Myogenic Motor-evoked Potentials Research Articles

Retrospective observational study of the effects of residual neuromuscular blockade and sugammadex on motor-evoked potential monitoring during spine surgery in Japan.

Given neuromuscular blockade (NMB) can affect the amplitude and detection success rate of motor-evoked potentials (MEP), sugammadex may be administered intraoperatively. We evaluated the factors affecting the degree of residual NMB (i.e., the train-of-four [TOF] ratio) and the relationship between TOF ratio and MEP detection success rate in Japanese patients undergoing spine surgery. This single-center retrospective observational study included adults who underwent spine surgery under propofol/remifentanil anesthesia, received rocuronium for intubation, and underwent myogenic MEP monitoring after transcranial stimulation. TOF ratios were assessed using electromyography. Sugammadex was administered after finishing the MEP setting and the TOF ratio was ≤0.7. To identify factors affecting the TOF ratio, TOF ratio and MEP detection success rate were simultaneously measured after finishing the MEP setting; to compare the time from intubation to the start of MEP monitoring after NMB recovery between sugammadex and spontaneous recovery groups, multivariable analyses were performed. Of 373 cases analyzed, sugammadex was administered to 221 (59.2%) cases. Age, blood pressure, hepatic impairment, and rocuronium dose were the main factors affecting the TOF ratio. Patients with higher TOF ratios (≥0.75) had higher MEP detection success rates. The time from intubation to the start of MEP monitoring after NMB recovery was significantly shorter in patients administered sugammadex versus patients without sugammadex (P < .0001). The MEP detection success rate was higher in patients with a TOF ratio of ≥0.75. Sugammadex shortened the time from intubation to the start of MEP monitoring after NMB recovery.

Successful recording of direct cortical motor-evoked potential from a pediatric patient under remimazolam anesthesia: a case report

BackgroundIntraoperative motor-evoked potential (MEP) monitoring reduces postoperative motor deficits. Propofol-based total intravenous anesthesia is the gold standard for intraoperative myogenic MEPs. Although there is no contraindication to administering propofol in adults with peanut, soy, or egg allergies, its safety in children with these allergies remains unclear.Case presentationA 12-year-old girl required general anesthesia under intraoperative direct cortical MEP (dc-MEP) monitoring due to supratentorial glioma. Remimazolam-based anesthesia was selected, instead of propofol, due to the patient’s egg hypersensitivity. Stable myogenic MEPs were recorded throughout the surgery with remimazolam at 0.9 mg/kg/h and remifentanil at 0.35 μg/kg/min, following adjustments of stimulation intensity and titration of remimazolam infusion. Neither intraoperative memory nor motor deficits were present after surgery.ConclusionsWe present a pediatric case whose dc-MEP was recorded under remimazolam anesthesia. The cardiovascular stability and avoidance of propofol infusion syndrome with remimazolam were superior to propofol.

Feasibility of intraoperative monitoring of motor evoked potentials obtained through transcranial electrical stimulation in infants younger than 3 months.

This study aimed to investigate the feasibility and safety of intraoperative motor evoked potential (MEP) monitoring in infants less than 3 months of age. The authors investigated 25 cases in which infants younger than 3 months (mean age 72.8 days, range 39-87) underwent neurosurgery between 2014 and 2017. Myogenic MEPs were obtained through transcranial electrical stimulation. In all cases, surgery was performed under total intravenous anesthesia, maintained with remifentanil and propofol. MEPs were documented in 24 infants, the sole exception being 1 infant who was lethargic and had 4-limb weakness before surgery. The mean stimulation intensity maintained during monitoring was 596 ± 154 V (range 290-900 V). In 19 of 24 infants MEP signals remained at ≥ 50% of the baseline amplitude throughout the operation. Among 5 cases with a decrease in intraoperative MEP amplitude, the MEP signal was recovered in one during surgery, and in the other case a neurological examination could not be performed after surgery. In the other 3 cases, 2 infants had relevant postoperative weakness and the other did not show postoperative neurological deficits. Postoperative weakness was not observed in any of the 20 infants who had no deterioration (n = 19) or only temporary deterioration (n = 1) in MEP signal during surgery. Transcranial electrical MEPs could be implemented during neurosurgery in infants between 1 and 3 months of age. Intraoperative MEP monitoring may be a safe adjunct for neurosurgical procedures in these very young patients.

The Correlation Between Recordable MEPs and Motor Function During Spinal Surgery for Resection of Thoracic Spinal Cord Tumor.

Motor evoked potentials (MEPs) are commonly used during surgery for spinal cord tumor resection. However, it can be difficult to record reliable MEPs from the muscles of the lower extremities during surgery in patients with preoperative weakness due to spinal cord compression. In this study, motor function of patients' lower extremities and their association with intraoperative MEP recording were compared. Patients undergoing thoracic spinal cord tumor resection were studied. Patients' motor function was checked immediately before the surgical procedure. MEP responses were recorded from the tibialis anterior and foot muscles, and the hand muscles were used as control. Electrical current with train of eight pulses, 200 to 500 V was delivered through 2 corkscrews placed at C3' and C4' sites. Anesthesia was maintained by total intravenous anesthesia using a combination of propofol and remifentanil after induction with intravenous propofol, remifentanil, and rocuronium. Rocuronium was not repeated. Bispectral Index was maintained between 40 to 50. From 178 lower limbs of 89 patients, myogenic MEPs could be recorded from 100% (105/105) of the patients with 5 of 5 motor strength in lower extremity; 90% (36/40) from the patients with 4/5 motor strength; only 25% (5/20) with 3/5; and 12.5% (1/8) with 2/5 motor strength; none (0/5) were able to be recorded if the motor strength was 1/5. The ability to record myogenic MEPs is closely associated with the patient's motor function. They are difficult to obtain if motor function is 3/5 motor strength in the lower extremity. They are almost impossible to record if motor function is worse than 3/5.

Quantitative analysis of motor evoked potentials in the neonatal lamb

Evoking motor potentials are an objective assessment method for neuromotor function, yet this was to our knowledge never done in neonatal lambs. There is neither a method for standardized quantification of motor evoked potentials (MEPs). We first aimed to evaluate the feasibility of MEP recording in neonatal lambs and test its validity. Second we aimed to develop an algorithm for its quantification and test its reliability since manual input is required. We recorded myogenic MEPs after transcranial motor cortex stimulation in 6 lambs aged 1–2 days. MEPs were also measured in one lamb undergoing Neuro-Muscular Blockade (NMB) and another undergoing lumbar spinal cord (SC) transection, both serving as controls. We computed 5 parameters using a custom-made algorithm: motor threshold, latency, area-under-the-curve, peak-to-peak amplitude and duration. Intra- and inter-observer reliability was analyzed. MEPs could be easily recorded, disappearing after NMB and SC transection. The algorithm allowed for analysis, hence physiologic readings of the parameters in all 4 limbs of all lambs were obtained. Our method was shown to have high intra- and inter-observer ( ≥70%) reliability for latency, area-under-the-curve and peak-to-peak amplitude. These results suggest that standardized MEP recording and analysis in neonatal lambs is feasible, and can reliably assess neuromotor function.

Influence of Tumor Location and Other Variables on Predictive Value of Intraoperative myogenic Motor-Evoked Potentials in Spinal Cord Tumor Surgery

To study the influence of tumor location (cervical vs. thoracic; extramedullary vs. intramedullary) on predictive value of intraoperative myogenic motor-evoked potentials (iMEP) changes in patients undergoing surgery for spinal cord tumors. Three hundred patients retrospective data (91 intramedullary) and 209 (intradural extramedullary) with successful iMEP recordings were analyzed. Responses to transcranial electrical stimulation were recorded from the lower limb muscles. Preoperative clinical variables, iMEPs changes, and postoperative neurologic deficits were noted. Associations between categorical variables and outcome were analyzed with the Fisher exact test. Of the 300 patients 28 (9.3%) had significant intraoperative worsening of iMEPs. New postoperative deficits occurred in 23 of these 28 patients. False-positive decreases in iMEPs were observed in 5 patients. There was a significant association between changes in iMEP and postoperative new motor deficits (P ≤ 0.0001). Multivariate analysis showed that patients with changes in iMEP undergoing surgery for thoracic segment tumors, with longer duration of symptoms (>12 months) and older age (≥21.5 years) were more likely to suffer postoperative neurological decline (odds ratio 4.1, P ≤ 0.001 and odds ratio 5.4 P ≤ 0.0001, respectively). The sensitivity of iMEPs was 100% and specificity 98.2%. The positive and negative predictive values were 82% and 100%; however, the sensitivity and specificity is similar in thoracic intramedullary (TIM) (n= 53) and cervical intramedullary tumors (n= 38) (both were 100% and 97%). The positive predictive value was significantly greater for TIM tumors (93% vs. 50%). A strong association was observed between worsening of iMEPs and postoperative new neurological deficits in patients with TIM tumor.

Improvement of motor evoked potentials monitoring is required during thoracic or thoracoabdominal aortic aneurysm surgery under hypothermic cardiopulmonary bypass

Keywords Motor evoked potential Thoracoabdominalaortic aneurysm HypothermiaMonitoring of motor evoked potentials (MEPs) can provideinformation about the functional integrity of a motorpathway and, therefore, has been widely used during sur-gery in which a motor pathway is at risk of injury,including craniotomy, spine/spinal surgery, and thoracic orthoracoabdominal aortic aneurysm (TAA or TAAA) sur-gery [1, 2]. Because myogenic MEPs are very sensitive tosuppression by anesthetic agents and neuromuscularblockade, careful attention should be paid to anestheticmanagement during monitoring of MEPs. In general, totalintravenous anesthesia with propofol and opioids is used,although ketamine-based anesthesia may be applied incases in which recording of myogenic MEPs is not reliablebecause of suppression by anesthetic agents or influence bypreoperative motor dysfunction.Of the cases in which MEPs monitoring is required,TAA/TAAA surgery is the most challenging for reliableMEP monitoring. In addition to the effects of anesthetics,a variety of factors can affect the recording of MEPs,including hypothermia, aortic clamp, and induction ofcardiopulmonary bypass (CPB). Hypothermia may havedirect effects on the MEP itself and also indirect effects onMEPs by changing the pharmacokinetics and pharmaco-dynamics of anesthetic agents, modified by the effects ofaortic clamp and the induction of CPB. Leslie et al. [3]demonstrated that a temperature reduction of 3 C increasedblood propofol concentration by 30 C during a constantrate infusion. Kakinohana et al. [4] reported that plasmapropofol concentration increased and bispectral indexvalues decreased rapidly after aortic cross-clamping inthoracic aortic aneurysm repair surgery during propofolanesthesia. Yoshitani et al. [5] demonstrated that, in patientsreceiving propofol infusion at a rate of 5 or 6 mg/kg/h,plasma propofol concentration increased and burst sup-pression rate on electroencephalogram increased after theinduction of normothermic CPB, compared with thosevalues before CPB. These findings suggest that interpre-tation of MEP changes could be difficult because anestheticlevel can easily fluctuate during TAAA surgery withhypothermic CPB.Data regarding the effects of hypothermia on myogenicMEPs have been limited to animal experiments. Meylaertset al. [6] investigated the effects of regional spinal cordhypothermia on myogenic MEPs induced by transcranialtrain-pulse stimulation in pigs anesthetized with ketamine,sufentanil, clonidine, and nitrous oxide. They demonstratedthat progressive cooling resulted in an increase of MEPs at28 –30 C and was followed by a progressive decrease inMEP amplitude. MEP amplitude decreased below 25% atapproximately 14 C. Sakamoto et al. [7] also investigatedthe effects of systemic hypothermia on myogenic MEPsinduced by a single pulse and a train of pulses under

Spinal somatosensory evoked potential combined with motor evoked potential in monitoring patients undergoing surgery for cervical spondylosis

Objective To explore the value of cortical somatosensory evoked potential(CSEP)combined with myogenic motor evoked potential(MMEP)in monitoring patients undergoing surgery for cervical spndylosis. Methods CSEP and MMEP were simultaneously performed on 35 patients with spinal type cervical spndylosis during spinal surgery. The accuracy of CSEP and MMEP were assessed according to the signal recording at different stages combined with postoperative spinal cord function. ResultsThe success rate was 100% for CSEP and 85.7% for MMEP recording. CSEP reached the warning standards in one patient intraoperatively, warning the surgeons to discontinue the procedure temporarily until the influencing factors were eliminated and the amplitudes of CSEP returned gradually. The wave tape of MMEP disappeared in 2 patients transitorily. The amplitudes of CSEP increased in 28 patients after decompression; CSEP and MMEP had little change in 3 patients. No nerve injury symptoms were worsened in all the patients postoperatively. Conslusions During cervical spinal surgery, CSEP and MMEP can reflect the physiological and pathological conditions of the spinal cord after the interfering factors are excluded. The combined application of CSEP and MMEP improve the monitoring effect and guarantee patient safety. Key words: Somatosensory evoked potential; Motor evoked potential; Cervical spndylosis; Spinal cord monitoring

Evaluation of Posttetanic Motor Evoked Potentials—The Influences of Repetitive Use, the Residual Effects of Tetanic Stimulation to Peripheral Nerve, and the Variability

Recently, we developed a new technique to augment myogenic motor evoked potentials (MEPs), called as posttetanic MEPs (p-MEPs), in which tetanic stimulation is applied to peripheral nerve before transcranial stimulation. However, the data on p-MEPs are limited. This study was conducted; (1) to evaluate the influences of repetitive use of p-MEPs on p-MEP amplitudes, (2) to evaluate the residual effects of use of p-MEPs on subsequent conventional MEPs (c-MEPs), and (3) to compare the variability of p-MEPs with that of c-MEPs. Sixty patients under propofol/fentanyl anesthesia with partial neuromuscular blockade were enrolled. For p-MEP measurements, tetanic stimulation was applied to posterior tibial nerve 1 second before transcranial stimulation. In study 1, p-MEPs were repetitively recorded with intervals of 10 or 60 seconds. In study 2, the amplitudes of c-MEPs recorded 15, 30, 60, and 120 seconds after p-MEP recordings were compared with those of control. In study 3, the coefficients of variation of c-MEP and p-MEP responses were compared. The repetitive use of p-MEP with an interval of 10 seconds, but not 60 seconds, induced a significant reduction of p-MEP amplitude. Amplitudes of c-MEP were significantly increased when applied within 60 seconds after p-MEP recordings. The coefficient of variations of p-MEPs was similar to those of c-MEPs. The results indicated that the amplitudes of p-MEP and c-MEP might be affected when applied with a short interval after p-MEP recording.

Intraoperative preservation of corticospinal function in the brainstem

Background:The corticospinal tract features a largely exposed course through the brainstem, and is therefore at risk in many brainstem-related procedures. No large case series on motor-evoked potential (MEP) monitoring during...

Recent advances in the monitoring of myogenic motor-evoked potentials: development of post-tetanic motor-evoked potentials

Recent advances in the monitoring of myogenic motor-evoked potentials: development of post-tetanic motor-evoked potentials

Delayed response of transcranial myogenic motor-evoked potential monitoring to spinal cord ischemia during repair surgery for descending thoracic aortic aneurysm

The efficacy of transcranial myogenic motor-evoked potential (tc-MEP) monitoring during thoracic aortic surgery has been the subject of some reports, because tc-MEP monitoring can rapidly reflect changes in spinal cord blood flow during thoracic aortic cross-clamping. In this article, we present a case in which delayed loss of tc-MEP signals was observed after cross-clamping of the descending thoracic aorta. We must be aware that tc-MEPs recorded from the lower extremities can fail to provide rapid detection of spinal cord ischemia in the upper thoracic level after cross-clamping of the descending thoracic aorta.

The Application of Tetanic Stimulation of the Unilateral Tibial Nerve Before Transcranial Stimulation Can Augment the Amplitudes of Myogenic Motor-Evoked Potentials from the Muscles in the Bilateral Upper and Lower Limbs

Recently, we reported a new technique to augment motor-evoked potentials (MEPs) under general anesthesia, posttetanic MEP (p-MEP), in which tetanic stimulation of the peripheral nerve before transcranial stimulation enlarged amplitudes of MEPs from the muscle innervated by the nerve subjected to tetanic stimulation. In the present study, we tested whether tetanic stimulation of the left tibial nerve can also augment amplitudes of MEPs from the muscles which are not innervated by the nerve subjected to tetanic stimulation. Thirty patients undergoing spinal surgery under propofol-fentanyl anesthesia with partial neuromuscular blockade were examined. For conventional MEP (c-MEP) recording, transcranial stimulation with train-of-five pulses was delivered to C3-4, and the compound muscle action potentials were bilaterally recorded from the abductor pollicis brevis, abductor hallucis (AH), tibialis anterior, and soleus muscles. For p-MEP recording, tetanic stimulation (50 Hz, 50 mA of stimulus intensity) with a duration of 5 s was applied to the left tibial nerve at the ankle 1 s before transcranial stimulation. Transcranial stimulation and recording of compound muscle action potentials were performed in the same manner as c-MEP recording. Amplitudes of c-MEP and p-MEP were compared using Wilcoxon's signed rank test. Amplitudes of p-MEPs from the left AH muscle innervated by the left tibial nerve with tetanic stimulation were significantly larger compared with those of c-MEPs. Amplitudes of p-MEPs from the bilateral abductor pollicis brevis and soleus muscles and right AH and tibialis anterior muscles, which were not innervated by the left tibial nerve with tetanic stimulation, were also significantly larger compared with those of c-MEPs. In patients under propofol and fentanyl anesthesia with partial neuromuscular blockade, the application of tetanic stimulation to the left tibial nerve augmented the amplitudes of MEPs from the muscles without tetanic nerve stimulation and those with stimulation.

Interference of anesthesia on intraoperative monitoring of spinal motor function with motor-evoked potentials

As a neurophysiological monitoring technique, motor-evoked potentials (MEPs) has been widely used for intraoperatively monitoring motor function of spinal cord. Myogenic MEPs elicited by transcranial electrical stimulation are commonly used to monitor spinal function. However, myogenic MEPs may be interfered by most anesthetic agents and hypothermia. Muhipulse electric stimulation is the recommended tool for intraoperative monitoring of spinal motor function. Key words: motor-evoked potentials; multipulse electric stimulation; anesthetic agents; hypothermia; spinal motor function monitoring

The Effects of the Neuromuscular Blockade Levels on Amplitudes of Posttetanic Motor-Evoked Potentials and Movement in Response to Transcranial Stimulation in Patients Receiving Propofol and Fentanyl Anesthesia

Patient movement in response to transcranial stimulation during monitoring of myogenic motor-evoked potentials (MEPs) may interfere with surgery. We recently reported a new technique to augment the amplitudes of myogenic MEPs, called "post-tetanic MEPs (p-MEPs)," in which tetanic stimulation of a peripheral nerve was applied prior to transcranial stimulation. We conducted the present study to determine an appropriate level of neuromuscular blockade during the monitoring of p-MEPs with a focus on patient movement. In 15 patients under propofol/fentanyl anesthesia, conventional MEPs (c-MEPs) and p-MEPs in response to transcranial electrical stimulation were recorded from the abductor hallucis muscle. For p-MEP recording, tetanic stimulation to the posterior tibial nerve at an intensity of 50 mA for 5 s was started 6 s prior to transcranial stimulation. The level of neuromuscular blockade was assessed by recording the amplitude of compound muscle action potentials (T1) from the abductor hallucis brevis muscle in response to supramaximal electrical stimulation of the median nerve at the wrist. After the baseline recordings of c-MEP and p-MEP at a T1 of 50% of control, 0.1 mg/kg of vecuronium was injected and the amplitudes of c-MEPs and p-MEPs were recorded. Patient movement was also assessed with the movement score ranging from 1 to 4 (1 = no movement, 4 = severe movement). T1, %T1, the amplitudes of c-MEPs and p-MEPs, and the movement score changed in parallel after the administration of vecuronium. The amplitudes of p-MEPs before and 15-45 min after the administration of vecuronium were significantly higher than those of c-MEPs. When T1 and %T1 were less than and equal to 1 mV and 10%, respectively, the movement score was 1 or 2 in all patients, indicating that microscopic surgery was possible without the interruption of surgical procedures. When T1 was around 1 mV (0.8-1.2 mV), the success rates of recording of c-MEPs and p-MEPs were 73% (11 of 15) and 100% (15 of 15), respectively. Under propofol/fentanyl anesthesia, p-MEP could be recorded at a T1 of 1 mV, in which patient movement in response to transcranial stimulation did not interfere with surgery. This technique may be used in patients without preoperative motor deficits, in which patient movement during surgical procedures is not preferable.

The Effects of Dexmedetomidine on Myogenic Motor Evoked Potentials in Rabbits

Dexmedetomidine is used in the perioperative management of patients, including as an intraoperative adjuvant. The effects of dexmedetomidine on myogenic motor evoked potentials (MEPs) remain undetermined. We conducted the present study to investigate the effects of dexmedetomidine on myogenic MEPs in rabbits. New Zealand white rabbits were used for the studies. First, to determine appropriate doses of dexmedetomidine as an adjunct for anesthesia in rabbits, the level of anesthesia was evaluated by testing the palpebral and limb withdrawal reflexes, and the reactions to ear pinching and tail clamp at 5, 25, 50, 100 microg/kg/h. Second, in 10 rabbits under ketamine and fentanyl anesthesia, myogenic MEPs in response to single pulse and a train-of-five pulses were recorded from the soleus muscle before, during, and after the administration of dexmedetomidine at 5, 25, and 50 microg/kg/h. At 50 microg/kg/h of dexmedetomidine, palpebral reflex, limb reflex, and reaction to ear pinching were inhibited in >50% of animals, but the reaction to tail clamp was not reduced. Dexmedetomidine suppressed myogenic MEPs in a dose-dependent manner, but when multipulses were used for stimulation, myogenic MEPs could be recorded in all animals at 50 microg/kg/h. As long as multipulse is used for stimulation, the recording of myogenic MEPs is feasible in rabbits under ketamine and fentanyl anesthesia during the administration of dexmedetomidine at doses that are an adjunct to anesthesia.

Success Rate of Motor Evoked Potentials for Intraoperative Neurophysiologic Monitoring: Effects of Age, Lesion Location, and Preoperative Neurologic Deficits

Transcranial electrical stimulation with myogenic motor evoked potential (MEP) recording was used for intraoperative neurophysiologic monitoring in 341 consecutive "high-risk" neurosurgical or orthopedic procedures. Overall, the success rate for establishing reliable MEP response was 94.8% for upper extremities and 66.6% for lower extremities. The rate was only 39.1% for lower extremities in patients with preoperative motor deficit and up to 81% in neurologically intact adults. Further analysis demonstrated that extremes of age or the presence of a lesion in the spinal cord and motor deficit contributed to failure in obtaining reliable MEPs.

The Effects of Xenon on Myogenic Motor Evoked Potentials in Rabbits: A Comparison with Propofol and Isoflurane

We compared the effects of xenon on myogenic motor evoked potentials (MEPs) with those of propofol and isoflurane in rabbits under ketamine/fentanyl anesthesia. Thirty animals were randomly allocated to one of 3 groups (n = 10 in each group). In the propofol group, propofol was administered at a rate of 0.4 mg x kg(-1) x min(-1) (small) and 0.8 mg x kg(-1) x min(-1) (large). In the isoflurane group, isoflurane was administered at 0.8% (small) and 1.6% (large). In the xenon group, xenon was administered at 35% (small) and 70% (large). Myogenic MEPs in response to stimulation with single pulse and a train of 5 pulses were recorded from the soleus muscle before, during (at small and large doses), and after the administration of each anesthetic. With single-pulse stimulation, MEPs were recorded in 90% and 50% of animals at small and large doses of xenon, respectively, and MEP amplitudes in the xenon and isoflurane groups were significantly lower compared with those in the propofol group. With train pulse stimulation, MEPs were recorded in 100% and 90% of animals at small and large doses of xenon, respectively, and a reduction in MEP amplitudes by xenon was more prominent than by propofol but less than isoflurane at large doses. These results suggest that MEP recording may be feasible under xenon anesthesia if multipulse stimulation is used, although xenon has suppressive effects on myogenic MEPs.

The Effects of Volatile Anesthetics on Intraoperative Monitoring of Myogenic Motor-Evoked Potentials to Transcranial Electrical Stimulation and on Partial Neuromuscular Blockade During Propofol/Fentanyl/Nitrous Oxide Anesthesia in Humans

The aim of the present study was to compare the influence of volatile anesthetics on transcranial motor-evoked potentials (tcMEP) in humans anesthetized with propofol/fentanyl/nitrous oxide and on partial neuromuscular blockade (NMB). The authors studied 35 ASA I and II patients who were undergoing elective craniotomy and brain tumor resection. The patients were randomized to one of three groups to receive halothane (HAL), isoflurane (ISO), or sevoflurane (SEV). Anesthetic depth was initially adjusted using the bispectral index to 40+/-5, and NMB was adjusted to 40%-50% of one twitch of train of four (T1) after recovery from intubation. MEPs with train of five square-wave pulses were elicited using screw electrodes placed in the skull over C3-C4. After craniotomy, the inhalational agent was introduced at 0.5 MAC and then 1.0 MAC (20 minutes each), and the effects on MEPs, NMB, and hemodynamic variables were studied. A decrease in BIS and systolic blood pressure was observed with all agents. Both SEV and ISO at 1.0 MAC significantly decreased train-of-four ratio from 38.4+/-18.1 at control to 19.0+/-9.7 and from 35.3+/-12.4 to 26.1+/-13.7, respectively (P<0.001), but not HAL at 1.0 MAC. The amplitudes of tcMEPs were significantly reduced by all agents at 1.0 MAC, with the effect being less in HAL at 0.5 MAC. We have shown that HAL had a lesser suppressive effect on MEPs than either ISO or SEV at 0.5 MAC, which was partially due to a lesser degree of NMB.

Tetanic Stimulation of the Peripheral Nerve before Transcranial Electrical Stimulation Can Enlarge Amplitudes of Myogenic Motor Evoked Potentials during General Anesthesia with Neuromuscular Blockade

Neuromuscular blockade can suppress myogenic motor evoked potentials (MEPs). The authors hypothesized that tetanic stimulation (TS) of the peripheral nerve before transcranial stimulation may enhance myogenic MEPs during neuromuscular blockade. In the current study, the authors evaluated MEP augmentations by TS at different levels of duration, posttetanic interval, neuromuscular blockade, and stimulus intensity. Thirty-two patients undergoing propofol-fentanyl-nitrous oxide anesthesia were examined. Train-of-five stimulation was delivered to C3-C4, and MEPs were recorded from the abductor hallucis muscle. In study 1, TS with a duration of 1, 3, or 5 s was delivered at 50 Hz to the tibial nerve 1, 3, or 5 s (interval) before transcranial stimulation, and the effects of TS on MEP amplitude were evaluated. In study 2, TS-induced MEP augmentations were evaluated at the neuromuscular blockade level (%T1) of 50% or 5%. In study 3, MEP augmentations by TS at stimulus intensities of 0, 5, 25, and 50 mA were evaluated. The application of TS significantly enlarged the amplitudes of MEPs at the combinations of duration (3, 5 s) and interval (1, 3, 5 s) compared with those without TS. TS-induced MEP augmentations were similarly observed at %T1 of both 50% and 5%. TS-induced MEP augmentations were observed at stimulus intensities of 25 and 50 mA. The results indicate that TS of the peripheral nerve before transcranial stimulation can enlarge the amplitude of MEPs during general anesthesia with neuromuscular blockade. TS of the peripheral nerve can be intraoperatively applied as a method to augment myogenic MEP responses.