The airway assessment was conducted in two phases: evaluation of clinical parameters and ultrasound parameters. Basics demographics such as age, gender, height, weight, BMI and comorbidities were noted along with the ASA classification and urgent or elective nature of the surgery. Clinical screening tests were included to define a difficult laryngoscopy such as history of difficult or impossible intubation, NC, MMS, retrognathia, protrusion of the upper incisors, TMD, mouth opening, dentition, macroglossia, presence of a beard, UBLT, spine mobility and palm print test for diabetes.
Ultrasound airway assessment was performed using a sonosite portable sonography machine with a high-frequency linear probe (10MHz) as well as a convex probe (3.5-5 MHz), with the patient lying supine and keeping the head in a neutral position. Three parameters were measured to calculate the scores: Tongue thickness (TT) in coronal plane, Distance from the skin to hyoid bone (SHB), Distance from the skin to the thyrohyoid membrane (STM). TT was measured with a low-frequency convex probe which was places submandibularly in a perpendicular plane, the cross-sectional plane was the one passing through the two lingual arteries, we considered the widest diameter. SHB was measured by placing the linear high-frequency ultrasound probe transversely over the hyoid bone. Similarly, distance from skin to the thyrohyoid membrane (STM) was measured midway between hyoid and thyroid cartilage at the level of the epiglottis.
The hyoid bone was identified as a curved echogenic structure with posterior acoustic shadow and epiglottis was identified as a curvilinear hypoechoic structure with a bright posterior air mucosal interface and hyperechoic pre-epiglottic space. Two scores were developed from a study conducted in 2019. The first score, to predict a difficult laryngoscopy, uses two parameters: the modified MALLAMPATI class (MMS) and the STM. A score strictly greater than 2 is predictive of a difficult laryngoscopy. The second score, to predict difficult ventilation, uses four parameters: BMI, NC, TT in coronal plane, and SHB. A score strictly greater than 20 is predictive of difficult ventilation. The patient was then shifted to the operating room; electrocardioscope, pulse oximetry, gas analyzer, capnography monitoring, non-invasive blood pressure, and neuromuscular blockade monitoring were connected. Other monitoring methods could be used depending on the patient condition and surgical risk, and their choice was left to the discretion of the anesthesiologist in charge of the patient. After pre-oxygenation with 100% oxygen, they were induced with intravenous injections of Sufentanil 0.3µg/kg, Propofol 3mg/kg and cisatracurium 0.15mg/kg or succinylcholine in case of a CRUSH induction. A well-experienced anesthesiologist performed a direct laryngoscopy and graded it as Cormack-Lehane's grading.
Difficulty in intubation and/or mask ventilation was managed according to the 2017 SFAR guidelines.
