The Effect of Age on Motor Evoked Potentials in Children Under Propofol/Isoflurane Anesthesia 丙泊酚/异氟醚麻醉下 年龄对儿童运动诱发电位的影响 Jeremy A, Russ Lyon, John Feiner, et al. Background • MEPs may identify and help prevent injury to motor pathways. • MEPs can be detected intraoperatively in neurologically intact children , but these signals may be missed in 32%–61% of children with preexisting neurologic deficits. Background • The depression of MEPs caused by inhaled anesthetics or by the sedative-hypnotic propofol. • Age may also influence our ability to elicit MEPs. • We retrospectively analyzed data obtained from pediatric patients (2–18 yr of age) who underwent spine surgery while using transcranial electric stimulation MEPs monitoring. • We specifically evaluated how age affected stimulation variables necessary to elicit MEPs to during general anesthesia. Methods • 56 neurologically intact subjects who underwent correction for idiopathic scoliosis and who were monitored using transcranial myogenic MEPs. • excluded patients who had myelopathic, neuromuscular, and congenital spine pathologies. Sevoflurane Methods Propofol Muscle relaxant endotracheal intubation Adjusted anesthetic levels as needed for Isoflurane (0.75%~1.0%) clinical reasons. Propofol(50~75µg/kg·min) and Fentanyl No BIS. no Muscle relaxant Stimuli locus:C3,C4 started at 150 V and Record locus: thenar-hypothenar muscles was increased by 25 V tibialis anterior increments up to extensor hallucis longus, amaximum of 400 V abductor hallucis muscles Stimulation intensity:150~400V Interstimulus Interval:2ms Train stim: 5 pulses Duration: 0.5ms after increments of Recording and filtering parameters were typically 50–75 V, an additional 30–1000 Hz, with a time base of 100 ms. pulse was added reproducible response waveforms with amplitudes of at least 50 V in all muscle groups. If MEP responses were unobtainable after all technical adjustments had been exhausted, the anesthesiologist will reduce the dose of the volatile anesthetic drug. All recordings were done before surgical incision . Methods • Data analysis: – Statistical analysis was performed using JMP 4.0. – We performed multiple linear regression to develop a model of the change in threshold voltage and to investigate the contribution of multiple factors, including age, height,weight, gender, BMI, BSA, and anesthetic dose. – χ2analyses were used to compare categorical variables across groups. Result • MEP stimulation threshold voltage was higher in younger patients and decreased with increasing age (P ＜ 0.0001). • There was a similar, but weaker relationship between threshold voltage and increasing BSA ([R2=0.42 versus 0.53 for age], P ＜0.0001), weight ( [R2=0.36], P＜0.0001) and height ([R2=0.27], P＜0.0001). Result • BMI was weakly associated with threshold voltage (data not shown; [R2=0.12], P＜0.05). Gender was not a significant factor. • Younger patients also required more stimulating pulses, as compared with older patients, to elicit reproducible MEP responses (R2=0.53, P＜0.0001). Result • Younger patients received smaller isoflurane doses, measured as both absolute end-tidal isoflurane concentration (P＜0.0001). • MEP stimulation threshold was higher in subjects receiving smaller isoflurane doses (R2=0.24, P＜ 0.001). Result (Multivariate analysis) • Age was the dominant predictor of threshold voltage (P＜ 0.0001). • Isoflurane dose, Weight, height, BMI, and BSA were not statistically significant, after accounting for age. Discussion • The age-related difference in stimulating voltage cannot be attributed to larger concentrations of suppressive anesthetics administered to younger subjects. – Significantly less isoflurane was administered to the younger subjects. – Range of propofol dosage has not been shown to substantially depress MEP responses. Discussion (limitation) • Total anesthetic depth is not known. No BIS. • Arterial blood pressure was not consistently recorded. • Temperature, another physiologic factor that affects MEP responses , did not differ by age at the time MEP measurements were made. Discussion (limitation) • Perhaps the key limitation of this study is that it was not designed to test maturational effects of MEP responses. • Our analysis focused on defining the minimum parameters needed to generate interpretable responses rather than on examining the characteristics of the responses. Discussion (other supports) • Higher thresholds are needed when using transcranial magnetic MEPs in awake children versus adults. • Parano et al. observed diminished evoked response amplitudes in infancy and childhood compared to adults; this difference was more pronounced in the first 2 years of life. • Reliable MEP responses may be unobtainable in children younger than 6 years, even when they are awake. Discussion (explain) • Our study results can best be explained by immaturity of the central nervous system. – Cortical changes with aging may affect MEP responses. – Hagelthorn et al. observed decreased evoked potential inter-hemispheric transmission time with increasing age (7–17 years), suggesting increased corpus callosal myelination and integration during childhood. Discussion (explain) • Spinal cord motor pathways also undergo a prolonged period of maturation. – Nezu et al. estimated that electrophysiologic maturity of the corticospinal tracts (CST) innervating the hand muscles was complete by 13 years of age. – At birth, the conduction velocity of central motor fibers of the spinal cord are approximately 10 m/s, whereas adult values are in the range of 50–70 m/s. Discussion • We cannot conclude that these observations are attributable solely to maturation effects. – Younger patients may have enhanced sensitivity to suppression by volatile anesthetics or propofol. – Technical challenges. Conclusion • younger children are at greater risk for diminished or lost MEP responses during surgery when using this anesthetic regimen. – 90% of our patients 2 to 10 years old----above 300V – 30% of children older than 10 years required 300V • Threshold voltage requirements may further increase during the course of surgery (20,37), especially with changes in physiologic variables, such as arterial blood pressure, temperature, and hematocrit. Conclusion • The selection of the anesthetic regimen for any child must consider all of the desired anesthetic and surgical goals, including the effects on MEPs. – Combined propofol and fentanyl anesthesia has been used successfully for obtaining both myogenic and epidurally recorded MEPs in a pediatric population as young as 8 to 12 months. – Improved MEP stimulation techniques may be especially useful for obtaining MEP responses in very young subjects . Thank you ! You’re over the hill!
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