Measuring Upper Airway Cross Sectional Areas During Residual Neuromuscular Blockade and After Reserval

Introduction

Postoperative residual neuromuscular can cause severe respiratory complications. One of these is pharyngeal dilator muscle weakness, which can lead to airway obstruction. One way to prevent this is to have objective neuromuscular monitors to track the degree of muscle relaxation and use the measurements to guide the the suspension of neuromuscular block. Monitoring the muscle relaxant effect is not a mandatory component of anaesthesia and is therefore often omitted in anaesthesiologists. In our study, the investigators investigated whether, at the end of surgery, only patients extubated at the end of surgery, based on clinical signs, have an airway diameter reduction and how this relates to the degree of residual muscle relaxation. Our single-centre, prospective study included 20 patients. The patients' narcosis by the anaesthetist, as is often done in routine anaesthesia work, is without monitoring neuromuscular block and on the basis of clinical signs deciding on extubation.

Sample size calculation

In a previous study, Eikermann and associates (Eikermann M, Vogt FM, Herbstreit F, Vahid-Dastgerdi M, Zenge MO, Ochterbeck C, de Greiff A, Peters J. The predisposition to inspiratory upper airway collapse during partial neuromuscular blockade. Am J Respir Crit Care Med. 2007;175(1):9-15.) using MRI technique described a minimal retroglossal diameter after a total neuromuscular recovery of 20.2±5.2 mm and a 20% decrease of this diameter at TOFR=0.8. the investigators hypothesised that in the present study, the decrease of the retroglossal cross-section area during inspiration will decrease by 30% in patients with residual neuromuscular block of any severity. Using and alpha of 0.05 and a power of 90%, 8 patients were calculated to be necessary to prove our hypothesis. In a previous study, it was found that in our working group the amount of residual neuromuscular block (as defined TOFR <90%) during spontaneous recovery at the end of surgery approximates 45% of all cases (Nemes R, Fülesdi B, Pongrácz A, Asztalos L, Szabó- Maák Z, Lengyel S, Tassonyi E. Impact of reversal strategies on the incidence of postoperative residual paralysis after rocuronium relaxation without neuromuscular monitoring: A partially randomised placebo controlled trial. Eur J Anaesthesiol. 2017;34(9):609-616.). With respect to this, the investigators planned to include 18 patients. the investigators also calculated with eventual dropouts and finally included 20 patients.

Procedure of the investigation

During the operation, the anaesthetist will routinely administer anaesthesia, including the selection of muscle relaxant and the timing of extubation. The latter is based solely on clinical signs and the anaesthetist performing narcosis does not use a neuromuscular monitor. However, the patients are not left without monitoring of the effect of the muscle relaxant, as an independent anaesthesiologist performs continuous electromyographic neuromuscular monitoring of the anaesthetised patients, the actual values of which are not known to the anaesthetist.

Medication of the patient, surgical procedure

As part of the balanced anaesthesia routinely used at the institute, the patient is first premedicated with 7.5 mg midazolam 60 minutes before the onset of anaesthesia. A peripheral vein is secured in one arm of the patient and infusion with Ringer's lactate solution is started. The other arm is left completely free for the independent anaesthetist, on which neuromuscular monitoring is performed throughout the operation. Continuous monitoring of the patient's physiological parameters is ensured by the use of precordial ECG, pulse oximetry, blood pressure measurement, central body temperature measurement, end-expiratory oxygen and CO2 measurement throughout the duration of the operation.

Induction of anaesthesia and maintenance of narcosis is achieved with total intravenous anaesthesia, propofol anaesthesia, TCI perfusion using the Schnider model. During induction, the propofol plasma concentration is set at 4-6 micrograms/ml. Before the administration of opioids and muscle relaxants, a control pharyngoscopy is performed, at which time the BIS index ranges between 60-70%. Patients are then asleep but spontaneous breathing is maintained. Once the recording is complete, the patient is given the type and dose of muscle relaxant chosen by the anaesthetist, fentanyl is administrated and the anethesist intubate the trachea. Subsequently, anaesthesia is deepened for the duration of the operation, with a BIS index of between 40-60%. The target concentration is then changed to 2.5-4 µg/ml to maintain narcosis. At the end of the operation, the non-monitoring anaesthetist performing the anaesthesia extubates the patient by observing the clinical signs, at which time the independent anaesthetist records the TOF value at the time of extubation. Fentanyl is antagonised while the patient is asleep. Pharyngoscopy is performed on the extubated patient. If necessary, i.e. when the TOF rate is below 90%, rescue medication is administered.

Neuromuscular monitoring

The anaesthetist did not monitor the patient, but the independent anaesthetist monitored the effects of the muscle relaxant throughout the operation. To do this, he uses a Tetragraph® electromyograph, which stimulates the ulnar nerve and detects the direct action potential of the adductor pollicis muscle. The application of the measuring instrument is facilitated by self-adhesive electrodes: the stimulating electrodes are placed on the volar surface of the wrist according to the course of the nerve mentioned above, and the sensing electrodes are placed towards the adductor pollicis muscle. The device is started with the patient already asleep, thus avoiding any discomfort due to stimulation. Once started, the device performs an autocalibration to determine the supramaximal excitation current to ensure muscle contraction. The electromyography uses the train of four -TOF stimulation pattern.

Offline analysis of pharnygoscopy and airway areas

During the examination of a patient, two pharyngoscopies are performed. The first one (called control pharnygoscopy) is performed under propofol but before the administration of opioid and muscle relaxant to ensure that the patient is breathing spontaneously. This is also to exclude the negative effect of opiate analgesia on the pharyngeal muscles. The second admission is performed after extubation. During the pharnygoscopy, a continuous chin lift, Escmarch-Heiberg manoeuvre, is used to guide the Ambu® aScope™ 4 Rhino Laryngo Slim rhino-laryngoscope down the nose to the vocal cord. The vocal fold was used as a landmark. From here, the camera was carefully retracted to locate the narrowest part of the pharynx, which corresponded to the retroglossal region. The found position is marked on the instrument with a marker, making it easier to find the same part of the pharynx for the second recording. During the pharyngoscopic examinations, moving images were also taken during the inhalation and exhalation phases, and these are analysed offline. Using the online available software Image J (Rasband WS, U.S. National Institutes of Health, Bethesda, Maryland, USA, https://imagej.net/ij/), the size of the airway areas is determined in pixels. From the resulting data set, the investigators can compare the data from control and end-of-operation pharyngoscopies and subsequently perform statistical calculations. Whenever possible, the investigators use analysis of variance (ANOVA) to estimate the parametric tests. Otherwise, the non-parametric Kruskal-Wallis test is used to compare each group. The significance level is defined as usual, p<0.05. Continuous variables are characterized by means and standard deviations.

Rescue medication

After pharyngoscopy, rescue medication is given if necessary, i.e. below 90% TOF, depending on the type of muscle relaxant used. If an aminosteroid muscle relaxant is used, the patient is given 2 mg/kg sugammadex, while if a benzylisoquinoline muscle relaxant is used, 0.05 mg/kg neostigmine and 0.015 mg/kg atropine are administrated to antagonise the drug effect.