A comparison of myogenic motor evoked responses to electrical and magnetic transcranial stimulation during nitrous oxide/opioid anesthesia

Transcranial motor evoked potentials (tc-MEPs) are used to monitor spinal cord integrity intraoperatively. We compared myogenic motor evoked responses with electrical and magnetic transcranial stimuli during nitrous oxide/opioid anesthesia. In 11 patients undergoing spinal surgery, anesthesia was induced with i.v. etomidate 0.3 mg/kg and sufentanil 1.5 microg/kg and was maintained with sufentanil 0.5 microg x kg(-1) x h(-1) and N2O 50% in oxygen. Muscle relaxation was kept at 25% of control with i.v. vecuronium. Electrical stimulation was accomplished with a transcranial stimulator set at maximal output (1200 V). Magnetic transcranial stimulation was accomplished with a transcranial stimulator set at maximal output (2 T). Just before skin incision, triplicate responses to single stimuli with both modes of cortical stimulation were randomly recorded from the tibialis anterior muscles. Amplitudes and latencies were compared using the Wilcoxon signed rank test. Bilateral tc-MEP responses were obtained in every patient with electrical stimulation. Magnetic stimulation evoked only unilateral responses in two patients. With electrical stimulation, the median tc-MEP amplitude was 401 microV (range 145-1145 microV), and latency was 32.8 +/- 2.3 ms. With magnetic stimulation, the tc-MEP amplitude was 287 microV (range 64-506 microV) (P < 0.05), and the latency was 34.7 +/- 2.1 ms (P < 0.05). We conclude that myogenic responses to magnetic transcranial stimulation are more sensitive to anesthetic-induced motoneural depression compared with those elicited by electrical transcranial stimulation. IMPLICATIONS: Transcranial motor evoked potentials are used to monitor spinal cord integrity intraoperatively. We compared the relative efficacy of electrical and magnetic transcranial stimuli in anesthetized patients. It seems that myogenic responses to magnetic transcranial stimulation are more sensitive to anesthetic-induced motoneural depression compared with electrical transcranial stimulation.     

Motor and Somatosensory Evoked Potentials: Their Role in Predicting Spinal Cord Ischemia in Patients Undergoing Thoracoabdominal Aortic Aneurysm Repair with Regional Lumbar Epidural Cooling

Background: Paraplegia is a devastating complication for patients undergoing repair of thoracoabdominal aortic aneurysms. A monitor to detect spinal cord ischemia is necessary if anesthesiologists are to intervene to protect the spinal cord during aortic aneurysm clamping.

Methods: The medical records of 60 patients who underwent thoracoabdominal aortic aneurysm repair with regional lumbar epidural cooling with evoked potential monitoring were reviewed. The authors analyzed latency and amplitude of motor evoked potentials, somatosensory evoked potentials, and H reflexes before cooling and clamping, after cooling and before clamping, during clamping, and after release of aortic cross clamp.

Results: Twenty minutes after the aortic cross clamp was placed, motor evoked potentials had 88% sensitivity and 65% specificity in predicting spinal cord ischemia. The negative predictive value of motor evoked potentials at 20 min after aortic cross clamping was 96%.     

Effects of altering core body temperature on somatosensory and motor evoked potentials in rats.

The effects of core temperature on three potentials--somatosensory spinal evoked potential, somatosensory cortical evoked potential, and spinal motor evoked potential--were studied in rats. Hyperthermia reduced the latency and increased the conduction velocity of all three potentials. Somatosensory spinal evoked potential amplitude was unchanged, whereas somatosensory cortical and spinal motor evoked potentials deteriorated above 42 C. Hypothermia increased latency and decreased conduction velocity in all three potentials. The amplitude of the spinal motor evoked potential decreased, and the somatosensory cortical and spinal motor evoked potentials disappeared below 28 C. Hyperthermia and hypothermia caused significant changes in the latency of all three potentials. The latency change of all three potentials became significant at 2-2.5 C above or below baseline, suggesting a range within which evoked potential studies should be performed.     

Effects of dexmedetomidine on intraoperative motor and somatosensory evoked potential monitoring during spinal surgery in adolescents

Background: Dexmedetomidine may be a useful agent as an adjunct to an opioid–propofol total intravenous anesthesia (TIVA) technique during posterior spinal fusion (PSF) surgery. There are limited data regarding its effects on somatosensory (SSEPs) and motor evoked potentials (MEPs). Methods: The data presented represent a retrospective review of prospectively collected quality assurance data. When the decision was made to incorporate dexmedetomidine into the anesthetic regimen for intraoperative care of patients undergoing PSF, a prospective evaluation of its effects on SSEPs and MEPs was undertaken. SSEPs and MEPs were measured before and after the administration of dexmedetomidine in a cohort of pediatric patients undergoing PSF. Dexmedetomidine (1 μg·kg^?1 over 20 min followed by an infusion of 0.5 μg·kg^?1·h^?1) was administered at the completion of the surgical procedure, but prior to wound closure as an adjunct to TIVA which included propofol and remifentanil, adjusted to maintain a constant depth of anesthesia as measured by a BIS of 45–60. Results: The cohort for the study included nine patients, ranging in age from 12 to 17 years, anesthetized with remifentanil and propofol. In the first patient, dexmedetomidine was administered in conjunction with propofol at 110 μg·kg^?1·min^?1 which resulted in a decrease in the bispectral index from 58 to 31. Although no significant effect was noted on the SSEPs (amplitude or latency) or the MEP duration, there was a decrease in the MEP amplitude. The protocol was modified so that the propofol infusion was incrementally decreased during the dexmedetomidine infusion to achieve the same depth of anesthesia. In the remaining eight patients, the bispectral index was 52 ± 6 at the start of the dexmedetomidine loading dose and 49 ± 4 at its completion (P = NS). There was no statistically significant difference in the MEPs and SSEPs obtained before and at completion of the dexmedetomidine loading dose. Conclusion: Using the above‐mentioned protocol, dexmedetomidine can be used as a component of TIVA during PSF without affecting neurophysiological monitoring.     

Volatile Anesthetics Depress Spinal Motor Neurons

Background: Depression of spinal alpha-motor neurons apparently plays a role in the surgical immobility induced by isoflurane. Using the noninvasive technique of F-wave analysis, the authors tested the hypothesis that depressed motor neuron excitability is an effect common to other clinically relevant inhaled anesthetics.

Methods: The authors measured F-wave amplitude in rats anesthetized with desflurane, enflurane, halothane, or sevoflurane. Each animal received one anesthetic at five equipotent anesthetic concentrations (0.6, 0.8, 1.2, and 1.6 minimum alveolar concentration [MAC] and 0.8 MAC with 65% N2 O). F waves were detected as late potentials in electromyographic responses evoked in the intrinsic muscles of the hind paw after monopolar stimulation of the ipsilateral posterior tibial nerve.

Results: All tested inhaled anesthetics depressed F-wave amplitude but not M-wave (orthodromic, early muscle activation) amplitude, and increased M-F latency in a dose-dependent manner. At 1.0 MAC, the estimated F/M ratio was 70+/-13% SD of that at baseline (0.6 MAC). Nitrous oxide added to 0.8 MAC of the potent vapors depressed F/M ratio by 63+/-17%.     

Propofol and Sevoflurane in Subanesthetic Concentrations Act Preferentially on the Spinal Cord: Evidence from Multimodal Electrophysiological Assessment

Background: Animal experiments in recent years have shown that attenuation of motor responses by general anesthetics is mediated at least partly by spinal mechanisms. Less is known about the relative potency of anesthetic drugs in suppressing cortical and spinal electrophysiological responses in vivo in humans, particularly those, but not only those, connected with motor responses. Therefore, we studied the effects of sevoflurane and propofol in humans using multimodal electrophysiological assessment.

Methods: We studied nine healthy volunteers in two sessions during steady state sedation with 0.5, 1.0, and 1.5 [mu]g/l (targeted plasma concentration) propofol or 0.2 and 0.4 vol% (end-tidal) sevoflurane. Following a 15-min equilibration period, motor responses to transcranial magnetic stimulation and peripheral (H-reflex, F-wave) stimulation were recorded, while electroencephalography and auditory evoked responses were recorded in parallel.

Results: At concentrations corresponding to two thirds of C50 awake, motor responses to transcranial magnetic stimulation were reduced by approximately 50%, H-reflex amplitude was reduced by 22%, F-wave amplitude was reduced by 40%, and F-wave persistence was reduced by 25%. No significant differences between sevoflurane and propofol were found. At this concentration, the Bispectral Index was reduced by 7%, and the middle-latency auditory evoked responses were attenuated only mildly (Nb latency increased by 11%, amplitude PaNb did not change). In contrast, the postauricular reflex was suppressed by 77%.     

术中无肌松全静脉麻醉用于联合神经电生理监护脊柱手术的可行性

［摘要］ 目的 比较全静脉麻醉下术中使用或不使用肌松药对脊柱手术中联合神经电生理监测结果的影响,探讨安全有效的神经电生理监测麻醉方案。方法 选择拟行联合神经电生理监测的择期脊柱手术病人 40 例,分为A、B两组。两组病人均采用丙泊酚、瑞芬太尼和右美托咪定全凭静脉麻醉,A组病人术中使用小剂量阿曲库铵维持肌松,B组病人术中不使用肌松药。同时监测体感诱发电位（SEP）和运动诱发电位（MEP）评判脊髓功能。记录术中不同时间点两组病人的生命体征和SEP和MEP的波幅和潜伏期,同时记录经颅电刺激时病人是否出现剧烈体动和自主呼吸。比较两组病人术毕后麻醉苏醒时间和质量。结果 两组病人不同时间点的生命体征差异无统计学意义。两组病人的SEP的波幅和潜伏期差异无统计学意义,MEP的潜伏期差异无统计学意义,MEP的波幅差异有显著性统计学意义。两组病人在电刺激时均无自主呼吸和剧烈体动发生。结论 术中不使用肌松药的全静脉麻醉方案可安全有效地用于行神经电生理监测的脊柱手术,并且在电生理监测信号质量和术后苏醒方面具有明显优势。     

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.     

脊髓损伤后早期减压对诱发电位影响的实验研究

[目的]观察脊髓损伤后早期减压对体感诱发电位及经颅磁刺激运动诱发电位的影响，以探讨诱发电位在判断手术时机及预后中的应用价值。[方法]日本大耳兔32只随机分4组。A组为对照组，不造成脊髓损伤。B、C、D组为脊髓损伤组。对每组动物于不同时间分别检测SEP、MEP。分析波形的潜伏期、峰问波幅。用后肢的Tarlov分级法作伤后运动功能评分。取脊髓标本，行组织学观察。[结果]随着脊髓压迫时间的延长，SEP、MEP的潜伏期逐渐延长，波幅逐渐减小．波幅变化较潜伏期更为敏感。在恢复过程中，脊髓受压时间越短，诱发电位恢复越早。潜伏期恢复早于波幅，而且SEP恢复早于MEP，MEP的恢复早于功能评分。[结论]SEP与TMS-MEP对脊髓损伤十分敏感，能较早反映脊髓损伤程度，可用于指导临床手术治疗和判断预后。     

Comparison of transcranial electric motor and somatosensory evoked potential monitoring during cervical spine surgery

BACKGROUND: There has been little enthusiasm for somatosensory evoked potential monitoring in cervical spine surgery as a result, in part, of the increased risk of motor tract injury at this level, to which somatosensory monitoring may be insensitive. Transcranial electric motor evoked potential monitoring allows assessment of the motor tracts; therefore, we compared transcranial electric motor evoked potential and somatosensory evoked potential monitoring during cervical spine surgery to determine the temporal relationship between the changes in the potentials demonstrated by each type of monitoring and neurological sequelae and to identify patient-related and surgical factors associated with intraoperative neurophysiological changes. METHODS: Somatosensory evoked potential and transcranial electric motor evoked potential data recorded for 427 patients undergoing anterior or posterior cervical spine surgery between January 1999 and March 2001 were analyzed. All patients who showed substantial (at least 60%) or complete unilateral or bilateral amplitude loss, for at least ten minutes, during the transcranial electric motor evoked potential and/or somatosensory evoked potential monitoring were identified. RESULTS: Twelve of the 427 patients demonstrated substantial or complete loss of amplitude of the transcranial electric motor evoked potentials. Ten of those patients had complete reversal of the loss following prompt intraoperative intervention, whereas two awoke with a new motor deficit. Somatosensory evoked potential monitoring failed to identify any change in one of the two patients, and the change in the somatosensory evoked potentials lagged behind the change in the transcranial electric motor evoked potentials by thirty-three minutes in the other. No patient showed loss of amplitude of the somatosensory evoked potentials in the absence of changes in the transcranial electric motor evoked potentials. Transcranial electric motor evoked potential monitoring was 100% sensitive and 100% specific, whereas somatosensory evoked potential monitoring was only 25% sensitive; it was, however, 100% specific. CONCLUSIONS: Transcranial electric motor evoked potential monitoring appears to be superior to conventional somatosensory evoked potential monitoring for identifying evolving motor tract injury during cervical spine surgery. Surgeons should strongly consider using this modality when operating on patients with cervical spondylotic myelopathy in general and on those with ossification of the posterior longitudinal ligament in particular.     

Propofol and sevoflurane in subanesthetic concentrations act preferentially on the spinal cord: evidence from multimodal electrophysiological assessment

BACKGROUND: Animal experiments in recent years have shown that attenuation of motor responses by general anesthetics is mediated at least partly by spinal mechanisms. Less is known about the relative potency of anesthetic drugs in suppressing cortical and spinal electrophysiological responses in vivo in humans, particularly those, but not only those, connected with motor responses. Therefore, we studied the effects of sevoflurane and propofol in humans using multimodal electrophysiological assessment. METHODS: We studied nine healthy volunteers in two sessions during steady state sedation with 0.5, 1.0, and 1.5 microg/l (targeted plasma concentration) propofol or 0.2 and 0.4 vol% (end-tidal) sevoflurane. Following a 15-min equilibration period, motor responses to transcranial magnetic stimulation and peripheral (H-reflex, F-wave) stimulation were recorded, while electroencephalography and auditory evoked responses were recorded in parallel. RESULTS: At concentrations corresponding to two thirds of C(50 awake), motor responses to transcranial magnetic stimulation were reduced by approximately 50%, H-reflex amplitude was reduced by 22%, F-wave amplitude was reduced by 40%, and F-wave persistence was reduced by 25%. No significant differences between sevoflurane and propofol were found. At this concentration, the Bispectral Index was reduced by 7%, and the middle-latency auditory evoked responses were attenuated only mildly (N(b) latency increased by 11%, amplitude P(a)N(b) did not change). In contrast, the postauricular reflex was suppressed by 77%. CONCLUSIONS: The large effect of both anesthetics on all spinal motor responses, compared with the small effect on electroencephalography and middle-latency auditory evoked responses, assuming that they represent cortical modulation, may suggest that the suppression of motor responses to transcranial magnetic stimulation is largely due to submesencephalic effects.     

体感诱发电位N20波能否反映全麻中的镇痛成份

目的：比较非镇生麻醉药异丙酚与镇痛性麻醉药芬太尼对上肢短潜伏期体感诱发电位（ＳＬＳＥＰ）Ｎ２０波影响的差异。方法：２０例择期全麻患者依异丙酚和芬太尼的给药顺序不同分为两组监测ＳＬＳＥＰＮ２０波的变化。结果：异丙酚使Ｎ２０波潜伏期显著延长，波幅显著升高。芬太尼对Ｎ２０波潜伏期及波幅均无显著影响。异丙酚－芬太尼复合使用时Ｎ２０波潜伏期显著延长，波幅则无明显改变。结论：ＳＬＳＥＰＮ２０波不能反映全麻中的     

右美托咪啶-异丙酚-芬太尼复合麻醉对颈椎手术病人体感诱发电位及运动诱发电位的影响

目的 评价右美托咪啶-异丙酚-芬太尼复合麻醉对颈椎手术病人体感诱发电位及运动诱发电位的影响.方法 择期全麻下行颈椎手术的病人36例,随机分为2组(n=18):异丙酚-芬太尼复合麻醉组(C组)和右美托咪啶-异丙酚-芬太尼复合麻醉组(D组).麻醉诱导:TCI异丙酚,血浆靶浓度为2μg/ml,静脉注射芬太尼1～2μg/kg,意识消失后经口置入喉罩进行通气.意识消失后D组经10 min静脉注射右美托咪啶0.5μg/kg,随后以0.5μg·kg-1·h-1的速率静脉输注至术毕,C组给予等容量生理盐水.分别于给予右美托咪啶前及静脉输注右美托咪啶10 min时记录体感诱发电位P15-N20波的波幅和潜伏期,并记录运动诱发电位的未引出情况.结果 与C组比较,D组P15-N20波的波幅和潜伏期差异无统计学意义(P＞0.05);两组运动诱发电位未引出率均为0.结论 右美托咪啶-异丙酚-芬太尼复合麻醉对颈椎手术病人体感诱发电位及运动诱发电位无影响.     

Effects of Isoflurane and Desflurane on Neurogenic Motor- and Somatosensory-evoked Potential Monitoring for Scoliosis Surgery

Background: Most techniques used to monitor spinal cord tracts are sensitive to the effects of anesthesia, particularly to volatile anesthetic agents. The aim of this prospective study was to show that evoked potentials recorded from the peripheral nerves after spinal cord stimulation, so-called neurogenic motor evoked potentials, are resistant to clinical concentrations of isoflurane or desflurane, compared with somatosensory-evoked potentials.

Methods: Twenty-three patients were studied during surgery to correct scoliosis. The background anesthetic consisted of a continuous infusion of propofol. Isoflurane (n = 12) or desflurane (n = 11) were then introduced to achieve 0.5 and 1.0 end-tidal minimum alveolar concentrations (MAC), both in 50% oxygen-nitrous oxide and in 100% oxygen. Somatosensory-evoked potentials were elicited and recorded using a standard method, defining cortical P40 and subcortical P29. Neurogenic motor-evoked potentials were elicited by electric stimulation of the spinal cord via needle electrodes placed by the surgeon in the rostral part of the surgical field. Responses were recorded from needle electrodes inserted in the right and left popliteal spaces close to the sciatic nerve. Stimulus intensity was adjusted to produce a supramaximal response; that is, an unchanged response in amplitude with subsequent increases in stimulus intensity. Measurements were obtained before introducing volatile agents and 20 min after obtaining a stable level of each concentration.

Results: Isoflurane and desflurane in both 50% oxygen-nitrous oxide and 100% oxygen were associated with a significant decrease in the amplitude and an increase in the latency of the cortical P40, whereas subcortical P29 latency did not vary significantly. Typical neurogenic motor-evoked potentials were obtained in all patients without volatile anesthetic agents, consisting of a biphasic wave, occurring 15 to 18 ms after stimulation, with an amplitude ranging from 1.3 to 4.1 micro Volt. Latency or peak-to-peak amplitude of this wave was not significantly altered with isoflurane and desflurane, either in the presence or in the absence of nitrous oxide.     

Variability of somatosensory cortical evoked potentials during spinal surgery. Effects of anesthetic technique and high-pass digital filtering

The effects of anesthetic technique (nitrous oxide or propofol) and high-pass digital filtering on within-patient variability of posterior tibial nerve somatosensory cortical evoked potentials (PTN-SCEP) were compared prospectively in two groups of 20 patients undergoing spinal surgery. Average P1N1 amplitude was significantly higher and P1N1 amplitude variability lower during propofol/alfentanil anesthesia than during nitrous oxide/alfentanil anesthesia. Off-line 30-Hz high-pass digital filtering significantly reduced P1N1 amplitude variability without decreasing P1N1 amplitude. In 93 patients studied retrospectively, a significant negative logarithmic correlation (r = -0.77) was observed between P1N1 amplitude and P1N1 amplitude variability. This study shows the importance of maintaining the highest possible PTN-SCEP amplitudes during spinal surgery. Propofol/opioid anesthesia may be an alternative anesthetic technique to nitrous oxide/opioid anesthesia during spinal cord function monitoring.     

Effects of hypovolemic hypotensive shock on somatosensory and motor evoked potentials

The utility of evoked potentials in monitoring spinal cord and cerebral function in various neurosurgical and orthopedic operations has now been established. To study the effects of graded hypotension upon spinal and cortical somatosensory evoked potentials (SMEPs), and spinal motor evoked potentials (SMEPs), 12 anesthetized cats were subjected to graded hypotension ranging from a mean arterial blood pressure of 100 mmHg to 30 mmHg or less. Hypotension causes a progressive increase in onset latency and a decrease in amplitude and conduction velocity of SEPs and SMEPs. Cortical SEPs and SMEPs were sensitive to profound hypotension (MAP less than 30 mmHg). Spinal SEPs showed more resistance and disappeared at lower levels of hypotension. Immediate blood transfusion caused resumption of cortical SEPs and SMEPs within 30 minutes after infusion; however, the latency and amplitude of responses did not reach the baseline values within 1 hour after transfusion. The sequential recovery of evoked responses was dependent upon the length of hypotension. When 15 minutes elapsed between loss of responses and transfusion, cortical SEPs and SMEPs did not resume within 1 hour after infusion. No return of signals occurred if 30 minutes elapsed between the loss of evoked responses and blood reperfusion. These findings suggest that ischemia associated with profound systemic hypotension can alter or obliterate evoked responses.     

Temperature effects on feline cortical and spinal evoked potentials

To evaluate the effects of hypothermia on the somatosensory evoked potentials, baseline cortical and spinal evoked responses were obtained following induction of anesthesia at normal body temperature in five cats. The body temperature was lowered between 5 degrees and 6 degrees C and repeat cortical and spinal evoked responses were obtained. The cats were warmed to their original normal temperatures and the cortical and spinal evoked responses were repeated. After cooling the spinal evoked responses showed an average 40% increase in the latency (range 29-51%). Three of the animals showed a change in the normal waveform with the development of two distinct peaks instead of a single waveform. This was thought to be due to the varied vulnerability of the different cortical tracts to the lower temperature. Upon rewarming, the average latency returned to within 2.7% of the initial value, and the double waveform reunited to form a single wave. With cooling, the cortical evoked responses showed a similar consistent increase in latency. However, there was a much larger variability in the appearance of the waveforms. There was a range from an almost nonexistent wave in two cats to various waveform changes in the other cats. After rewarming there was a much slower return to normal latency in waveform than was found with the spinal evoked potentials. These experimental findings revealed an apparent deleterious, although reversible, effect on the somatosensory evoked potentials. Until the physiologic effects of cooling on the spinal cord are better understood, it is recommended that the temperature of patients undergoing spinal instrumentation be kept as close to normal as possible.     

The effect of anesthetic agents on descending spinal cord evoked potential and the compound muscle action potentials elicited by stimulation at the cerebral motor cortex and the spinal cord]

Recently, intraoperative monitoring of the motor tract by descending spinal cord motor evoked potentials (MEP) and compound muscle action potentials (CMAP) has been applied in clinical testing. Since several reports have mentioned the vulnerability of these potentials to anesthetic agents, experimental studies were carried out on the relationship between these potentials and anesthesia using 41 adult cats. The effects of anesthesia on changes in amplitude of the direct wave (D wave) and indirect wave (I wave) of the MEP and CMAPs were investigated. These potentials were generated by stimulation of the spinal cord and the motor cortex, respectively. Enflurane (2%), halothane (1%) and isoflurane (1.5%) with pure oxygen decreased the amplitude of the I wave to less than 50% of the control level. The CMAP after stimulation of the spinal cord was degraded to less than 30%, and the CMAP after cortical stimulation vanished completely. Only the D wave was stable against inhalational anesthetic agents. Sixty-seven percent nitrous oxide with the above concentrations of these inhalational anesthetic agents decreased the amplitude of the I wave to less than 30% and the CMAP evoked by spinal cord stimulation vanished. The effect of modified NLA (diazepam and pentazocine) on these potentials was weaker than that of the inhalational anesthetic agents.     

Failure of motor evoked potentials to predict neurologic outcome in experimental thoracic aortic occlusion

Motor evoked potential monitoring was tested as an alternative to somatosensory evoked potential monitoring in evaluating spinal cord function during thoracic aortic occlusion in dogs. Twenty-seven animals underwent 60 minutes of cross-clamping of the proximal descending thoracic aorta with (n = 18) or without (n = 9) cerebrospinal fluid drainage. Spinal cord blood flow was measured with microspheres, and neurologic outcome was evaluated at 24 hours with Tarlov's scoring system. Cerebrospinal fluid drainage improved neurologic outcome (p less than 0.05). Motor evoked potentials recorded over the lumbar spinal cord were lost in 9 of 20 dogs with ischemic cord injury and were not lost in any of the 7 dogs that were neurologically normal. Somatosensory evoked potential were lost in 19 of 20 paraplegic/paraparetic dogs and lost in 3 of 7 normal dogs (p less than 0.01). After reperfusion, motor evoked potentials returned in all nine neurologically injured dogs that lost the potentials and were still present at 24 hours. Changes in amplitude, latency, or time until loss or return of motor evoked potentials or somatosensory evoked potentials did not predict neurologic injury. Loss of somatosensory evoked potentials had a high sensitivity (95%) but had low specificity (67%) because of peripheral nerve ischemia. Loss of motor evoked potentials recorded from the spinal cord had high specificity (100%) but a low sensitivity (46%) and was therefore not a reliable predictor of neurologic injury. Return of motor evoked potentials during reperfusion did not correlate with functional recovery. Motor evoked potentials stimulated in the cortex and recorded from the spinal cord had low overall accuracy (59%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Transtracheal electrical stimulation of the spinal cord for intraoperative monitoring of the motor pathway

Because of the suppressant effects of anesthetic drugs and muscle relaxants on motor responses elicited by either magnetic or electrical transcranial stimulation, intraoperative monitoring of the motor system, and especially monitoring of lower limb function, presents many difficulties. The upper part of the spinal cord was stimulated in 14 anesthetized and relaxed dogs with a cathode attached to the intratracheal tube and an anode fixed above the upper cervical spinous processes. Action potentials evoked by single and serial stimuli were recorded from the exposed right femoral nerve and quadriceps muscle Averaging was necessary for serial stimulations. Reproducible early and late responses to both single and serial stimulations were recorded during regular anesthesia. The origin of the different responses is discussed. Transtracheal stimulation of the spinal cord is easy to perform and the responses recorded from the peripheral nerve or limb muscle are well reproducible in regular anesthesia. The method seems to be appropriate for intraoperative monitoring of the thoracolumbar spine.