Perioperative management

The IONM setup was prepared by applying Dragonfly^® Single Channel Laryngeal Surface Electrode (Electrode LSE 500Ms; Neurovision Medical Products, Ventura, CA, USA) to a #7 cuffed ETT (Medline Industries, Mundelein, IL, USA) according to the manufacturer’s instructions. The nerve locator system was tested as per their instructions.

When the patient was received in the operating room, in addition to standard ASA monitors, a radial arterial line was placed under local analgesia for closer hemodynamic monitoring. Lidocaine 1 mg/kg was administered as an IV bolus for all the patients prior to induction. The SM received from the pharmacy in prefilled coded syringe was set up to deliver at the volume equivalent of 1.5 mg/kg/h through an infusion pump. The patients were preoxygenated and induced with a bolus of midazolam 2 mg and fentanyl 1.25 µg/kg followed by propofol 1.5 mg/kg and cisatracurium 4 mg. Intubating conditions were established by succinylcholine 1.2 mg IV bolus and the airway was secured with the pre-prepared surface electrode mounted ETT by direct vision laryngoscopy. Anesthesia was maintained with air, oxygen and sevoflurane at the desired minimum alveolar concentration levels titrated to effect. The placement of the electrode plates appropriately in contact with the vocal cords was verified with a Glidescope^® after final positioning the patient with the desired neck extension. A bispectral index (BIS) monitor (Model 1 A 2000; Aspect Medical Systems, Newton, MA, USA) was connected to guard against “recall” during lighter planes of anesthesia and a train of four (TOF) monitor (Microstim Plus, Neurotechnology, Houston, TX, USA) was employed to guard against inadvertent neuromuscular blockade (NMB). Hemodynamic fluctuations during surgery were treated with β-blockers and sympathomimetic agents as appropriate. The initial stimulation level was set at 1.5 mA and gradually reduced to 0.5 mA. The lowest strength of the StMC required to elicit a positive signal upon the RLN stimulation was the goal, while the visual confirmation of the RLN was the gold standard for the study. The DAIL at the end of surgery was recorded. Greater than 50% DAIL was considered significant indicator of loss of contact between the electrode sensors and vocal cords. Inability to elicit a signal at 2 mA was considered as equipment/placement failure. Additional doses of fentanyl were administered as needed to sustain basal narcosis. The SM infusion rate was tapered down and discontinued upon extubation.

Meticulous dissection and visual identification of the RLN were the primary focus during surgery (Figs. 1 and ​and2).2). Mobilizing the thyroid lobe medially until cricothyroid joint is palpable served as the initial starting point for the RLN location. The next step was to define the trachea esophageal groove (TEG) by lateral traction on the paratracheal tissue around the carotid and medial traction on the trachea. Identification of the inferior thyroid artery in the TEG serves as an additional guide to the RLN. Definitive confirmation of the RLN identity was by the IONM stimulation of the nerve resulting in the palpable and visual contraction of the posterior cricoarytenoid muscle. The number of stimuli delivered until confirmatory cricoarytenoid contraction depended upon the level of resident participation. The attending surgeon’s experience predated the IONM by many years, while residents needed frequent reassurances approaching the RLN and the number of IONM stimulations varied accordingly.

Right recurrent laryngeal nerve dissection.

Left recurrent laryngeal nerve dissection.

The patients stayed overnight in the PACU for observation. Their vital signs and study parameters were recorded at appropriate intervals. PACU data were determined and documented by the PACU staff not involved with the study.