Effect of desflurane anesthesia on transcortical motor evoked potentials

Propofol reduces the amplitude of transcranial electrical motor-evoked potential without affecting spinal motor neurons: a prospective, single-arm, interventional study

PURPOSE

Propofol inhibits the amplitudes of transcranial electrical motor-evoked potentials (TCE-MEP) in a dose-dependent manner. However, the mechanisms of this effect remain unknown. Hence, we investigated the spinal mechanisms of the inhibitory effect of propofol on TCE-MEP amplitudes by evaluating evoked electromyograms (H-reflex and F-wave) under general anesthesia.

METHODS

We conducted a prospective, single-arm, interventional study including 15 patients scheduled for spine surgery under general anesthesia. Evoked electromyograms of the soleus muscle and TCE-MEPs were measured at three propofol concentrations using target-controlled infusion (TCI: 2.0, 3.0, and 4.0 µg/mL). The primary outcome measure was the left H-reflex amplitude during TCI of 4.0- compared to 2.0-µg/mL propofol administration.

RESULTS

The median [interquartile range] amplitudes of the left H-reflex were 4.71 [3.42-6.60] and 5.6 [4.17-7.46] in the 4.0- and 2.0-μg/mL TCI groups (p = 0.4, Friedman test), respectively. There were no significant differences in the amplitudes of the right H-reflex and the bilateral F-wave among these groups. However, the TCE-MEP amplitudes significantly decreased with increased propofol concentrations (p < 0.001, Friedman test).

CONCLUSION

Propofol did not affect the amplitudes of the H-reflex and the F-wave, whereas TCE-MEP amplitudes were reduced at higher propofol concentrations. These results suggested that propofol can suppress the TCE-MEP amplitude by inhibiting the supraspinal motor pathways more strongly than the excitability of the motor neurons in the spinal cord.

Desflurane Preconditioning Protects Against Renal Ischemia-Reperfusion Injury and Inhibits Inflammation and Oxidative Stress in Rats Through Regulating the Nrf2-Keap1-ARE Signaling Pathway

OBJECTIVE

Kidney is sensitive to ischemia-reperfusion (I/R) injury because of its special structure and function. In this study, we aimed to explore the mechanism of desflurane (DFE) preconditioning effecting on renal I/R injury in rats.

METHODS

Renal I/R injury rats model was constructed, and the expressions of serum renal function parameters (blood urea nitrogen (BUN) and serum creatinine (SCr)) and lipid peroxidation-related factors were detected using corresponding commercial kits to assess the degrees of renal functional damage and oxidative stress. Hematoxylin--eosin (HE) staining and Masson trichrome staining were applied to measure the renal histologic damage. The expressions of inflammation-related factors were determined by ELISA assay. The cell apoptosis was analyzed using TUNEL, Western blot and immunohistochemistry (IHC). IHC was also used to detect the number of myeloperoxidase (MPO)-positive cells. The expressions of proteins associated with the Nrf2-Keap1-ARE pathway were assessed by Western blot and IHC.

RESULTS

DFE preconditioning inhibited I/R injury-induced BUN and SCr increase and renal histologic injury in rats. Also, DFE suppressed the inflammation, apoptosis and oxidative stress caused by renal I/R injury in vivo. In addition, DFE preconditioning repressed peroxide-related factors (MDA, MPO and NO) expressions and promoted antioxidant-related factors (GSH, SOD, GPx and CAT) expressions. In addition, DFE promoted Nrf2-Keap1-ARE-related proteins including Nrf2, NQO1, HO-1, γ-GCS, GSR and GCLc expressions.

CONCLUSION

DFE preconditioning protected the kidney as well as inhibited the inflammation, cell apoptosis and oxidative stress in renal I/R injury rats by activating the Nrf2-Keap1-ARE signaling pathway.

A practical guide for anesthetic management during intraoperative motor evoked potential monitoring

Postoperative motor dysfunction can develop after spinal surgery, neurosurgery and aortic surgery, in which there is a risk of injury of motor pathway. In order to prevent such devastating complication, intraoperative monitoring of motor evoked potentials (MEP) has been conducted. However, to prevent postoperative motor dysfunction, proper understanding of MEP monitoring and proper anesthetic managements are required. Especially, a variety of anesthetics and neuromuscular blocking agent are known to attenuate MEP responses. In addition to the selection of anesthetic regime to record the baseline and control MEP, the measures to keep the level of hypnosis and muscular relaxation at constant are crucial to detect the changes of MEP responses after the surgical manipulation. Once the changes of MEP are observed based on the institutional alarm criteria, multidisciplinary team members should share the results of MEP monitoring and respond to check the status of monitoring and recover the possible motor nerve injury. Prevention of MEP-related adverse effects is also important to be considered. The Working Group of Japanese Society of Anesthesiologists (JSA) developed this practical guide aimed to help ensure safe and successful surgery through appropriate anesthetic management during intraoperative MEP monitoring.

The influence of depth of anesthesia on motor evoked potential response during awake craniotomy

OBJECTIVE

Motor evoked potentials (MEPs) are a critical indicator for monitoring motor function during neurological surgery. In this study, the influence of depth of anesthesia on MEP response was assessed.

METHODS

Twenty-eight patients with brain tumors who underwent awake craniotomy were included in this study. From a state of deep anesthesia until the awake state, MEP amplitude and latency were measured using 5-train electrical bipolar stimulations on the same site of the precentral gyrus each minute during the surgery. The depth of anesthesia was evaluated using the bispectral index (BIS). BIS levels were classified into 7 stages: < 40, and from 40 to 100 in groups of 10 each. MEP amplitude and latency of each stage were compared. The deviation of the MEP measurements, which was defined as a fluctuation from the average in every BIS stage, was also considered.

RESULTS

A total of 865 MEP waves in 28 cases were evaluated in this study. MEP amplitude was increased and latency was decreased in accordance with the increases in BIS level. The average MEP amplitudes in the > 90 BIS level was approximately 10 times higher than those in the < 40 BIS level. Furthermore, the average MEP latencies in the > 90 BIS level were 1.5–3.1 msec shorter than those in the < 60 BIS level. The deviation of measured MEP amplitudes in the > 90 BIS level was significantly stabilized in comparison with that in the < 60 BIS level.

CONCLUSIONS

MEP amplitude and latency were closely correlated with depth of anesthesia. In addition, the deviation in MEP amplitude was also correlated with depth of anesthesia, which was smaller during awake surgery (high BIS level) than during deep anesthesia. Therefore, MEP measurement would be more reliable in the awake state than under deep anesthesia.

Effects of intrathecal morphine on transcranial electric motor-evoked potentials in adolescents undergoing posterior spinal fusion

BACKGROUND

Intrathecal morphine (ITM) provides effective analgesia after posterior spinal fusion (PSF). Although most anesthetic drugs have well-characterized effects on evoked potentials, there is little data on the effects of ITM on transcranial electric motor-evoked potentials (tceMEPs). We performed this study to assess the effects of ITM on tceMEPs in the first 30 minutes after administration. We hypothesized that administration of ITM in doses currently used at our institution would not significantly affect mean tceMEP amplitudes and latencies of an ITM study group relative to control patients who did not receive the drug.

METHODS

tceMEPs were recorded before ITM injection and 5, 10, 20, and 30 minutes after injection in 14 subjects ages 11 through 18 years undergoing PSF. These recordings were compared to an age-matched control group undergoing PSF in which ITM was not injected. The effects of ITM on tceMEP amplitude and latency were compared between the 2 groups.

RESULTS

Fourteen subjects were enrolled in the ITM group and 16 served as controls. There were no significant differences in the baseline mean response amplitudes of the 2 groups for any of the 8 muscles studied. Mean response amplitudes over the 30-minute posttreatment period in the ITM group did not differ significantly from those of the control subjects. Average response amplitudes collapsed across all muscles for each subject were not significantly different during the baseline period (95% CI = -38% to 45%; P = 0.783), nor were they significantly different between the 2 groups during the posttreatment period (95% CI = -30% to 78%; P = 0.640). There also were no significant differences in the mean response latencies of the 2 groups in either the baseline or posttreatment periods. Average response latencies collapsed across all muscles for each subject were 4% larger for the ITM group than for controls during the baseline period (95% CI = -5% to 13%; P = 0.377), and 3% larger for the ITM group than for controls during the posttreatment period (95% CI = -4% to 12%; P = 0.359).

CONCLUSIONS

Administration of ITM in doses currently used at our institution did not cause more than a 70% attenuation of mean tceMEP amplitudes or latency changes of an ITM study group relative to control subjects during the 30-minute period after injection. Further studies are required to determine if there are delayed effects after this initial time period.

Noninvasive model of sciatic nerve conduction in healthy and septic mice: reliability and normative data

Neuromuscular disorders frequently complicate sepsis and other critical illnesses in patients. Mice are the major species used as a model for sepsis. Nerve conduction studies (NCS), the primary tool for noninvasive assessment of nerve and muscle function, is challenging to perform in small animals. A reliable method for noninvasive, repeated NCS testing has not been reported in mice. We developed and validated a method for the repeated measurement of mouse sciatic nerve conduction in normal and septic mice. Our sedated and awake NCS system enabled minimally invasive long-term repeated measurements. The mean compound muscle action potential (CMAP) amplitude and latency were 17.4 mV and 1.11 ms, respectively (n = 59). There was an excellent intertester reproducibility by linear regression in both normal (r = 0.95) and septic (r = 0.98) mice. We also showed significant, time-dependent isoflurane-induced CMAP suppression in all animals, which was further exacerbated in septic mice. This study provides a new tool for the assessment of peripheral nerve/muscle function in mouse neuromuscular disease models that require repeated, long-term, and minimally invasive monitoring.

Functional abnormalities of the motor tract in the rat after portocaval anastomosis and after carbon tetrachloride induction of cirrhosis

INTRODUCTION

Hepatic encephalopathy is a neurologic syndrome secondary to liver failure that causes cognitive and motor abnormalities. Impairment in the function of the first neuron of the motor tract (corticospinal tract) has been demonstrated in patients with cirrhosis and minimal hepatic encephalopathy.

AIM

Investigate the function of the first neuron of the motor tract in experimental models of minimal hepatic encephalopathy.

MATERIAL AND METHODS

Rats with portocaval anastomosis (n = 8) and rats with carbon tetrachloride induced cirrhosis (n = 11) underwent neurophysiological recording under light anesthesia with propofol. Motor evoked potentials were elicited applying a transcranial electric pulse and were recorded in the tibialis anterior muscle. The effect of the dose of anesthesia was assessed in a group of normal rats (n = 10).

RESULTS

Rats with portocaval anastomosis exhibited a decrease in motor evoked potentials amplitude following surgery (67 +/- 11 to 41 +/- 16%, P < 0.001). Cirrhotic rats exhibited an increase in motor evoked potentials latency after the appearance of ascites (4.65 +/- 0.43 to 5.15 +/- 0.67 ms., P = 0.04). Increasing doses of propofol produced a decrease in the amplitude and an increase in the latency of motor evoked potentials.

CONCLUSION

It is possible to reproduce functional abnormalities of the central motor tract in rats with portocaval anastomosis and carbon tetrachloride induced cirrhosis. The development of motor abnormalities in experimental models of minimal hepatic encephalopathy offers the possibility to investigate the mechanisms involved in the pathogenesis of hepatic encephalopathy and test therapeutic strategies.

Intraoperative Motor-evoked Potential Monitoring in Scoliosis Surgery: Comparison of Desflurane/Nitrous Oxide With Propofol Total Intravenous Anesthetic Regimens

STUDY DESIGN

A prospective, randomized study in a large general hospital setting.

BACKGROUND

During spinal surgery, monitoring motor-evoked potentials (MEPs) is a means of assessing the intraoperative integrity of corticospinal pathways. However, MEPs are known to be sensitive to the effects of anesthetic agents.

OBJECTIVE

To compare the use of desflurane or total intravenous anesthetic regimens (TIVA) with multipulse cortical stimulation for intraoperative monitoring (IOM).

METHODS

Twenty consecutive patients (10 in each arm) undergoing scoliosis correction surgery were randomly assigned to 2 equal groups receiving desflurane or TIVA. Inhalational anesthesia was maintained using 66% nitrous oxide in oxygen and a mean end-tidal desflurane concentration of 3.4%. For TIVA, continuous intravenous infusion of propofol was used. For analgesia, fentanyl and morphine were given when required for both groups. Cortical stimulation was achieved with 2 bipolar direct current stimulators connected in parallel by jumper cables. Five equivalent pulses 0.5 ms in duration at 4 ms intervals were delivered at C1C2 positions. MEP recordings were made in the abductor hallucis (AH) and tibialis anterior (TA) with needle electrodes.

RESULTS

Reproducible MEPs were obtained throughout the operation in all 20 cases, with up to 80 mA per stimulator. Before insertion of pedicle screws, mean MEP amplitudes (SD) obtained were 85 (19) and 21.7 (10.8) mV for AH and TA, respectively, using desflurane. With TIVA, amplitudes were 56.7 (28.4) and 59.1 (24.5) mV, respectively. Both muscle MEP amplitudes were significantly different using different anesthetic regimens (P < 0.05 for all). AH MEP amplitudes obtained with desflurane were significantly larger than TA amplitudes (P < 0.0001). No complications were reported intraoperatively and postoperatively.

CONCLUSIONS

This is the first study comparing the use of desflurane and TIVA showing that both anesthetic regimens allowed successful intraoperative monitoring useage throughout the procedures. For MEP recording, the AH was the preferred muscle with a desflurane anesthetic regimen.

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.

The Effects of Stimulation Pattern and Sevoflurane Concentration on Intraoperative Motor-Evoked Potentials

The usefulness of intraoperative monitoring of motor-evoked potentials (MEPs) during inhaled anesthesia is limited by the suppressive effects of volatile anesthetics on MEP signals. We investigated the effects of different stimulation patterns and end-tidal concentrations of sevoflurane on intraoperative transcranial electrical MEPs. In 12 patients undergoing craniotomy, stimulation patterns (300-500 V, 100-1000 Hz, 1-5 stimuli) and multiples (0.5, 0.75, and 1.0) of minimum alveolar concentration (MAC) of sevoflurane were varied randomly while remifentanil was administered at a constant rate of 0.2 microg x kg(-1) x min(-1). MEPs were recorded from thenar and hypothenar muscles and analyzed without knowledge of the respective MAC. Three-way analysis of variance revealed significant main effects for increasing stimulation intensity, frequency, and number of stimuli on MEP amplitude (P < 0.05). Maximum MEP amplitudes and recording success rates were observed during 4 stimuli delivered at 1000 Hz and 300 V. A significant main effect of sevoflurane concentration (0.5 versus 0.75 and 1 MAC multiple) on MEP amplitude was observed at the thenar recording site only (P < 0.05). In conclusion, MEP characteristics varied significantly with changes in stimulation pattern and less so with changes in sevoflurane concentration. The results suggest that high frequency repetitive stimulation allows intraoperative use of MEP monitoring during up to 1 MAC multiple of sevoflurane and constant infusion of remifentanil up to 0.2 microg x kg(-1) x min(-1).

Intra-operative monitoring in scoliosis surgery with multi-pulse cortical stimuli and desflurane anesthesia

STUDY DESIGN

Prospective, observational study.

SETTING

Country General Hospital, Singapore.

OBJECTIVE

Intraoperative monitoring (IOM) with motor-evoked potentials (MEPs) assesses the integrity of cortical spinal tracts during scoliosis surgery. MEPs are sensitive to the effects of inhalational anesthetic agents. We evaluate the use of desflurane in combination with multipulse cortical stimulation in this study.

METHODS

In all, 10 consecutive neurologically normal subjects underwent scoliosis surgery with desflurane anesthesia (0.5 maximum alveolar concentration) and five pulse cortical stimulation (250 Hz) from two stimulators in parallel configuration, delivering a maximum intensity of 160 mA.

RESULTS

Consistent MEPs were obtained from the abductor hallucis and tibialis anterior in nine of ten and five of five of subjects, respectively. Baseline coefficients of variations were below 16% for both muscles.

CONCLUSION

This combination of anesthetic and stimulation protocols is efficacious for IOM during spinal cord surgery. Our findings support the use of desflurane for successful acquisition of MEPs during scoliois surgery as an alternative anesthetic regime.

Electrophysiologic intraoperative monitoring for spine procedures

The advent of equipment capable of performing SEPs, MEPs, and EMG in a multiplexed manner and in a timely fashion brings a new level of monitoring that far exceeds the previous basic monitoring done with SEPs only. Whether this more comprehensive monitoring will result in greater protection of the nervous system awaits future analysis. In any event, monitoring of the spinal cord with SEPs is an accepted standard of care for cases that place the spinal cord at risk. Likewise, nerve root monitoring with EMG is a widely practiced form of monitoring and shows great benefit. MEPs and reflex monitoring, which address the descending pathways and the interneuronal connections, is efficacious in detecting abnormalities that may be missed by SEPs.

Effects of Anesthetic Agents and Physiologic Changes on Intraoperative Motor Evoked Potentials

Motor evoked potentials (MEPs) have shown promise as a valuable tool for monitoring intraoperative motor tract function and reducing postoperative plegia. MEP monitoring has been reported to contribute to deficit prevention during resection of tumors adjacent to motor structures in the cerebral cortex and spine, and in detecting spinal ischemia during thoracic aortic reconstruction. Many commonly used anesthetic agents have long been known to depress MEP responses and reduce MEP specificity for motor injury detection. Although new stimulation techniques have broadened the spectrum of anesthetics that can be used during MEP monitoring, certain agents continue to have dose-dependent effects on MEP reliability. Understanding the effects of anesthetic agents and physiologic alterations on MEPs is imperative to increasing the acceptance and application of this technique in the prevention of intraoperative motor tract injury. This review is intended as an overview of the effects of anesthetics and physiology on the reproducibility of intraoperative myogenic MEP responses, rather than an analysis of the sensitivity and specificity of this monitoring method in the prevention of motor injury.

Effects of desflurane on spinal somatosensory-evoked potentials and conductive spinal cord evoked potential

STUDY DESIGN

Spinal somatosensory-evoked potential (interspinous-space-recorded evoked potentials after peripheral nerve or dermatomal stimulation) and conductive spinal cord evoked potential (interspinous-space-recorded evoked potentials after spinal cord stimulation) were analyzed in rats under different concentrations of the anesthetic desflurane.

OBJECTIVES

To investigate and compare the effects of a new volatile anesthetic, desflurane, on the common intraoperative neuromonitoring models.

SUMMARY OF BACKGROUND DATA

Intraoperative evoked potentials are sensitive to most anesthetics. Interpretation of the data becomes complicated because of a suppression effect caused by the anesthesia. Desflurane has become a valuable anesthetic in neurosurgery because of its pharmacokinetic advantages.

METHODS

Fifteen rats were placed under general anesthesia, and vital signs were closely monitored. Needle recording electrodes were placed stereotactically into the thoracolumbar interspinous ligament; dermatomal somatosensory-evoked potential by L5 dermatome, mixed-nerve somatosensory-evoked potential by sciatic nerve stimulation, and spinal cord evoked potential of the same recording electrodes elicited by C2-C3 interspinous stimulation were obtained. The effects of desflurane were examined at end-tidal concentrations of 6% (1.05 minimal alveolar concentration), 9% (1.57 minimal alveolar concentration), and 12% (2.10 minimal alveolar concentration).

RESULTS

Amplitude decreased and latency was delayed in all three kinds of potentials, and the more so with higher concentrations. Comparing 9% with 6% desflurane, the amplitude in dermatomal somatosensory-evoked potential, mixed-nerve somatosensory-evoked potential, and spinal cord evoked potential decreased to 84.3%, 88.9%, and 70.8%, respectively, values with no statistically significant difference. However, at 12%, again compared with 6%, the amplitude decreased further to 64.4%, 70.3%, 41.8%, respectively; mixed-nerve somatosensory-evoked potential and dermatomal somatosensory-evoked potential were significantly more preserved than spinal cord evoked potential (P = 0.04).

CONCLUSIONS

The concentration of desflurane alters the amplitude of somatosensory-evoked potential and spinal cord evoked potential, and, to a lesser degree, delays the latency; spinal cord evoked potential is more liable to be suppressed than somatosensory-evoked potential. The dose-dependent suppression effect on amplitude should be considered when interpreting changes during surgery. Furthermore, the potential benefit of somatosensory-evoked potential elicited by direct major nerve stimulation should be considered because of its large amplitude and higher resistance, even with a greater concentration of volatile anesthetics.

Intraoperative monitoring of myogenic motor-evoked potentials from the external anal sphincter muscle to transcranial electrical stimulation

STUDY DESIGN

Motor-evoked potentials from the external anal sphincter were analyzed using transcranial electrical stimulation during spinal surgery in patients under general anesthesia.

OBJECTIVE

To investigate whether motor-evoked potentials from the external anal sphincter could be elicited by transcranial electrical stimulation under general anesthesia.

SUMMARY OF BACKGROUND DATA

Lumbosacral surgery often places nerve rootlets at risk for injury during operative dissection. Specifically, injury for sacral rootlets can result in bowel and bladder dysfunction, but the techniques for monitoring bowel and bladder function are limited.

METHODS

Thirty patients who underwent elective spinal surgery were studied. Patients were anesthetized with 50% nitrous oxide in oxygen, fentanyl, and 4 mg/kg/h of propofol (n = 19) or 1 mg/kg/h of ketamine (n = 11). The level of neuromuscular blockade, assessed by recording the M-response from the right abductor pollicis brevis muscle, was maintained at an M-response amplitude of 40-50% of control. Motor-evoked potentials in response to a multipulse transcranial electrical stimulation at stimulus sites of C3-C4 or Fz-Cz were recorded from the skin over the subcutaneous part of the external anal sphincter using a plug-type electrode probe. The success rate of motor-evoked potentials' recording and peak-to-peak amplitude and the onset latency of motor-evoked potentials were evaluated.

RESULTS

Success rates of motor-evoked potentials from the external anal sphincter were 73% and 53% after transcranial stimulation at stimulus sites of C3-C4 and Cz-Fz, respectively. Amplitudes of motor-evoked potentials after C3-C4 stimulation were significantly greater than those after Cz-Fz stimulation. Motor-evoked potential latency from the external anal sphincter was 18.6 +/- 1.5 and 19.0 +/- 2.7 msec after C3-C4 and Cz-Fz stimulation, respectively.

CONCLUSIONS

The results suggest that, using a transcranial multipulse stimulation, monitoring of motor-evoked potentials from the external anal sphincter is feasible during ketamine- and propofol-based anesthesia. However, further improvement of techniques would be required for intraoperative elicitation of motor-evoked potentials from the external anal sphincter.

Amplitudes and intrapatient variability of myogenic motor evoked potentials to transcranial electrical stimulation during ketamine/N2O- and propofol/N2O-based anesthesia

The aim of the current study was to investigate whether there are differences in amplitudes and intrapatient variability of motor evoked potentials to five pulses of transcranial electrical stimulation between ketamine/N2O- and propofol/N2O-based anesthesia. Patients in the propofol group (n = 13) and the ketamine group (n = 13) were anesthetized with 50% N2O in oxygen, fentanyl, and 4 mg/kg/hr of propofol or 1 mg/kg/hr of ketamine, respectively. The level of neuromuscular blockade was maintained at an M-response amplitude of approximately 50% of control. Motor evoked potentials in response to multipulse transcranial electrical stimulation were recorded from the right adductor pollicis brevis muscle, and peak-to-peak amplitude and onset latency of motor evoked potentials were evaluated. To estimate intrapatient variability, the coefficient of variation (standard deviation/mean x 100%) of 24 consecutive responses was determined. Motor evoked potential amplitudes in the ketamine group were significantly larger than in the propofol group (mean, 10th-90th percentile: 380 microV, 129-953 microV; 135 microV, 38-658 microV, respectively; P <.05). There were no significant differences in motor evoked potential latency (mean +/- standard deviation: 20.9 +/- 2.2 msec and 21.4 +/- 2.2 msec, respectively) and coefficient of variation of amplitudes (median [range]: 32% [22-42%] and 26% [18-41%], respectively) and latencies (mean +/- standard deviation: 2.1 +/- 0.7% and 2.1 +/- 0.7%, respectively) between the ketamine and propofol groups. In conclusion, intrapatient variability of motor evoked potentials to multipulse transcranial stimulation is similar between ketamine/N2O- and propofol/N2O-based anesthesia, although motor evoked potential amplitudes are lower during propofol/N2O-based anesthesia than ketamine/N2O-based anesthesia.

Total intravenous anesthesia for intraoperative monitoring of the motor pathways: an integral view combining clinical and experimental data

OBJECT

Monitoring of descending corticospinal pathways by using motor evoked potentials (MEPs) has proven to be useful in preventing permanent neurological deficits during cranial and spinal procedures. Difficulties in interpretation of intraoperative changes in potentials may largely be attributed to the effects of anesthesia. Development of suitable intravenous anesthesia protocols specifically tailored for MEP monitoring, including plasma level target-controlled infusion (TCI), requires precise knowledge of the specific neurophysiological properties of the various agents.

METHODS

The effects of alfentanil, sufentanil, fentanyl, remifentanil, thiopental, midazolam, etomidate, ketamine, and propofol on neurogenic and myogenic MEPs were evaluated in an integral study combining clinical data obtained in 40 patients and experimental investigations conducted in 140 animals. The dose-dependent modulation of MEPs after electrical and magnetoelectrical stimulation of the motor cortex was recorded from peripheral muscles and the spinal cord. The results were as follows: opioids, propofol, and thiopental suppressed myogenic, but not neurogenic MEPs in a dose-dependent fashion; remifentanil exerted the least suppressive effects. Etomidate and midazolam did not suppress myogenic MEP, even at plasma concentrations sufficient for anesthesia. Ketamine induced moderate reduction of compound muscle action potential amplitudes only at high doses. Remifentanil and propofol administered via TCI systems allowed recording of myogenic potentials within a defined target plasma concentration range.

CONCLUSIONS

Development of standardized total intravenous anesthesia/TCI protocols by using anesthetic agents such as propofol, remifentanil, ketamine, and midazolam, which have favorable pharmacokinetic and neurophysiological properties, will enhance the quality of intraoperative MEPs and promote the use of MEP monitoring as a useful tool to reduce surgery-related morbidity.

Low dose propofol as a supplement to ketamine-based anesthesia during intraoperative monitoring of motor-evoked potentials

STUDY DESIGN

Motor-evoked potentials (MEPs) were analyzed using transcranial electrical stimulation during spinal surgery in patients under ketamine-based anesthesia, with and without propofol.

OBJECTIVE

To investigate the effects of propofol on MEPs and ketamine-induced adverse effects during spinal surgery in patients under ketamine-based anesthesia.

SUMMARY OF BACKGROUND DATA

Intraoperative monitoring of transcranial motor-evoked responses provides a method for monitoring the functional integrity of descending motor pathways. However, because these responses are sensitive to suppression by most anesthetic agents, anesthetic technique is limited during the monitoring of MEPs. Ketamine has been reported to have little effect on MEPs but may produce adverse effects such as psychedelic effect and hypertension. Recently, it has been reported that propofol may be able to inhibit ketamine-induced adverse effects.

METHODS

Intraoperative monitoring of MEPs was performed in 58 patients who underwent elective spinal surgery. Anesthesia was maintained with nitrous oxide-fentanyl-ketamine without or with low-dose (1-3 mg/kg/hr) of propofol (K group; n = 34, KP group; n = 24, respectively). Transcranial stimulation with single or paired pulses or a train of three or five pulses (interstimulus interval, 2 msec) were delivered to the scalp, and compound muscle action potentials were recorded from the left and right tibialis anterior muscles. To investigate the dose effects of propofol on MEPs, propofol was administered at an infusion rate of 6, 4, and 2 mg/kg/hr and then discontinued in 14 patients.

RESULTS

Results of MEPs were comparable between the K and KP groups. The incidence of postoperative psychedelic effect was significantly less in the KP group (14%) than in the K group (41%). Although propofol inhibited MEPs dose dependently, the use of a train of pulses for stimulation could overcome such inhibition.

CONCLUSIONS

If a train of pulses were used for transcranial stimulation, low-dose propofol can be effectivelyused as a supplement to ketamine-based anesthesia during intraoperative monitoring of myogenic MEPs. Addition of propofol significantly reduced the ketamine-induced psychedelic effects.