Airway Assessment: Comprehensive Review with Recent Updates & Ultrasound Assessment

1. WHY AIRWAY ASSESSMENT MATTERS

• Difficult mask ventilation
• Difficult supraglottic airway (SGA) placement
• Difficult laryngoscopy
• Difficult tracheal intubation
• Difficult surgical airway (front-of-neck access)

2. STANDARD CLINICAL AIRWAY ASSESSMENT

A. History

• Prior anesthetic records (previous difficult airway)
• Symptoms: dysphagia, odynophagia, stridor, positional dyspnea, changes in voice
• Diseases: rheumatoid arthritis (atlanto-axial instability), obesity, diabetes (stiff joint syndrome), previous neck surgery, radiotherapy, burns
• Neck masses, obstructive sleep apnea

B. Physical Examination - Bedside Tests

1. Mouth Opening (Inter-incisor Distance)

• Normal: >3 cm (3 fingerbreadths) in adults
• <3 cm predicts difficulty

2. Mallampati Classification (Modified Samsoon & Young)

• Class III/IV predicts difficult intubation (corresponds to Cormack-Lehane Grade III/IV on laryngoscopy)
• Sensitivity ~39%, Specificity ~86% (2024 meta-analysis, 686,089 patients)

3. Thyromental Distance (TMD / Patil's test)

• Measured from the mentum to superior thyroid notch with neck fully extended
• Normal: >6.5 cm (3 fingerbreadths)
• <6 cm = difficult intubation likely
• Sensitivity ~38%, Specificity ~83%

4. Sternomental Distance

• Chin to sternal notch, neck extended
• Normal: >12.5 cm

5. Upper Lip Bite Test (ULBT)

• Class III = difficult intubation; Sensitivity ~52%, Specificity ~84% (best single predictor among physical exam tests per 2024 meta-analysis)

6. Neck Circumference

• >40 cm associated with difficult bag-mask ventilation
• >17 inches (43 cm) associated with difficult glottic visualization

7. Neck Mobility

• Normal: flexion/extension 90°+
• Reduced in rheumatoid arthritis, ankylosing spondylitis, prior fusion, halo traction

8. 3-3-2 Rule (for laryngoscopy geometry)

• 3 fingers = inter-incisor gap
• 3 fingers = hyoid-to-chin distance (mandibular space)
• 2 fingers = hyoid to thyroid notch (to ensure larynx is in submandibular space)

9. Cormack-Lehane Grading (at direct laryngoscopy)

3. MULTIVARIATE SCORING SYSTEMS

LEMON Score (Emergency Medicine Standard)

• Score 0-1: lower risk; Score ≥3: high risk for difficult intubation
• Widely used in emergency settings; higher scores predict difficult laryngoscopy

Wilson Risk Score (5 factors, score ≥2 = difficult intubation risk)

1. Weight >90 kg
2. Head & neck movement <90°
3. Jaw movement limited
4. Receding mandible
5. Buck teeth

El-Ganzouri Score (multiparameter, range 0-12)

4. PREDICTORS OF DIFFICULT BAG-MASK VENTILATION

• M - Mask seal (beard, facial abnormality)
• O - Obesity/Obstruction
• A - Age >55 years
• N - No teeth (edentulous)
• S - Snoring/Stiff lungs (COPD, pregnancy)

5. THE DIFFICULT AIRWAY ALGORITHM (ASA/DAS)

ASA Task Force - 4 Core Steps:

1. Assess likelihood of difficult: cooperation/consent, mask ventilation, SGA, laryngoscopy, intubation, surgical airway
2. Actively pursue supplemental O2 throughout
3. Consider basic choices: Awake vs. asleep intubation; non-invasive vs. invasive; video laryngoscopy as initial approach; preserve vs. ablate spontaneous ventilation
4. Primary + alternative strategies ready before induction

• If face mask ventilation adequate → Nonemergency pathway → alternative intubation devices
• If face mask ventilation NOT adequate → Attempt SGA → if SGA adequate = nonemergency pathway; if SGA fails → Emergency pathway → Front-of-Neck Access (FONA)

6. ULTRASOUND (USG/POCUS) AIRWAY ASSESSMENT

Why USG? The Paradigm Shift

USG Applications in Airway Assessment

A. Prediction of Difficult Laryngoscopy

• Linear high-frequency probe (7-12 MHz), transverse view at level of thyroid cartilage
• Pooled Sensitivity: 0.84 (95%CI 0.74-0.91)
• Pooled Specificity: 0.81 (95%CI 0.61-0.92)
• AUROC: 0.87 - HIGH certainty of evidence

B. Prediction of Difficult Tracheal Intubation

• Transverse probe at hyoid level, scan down to visualize epiglottis
• Sensitivity: 0.80, Specificity: 0.86 - HIGH certainty of evidence
• Increased DSE = more soft tissue = more difficult intubation

• Highest AUROC: 0.86 (95%CI 0.73-0.92) - moderate certainty
• Sensitivity 0.78, Specificity 0.81

C. Prediction of Difficult Bag-Mask Ventilation

• Tongue thickness (assessed at floor-of-mouth): increased thickness associated with difficult mask ventilation
• Useful in obese patients where clinical assessment is unreliable

D. Cricothyroid Membrane (CTM) Identification

• Gold standard application with strongest evidence
• USG vs. palpation for CTM identification - OR: 3.61 (95%CI 2.20-5.92) - moderate-high certainty
• DAS 2025 guidelines explicitly recommend USG (or visual assessment) for CTM identification during preoperative airway assessment, especially in obese patients or those with difficult anatomy
• Technique: Linear probe, midline neck, longitudinal scan - identify thyroid cartilage (hyperechoic) → CTM (hypoechoic band between thyroid and cricoid cartilages) → cricoid ring

E. Endotracheal Tube Confirmation

• Trachea: hyperechoic horseshoe-shaped cartilage, posterior acoustic shadowing (air artifact)
• On intubation: single echogenic line (comet-tail artifact) in trachea = correct placement
• Esophageal intubation: second lumen appears posterior/lateral to trachea ("double trachea sign")
• Useful when capnography unreliable (low cardiac output, cardiac arrest)

F. Percutaneous Tracheostomy Guidance

• USG use associated with higher first-pass success: OR 3.9 (95%CI 2.1-7.1) - low-moderate certainty
• Identifies tracheal rings, avoids inadvertent vessel injury, confirms Seldinger wire position

USG Technique Summary

7. RECENT UPDATES (2024-2025)

DAS 2025 Guidelines (Difficult Airway Society)

1. Video laryngoscopy as the initial approach for all anticipated and unanticipated difficult airways
2. Ultrasound identification of the CTM is now recommended as a standard step during airway assessment - ideally with neck in neutral or extended position preoperatively
3. Maximum 3 attempts at laryngoscopy before escalating (same operator); call for help early
4. Physiologically difficult airway concept: hemodynamically unstable, hypoxic, or acidotic patients - POCUS cardiac/gastric assessment integrated into airway planning
5. Front-of-neck access (FONA) = scalpel cricothyrotomy preferred over needle technique in emergency

2024 Meta-Analysis Findings (Wang Z et al., BMC Anesthesiology, 2024 - 686,089 patients)

• No single test is superior
• Ultrasound DSE outperforms all traditional tests with Sn 0.80, Sp 0.77
• ULBT performs better than Mallampati alone
• Composite/multimodal assessment mandatory - no single test sufficient
• AI and deep learning for real-time airway prediction emerging

2025 POCUS Meta-Analysis (Soni VM et al., Anaesthesia, 2025 - 10,580 patients)

• Skin-to-vocal cords = best single predictor for difficult laryngoscopy (AUROC 0.87)
• Skin-to-epiglottis = best for difficult intubation (Sn 0.80, Sp 0.86)
• USG CTM identification superior to palpation (OR 3.61)
• POCUS should be integrated with standard tests - not used in isolation

8. SPECIAL SITUATIONS

Obese Patients

• Neck circumference >40 cm = strong predictor of difficult mask ventilation
• Ramp position ("sniffing" extended with shoulders elevated) mandatory
• USG tongue thickness and DSE especially useful when standard exam unreliable

Obstetric Airway

• Mallampati score worsens during labor (edema, fluid shifts)
• Higher risk of difficult and failed intubation than general surgical population
• Failed intubation rate ~1:300 in obstetrics vs. ~1:2000 general

Trauma / Emergency Airway

• LEMON score used to stratify risk quickly
• Assume cervical spine injury = manual in-line stabilization during laryngoscopy
• Blood/secretions = SALAD technique (Suction-Assisted Laryngoscopy Airway Decontamination)
• POCUS in trauma: predicts difficult airway, confirms ETT, identifies CTM in distorted anatomy

Cannot Intubate, Cannot Oxygenate (CICO)

1. Call for help immediately
2. SGA attempt
3. If SGA fails → Scalpel cricothyrotomy (DAS 2025: preferred over needle technique)
4. Ultrasound pre-marking of CTM reduces time to surgical airway in CICO

9. KEY TAKEAWAYS

• Morgan & Mikhail's Clinical Anesthesiology, 7e (pp. 570-573, 608-609)
• Miller's Anesthesia, 10e (p. 5081)
• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e (p. 4528)
• Tintinalli's Emergency Medicine, Rosen's Emergency Medicine
• Soni VM et al., Anaesthesia, 2025 (PMID 40891437) - POCUS airway meta-analysis (60 studies, 10,580 patients)
• Wang Z et al., BMC Anesthesiology, 2024 (PMID 39020308) - Airway prediction methods meta-analysis (227 studies, 686,089 patients)
• Ahmad I et al., Br J Anaesth, 2025 - DAS 2025 Guidelines for unanticipated difficult intubation