Tetanic 100 or 200 Hz fade ratio to detect residual neuromuscular block.

The study protocol was designed to determine the potential benefit of 100 and 200 Hz tetanic stimulation –compared to train-of-four (TOF) ratio 0.9- to detect a residual neuromuscular block (NMB) in anaesthetized patients. A new mechanomyographic device was developed to record the muscle responses in the clinical setting compared to acceleromyography on the contralateral hand. The Isometric Thumb Force (ITF) monitoring system measures the isometric forces resulting from the thumb muscular activities elicited by electrical stimulations applied by surface electrodes placed above the ulnar nerve location at the wrist. A nerve stimulator (TOF-Watch, Organon-Technika, Copenhagen, Denmark) was dedicated to deliver TOF, PTC and different rates (100, 150, 200 Hz) of 5 seconds tetanic stimulations. In ITF handgrip, the thumb is placed in a tunnelled cradle surmounting a circular handle. The thumb and the rest of the hand are maintained in place by non-elastic straps. The transducer, inserted into the handgrip, transmits linear signals for compression forces ranging from 5 to 22000 N. The study plan to include a cohort of 20 patients with American Society of Anesthesiologists (ASA) grades I to II (aged 18 to 80 years), who were scheduled to undergo rhinoplasty or rhinoseptoplasty under general anaesthesia. The patient's height, weight, age, and sex were recorded in the protocol, as was their dominant hand (left or right) to define the population investigated. All patients received oral alprazolam 0.5 mg 60 minutes before arriving in the operating theatre. They were conventionally monitored with a pulse oximeter, a three-lead electrocardiogram, and a non-invasive blood pressure scheduled in automatic mode at 5-minute intervals. An intravenous catheter was inserted into their forearm for crystalloid infusion (side at random). Neuromuscular monitoring was set up according to a specific protocol. A TOF-Watch SX (Alvesia Pharma, France) (TWX) was set on the left hand. The AMG transducer was taped on the thumb’s pulp and the hand was inserted and held inside a SL TOF-Tube. Another TOF-Watch designed to deliver high frequencies tetanic stimulation (including 100 and 200 Hz) was connected to the electrodes on the right arm which was equipped with the ITF handgrip. Both arms were positioned alongside the body on soft padding to protect nerve structures. Anaesthesia was induced with continuous intravenous infusion of remifentanil 0.25 µg/kg/min and continuous infusion of propofol 1% to obtain a theoretical plasma concentration of 3 to 6 µg/mL (Diprifusor Cardinal Health, Basingstoke, UK). Lidocaine 1 mg/kg was given as an intravenous bolus. After loss of consciousness, manual ventilation was provided during Neuromuscular Transmission Monitoring (NMTM) calibration and the measurement of the initial baseline. Supra-maximal stimulation and initial calibration were obtained using the TOF-Watch SX internal automatic sequence, and the current intensity was recorded and applied on both sides. Then, TOF stimulations were applied at 15-second intervals during a short stabilization period. On the TWX side, four T4/T1 ratios were recorded to determine the initial baseline. We calculated 90% of the mean value to determine the normalized TOF ratio 0.9 recovery threshold for each monitoring module using the following formula: normalized TOF ratio 0.9 = sum of 4 TOF ratio × 9/40 with a result rounded up. On the ITF side were applied two consecutive tetanic stimulations (100 and 200 Hz at random) with a two minutes interval to avoid any potentiation. The force applied on the mechanomyographic sensor was recorded by ITF monitoring system. The residual force at the end of the contraction (F res) was divided by the maximal value (F max, obtained at the beginning of the contraction) to provide baseline tetanic fade ratios (TFR). Then, consecutive TOF stimulations every 15 seconds were applied on both hands to monitor the NMB, from the onset and during surgery. Rocuronium 0.45 mg/kg was administered intravenously. Non-invasive automatic blood pressure measurement was suspended during the neuromuscular onset so as not to limit the distribution of the rocuronium in either of the two arms. Onset time was recorded when the TOF count reached zero with both monitors and tracheal intubation was performed. Automatic non-invasive blood pressure measurement was reactivated. Mechanical ventilation (closed circuit, 40% oxygen in air) maintained end-tidal CO2 within the normal range. Anaesthesia was maintained with the continuous intravenous administration of remifentanil 0.15 µg/kg/min and propofol with a target plasma concentration of 2 to 4 µg/mL. Blankets prevented heat loss from the body, and the oropharyngeal temperature was kept stable. When the spontaneous NMB recovery reached a normalized TOF ratio 0.9 on the TWX side, both tetanic stimulations (same order than previously) were applied again with 2 minutes interval and the F max and TFR were recorded by ITF. Then, to accelerate NMB final recovery, 2 mg/kg of sugammadex was administered. After 3 minutes, two last tetanic stimulations (ITF side) and four consecutive TOF ratio measurements (TWX side) were recorded to determine the final baselines. The investigation did not interfere with intra- and postoperative care.