains higher SpO₂ during intubation in critically ill patients.

PART 2: AIRWAY MANAGEMENT

Physiological Basis

General anaesthesia causes loss of pharyngeal muscle tone, resulting in airway obstruction. Muscle relaxants eliminate spontaneous breathing. The anaesthetist must:

Maintain a patent airway
Ensure adequate ventilation
Protect lungs from aspiration

Airway Management Progression

Step 1 - Basic manoeuvres:

Head-tilt / chin-lift: extends atlanto-occipital joint, lifts tongue off posterior pharynx
Jaw thrust: displaces mandible anteriorly, most effective for airway opening

Step 2 - Airway adjuncts:

Oropharyngeal Airway (Guedel)

Three oropharyngeal airways (Guedel) in different sizes - red (large), orange (medium), green (small)

FIGURE: Oropharyngeal airways in three sizes (Bailey & Love 28e)

Curved rigid device; displaces tongue forward
Available in multiple sizes (measured from centre of mouth to earlobe)
Holds the tongue forward but does not prevent aspiration
Requires adequate depth of anaesthesia to tolerate (stimulates gag reflex if awake)

Nasopharyngeal airway: better tolerated in semi-conscious patients; contraindicated in base of skull fractures.

Supraglottic Airway Devices (SADs)

Laryngeal Mask Airway (LMA)

Developed by Dr Archie Brain (UK). The most important supraglottic airway device.

Step-by-step LMA insertion: A - deflated cuff ready; B - insertion against hard palate; C - advancing into hypopharynx; D - final positioning with gentle downward pressure

FIGURE: LMA insertion technique (Morgan & Mikhail 7e)

Structure: Wide-bore tube with a proximal 15-mm connector and distal inflatable elliptical cuff.

Positioning: Cuff settles with:

Base of tongue - superiorly
Pyriform sinuses - laterally
Upper oesophageal sphincter - inferiorly

Insertion steps (Table 19-2, Morgan & Mikhail):

Choose correct size; check for leaks
Deflate cuff with rim wrinkle-free, facing away from aperture
Lubricate only the back of the cuff
Ensure adequate anaesthesia
Place patient in sniffing position
Use index finger to guide insertion along hard palate

Three LMA devices (sizes 3, 4, 5) showing the inflatable elliptical cuffs used for perilaryngeal seal

FIGURE: LMA sizes 3, 4, 5 (i-gel style supraglottic airways - Bailey & Love 28e)

LMA Size Selection:

Size	Patient Weight
1	Neonates / infants up to 5 kg
1.5	Infants 5-10 kg
2	Infants/children 10-20 kg
2.5	Children 20-30 kg
3	Small adults 30-50 kg
4	Adults 50-70 kg
5	Large adults >70 kg

Advantages:

Less invasive than intubation; less haemodynamic response
Can be taught to non-anaesthetists and paramedics
Useful as rescue device in "can't intubate" scenario
Partial protection from pharyngeal secretions (not gastric regurgitation)
Remove only when patient regains airway reflexes (cough, mouth opening on command)

Contraindications / Limitations:

Not suitable for high BMI patients (inadequate seal at raised pressures)
Does not protect against aspiration of gastric contents
Anatomic variations may prevent adequate seal

LMA Generations and Variants

Device	Generation	Features
Classic LMA	1st	Standard inflatable cuff
Flexible LMA	1st	Reinforced tube for airway surgery
ProSeal LMA	2nd	Oesophageal drain tube; tolerates higher inflation pressures; suitable for higher BMI
i-gel	2nd	Thermoplastic gel cuff (no inflation needed); built-in bite block; oesophageal drain
Fastrach (ILMA)	-	Intubating LMA; allows blind tracheal tube insertion
LMA CTrach	-	Camera-equipped; facilitates tube passage

There is increasing evidence that second-generation devices should replace first-generation devices due to superior safety and efficacy profile (Bailey & Love 28e).

Esophageal-Tracheal Combitube

Two fused tubes; inserted blindly through the mouth
Sits in oesophagus ~95% of time; ventilates through side perforations into larynx
Tracheal placement: use the clear (open-tipped) tube
Useful in emergency / prehospital settings

King Laryngeal Tube

Single tube with a small oesophageal balloon and larger hypopharyngeal balloon
Both inflate via one inflation line; ventilation occurs between the two balloons
Suction port allows gastric decompression
If ventilation is poor: tube is likely too deep - withdraw slowly

Endotracheal (Tracheal) Intubation

The gold standard for definitive airway control.

Indications:

Aspiration risk (full stomach, GORD, pregnancy)
Need for positive pressure ventilation at high pressures
Procedures requiring muscle relaxation
Thoracic / abdominal surgery
Head and neck surgery
Prolonged operations

The cuffed endotracheal tube:

Cuff inflation facilitates positive pressure ventilation
Protects lungs from aspiration of regurgitated gastric contents
Cuff pressure should be checked - target 20-30 cmH₂O

Standard tube sizes:

Adult female: 7.0-7.5 mm ID
Adult male: 8.0-8.5 mm ID
Paediatric formula: (age/4) + 4 mm (uncuffed); (age/4) + 3.5 mm (cuffed)

Confirmation of correct placement:

Visualisation through vocal cords
Bilateral chest expansion
Auscultation (bilateral air entry, no gastric sounds)
End-tidal CO₂ (capnography - gold standard)
SpO₂ maintenance

Specialist tubes:

Double-lumen tubes / endobronchial tubes: used for thoracoscopic, pulmonary, and oesophageal surgery; allow single-lung ventilation for surgical access; essential in bronchopleural fistula and empyema.

Difficult Airway Management

Endotracheal intubation is feasible in most patients, but a proportion will be difficult or impossible. The combination of difficult intubation AND difficult mask ventilation can be catastrophic.

Predictors of difficult intubation (LEMON score):

Look externally (beard, obesity, small mouth)
Evaluate 3-3-2 rule (mouth opening 3 fingers, hyoid-chin 3 fingers, hyoid-thyroid notch 2 fingers)
Mallampati score (class III/IV)
Obstruction (stridor, tumour)
Neck mobility (limited in ankylosing spondylitis, C-spine injury)

Devices for anticipated difficult intubation:

Device	Description
McGrath / C-MAC video laryngoscope	Indirect laryngoscopy with camera; improves glottic view
Airtraq	Optical laryngoscope; channel for tube guidance
McCoy laryngoscope	Hinged blade tip; useful in anterior larynx
Flexible fibreoptic bronchoscope	Gold standard for awake intubation; tube threaded over scope under direct vision

Fibreoptic intubation: Anaesthetist places bronchoscope into trachea, then threads ET tube over it. Can be performed:

Awake with topical local anaesthetic (in anticipated very difficult airway)
Under GA in anticipated but not critical difficulty

Surgical airway:

Cricothyroidotomy: emergency access through cricothyroid membrane; used in "can't intubate, can't oxygenate" scenario
Tracheostomy: elective surgical airway; preferred for prolonged ventilation

Summary Box - Techniques for Maintaining an Airway (Bailey & Love 28e):

Technique	Key Feature
Chin lift / jaw thrust	Basic; no equipment
Guedel (oral) airway	Holds tongue forward; does not prevent aspiration
LMA (1st generation)	Low-pressure perilaryngeal seal; easy to insert
2nd generation SAD (ProSeal, i-gel)	Higher sealing pressure; oesophageal drain; preferred
Endotracheal tube	Definitive airway; protects against aspiration; allows IPPV
Double-lumen tube	Lung isolation for thoracic surgery
Fibreoptic intubation	Difficult/anticipated difficult airway; awake technique
Cricothyroidotomy	Emergency surgical airway

Summary Table: Breathing Systems Comparison

System	FGF Needed	CO₂ absorption	Rebreathing	Portability	Best Use
Mapleson A	= MV (spont)	No	Minimal	Good	Adult spontaneous ventilation
Mapleson D/Bain	2-3x MV	No	Minimal	Good	Adult/child controlled ventilation
Mapleson F (Jackson-Rees)	High	No	Minimal	Excellent	Paediatrics
Circle system	≤1 L/min	Yes (soda lime)	Controlled	Poor	Prolonged adult GA
AMBU bag	100% O₂	No	No	Excellent	Emergency/resuscitation

Key references: Morgan & Mikhail's Clinical Anesthesiology 7e (Ch 3, 19); Bailey & Love's Short Practice of Surgery 28e (Ch 23); Miller's Anesthesia 10e (Ch 6)

Anaesthesia equipment -Breathing system

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Anaesthesia Equipment: Breathing Systems

Sources: Morgan & Mikhail's Clinical Anesthesiology 7e, Barash Clinical Anesthesia 9e, Miller's Anesthesia 10e

Introduction

A breathing system (breathing circuit) forms the final conduit between the anaesthesia machine and the patient's airway. Its functions are:

Deliver O₂ and anaesthetic gases to the patient
Eliminate CO₂ (by washout or chemical absorption)
Conserve heat and humidity
Allow monitoring of ventilation

The major systems are: Insufflation → Draw-over → Mapleson circuits → Circle system → Resuscitation bags

1. Insufflation

The simplest technique - anaesthetic gases are blown across the patient's face without direct connection to the airway.

Used in children who resist face masks during inhalational induction
Used in ophthalmic surgery under local anaesthesia (to avoid CO₂ accumulation under drapes) - air blown at >10 L/min
No rebreathing if flow is sufficient
Cannot control ventilation; inspired concentration is unpredictable due to entrainment of atmospheric air

2. Draw-Over Anaesthesia

Uses the patient's own inspiratory effort to draw ambient air through a low-resistance vaporizer.

Draw-over anaesthesia device: patient's inspiratory effort draws air through vaporizer, with bag, tubing, valve and mask

FIGURE: Schematic of a draw-over anaesthesia device (Morgan & Mikhail 7e)

Key properties:

Portable; no compressed gases required (battlefield, remote settings)
Non-rebreathing circuit; ambient air is carrier gas
Supplemental O₂ can be added (1 L/min → FiO₂ 30-40%; 4 L/min → FiO₂ 60-80%)
Can be adapted for IPPV, CPAP, PEEP
Patients breathing room air alone often have SpO₂ <90% - supplemental O₂ essential

3. Mapleson Circuits

Mapleson (1954) classified semi-open circuits by the arrangement of four key components: fresh gas inlet (FGI), APL valve, reservoir bag, corrugated breathing tube. The relative position of these components determines performance.

Components

FIGURE 3-5: Components of a Mapleson circuit. Tube volume must be ≥ tidal volume to prevent rebreathing (Morgan & Mikhail 7e)

A. Breathing Tubes (Corrugated tubing)

22 mm diameter - low resistance
Volume should be ≥ patient's tidal volume to prevent rebreathing
High compliance → tidal volume "lost" to circuit during IPPV (e.g., circuit compliance of 8 mL/cmH₂O at 20 cmH₂O = 160 mL lost)

B. Fresh Gas Inlet (FGI)

Continuously delivers O₂ + anaesthetic from the machine
Its position relative to the APL valve is the key differentiator between Mapleson types

C. APL (Adjustable Pressure-Limiting) Valve

Also called pop-off valve or pressure-relief valve
Vents excess gas to the scavenger system
During spontaneous breathing: kept fully open
During controlled ventilation: partially closed to build circuit pressure
Spring-loaded; opening pressure is adjustable

D. Reservoir Bag

Acts as gas reservoir between patient breaths
Allows manual positive-pressure ventilation
Visual indicator of ventilation (moves with each breath)
Standard adult size: 2 L

Mapleson Classification Table

TABLE 3-2: Classification and characteristics of Mapleson circuits (Morgan & Mikhail 7e)

Circuit	Other Name	FGI Position	APL Position	FGF: Spontaneous	FGF: Controlled	Best Use
A	Magill attachment	Near bag	Near patient (mask end)	= Minute ventilation (~80 mL/kg/min)	Very high, unpredictable	Adult spontaneous ventilation
B	-	Near patient	Near patient	2× MV	2-2.5× MV	Rarely used
C	Waters' to-and-fro	Near patient	Near patient	2× MV	2-2.5× MV	Rarely used
D	Bain circuit	Near patient	Near bag	2-3× MV	1-2× MV	Controlled ventilation
E	Ayre's T-piece	Near patient	None (open end)	2-3× MV	3× MV	Paediatrics
F	Jackson-Rees	Near patient	Bag tail	2-3× MV	2× MV	Paediatrics, neonates

Memory rule:

Mapleson A = most efficient for Spontaneous ventilation (A for Anaesthetist breathing spontaneously)
Mapleson D = most efficient for Controlled ventilation

The Bain Circuit (Mapleson D Coaxial)

Bain circuit: fresh gas tubing runs inside the outer corrugated breathing tube. APL valve at left (near bag), breathing tube, mask at right.

FIGURE 3-7: The Bain circuit - a Mapleson D modification with fresh gas tubing inside the corrugated outer tube (Morgan & Mikhail 7e)

Fresh gas tubing runs inside the outer corrugated tube (coaxial design)
FGI at patient end; APL valve and bag at machine end
Advantages: less bulk; partial heat & humidity conservation by counter-current exchange with expired gases
Disadvantage: kinking or disconnection of inner tube can cause unrecognised rebreathing - periodic inspection mandatory
FGF for controlled ventilation: 70 mL/kg/min (or 1-2× MV)

Ayre's T-Piece (Mapleson E) & Jackson-Rees (Mapleson F)

Ayre's T-piece: No APL valve, no bag. T-shaped connector with FGI on one limb and open expiratory limb. Used in children <20 kg. Scavenging is difficult.
Jackson-Rees modification: Adds an open-tailed breathing bag to the expiratory limb of the T-piece. Allows controlled ventilation, PEEP, and scavenging. Standard paediatric circuit for infants and neonates.

4. The Circle System

The most widely used breathing system in modern anaesthesia. Adds CO₂ absorber + unidirectional valves to allow rebreathing at low fresh gas flows, overcoming the main drawback of Mapleson circuits.

Circle System Diagram

FIGURE 3-8: The circle system - showing all major components (Morgan & Mikhail 7e)

Circle system schematic: circular arrangement showing inspiratory limb, expiratory limb, Y-piece, inspiratory and expiratory unidirectional valves, fresh gas inlet, CO₂ canister, APL valve, reservoir bag (B), ventilator (V) and bag/vent selector switch

FIGURE 25-32: Circle breathing system components showing bag/vent selector switch (Barash Clinical Anesthesia 9e)

Components

The circle system consists of 7 primary components:

#	Component	Function
1	Fresh gas inlet	Delivers O₂ + anaesthetic from common gas outlet
2	Inspiratory unidirectional valve	Directs gas flow toward patient only
3	Inspiratory breathing tube	Carries fresh + recirculated gas to Y-piece
4	Y-piece connector	Connects circuit to patient's airway device
5	Expiratory breathing tube	Carries exhaled gas from Y-piece
6	Expiratory unidirectional valve	Directs flow away from patient only
7	APL valve (pop-off valve)	Vents excess gas to scavenger; adjustable spring-loaded
8	Reservoir bag	Gas storage; visual ventilation monitor; manual IPPV
9	CO₂ absorber	Chemical neutralisation of exhaled CO₂

A. CO₂ Absorber and Soda Lime

Rebreathing exhaled gas conserves heat and humidity, but CO₂ must be removed to prevent hypercapnia.

Soda lime composition:

Calcium hydroxide Ca(OH)₂: 80% (main component)
Sodium hydroxide NaOH
Potassium hydroxide KOH (small amount)
Water: 14-19%
Silica (added to increase hardness, reduce dust)

Chemical reactions:

CO₂ + H₂O → H₂CO₃              (carbonic acid formation)
H₂CO₃ + 2NaOH → Na₂CO₃ + 2H₂O + Heat    (fast reaction)
Na₂CO₃ + Ca(OH)₂ → CaCO₃ + 2NaOH         (slow reaction)

Water and NaOH are regenerated - soda lime is self-regenerating to a degree
End products: heat, water, calcium carbonate (CaCO₃)
Capacity: 23 L CO₂ per 100 g absorbent
Granule size: 4-8 mesh (balance between surface area and flow resistance)

Exhaustion indicator:

pH indicator dye (ethyl violet): white → purple as absorbent is exhausted
Replace when 50-70% has changed colour
Exhausted granules may revert to white on resting, but absorptive capacity does NOT recover

Volatile anaesthetic degradation hazards:

Agent	Product	Condition
Desflurane	Carbon monoxide (most)	Dry absorbent
Sevoflurane	Compound A (nephrotoxic in animals)	High temperature
Isoflurane/Enflurane	Carbon monoxide	Dry absorbent

Amsorb (calcium hydroxide + calcium chloride): more inert than soda lime; less volatile agent degradation

B. Unidirectional Valves

Ensure one-way gas flow through the circuit
Inspiratory valve: opens on inspiration (gas flows to patient)
Expiratory valve: opens on expiration (gas flows to absorber)
Optimal placement: close to patient but NOT at Y-piece (difficult to confirm function at Y-piece)

C. APL Valve

Spring-loaded valve with adjustable spring tension
Gas vented to scavenger system (not to atmosphere)
Manual ventilation: partially close to build circuit pressure
Spontaneous ventilation: fully open

D. Optimal Component Arrangement

The arrangement matters for efficiency and safety:

Fresh gas inlet → between CO₂ absorber and inspiratory valve
- Downstream of inspiratory valve would waste fresh gas during exhalation
- Between expiratory valve and absorber would dilute gas and interact with soda lime
APL valve → between CO₂ absorber and expiratory valve (close to reservoir bag)
- Conserves absorber capacity; minimises venting of fresh gas
Unidirectional valves → close to patient
- Prevents backflow if circuit leak develops
Reservoir bag → in expiratory limb
- Decreases resistance to exhalation

Performance Characteristics

A. Fresh Gas Flow (FGF) Requirements

FGF	System Mode	Notes
= Patient O₂ uptake	Closed system	Complete rebreathing; FGF matches consumption only
≤1 L/min	Semi-closed (low flow)	CO₂ absorber essential; economical, heat-conserving
1-5 L/min	Semi-closed (moderate flow)	Most common in practice
>5 L/min	Semi-open (high flow)	CO₂ absorber unnecessary; rapid changes in concentration

B. Dead Space

Apparatus dead space = only the volume distal to Y-piece
Unidirectional valves prevent tube dead space (unlike Mapleson circuits)
Corrugated tube length does NOT affect dead space (unlike Mapleson), but DOES affect circuit compliance

C. Resistance

Increased by valves and absorber
Still safe for premature neonates
Worse at high respiratory rates and large tidal volumes

D. Heat and Humidity Conservation

High FGF → low relative humidity (fresh gas is cold and dry)
Low FGF → greater heat and humidity conservation from rebreathing
CO₂ absorbent granules themselves are a significant source of heat and moisture (exothermic reaction)

E. Bacterial Contamination

Minimal but theoretically possible
Bacterial filters recommended at Y-piece, or in inspiratory/expiratory limbs

Modes of the Circle System

Mode	FGF	Absorber needed	Rebreathing
Semi-open	Very high	No	Negligible
Semi-closed	Moderate-low	Yes	Partial
Closed	= Uptake only	Yes	Complete (after CO₂ absorption)

Circle System Monitoring Components

Oxygen Analyser

Mandatory for every general anaesthetic
Types: Polarographic (Clark electrode), Galvanic (fuel cell), Paramagnetic
Placed in inspiratory or expiratory limb (not fresh gas line)
Must have automatic low-level alarm on machine start-up
Paramagnetic type: self-calibrating, no consumable parts, fast enough to differentiate inspired vs. expired O₂

Spirometer (Wright Respirometer)

Measures exhaled tidal volume
Placed near expiratory valve
Common type: rotating vane anemometer
Errors: under-reads at low flows; over-reads at high flows; water condensation causes errors
"Exhaled" volumes at expiratory limb include gas that expanded breathing tubes (not delivered to patient)

Capnography

Measures end-tidal CO₂
Confirms circuit integrity, tracheal tube position, and ventilation adequacy
Normal end-tidal CO₂: 35-45 mmHg
Absent ETCO₂ indicates oesophageal intubation or circuit disconnection

Disadvantages of the Circle System

Larger and less portable than Mapleson circuits
Greater complexity → higher risk of disconnection or malfunction
CO₂ absorbent-related complications (CO production, Compound A)
Unpredictable inspired concentrations at low FGF
Increased resistance from valves and absorber

5. Resuscitation Breathing Systems (AMBU Bag / Bag-Mask Unit)

Used for emergency ventilation - simple, portable, and can deliver nearly 100% oxygen.

Key difference from other circuits: Contains a non-rebreathing valve (unlike Mapleson which has an APL valve, or circle which has unidirectional valves directing flow through the absorber).

How it works:

Patient valve opens during inspiration → gas flows from bag to patient
Exhaled gas vented to atmosphere through exhalation ports (no rebreathing)
Self-refilling compressible bag with intake valve
Reservoir attached to intake valve prevents room air entrainment
FiO₂ ∝ O₂ flow rate / minute ventilation delivered

Disadvantages:

High FGF needed for high FiO₂
Exhaled moisture can cause valve sticking
Exhaled gas vented to environment (infection risk)
Cannot scavenge waste anaesthetic

Comparison of All Breathing Systems

System	Complexity	FGF Required	CO₂ Removal	Rebreathing	Heat/Humidity	Portability	Best Use
Insufflation	Minimal	Very high	Washout	None	No	Excellent	Paediatric induction; ophthalmic surgery
Draw-over	Simple	Low	Washout	None	No	Excellent	Remote/battlefield; no compressed gas
Mapleson A	Simple	= MV (spont)	Washout	Minimal	No	Good	Adult spontaneous ventilation
Mapleson D/Bain	Simple	1-2× MV (ctrl)	Washout	Minimal	Partial	Good	Adult/paediatric controlled ventilation
Mapleson E/F	Simple	2-3× MV	Washout	Minimal	No	Excellent	Neonates and infants
Circle (low flow)	Complex	≤1 L/min	Soda lime	Controlled	Yes	Poor	Prolonged adult GA; low-flow/closed
AMBU bag	Simple	High	None	None	No	Excellent	Emergency; resuscitation

Clinical Pearls

The circle system is the standard for adult GA in theatre; low-flow anaesthesia (≤1 L/min) is economical and environmentally responsible.
Mapleson A requires only MV during spontaneous breathing - the most efficient semi-open circuit for this purpose.
Bain circuit inner tube disconnection causes rebreathing - test by occluding inner tube with a finger (Pethick test): if flow stops at rotameter it is patent.
Soda lime exhaustion presents as rising ETCO₂ despite adequate ventilation.
Dry soda lime degrades desflurane to CO most readily - check absorbent is not desiccated after machine has been idle.
High FGF (>5 L/min) in circle system: allows rapid changes in delivered anaesthetic concentration (useful for induction and emergence); CO₂ absorber is not needed.

References: Morgan & Mikhail's Clinical Anesthesiology 7e, Ch 3 (pp 78-101) | Barash, Cullen & Stoelting's Clinical Anesthesia 9e, Ch 25 | Miller's Anesthesia 10e, Ch 6

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