VAGAL NERVE MONITORING - A COMPARISON OF TECHNIQUES IN A CANINE MODEL

Hypothesis: An optimal technique exists for intraoperative, electrophy siologic vagal nerve monitoring. Background: Analogous to facial nerve monitoring during lateral skull base surgery, vagal nerve monitoring may be used at surgery involving the jugular foramen, the posterior cr anial fossa, the infratemporal fossa, the parapharyngeal space, and th e thyroid gland to decrease the incidence of iatrogenic injuries. Lary ngeal electromyography (EMG) is an accurate test of vagal nerve functi on: four applications have been described for use intraoperatively. Th e purpose of this study was to compare the sensitivities of these tech niques in a canine model in order to identify the optimal method of in traoperative vagal nerve monitoring. Methods: Four techniques of EMG v agal nerve monitoring were studied in dogs. The thyroarytenoid muscle (TA) was monitored directly in three techniques. Two methods used bipo lar hookwire electrodes (L.A. Diagnostics, Los Angeles, CA) inserted i n the TA percutaneously through the cricothyroid membrane or via direc t laryngoscopy (DL). The third TA monitoring technique involved the us e of an EMG endotracheal tube (Xomed-Treace, Jacksonville, FL). The fo urth technique used a laryngeal surface EMG electrode (RLN Systems, Je fferson City, MO), laryngoscopically placed in the postcricoid space. After placing each monitoring device, the vagus nerve was identified b ilaterally in the neck. The nerves were sequentially stimulated at a c onstant current of 4.1 Hz with increasing intensity (starting at 0.05 mAmps) to determine the minimum thresholds to stimulate vocal cord con traction, A positive response at the vocal cord was defined as a train of four contractions of greater than or equal to 50 mV. The lowest th reshold for each technique in each dog was recorded. Results: A positi ve response was obtained in 27 of 32 possible cases using a maximum bo undary of 0.5 mAmps for stimulus intensity. Survival analysis was then used to generate Kaplan-Meier survival curves, allowing a comparison of the mean time needed to obtain a response. Log-rank ii statistics s howed that the survival curves are inhomogenous (degrees of freedom [d f] = 3, chi = 15.58, p < 0.001). The laryngeal surface electrode appea rs to offer the most sensitive method for vagal nerve monitoring. Conc lusions: Four techniques of intraoperative, EMG vagal nerve monitoring were compared in a canine model. The results suggest that EMG recordi ngs can be obtained successfully through a variety of techniques and t hat the laryngeal surface electrode appears to be the most sensitive t echnique in the canine model.