Introduction
Intra-operative laryngeal nerve monitoring is increasingly being used as a surgical adjunct to prevent recurrent laryngeal nerve injury during thyroid surgery. One of the most commonly used recurrent laryngeal nerve monitoring systems is the Nerve Integrity Monitoring® system, which consists of an endotracheal tube with surface electrodes and an electromyography (EMG) recording device.
Accurate positioning of the endotracheal tube is of paramount importance for the proper monitoring of the recurrent laryngeal nerve. Malpositioning of the tube will result in Equipment malfunction, which could give misleading information and potentially increase the risk of nerve injury.Reference Lu, Chu, Tsai, Wu, Kuo and Chen1 In order to improve the visualisation of tube placement during intubation, we routinely use the GlideScope® video laryngoscope. In this article we describe our experience with the GlideScope and explain how it facilitates the accurate placement of the Nerve Integrity Monitoring endotracheal tube, allowing the active participation of the surgeon in the process.
Materials and methods
The Nerve Integrity Monitoring system uses a specialised endotracheal tube that has four stainless steel wire electrodes (two pairs) embedded in the silicone of the main shaft of the endotracheal tube. There is a 30-mm length of electrode exposure around the perimeter of the tube, with a midpoint approximately 90 mm above the distal tip of the tube. It is this area which must be in contact with the vocal folds to facilitate EMG monitoring during surgery. The multi-channel Nerve Integrity Monitoring EMG endotracheal tube device monitors activity from multiple muscles and provides visual and audible warnings to alert the surgeon regarding inadvertent stimulation of the nerve (usually due to traction or pressure).
Video laryngoscopes are similar to conventional laryngoscopes, but they incorporate a miniature digital camera that enables the operator to visualise the glottis on a high resolution video monitor. The GlideScope video laryngoscope is a commercially available video laryngoscope that has been advocated as an effective tool for the management of a difficult airway.
The GlideScope consists of: a plastic handle; a curved blade that is 14 mm at its widest, which bends at a 60 degree angle in the midline; and a camera, which is located midway along the bottom of the blade. A light-emitting diode solid-state light source assembly is mounted beside the camera to provide illumination. The image is displayed on a video monitor, which is positioned on a mobile stand.
The GlideScope offers better visualisation of the glottis compared with a standard laryngoscope due to higher magnification, reliable illumination and operator comfort. It has been shown to have a high success rate in primary airway management.Reference Sun, Warriner, Parsons, Klein, Umedaly and Moult2 As it improves visualisation of the larynx, we have used it routinely for the precise placement of the Nerve Integrity Monitoring endotracheal tube for laryngeal nerve monitoring during thyroid and parathyroid surgery.
An appropriately sized endotracheal tube is used to ensure electrode contact with the vocal folds. After pre-oxygenation and induction of general anaesthesia, the GlideScope is inserted. The first landmark on insertion into the mouth is the uvula; this ensures that the blade is in the midline and correctly oriented, thus avoiding trauma to the oropharyngeal mucosa. The GlideScope is then glided downward on the surface of the tongue following the anatomical curvature of the oropharynx, whilst sequentially visualising the base of the uvula, soft palate, tongue base, vallecula and epiglottis on the high resolution video monitor.
As the Nerve Integrity Monitoring endotracheal tube is softer, longer and more flexible than a standard endotracheal tube, a lubricated malleable stylet is used in place of the standard GlideScope stylet to both stiffen the tube and form a ‘J-shape’ to facilitate translaryngeal intubation. After advancing the tip of the Nerve Integrity Monitoring tube through the glottis, the stylet is withdrawn. The tube is then rotated to ensure accurate electrode contact with the true vocal folds; these can be visualised indirectly on the GlideScope video monitor by both the surgeon and anaesthesiologist. The 30-mm length of electrode exposure is located equally around the perimeter of the tube with a midpoint that is approximately 90 mm above the distal tip of the tube (Figure 1). Ideally, the plane of electrode contact with the true vocal folds must lie in the middle section of this colour-coded area of the tube. The GlideScope is then withdrawn after visual confirmation of correct tube placement.
Photograph of the Nerve Integrity Monitoring® endotracheal tube. Note the placement and position of the external contact electrodes within the blue area of the tube.
The patient is positioned for the surgery using a shoulder roll, and the endotracheal tube is then secured. Once the patient has been appropriately positioned we usually perform a second inspection using the GlideScope to verify the correct position of the tube and exclude any alteration in tube placement as a result of patient repositioning from a neutral to a hyperextended neck position.
Having been assured of the correct tube placement, the EMG monitor is used to measure electrode impedance and identify any significant imbalance. Impedance values for the endotracheal tube should usually be less than 10 kohm and the impedance imbalance between the positive and negative electrodes of the channel should be less than 1 kohm. With greater imbalance, one is likely to experience problems with artefacts and excessive background noise. An impedance mismatch of more than 1 kohm indicates excessive tube rotation and misalignment. We routinely check electrode impedance and imbalance levels at regular intervals throughout the procedure. Any sudden change in the impedance or imbalance levels during the procedure alerts the surgeon to the dislodgement of an electrode from the vocal fold due to excessive tube or neck movement. If this situation arises, the tube position is again confirmed using the GlideScope, and adjusted until the electrode impedance or imbalance levels return to acceptable values. At the end of the procedure, the ipsilateral vagus nerve is stimulated to check the integrity of the entire neural circuit.
Discussion
Since its commercial introduction in 2002, the GlideScope has been shown be effective and safe when used for orotracheal intubation in patients and simulators with both easy and difficult airways. The digital camera incorporated into the GlideScope is positioned only 3 cm away from the distal end of the blade, which shortens the distance between the viewing position and the laryngeal structures, thus circumventing the anterior blind area associated with the Macintosh laryngoscope (GVL; Verathon Inc., Bothell, Washington, USA). In addition, the blade of the GlideScope has a 60 degree angle at the midline, which provides laryngeal visualisation without alignment of the oral, pharyngeal and tracheal axis. This, coupled with the high resolution liquid crystal display video screen, contributes to the enhanced visualisation of the glottis.Reference Xue, Zhang, Liu, Li, Yang and Xu3
We have been using the GlideScope for the placement of the Nerve Integrity Monitoring endotracheal tube for the past 2 years; it has been used successfully in over 250 thyroid and parathyroid surgical procedures. As mentioned earlier, the effectiveness of the Nerve Integrity Monitoring system for monitoring the laryngeal nerves depends on the precise positioning of the electrodes, which are exposed at only the distal 30 mm of the tube. The superior magnified image of the glottis on the GlideScope high resolution video screen enables the anaesthesiologist to accurately position the endotracheal tube. In addition, no special training is required for GlideScope use for clinicians already familiar with Macintosh direct laryngoscopy, as both devices share some common features. Furthermore, GlideScope facilitates the active participation of the surgeon, who can be reassured that the endotracheal tube is properly inserted and positioned for reliable recurrent laryngeal nerve monitoring.
According to the international standard guidelines statement on electrophysiological monitoring during thyroid and parathyroid surgery, tube position should be verified after patient positioning to exclude any alteration in tube position as a result of subsequent neck hyperextension.Reference Randolph, Dralle, Abdullah, Barczynski, Bellantone and Brauckhoff4 Lu et al. Reference Lu, Chu, Tsai, Wu, Kuo and Chen1 found that the proper depth of the Nerve Integrity Monitoring tube relative to the corner of the mouth ensured adequate electrode contact with the vocal folds. However, this varies according to the gender and height of the patient. The authors proposed an ideal range of depth measurements to further assure proper Nerve Integrity Monitoring tube placement after neck repositioning following intubation. Nevertheless, we feel that direct visualisation with the GlideScope enables superior assessment and simple verification of tube position. The GlideScope also allows both the surgeon and anaesthesiologist to check the position of the tube during surgery if an impedance imbalance of the endotracheal tube raises a suspicion of displaced electrodes. It also enables the video recording of endotracheal tube intubation for teaching purposes.
Despite its ergonomic design and relative ease of implementation, various problems have been described in the use of the GlideScope. The inherent 60-degree curvature of the GlideScope blade can initially make intra-oral insertion difficult. Excessively deep insertion of the GlideScope can also be a problem, but this can be avoided with gradual midline insertion of the blade and sequential visualisation of the anatomy on the video monitor.Reference Xue, Zhang, Liu, Li, Yang and Xu3
Despite these caveats, we have found the GlideScope to be an indispensable adjunct for ensuring reliable intra-operative laryngeal nerve monitoring during thyroid and parathyroid surgery. The collaboration of the surgeon and anaesthesiologist during intubation further assures the reliability of the monitoring system. The use of the GlideScope should be encouraged in all surgical procedures requiring placement of the Nerve Integrity Monitoring endotracheal tube for intra-operative laryngeal nerve monitoring.
