Can Pre-operative Airway USG Predict Whether Mask Ventilation Will Improve After Muscle Relaxant?

Is This a Valid Research Question?

What the Existing Evidence Shows

1. Airway USG Can Predict Difficult Mask Ventilation (DMV)

2. The Muscle Relaxant - Mask Ventilation Link

• Bianchini et al. (2021) found that hyomental distance with hyperextension (DIMs) was a protective (predictive) parameter specifically in curarized patients - a hint that USG findings differ between pre- and post-NMB ventilation states.
• A Turkish study (JARSS, 2021) explicitly mentioned "difficulty in mask ventilation after neuromuscular blockade" as a gap needing further research.

Your Research Proposal - Design Guidance

Research Question (PICO)

• P: Adult patients undergoing general anesthesia
• I: Pre-operative airway USG measurements (tongue thickness, DLA, LPWT, ANS thickness, hyomental distance)
• C: Patients who do NOT improve mask ventilation grade after NMB vs. those who do
• O: Prediction of improvement in Han mask ventilation grade after succinylcholine/rocuronium

Study Design

1. Pre-induction: Measure USG parameters (tongue thickness, DLA, LPWT, ANS thickness, DIMs)
2. Post-induction, pre-NMB: Assess mask ventilation grade (Han scale, Grade 1-4)
3. Post-NMB (at full relaxation - confirmed by peripheral nerve stimulator): Re-assess mask ventilation grade with the same grader, blinded to USG findings
4. Primary outcome: Which USG parameter(s) predict a ≥1-grade improvement in Han score after NMB?

Key Variables to Measure

• Han mask ventilation grade at two time points (pre-NMB vs. post-NMB)
• USG: SBL/tongue thickness, DLA, LPWT, ANS thickness (4 levels), hyomental distance (resting + DIMs), PE/E-VC ratio
• Confounders: BMI, neck circumference, Mallampati score, STOP-BANG, age, sex, OSA diagnosis, edentulous state

Sample Size Estimate

Novelty and Significance

• It separates "difficult mask ventilation due to muscle tone/laryngospasm" from "difficult mask ventilation due to anatomical obstruction" - muscle relaxants fix the former but not the latter
• If a specific USG parameter (e.g., very large tongue) predicts non-improvement after NMB, this has direct clinical impact - it warns the anesthesiologist that NMB will not rescue a difficult airway situation

Key Existing Papers to Cite in Your Protocol

1. Bianchini A et al., Minerva Anestesiol 2021 (PMID 33054014) - USG in predicting DMV; found SBL >50mm and DIMs important; specifically noted the curarized vs. non-curarized difference
2. Akin S et al., Turk J Med Sci 2024 (PMID 38812631) - USG parameters for DMV in morbidly obese; ANS thickness and PE/E-VC ratio
3. Lin HY et al., Ultrasound Med Biol 2021 - Submental USG predicts DMV but not difficult laryngoscopy
4. JARSS 2021 Turkish study - DLA and TT predict DMV; explicitly mentions NMB research gap
5. NCT07243106 - Ongoing trial (2026) comparing airway USG vs. clinical predictors; check for overlap with your protocol

Bottom Line

USG Parameters Most Likely to Predict Mask Ventilation Improvement After Succinylcholine

The Core Physiological Logic First

• Muscle tone/active obstruction → will improve after succinylcholine
• Anatomical tissue bulk (large tongue, excess soft tissue) → will NOT improve, or will even worsen (flaccid tongue falls back further)

Parameter-by-Parameter Analysis

1. Hyomental Distance with Jaw Subluxation (DIMs) - MOST LIKELY to predict improvement

• DIMs reflects the functional space available when the jaw is actively displaced forward - a maneuver that is enhanced when the genioglossus and geniohyoid are relaxed and the jaw can be more easily subluxated
• Bianchini et al. (2021, PMID 33054014) found DIMs was a protective (predictive) parameter specifically in curarized patients - not in non-curarized patients. This is the strongest existing clue that DIMs behavior changes after NMB
• A short DIMs means there is intrinsically limited submandibular space - NMB will not create space that is anatomically absent

2. Geniohyoid Muscle Thickness (GMT) - STRONG CANDIDATE

• The geniohyoid is a direct skeletal muscle that pulls the hyoid anteriorly and holds the tongue base forward
• A thicker geniohyoid implies more muscle bulk contributing to active airway narrowing under induction (loss of spontaneous tone without NMB)
• After succinylcholine, this muscle goes completely flaccid - but in a patient with a large geniohyoid and adequate mandibular space (normal DIMs), the jaw-thrust maneuver will be far more effective
• Counter-intuitively: in a patient with small GMT but huge tongue bulk, the succinylcholine removes the last protective muscle holding the tongue base forward

3. Tongue Thickness / Submental Base of Tongue (SBL/TT) - PREDICTS NON-IMPROVEMENT (inverse)

• A thick tongue is pure anatomical bulk - it is not primarily a muscle tone issue
• Bianchini et al. found SBL >50mm predicted DMV in both curarized and non-curarized patients equally - this tells you tongue thickness is a fixed obstruction that succinylcholine cannot resolve
• After succinylcholine, the tongue actually becomes more passive/floppy and may fall further posteriorly, potentially worsening obstruction in the supine patient
• This is the control/negative predictor in your study - if SBL is large, expect no improvement

4. Lateral Pharyngeal Wall Thickness (LPWT) - MODERATE CANDIDATE for non-improvement

• Lateral pharyngeal wall fat and soft tissue is passive bulk, not muscle-dependent
• It cannot be recruited away by NMB
• However, in OSA patients, lateral wall muscle tone (tensor palatini, pharyngeal constrictors) does contribute to dynamic collapse - NMB removes this and may worsen lateral wall collapse

5. Distance Between Lingual Arteries (DLA) - SECONDARY CANDIDATE

• DLA reflects the transverse width of the tongue base
• Wider DLA implies more lateral tongue bulk filling the oropharynx
• Similar to tongue thickness - this is anatomical bulk rather than muscle tone
• A wider DLA alongside a normal or low SBL may indicate a tongue that is wide but not thick - in this configuration, jaw thrust + NMB may still help by lifting the tongue anteriorly

Summary Table - Predicted Associations

Proposed Study Hypothesis

"In patients with difficult mask ventilation (Han Grade 2-3) after induction, a larger DIMs and greater geniohyoid muscle thickness, but not tongue thickness or LPWT, will predict improvement in mask ventilation grade following succinylcholine administration."

One Important Practical Point

1. Before induction (awake, baseline)
2. After induction agent (propofol/thiopental) but before succinylcholine
3. After full succinylcholine effect (confirmed by absence of twitches on TOF/nerve stimulator)