The present prospective feasibility study was performed at the University Hospital of Erlangen and was evaluated and approved by the institutional ethics committee (reference number: 4590). Patient recruitment and data retrieval was according to the good clinical practice guidelines. All patients provided written informed consent. Following inclusion into the study, no further selection was performed.
The University Hospital of Erlangen provides the full spectrum of thoracic surgery. Anesthesia induction, maintaining and IONM were performed according to the standard operating procedures (SOPs) of the hospital, using a total intravenous anesthesia (TIVA). After neuromuscular block for intubation, neuromuscular transmission was monitored with acceleromyography at the adductor pollicis muscle (TOF-Watch SX, Organon Ireland Ltd., a division of Merck and Co. Inc., Swords, Co. Dublin, Ireland) to exclude a persistent neuromuscular block during IONM.
All patients were intubated orotracheally with a NIM EMG ETT (Medtronic Xomed, Jacksonville, Florida, USA) (7 mm I.D. for female patients, 8 mm I.D. for male patients). The NIM EMG ETT is a flexible silicone elastomer single-lumen ETT with an inflatable cuff. The tube is fitted with four stainless steel wire electrodes (two pairs) which are embedded in the silicone of the main shaft of the ETT and exposed only for a short distance (28 mm), slightly superior to the cuff, for contacting the true vocal cords. Correct positioning of the surface electrodes is essential to enable a side related EMG signal monitoring. Therefore the NIM EMG ETT was placed with the middle of the exposed electrodes well in contact with the true vocal cords (Fig. 1) using video laryngoscopy (Glidescope, Verathon Medical, Rennerod, Germany or C-MAC Karl Storz, Tuttlingen, Germany). After the patient had been placed in right decubitus position, a grounding electrode and an anode electrode were placed in the subcutaneous tissue of the left shoulder. All electrodes were connected to the NIM-Response 3.0 monitor (Medtronic Xomed, Jacksonville, Florida, USA). Correct positioning of the laryngeal surface electrodes was confirmed by transcutaneous cervical suprathreshold vagal stimulation using an electrical nerve stimulator (NS 252, Fisher and Paykel Healthcare Electronics Ltd., Auckland, New Zealand). In addition, the NIM-Response 3.0 monitor was used to verify electrode impedance measures less than 5 KΩ. The event threshold was set at 100 μV and an activated evoked potential greater than 100 μV was considered as a positive EMG signal. A cutoff device (cylindrical clamp) was secured around the electrocautery wire to sense current passing through the wire with each electrocautery use. To identify and preserve the RLN during surgery, the course of the RLN was explored by using a monopolar stimulating electrode (Prass, Medtronic Xomed, Jacksonville, Florida, USA) (Fig. 2). The monopolar probe provides a more diffuse current spread and thus could facilitate mapping out the RLN when compared to bipolar nerve stimulator probe where the stimulation is localized at the point of contact [10]. The stimuli for mapping out were generated from the NIM-Response 3.0 monitor and the intensity was typically 1.0 mA for RLN nerve mapping. However, once the RLN is visualized, the stimulation current could be turned down. Stimulation of the RLN or vagal nerve results in an audible and visual EMG signal on the monitor screen. During surgery, a three-step procedure of IONM was used: To ensure that the EMG monitoring system is functioning correctly and that the RLN remains on a normal path, a sequential stimulation of the vagal nerve was performed before dissection. To preserve the RLN during surgery, the course of the RLN was labeled by using the monopolar probe. After the tumor and/or lymph nodes had been completely dissected and hemostasis had been completed, the integrity of the RLN was confirmed again by sequential stimulation of the vagal nerve.
One-lung ventilation was performed using the EZB (AnaesthetIQ, Rotterdam, The Netherlands). This 7-French, 75-cm, 4-lumina Y-shaped semirigid endobronchial blocker has two different colored distal extensions, both with an inflatable cuff and a small central lumen (Fig. 3). Two pilot balloons at the proximal part of the device serve to inflate/deflate the cuffs. Two additional lumina are available for suction or oxygen insufflation. The EZB is inserted through the designated port on the enclosed multiport adapter. The multiport adapter is designed to connect to an ETT (minimum 7 mm I.D.) and contains two additional upper ports, one for the blocker itself and the other for the bronchoscope. EZB use for lung separation was provided as follows: After the patient had been placed in right decubitus position, the multiport adapter was connected to the NIM EMG ETT. To enable proper deployment of the Y-shaped distal part of the EZB, a minimum of 4 cm distance between the distal end of the NIM EMG ETT and the carina was verified under direct bronchoscopic vision. The EZB was lubricated with silicone spray and introduced through the lockable center port of the multiport adapter with its cuffs completely deflated. Further advance was guided with a fiberoptic bronchoscope (FOB), placing the distal EZB ends into the right and left mainstem bronchi under direct bronchoscopic vision (Fig. 4). If there was less than 4 cm distance between the distal end of the NIM EMG ETT and the carina, the NIM EMG ETT was slightly retracted more proximal. With the EZB finally properly placed, the NIM EMG ETT was readvanced into the trachea as necessary. Both movements required a deflated NIM EMG ETT cuff. To test the bronchial sealing, the cuff of the EZB was inflated with an appropriate volume under direct bronchoscopic vision and deflated again. The insertion technique described was used for all patients and all EZBs were placed under supervision of an attending physician in accordance with the SOP. To facilitate unilateral lung collapse, a specific sequence of action was used after the parietal pleura had been opened by the thoracic surgeon: First, disconnection of the tube from the ventilator allows the operated lung to collapse. After 20 s, reinflation of the blocker cuff under direct bronchoscopic vision with the same volume of air as used before and reconnection of the ETT to the ventilator establishes ventilation of the dependent lung. The adequacy of lung collapse was clinically assessed by the thoracic surgeon. According to our SOP for one-lung ventilation, after removing the endobronchial blocker at the end of surgery, the mucosa of the tracheobronchial system was observed with the FOB for possible damage due to the endobronchial blocker.
Demographic data of the patients, Mallampati score, Cormack and Lehane (CML) classification, surgical characteristics, time-span of clinical experience of the responsible anesthesiologist, anesthesia drugs used for induction and maintenance, airway management, time for EZB placement, problems with EZB placement, clinical adequacy and duration of one-lung ventilation, occurrence and incidence of EZB dislocation or bronchial injury, possible decrease of oxygen saturation, ventilation parameters, need for postoperative ventilation and finally adverse events during surgery were recorded in addition to the described standard monitoring parameters. The data were stored in the electronic patient data management system (NarkoData; IMESO, Hüttenberg, Germany). The acquired data were anonymized and transferred to an Excel datasheet (Microsoft, Redmond, USA) for statistical analysis. Descriptive statistical analysis was done using Statistica version 6 (StatSoft (Europe) GmbH, Hamburg, Germany). Categorical variables were given as absolute numbers and percentages of their occurrence. Continuous variables were presented as medians and interquartile range (IQR).