Intraoperative monitoring has been useful in surgical procedures in which peripheral or cranial nerves are at risk. Intraoperative monitoring of the recurrent laryngeal nerve (a motor division of the vagus nerve) records the electromyographic activity of the vocalis muscle by using a commercially available specifically designed endotracheal tube with exposed recording wires or recording electrodes that adhere to a standard endotracheal tube. When properly placed, the recording wires or electrodes attached to the endotracheal tube make contact with the vocal cords. Thus when the recurrent laryngeal nerve is stimulated a compound muscle action potential is seen from the vocalis muscles.
Recurrent laryngeal nerve monitoring is most commonly used during thyroid surgery. It assists the surgeon of precisely identifying the recurrent laryngeal nerve and its path during dissection of the thyroid. The recurrent laryngeal nerve is identified by using a supra threshold electrical stimulus of 0.8 mA-1.0 mA prior to the removal of the gland. After the thyroid is removed, a lower stimulus is used to establish the threshold of the nerve at 0.3 mA-0.6 mA.
Several important technical factors are warranted in order for recurrent laryngeal nerve monitoring to be successful.
No muscle relaxants. Succinylcholine can be used to intubate the patient wears off quickly.
No local anesthetic gel to lubricate the tube or topical anesthetic to numb the vocal cords.
Once the endotracheal tube is placed, check the impedances in order to determine proper placement. Furthermore, tap each side of the surgical site. This should elicit responses and confirms proper placement of the electrodes.