Ultrasound vs Clinical Parameters for Difficult Airway Prediction in Obesity

Airway management is a critical component of clinical anesthesia. A difficult airway is defined as a situation in which a conventionally trained anesthesiologist experiences difficulty with mask ventilation, laryngoscopy, supraglottic airway ventilation, tracheal intubation, extubation, or invasive airway management. Unanticipated difficult laryngoscopy and difficult mask ventilation can lead to catastrophic outcomes. The incidence of difficult laryngoscopy and intubation is estimated at 1.5-13%, and approximately 28% of anesthesia-related deaths are attributable to failed mask ventilation or intubation.

Obesity (BMI ≥30 kg/m²) is associated with significant anatomical alterations in the upper airway and head and neck region. Accumulation of adipose tissue in the pharynx and hypopharynx narrows the airway lumen. A larger tongue is more difficult to displace into the submental space during laryngoscopy. Enlarged tonsils further reduce oropharyngeal space. Fat deposition in the neck, shoulders, and back complicates optimal patient positioning for laryngoscopy. Excessive soft tissue around the face and cheeks impairs effective mask seal and ventilation.

With advances in portable ultrasound technology, airway ultrasonography has become an increasingly utilized clinical tool for preoperative airway assessment. However, ultrasonographic criteria for predicting difficult airway have not yet been standardized.

Study Protocol:

Adult obese patients (BMI >30 kg/m², aged 18-65 years, ASA I-III) scheduled for elective surgery requiring orotracheal intubation under general anesthesia will be enrolled. Prior to the operating room, the following conventional airway parameters will be assessed: neck circumference, Mallampati score, and STOP-BANG scale. Airway ultrasonographic measurements will be performed preoperatively and will include:

Distance from skin to hyoid bone (DSHB) at the transverse view of the hyoid bone level Anterior neck soft tissue thickness at the thyrohyoid membrane level Hyomental distance ratio (HMDR): ratio of hyomental distance in extension (HMDe) to hyomental distance in neutral position (HMDn)

Upon arrival to the operating room, standard monitoring will be applied including electrocardiogram (ECG), non-invasive blood pressure, peripheral oxygen saturation (SpO2), body temperature, and bispectral index (BIS). Following monitoring and preoxygenation (2 minutes with 100% oxygen via face mask), standard anesthesia induction will be performed with intravenous propofol 2 mg/kg, fentanyl 2 mcg/kg, and rocuronium 0.6 mg/kg. After adequate neuromuscular blockade, orotracheal intubation will be performed.

Mask ventilation difficulty will be graded using the Han Scale during the mask ventilation phase. Laryngoscopy difficulty will be graded using the Cormack-Lehane classification at the time of intubation.

Mechanical ventilation will be maintained in volume control mode with tidal volume of 6-8 ml/kg, targeting end-tidal CO2 (EtCO2) of 35-40 mmHg. Anesthesia maintenance will be provided with 1.5-2% sevoflurane in 50% O2/air at 2 L/min and intravenous remifentanil infusion at 0.05-0.5 mcg/kg/min, targeting BIS 40-60. Normothermia will be maintained with an active forced-air warming device throughout the procedure.

Twenty minutes before the end of surgery, standard postoperative analgesia and prophylaxis will be administered: paracetamol 1 g, tramadol 1 mg/kg, and ondansetron 4 mg intravenously. Heart rate, blood pressure, EtCO2, and SpO2 will be documented every 30 minutes intraoperatively.

At the end of surgery, anesthetic agents will be discontinued. Sugammadex 2 mg/kg IV will be administered for reversal of neuromuscular blockade upon return of spontaneous respiratory effort. Patients with adequate respiratory effort and BIS ≥85 will be extubated and transferred to the post-anesthesia care unit (PACU) when conscious, with recovered muscle strength and hemodynamic stability. Standard monitoring will continue in the PACU prior to ward transfer.

Primary Outcome: Preoperative ultrasonographic assessment of anterior neck using DSHB and HMDR parameters and their correlation with intraoperative Cormack-Lehane grade.

Secondary Outcome: Determination of whether DSHB and HMDR can predict difficult airway in adult obese patients when correlated with Cormack-Lehane laryngoscopy grade and Han Scale mask ventilation score.