7.1 Face Masks, Supraglottic Airways (LMA Types) & Endotracheal Tubes

7.1 Face Masks, Supraglottic Airways (LMA Types) & Endotracheal Tubes

Expert mastery of airway devices, from basic non-invasive adjuncts to specialized endotracheal tubes, is foundational to nurse anesthesia practice. Airway management requires precise understanding of device anatomy, mucosal perfusion thresholds, sealing pressures, and the physical principles governing cuff expansion and airway trauma.

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1. Face Masks & Airway Adjuncts

Face Mask Ventilation & Mechanics

Mask ventilation is the initial rescue technique in general anesthesia and resuscitation. Effective face mask ventilation requires a continuous airtight seal and a patent upper airway passage.

• Technique: The single-handed "E-C clamp" technique utilizes the thumb and index finger to form a "C" over the body of the mask, pressing downward, while the third, fourth, and fifth fingers form an "E" on the bony mandible to pull the jaw forward (jaw thrust) and extend the atlanto-occipital joint. In difficult mask ventilation (e.g., edentulous, bearded, obese, Mallampati III/IV), a two-person, four-handed technique is superior.
• Apparatus Dead Space: Standard adult face masks add approximately $150 - 200 \text{ mL}$ of apparatus dead space between the circuit Y-piece and the patient's facial contour.

+-------------------------------------------------------------------------+
|                   AIRWAY ADJUNCT COMPARISON & SIZING                    |
+-----------------------------------+-------------------------------------+
| OROPHARYNGEAL AIRWAY (OPA)        | NASOPHARYNGEAL AIRWAY (NPA)         |
+-----------------------------------+-------------------------------------+
| • Sizing: Corner of mouth /       | • Sizing: Tip of nose to tragus     |
|   central incisors to the angle   |   of the ear / external auditory    |
|   of the mandible.                |   meatus (plus ~2.5 cm in adults).  |
| • Guedel: Central hollow lumen.   | • Soft rubber / plastic "Trumpet"   |
| • Berman: Solid central I-beam    |   with flared proximal flange.      |
|   with dual side channels.        | • Bevel facing the nasal septum to  |
| • Function: Depresses tongue base |   prevent turbinate laceration.     |
|   off the posterior pharynx.      | • Tolerated in lighter planes of    |
| • Patient State: UNCONSCIOUS only |   anesthesia than an OPA.           |
|   (triggers severe gagging,       | • Absolute Contraindication:        |
|   vomiting, laryngospasm if light)|   Basilar skull fracture / cribriform|
| • Nerve Injury Risk: Lingual or   |   plate disruption; severe          |
|   mandibular branch of CN VII.    |   coagulopathy or epistaxis risk.   |
+-----------------------------------+-------------------------------------+

NCE Clinical Pearl — Basilar Skull Fracture Signposts: Signs of basilar skull fracture include periorbital ecchymosis ("raccoon eyes"), mastoid ecchymosis ("Battle sign"), hemotympanum, and clear cerebrospinal fluid (CSF) rhinorrhea or otorrhea. In these patients, nasopharyngeal airways and nasogastric tubes are strictly contraindicated due to the catastrophic risk of intracranial penetration through a fractured cribriform plate.

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2. Supraglottic Airway Devices (SADs / LMAs)

Supraglottic airways bridge the gap between the face mask and the endotracheal tube. They sit in the hypopharynx, sealing the perilaryngeal area around the glottic aperture above the vocal cords.

+-------------------------------------------------------------------------+
|                    SUPRAGLOTTIC AIRWAY GENERATIONS                      |
+-----------------------------------+-------------------------------------+
| FIRST-GENERATION SADs             | SECOND-GENERATION SADs              |
+-----------------------------------+-------------------------------------+
| • LMA Classic (reusable silicone) | • LMA ProSeal (dorsal cuff, gastric |
| • LMA Unique (disposable PVC)     |   drainage channel, seal 30 cmH₂O)  |
| • LMA Flexible (wire-reinforced)  | • LMA Supreme (curved anatomy,      |
| • LMA Fastrach (Intubating LMA)   |   dual seal, gastric suction port)  |
| • Characteristics:                | • i-gel (thermoplastic elastomer cuff|
|   - Simple breathing tube + cuff  |   non-inflatable, gastric channel,  |
|   - NO gastric access port        |   integrated bite block)            |
|   - Oropharyngeal seal:           | • Characteristics:                  |
|     ~18 - 20 cmH₂O                |   - Dedicated gastric drain tube    |
|   - Lower protection against      |   - Integrated bite block           |
|     pulmonary aspiration          |   - Higher seal: 25 - 30 cmH₂O      |
+-----------------------------------+-------------------------------------+

Sizing, Weight Ranges & Maximum Inflation Volumes

Mucosal Perfusion Dynamics & Cuff Pressure Limits

• Capillary Perfusion Pressure of Pharyngeal Mucosa: Approximately $60 \text{ mmHg}$ ($,\approx 80 \text{ cmH}_2\text{O}$).
• Maximal Recommended Intracuff Pressure: $< 60 \text{ cmH}_2\text{O}$ (routinely measured with an analog or digital manometer).
• Consequences of Cuff Overinflation ($> 60 \text{ cmH}_2\text{O}$): Overinflating the cuff does not improve seal quality; it forces the silicone or PVC membrane against the hypopharyngeal mucosa, collapsing capillary beds and causing ischemic neuropraxia:
  1. Lingual Nerve: Compressed between the inner mandibular ramus and LMA tube; results in numbness and loss of taste on the anterior two-thirds of the tongue.
  2. Hypoglossal Nerve (CN XII): Compressed against the hyoid bone; results in ipsilateral tongue weakness, motor dysfunction, and deviation toward the side of injury.
  3. Recurrent Laryngeal Nerve (CN X): Compressed near the cricoid cartilage; results in vocal cord paralysis, hoarseness, and stridor.

Clinical Contraindications & Aspiration Caveats for LMAs

LMAs do not provide a 100% barrier against pulmonary aspiration. Contraindications include:

• Non-fasted status / "full stomach" (emergency trauma, unknown NPO status).
• Symptomatic gastroesophageal reflux disease (GERD) or active hiatal hernia.
• Morbid obesity with BMI $>35 - 40 \text{ kg/m}^2$ (elevated intragastric and intrathoracic pressures).
• Delayed gastric emptying (diabetic gastroparesis, bowel obstruction, acute opioid administration, pregnancy $\ge 14 - 16$ weeks).
• High peak airway pressures ($> 20 - 25 \text{ cmH}_2\text{O}$ required for ventilation, e.g., severe bronchospasm, restrictive lung disease, laparoscopy in steep Trendelenburg).

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3. Endotracheal Tubes: Anatomy, Materials & Cuff Dynamics

                    [15 mm Machine Connector]
                                |
                                v
    +=======================================================+
    |                      ETT TUBE BODY                    |   <-- Radiopaque Line
    +=======================================================+
          |                                      |       \
          |                                      |        \ [Murphy Eye]
    [Pilot Balloon]                        [HVLP Cuff]     \  (Bevel Angle)
          |                                                 v
    [One-Way Valve]                                    [Tube Tip]

Core Structural Components

1. Material: Transparent polyvinyl chloride (PVC), non-toxic, radiopaque stripe along the entire length for chest radiograph confirmation.
2. Universal 15-mm Connector: Connects the proximal ETT to standard anesthesia circuit elbows and HME filters.
3. Bevel: Left-facing oblique cut at the patient end that facilitates visualization of the vocal cords as the tube approaches the glottis from the right side of the oropharynx.
4. Murphy Eye: A side hole positioned opposite the bevel.
   • Critical Function: Provides a secondary, collateral gas conduit if the primary distal bevel orifice becomes occluded by mucous, blood clots, or adherence against the tracheal lateral wall or carina. (Magill-type tubes lack a Murphy eye).

High-Volume Low-Pressure (HVLP) vs. Low-Volume High-Pressure (LVHP) Cuffs

+-------------------------------------------------------------------------+
|                        ETT CUFF DYNAMICS & PRESSURE                     |
+-----------------------------------+-------------------------------------+
| HIGH-VOLUME LOW-PRESSURE (HVLP)   | LOW-VOLUME HIGH-PRESSURE (LVHP)     |
+-----------------------------------+-------------------------------------+
| • Thin, compliant, large resting  | • Thick, rigid, small resting       |
|   surface area.                   |   volume / diameter.                |
| • Distributes pressure over large | • Concentrates high sealing pressure|
|   tracheal contact area.          |   onto a small focal mucosal ring.  |
| • Standard in modern clinical ETTs| • Historically used / specialty.    |
| • Intracuff Pressure Target:      | • Very high risk of mucosal necrosis|
|   20 - 30 cmH₂O (15 - 22 mmHg)    |   and post-intubation stenosis.     |
| • Drawback: Forms longitudinal    | • Advantage: Better seal against    |
|   micro-folds that allow silent   |   aspiration, small external profile|
|   aspiration of subglottic fluid. |   during tube passage.              |
+-----------------------------------+-------------------------------------+

Tracheal Capillary Arteriolar Perfusion Pressure Targets

• Tracheal Mucosal Capillary Pressure: Normal mucosal capillary perfusion pressure is approximately $25 - 30 \text{ mmHg}$ ($34 - 40 \text{ cmH}_2\text{O}$). Venous capillary drainage pressure is $\approx 18 - 20 \text{ mmHg}$.
• Optimal Cuff Pressure Window: $20 - 30 \text{ cmH}_2\text{O}$ ($15 - 22 \text{ mmHg}$).
  • If Cuff Pressure $> 30 \text{ cmH}_2\text{O}$: Exceeds mucosal capillary arteriolar perfusion pressure $\to$ mucosal ischemia, epithelial desquamation, ulceration, tracheal ring chondritis, and long-term fibrotic tracheal stenosis or tracheomalacia.
  • If Cuff Pressure $< 20 \text{ cmH}_2\text{O}$: Insufficient seal $\to$ leak of positive-pressure tidal volume and silent microaspiration of contaminated oral secretions past cuff folds into the lower respiratory tract (leading to ventilator-associated pneumonia).

Nitrous Oxide ($N_2O$) Cuff Expansion Dynamics

Nitrous oxide rapidly expands air-filled closed gas spaces via differential solubility:

• Blood:Gas Partition Coefficient of $N_2O$: $0.47$ (highly diffusible).
• Blood:Gas Partition Coefficient of Nitrogen ($N_2$): $0.015$ (poorly diffusible).
• Mechanism: $N_2O$ diffuses into an air-filled ETT or LMA cuff down its concentration gradient 34 times faster than nitrogen can diffuse out of the cuff into surrounding tissues. Consequently, within 60–120 minutes of $N_2O$ administration, intracuff pressures can double or triple (often exceeding $50 - 70 \text{ cmH}_2\text{O}$).
• Preventative Strategies: Intermittent cuff manometer deflations, filling the pilot cuff with saline or sterile water (in laser cases), or filling the cuff with an oxygen-nitrous oxide mixture matching the inspired gas composition.

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4. Specialized Endotracheal Tubes

+------------------------------------+-------------------------------------+
| TUBE TYPE                          | CLINICAL DESIGN & NCE PEARLS        |
+------------------------------------+-------------------------------------+
| RAE Tube (Ring-Adair-Elwyn)        | • Preformed acute bend angle.       |
| (Oral RAE vs. Nasal RAE)           | • Oral RAE: Bends downward over     |
|                                    |   chin (ENT, cleft palate, dental). |
|                                    | • Nasal RAE: Bends upward over      |
|                                    |   forehead (mandibular, cosmetic).  |
|                                    | • Trap: Depth cannot be adjusted;   |
|                                    |   flexion causes mainstem intubation|
|                                    |   and extension causes extubation.  |
+------------------------------------+-------------------------------------+
| Reinforced / Armored Tube          | • Stainless steel or nylon spiral   |
| (Anode Wire-Embedded Tube)         |   wire embedded in silicone/PVC.    |
|                                    | • Resists kinking during acute neck |
|                                    |   flexion, prone spine, neurosurgery|
|                                    | • Trap: If patient bites the tube,  |
|                                    |   the wire permanently collapses    |
|                                    |   (MUST insert rigid bite block).   |
+------------------------------------+-------------------------------------+
| Laser-Resistant Tubes              | • Stainless steel (Laser-Flex) or   |
| (CO₂, Nd:YAG, KTP Lasers)          |   foil-wrapped silicone/rubber.     |
|                                    | • Dual Cuffs (proximal & distal).   |
|                                    | • Cuffs filled with STERILE SALINE  |
|                                    |   and METHYLENE BLUE.               |
|                                    | • Saline absorbs laser heat; dye    |
|                                    |   alerts immediately to perforation.|
+------------------------------------+-------------------------------------+

NCE Exam Trap — Airway Fire Safety & Laser Tubes: In airway laser surgery, the $FiO_2$ must be titrated to the lowest possible level ($< 0.30$) using an air-oxygen blend (avoiding $N_2O$ entirely, as $N_2O$ supports combustion like oxygen). If a laser strike punctures the outer cuff, the sterile saline extinguishes embers and the methylene blue stains the surgical field, signaling the surgeon to halt laser discharge immediately while the intact distal cuff continues to maintain the airway seal.