Supraglottic Airway Devices (SADs) - Comprehensive Overview

Definition and General Principles

• Spontaneously breathing and mechanically ventilated patients during anesthesia
• Rescue ventilation when bag-mask ventilation (BMV) and ETT intubation fail
• As conduits for tracheal intubation in the difficult airway
• Pre-hospital and resuscitation scenarios

Classification

First-Generation SADs

• LMA Classic
• LMA Unique (disposable version)
• LMA Flexible

Second-Generation SADs

• LMA ProSeal
• LMA Supreme
• i-gel
• Air-Q

Esophageal/Pharyngeal Airway Devices (distinct SAD subtype)

• Esophageal-Tracheal Combitube
• King Laryngeal Tube (King LT/LTS)

Specific Devices

1. LMA Classic (cLMA)

• Base of tongue - superiorly
• Pyriform sinuses - laterally
• Upper esophageal sphincter - inferiorly

1. Choose appropriate size; check cuff for leaks
2. Deflate cuff so the leading edge is wrinkle-free, facing away from the aperture
3. Lubricate the back side of cuff only
4. Ensure adequate anesthesia/muscle relaxation (slightly more than for an oral airway)
5. Place patient in sniffing position (neck flexed, head extended)
6. Hold device like a pen; index finger at junction of tube and cuff
7. Press the tip against the hard palate and slide along the palate down into the hypopharynx until resistance is felt
8. Inflate cuff; longitudinal black line should face cephalad (toward upper lip)
9. Secure with tape

• Successful ventilation in ~98% of adults with known difficult airways
• ~90-95% success in unexpectedly difficult airways
• 88-100% placement success rate overall

2. LMA ProSeal (first second-generation SAD)

• First insertion success slightly lower than cLMA
• Size down from cLMA equivalent
• Particularly advantageous for obese patients, intra-abdominal procedures

3. LMA Supreme (single-use ProSeal variant)

• Disposable, fixed-curvature device
• Supports inspiratory pressures >35 cm H₂O
• Gastric drain tube incorporated
• Rigid pre-curved shaft - does not require cuff inflation index-finger technique
• Available for adult and pediatric use

4. i-gel

• A drain tube runs from the distal tip (positioned over the esophageal inlet) to an outlet lateral to the airway connector - accepts up to a 14-French gastric tube
• Airway leak pressures: 24-30 cm H₂O in adults
• Lower incidence of postoperative sore throat than LMA
• Less visible blood on device at removal
• Modestly shorter insertion times vs LMA ProSeal, but slightly lower oropharyngeal leak pressures
• The solid mask does not change shape with body temperature (contrary to popular belief)

5. Intubating LMA (ILMA) / LMA Fastrach

• Rigid stainless steel handle for precise positioning
• Rigid curved airway tube (allows one-hand insertion)
• Epiglottic elevating bar (EEB) that lifts the epiglottis as the tube advances
• Wide enough shaft for a standard ETT
• Specialized straight wire-reinforced ETT with soft molded tip designed for atraumatic blind passage

1. Neutral head position (non-extended) - different from cLMA
2. Use the handle to rotate the mask into the pharynx
3. Stabilize ventilation after insertion
4. Chandy maneuver: (i) rotate ILMA in sagittal plane until ventilation resistance is minimal; (ii) gently lift from posterior pharyngeal wall just before ETT passage
5. The "up-down maneuver" corrects down-folded epiglottis: withdraw ILMA ~5-6 cm while cuff remains inflated, then slide back pressing against posterior pharynx

• Technically easier to use than cLMA in emergency settings
• More practical when definitive airway (ETT) is the goal
• Better performance with head in neutral position / in-line stabilization of the cervical spine
• Particularly useful in: beards, severe facial trauma, obesity (none of these inhibit placement)
• Useful when brisk supraglottic bleeding makes conventional intubation difficult

6. LMA CTrach

7. LMA Flexible (Reinforced LMA)

• Wire-reinforced flexible shaft resists kinking and compression
• Shaft can be moved out of the surgical field without displacing the cuff
• Useful for oral/dental/ENT surgery where the airway tube must be flexible
• Not suitable as an intubation conduit

8. Esophageal-Tracheal Combitube

• Longer blue tube: occluded distal tip, gas exits through side perforations
• Shorter clear tube: open tip, no side perforations
• Two inflatable cuffs: 100-mL proximal pharyngeal cuff + 15-mL distal cuff
• Both cuffs inflated after device is advanced until the two black rings lie between the teeth

• In ~95% of insertions, the distal end sits in the esophagus - ventilate through the blue (longer) tube, gas exits side perforations into the larynx
• If the tube enters the trachea (5%), ventilate through the clear (shorter/tracheal) tube
• The clear tube can also drain gastric contents
• Requires minimum mouth opening; not appropriate for patients <4 feet tall

9. King Laryngeal Tube (King LT / King LTS-D)

• Distal cuff ocstructs the upper esophagus
• Proximal cuff seals the oral and nasal pharynx
• Apertures between cuffs serve as ventilation orifices

10. Air-Q (Cookgas Perilaryngeal Airway)

• Functions as both an elective SGA and intubation conduit (blind or fiberoptic-assisted)
• Cuff grossly similar in shape to LMA Classic cuff
• Airway seal pressures: 25-30 cm H₂O
• Sizes 0.5 to 4.5
• Insertion technique similar to LMA
• Fill cuff with <10 mL air (poor seal often due to over-inflation)
• If airway obstruction occurs after insertion: up-down motion of barrel realigns epiglottis
• Self-pressurizing air-Q variant: no cuff insufflation required; intracuff pressure varies with airway pressure - may improve fiberoptic visualization

Comparison of Key SADs

Indications

• Elective anesthesia: Suitable for low-risk surgeries not requiring ETT (peripheral, ophthalmic, brief ENT procedures)
• Difficult airway rescue: When BMV and ETT intubation have both failed - "cannot intubate, cannot oxygenate" scenario
• Conduit for intubation: ILMA, air-Q, or cLMA with fiberoptic scope
• Prehospital/cardiac arrest: Where endotracheal intubation is not feasible or causes unacceptable pauses in CPR
• Pediatric airways: LMA Classic allows adequate ventilation in >98% of adults and is reliable in difficult pediatric airways
• Cervical spine immobilization: ILMA performs better than cLMA with head in neutral/in-line stabilization

Contraindications

• Limited mouth opening (<1.5-2 cm)
• Pharyngeal pathology (abscess, obstruction)
• Glottic or subglottic obstruction

• Full stomach / high aspiration risk (pregnancy, hiatal hernia, intestinal obstruction, delayed gastric emptying)
• High airway resistance or low pulmonary compliance (peak inspiratory pressures >30 cm H₂O) - second-generation devices with higher seal pressures are preferred if SAD is necessary
• Inadequate sedation/paralysis

Note: SADs may be used as an airway salvage technique even in patients with aspiration risk, when the alternative is hypoxia.

Advantages Over ETT

• Improved protection from blood and surgical debris
• Reduced cardiovascular responses (no tracheal stimulation)
• Reduced coughing on emergence
• Reduced intraoperative bronchospasm
• Reduced laryngospasm after removal
• Improved oxygen saturation after removal
• Ability to administer O₂ until full return of airway reflexes
• Postoperative sore throat: 4-50% (vs 30-60% with ETT)

Complications

• Aspiration of gastric contents (primary risk - reduced but not eliminated by 2nd-gen devices)
• Laryngospasm, bronchospasm, coughing, gagging
• Nerve injuries (rare): Hypoglossal, lingual, and/or recurrent laryngeal nerves - typically manifest within 48 hours, resolve in 1 hour to 18 months. Cause: pressure neuropraxia from tube/cuff. Risk factors include small masks, lidocaine lubrication, nitrous oxide, cuff over-inflation, difficult insertion, non-supine positioning, cervical joint disease
• Postoperative sore throat and dysphagia: No single device consistently lower; all better than ETT
• Partial or complete airway obstruction (usually due to epiglottic down-folding)
• Gastric distension if esophagus lies within the rim of the cuff
• Tongue and uvular ulceration (reported with King LT)

Removal

1. When the patient is deeply anesthetized (avoids stimulation in excitation stage), or
2. After airway reflexes have fully returned - patient opens mouth on command

SAD as Intubation Conduit

• ILMA/Fastrach: Highest success for blind intubation; Chandy maneuver optimizes position; FIS-guided intubation through ILMA achieves higher first-attempt success than blind technique
• cLMA + FIS: Possible but requires a microlaryngeal tube due to narrow shaft
• cLMA + Aintree catheter + FIS: The Aintree intubation catheter (hollow airway exchange catheter) fits over a standard FIS; allows exchange from cLMA to ETT
• Most newer SGAs: Shaft wide enough for standard-sized ETT with or without FIS guidance
• LMA CTrach: Built-in camera facilitates guided intubation (largely replaced by video laryngoscopy)

Special Populations

• LMA Classic is the most extensively tested in children; more reliable than bag-mask in 98% of cases
• Provides safer means of ventilation than BMV in neonates and infants prone to gastric distension
• King LT: not recommended for children <10 kg; less effective than LMA in children <10 years
• SADs are life-saving rescue devices in difficult pediatric airways

• SADs are not the primary airway of choice (aspiration risk)
• Acceptable as rescue device when intubation fails
• King LTS-D has been used successfully during failed RSI for cesarean section

• ILMA preferred over cLMA; performs better with head in neutral/in-line stabilization
• Less cervical spine movement than direct laryngoscopy

• ILMA: beard, severe facial trauma, and obesity do not inhibit placement
• LMA ProSeal/Supreme: higher seal pressures accommodate higher ventilation pressures often needed

• Morgan and Mikhail's Clinical Anesthesiology, 7e, pp. 581-587
• Miller's Anesthesia, 10e, pp. 5950-5951
• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e, pp. 2322-2327
• Roberts and Hedges' Clinical Procedures in Emergency Medicine, pp. 99-100
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 1, p. 384
• Tintinalli's Emergency Medicine, p. 219

Mapleson Breathing Systems - Comprehensive Overview

Historical Background

General Principles

What They Are

• Accept fresh gas flow (FGF) from an anesthesia machine
• Supply gas from a reservoir to meet inspiratory flow and volume requirements
• Eliminate CO₂

How They Differ From the Circle System

• Bidirectional gas flow (no unidirectional valves)
• No CO₂ absorber - CO₂ elimination depends entirely on washout by fresh gas inflow
• Therefore require higher FGF rates to prevent rebreathing
• No risk of volatile anesthetic degradation by CO₂ absorbent
• Simpler, lighter, fewer components

Key Concept: CO₂ Rebreathing

1. Fresh gas inflow rate
2. Patient's minute ventilation
3. Mode of ventilation (spontaneous vs. controlled)
4. Tidal volume and respiratory rate
5. I:E ratio and duration of the expiratory pause
6. Peak inspiratory flow rate
7. Volume of reservoir tube and breathing bag
8. Whether ventilation is via mask or ETT
9. CO₂ sampling site

Components of Mapleson Circuits

1. Breathing Tubes (Corrugated Reservoir Tubing)

• Large diameter (22 mm) - creates a low-resistance pathway and acts as a gas reservoir
• Volume should be ≥ patient's tidal volume in most circuits to minimize FGF requirements
• Compliance of the breathing tube largely determines circuit compliance; high compliance = significant tidal volume lost to circuit expansion
  • Example: circuit with compliance of 8 mL/cm H₂O pressurized to 20 cm H₂O loses 160 mL per breath to circuit expansion
• Not present in Mapleson C

2. Fresh Gas Inlet (FGI)

• Continuous inflow of anesthetic gases mixed with O₂/air from the anesthesia machine
• The relative position of the FGI is the key distinguishing feature among Mapleson circuits and is the primary determinant of efficiency

3. Adjustable Pressure-Limiting (APL) Valve (Pop-Off Valve, Pressure-Relief Valve)

• Controls pressure by allowing excess gas to vent to atmosphere or scavenging system
• During spontaneous ventilation: APL should be fully open (circuit pressure remains negligible)
• During assisted/controlled ventilation: partially closed to allow positive pressure build-up
• Position relative to FGI is the other key differentiating feature among circuits

4. Reservoir Bag

• Provides a gas reservoir to meet peak inspiratory flow demands
• Allows hand ventilation and monitoring of ventilation during spontaneous breathing
• Not present in Mapleson E

The Six Mapleson Systems

Mapleson A - "Magill Circuit" / "Magill Attachment"

• FGI enters near the reservoir bag (far from the patient)
• APL valve is positioned near the patient (at the mask end)
• Reservoir bag is at the machine end

• Enclosed Magill system: improves efficiency
• Coaxial Mapleson A (Lack breathing system): incorporates fresh gas tube inside the breathing tube; facilitates waste gas scavenging because the APL valve is displaced away from the patient

Mapleson B

• FGI and APL valve are both located near the patient (at the mask end)
• Reservoir bag at the opposite end

• Spontaneous ventilation: FGF = 2× minute ventilation
• Controlled ventilation: FGF = 2-2.5× minute ventilation

Mapleson C - "Waters' To-and-Fro" Circuit

• FGI and APL valve both near the patient (similar to B)
• No corrugated reservoir tubing (shortest of all Mapleson circuits)
• Reservoir bag is directly connected near the patient

• Spontaneous ventilation: FGF = 2× minute ventilation
• Controlled ventilation: FGF = 2-2.5× minute ventilation

Mapleson D

• FGI enters near the patient (at the mask end)
• APL valve is near the reservoir bag (away from the patient)
• Reservoir bag at the machine end
• This is the exact inverse of the Mapleson A

• Spontaneous ventilation: FGF = 2-3× minute ventilation
• Controlled ventilation: FGF = 1-2× minute ventilation (most efficient for controlled ventilation)

Mapleson E - "Ayre's T-Piece"

• FGI enters near the patient
• Open expiratory limb (reservoir tube) at the distal end - no APL valve, no reservoir bag
• The only Mapleson system without a breathing bag
• T-piece design: the patient connection, FGI, and expiratory limb form a T

• Spontaneous ventilation: FGF = 2-3× minute ventilation
• Controlled ventilation: FGF = 3× minute ventilation (I:E ratio 1:2 assumed; exhalation tubing must provide a volume larger than tidal volume to prevent rebreathing)

• Minimal dead space
• Very low resistance (no valves at all)
• Particularly suitable for pediatric patients

• Cannot support spontaneous ventilation without modification unless FGF is very high
• Controlled ventilation requires intermittent occlusion of the expiratory limb
• Scavenging is very difficult (open expiratory limb)
• No ability to monitor or assist ventilation (no bag)

Mapleson F - "Jackson-Rees Circuit" (Jackson-Rees Modification of Ayre's T-Piece)

• A Mapleson E (Ayre's T-piece) with a breathing bag added to the expiratory limb
• The bag has an open "tail" which may have an adjustable valve
• FGI enters near the patient
• APL valve is on the open tail of the bag (away from the patient)

• Spontaneous ventilation: FGF = 2.5-3× minute ventilation
• Controlled ventilation: FGF = 1.5-2× minute ventilation

• The bag allows monitoring of spontaneous ventilation
• Allows manual controlled ventilation (squeeze the bag, partially occlude the tail)
• Allows CPAP application
• Still has low resistance and minimal dead space

Summary: FGF Requirements

• A = Best for spontaneous (FGF = 1× MV)
• D/E/F = Best for controlled (FGF ~1.5-2.5× MV)
• B/C = Intermediate (FGF ~2-2.5× MV for both)

Functional Groupings (Barash)

1. Group A (alone) - unique behavior, best for spontaneous ventilation
2. Group B and C - similar behavior, intermediate efficiency for both modes
3. Group D, E, and F - the "T-piece group," best for controlled ventilation, most widely used today

Special Modification: The Bain Circuit

• Fresh gas flows through a narrow inner tube inside the outer corrugated hose
• The inner tube enters the corrugated hose near the reservoir bag but empties into the circuit at the patient end
• Exhaled gases pass down the outer corrugated hose, around the central tubing, and are vented through the pop-off valve near the reservoir bag

1. Lightweight and convenient - decreases circuit bulk
2. Better heat and humidity retention - exhaled warm gas in the outer tube warms the inspired fresh gas via countercurrent heat exchange
3. Easier scavenging - APL valve is away from the patient
4. Disposable versions available

• Kinking or disconnection of the inner fresh gas tube - most serious hazard; if unrecognized, causes significant rebreathing of exhaled CO₂ and potential hypercapnia
• The outer corrugated tube must be transparent for ongoing inspection of the inner tube

1. High-flow O₂ is fed into the circuit while occluding the patient end until the reservoir bag fills
2. Patient end is opened and O₂ is flushed into the circuit
3. If inner tube is intact: Venturi effect at the patient end decreases circuit pressure → reservoir bag deflates
4. If inner tube is leaking/disconnected: Fresh gas escapes into the expiratory limb → reservoir bag remains inflated

Advantages of Mapleson Systems (General)

1. Low resistance to gas flow - no unidirectional valves
2. Simple, few components - lightweight, easy to maintain
3. Rapid changes in inspired gas composition - no large circuit volume to equilibrate
4. No CO₂ absorber - no risk of volatile anesthetic degradation (e.g., compound A production with sevoflurane)
5. Small dead space - especially Mapleson E and F (important in pediatrics)
6. Useful for patient transport and ICU - portable, requires no electricity

Disadvantages

1. High FGF requirements (except Mapleson A for spontaneous ventilation) - less economical with carrier gas and volatile agent compared to circle system
2. Less heat and humidity conservation - FGF continuously removes moisture and heat (partial exception: Bain circuit)
3. Difficult waste gas scavenging - especially Mapleson A, E (APL valve near patient or absent)
4. Not suitable as primary anesthetic delivery in modern practice in high-resource settings (circle system preferred)
5. Some rebreathing always occurs unless FGF is very high
6. Mapleson A is a poor choice for controlled ventilation

Clinical Uses Today

Sources

• Morgan and Mikhail's Clinical Anesthesiology, 7e, pp. 83-88
• Miller's Anesthesia, 10e, pp. 2351-2354
• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e, pp. 2010-2013