Vagal nerve monitoring: a comparison of techniques in a canine model.

HYPOTHESIS

An optimal technique exists for intraoperative, electrophysiologic 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 cranial fossa, the infratemporal fossa, the parapharyngeal space, and the thyroid gland to decrease the incidence of iatrogenic injuries. Laryngeal electromyography (EMG) is an accurate test of vagal nerve function: four applications have been described for use intraoperatively. The purpose of this study was to compare the sensitivities of these techniques in a canine model in order to identify the optimal method of intraoperative vagal nerve monitoring.

METHODS

Four techniques of EMG vagal nerve monitoring were studied in dogs. The thyroarytenoid muscle (TA) was monitored directly in three techniques. Two methods used bipolar hookwire electrodes (L.A. Diagnostics, Los Angeles, CA) inserted in the TA percutaneously through the cricothyroid membrane or via direct laryngoscopy (DL). The third TA monitoring technique involved the use of an EMG endotracheal tube (Xomed-Treace, Jacksonville, FL). The fourth technique used a laryngeal surface EMG electrode (RLN Systems, Jefferson City, MO), laryngoscopically placed in the postcricoid space. After placing each monitoring device, the vagus nerve was identified bilaterally in the neck. The nerves were sequentially stimulated at a constant current of 4.1 Hz with increasing intensity (starting at 0.05 mAmps) to determine the minimum thresholds to stimulate vocal cord contraction. A positive response at the vocal cord was defined as a train of four contractions of > or = 50 mV. The lowest threshold for each technique in each dog was recorded.

RESULTS

A positive response was obtained in 27 of 32 possible cases using a maximum boundary 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 chi statistics showed that the survival curves are inhomogenous (degrees of freedom [df] = 3, chi = 15.58, p < 0.001). The laryngeal surface electrode appears to offer the most sensitive method for vagal nerve monitoring.

CONCLUSIONS

Four techniques of intraoperative, EMG vagal nerve monitoring were compared in a canine model. The results suggest that EMG recordings can be obtained successfully through a variety of techniques and that the laryngeal surface electrode appears to be the most sensitive technique in the canine model.