Correction to: Changes in transcranial motor evoked potentials during hemorrhage are associated with increased serum propofol concentrations.

In the original publication of the article, the corresponding author inadvertently omitted one of the co-authors in the author group. The corrected author group is given in this erratum.

Changes in transcranial motor evoked potentials during hemorrhage are associated with increased serum propofol concentrations.

Transcranial motor evoked potentials (TcMEPs) monitor the integrity of the spinal cord during spine surgery. Propofol-based anesthesia is favored in order to enhance TcMEP quality. During intraoperative hemorrhage, TcMEP amplitudes may be reduced. The serum concentration of propofol may increase during hemorrhage. No study has determined whether changes in TcMEPs due to hemorrhage are related to changes in propofol blood levels. We monitored TcMEPs, mean arterial pressure (MAP), and cardiac output (CO) and hemoglobin in pigs (n = 6) undergoing controlled progressive hemorrhage during a standardized anesthetic with infusions of propofol, ketamine, and fentanyl. We recorded TcMEPs from the rectus femoris (RF) and tibialis anterior (TA) muscles bilaterally. A pulmonary artery catheter was placed to measure CO. Progressive hemorrhage of 10% blood volume increments was done until TcMEP amplitude decreased by >60% from baseline. Serum propofol levels were also measured following removal of each 10% blood volume increment. TcMEP responses were elicited every 3 min using constant stimulation parameters. We removed between 20 and 50% of total blood volume in order to achieve the >60% reduction in TcMEP amplitude. MAP and CO decreased significantly from baseline. At maximum hemorrhage, TcMEP amplitude decreased in the RF and TA by an average of 73 and 62% respectively from baseline (P < 0.01). Serum propofol levels varied greatly among animals at baseline (range 410-1720 ng/mL) and increased in each animal during hemorrhage. The mean propofol concentration rose from 1190 ± 530 to 2483 ± 968 ng/mL (P < 0.01). The increased propofol concentration correlated with decreased CO. Multivariate analysis using hierarchical linear models indicated that the decline of TcMEP amplitude was primarily associated with rising propofol concentrations, but was also independently affected by reduced CO. We believe that the decrease in blood volume and CO during hemorrhage increased the serum concentration of propofol by reducing the volume of distribution and/or rate of hepatic metabolism of the drug. Despite wide acceptance of propofol as the preferred anesthetic when using TcMEPs, intravenous anesthetics are vulnerable to altered pharmacokinetics during conditions of hemorrhage and could contribute to false-positive TcMEP changes.

A Clinical Trial to Detect Subclinical Transfusion-induced Lung Injury during Surgery.

BACKGROUND: Transfusion-related acute lung injury incidence remains the leading cause of posttransfusion mortality. The etiology may be related to leukocyte antibodies or biologically active compounds in transfused plasma, injuring susceptible recipient's lungs. The authors have hypothesized that transfusion could have less severe effects that are not always appreciated clinically and have shown subtly decreased pulmonary oxygen gas transfer in healthy volunteers after transfusion of fresh and 21-day stored erythrocytes. In this study, the authors tested the same hypothesis in surgical patients. METHODS: Ninety-one patients undergoing elective major spine surgery with anticipated need for erythrocyte transfusion were randomly allocated to receive their first transfusion of erythrocytes as cell salvage (CS), washed stored, or unwashed stored. Clinicians were not blinded to group assignment. Pulmonary gas transfer and mechanics were measured 5 min before and 30 min after erythrocyte transfusion. RESULTS: The primary outcome variable, gas transfer, as assessed by change of PaO2/FIO2, with erythrocyte transfusion was not significant in any group (mean ± SD; CS: 9 ± 59; washed: 10 ± 26; and unwashed: 15 ± 1) and did not differ among groups (P = 0.92). Pulmonary dead space (VD/VT) decreased with CS transfusion (-0.01 ± 0.04; P = 0.034) but did not change with other erythrocytes; the change from before to after erythrocyte transfusion did not differ among groups (-0.01 to +0.01; P = 0.28). CONCLUSIONS: The authors did not find impaired gas exchange as assessed by PaO2/FIO2 with transfused erythrocytes that did or did not contain nonautologous plasma. This clinical trial did not support the hypothesis of erythrocyte transfusion-induced gas exchange deficit that had been found in healthy volunteers.

Effect of hemorrhage and hypotension on transcranial motor-evoked potentials in swine.

BACKGROUND: Transcranial motor-evoked potentials (TcMEPs) monitor spinal cord motor tract integrity. Using a swine model, the authors studied the effects of vasodilatory hypotension, hemorrhage, and various resuscitation efforts on TcMEP responses. METHODS: Twelve pigs were anesthetized with constant infusions of propofol, ketamine, and fentanyl. Animals were incrementally hemorrhaged, until bilateral tibialis anterior TcMEP amplitude decreased to less than 40% of baseline or until 50% of the blood volume was removed. Mean arterial pressure (MAP), cardiac output (CO), and oxygen delivery (DO2) were examined. Resuscitation with phenylephrine, epinephrine, and colloid were evaluated. In seven animals, vasodilatory hypotension was examined. Paired comparisons and multivariate analysis were performed. RESULTS: Hemorrhage significantly reduced (as a percentage of baseline, mean±SD) TcMEPs (left, 33±29%; right, 26±21%), MAP (60±17%), CO (49±12%), and DO2 (43±13%), P value less than 0.001 for all. Vasodilation reduced MAP comparably, but TcMEPs, CO, and DO2, were not significantly lowered. After hemorrhage, restoration of MAP with phenylephrine did not improve TcMEPs, CO, or DO2, but similar restoration of MAP with epinephrine restored (to percentage of baseline) TcMEPs (59±40%), and significantly increased CO (81±17%) and DO2 (72±19%) compared with both hemorrhage and phenylephrine, P value less than 0.05 for all. Resuscitation with colloid did not improve TcMEPs. Multivariate analysis revealed that changes in TcMEPs were more closely associated with changes in CO and DO2 as compared with MAP. CONCLUSIONS: Hypotension from hemorrhage, but not vasodilation, is associated with a decrease in TcMEP amplitude. After hemorrhage, restoration of TcMEPs with epinephrine but not phenylephrine indicates that CO and DO2 affect TcMEPs more than MAP. Monitoring CO may be beneficial in major spine surgery when using TcMEP monitoring.

Fresh and stored red blood cell transfusion equivalently induce subclinical pulmonary gas exchange deficit in normal humans.

BACKGROUND: Transfusion can cause severe acute lung injury, although most transfusions do not seem to induce complications. We tested the hypothesis that transfusion can cause mild pulmonary dysfunction that has not been noticed clinically and is not sufficiently severe to fit the definition of transfusion-related acute lung injury. METHODS: We studied 35 healthy, normal volunteers who donated 1 U of blood 4 weeks and another 3 weeks before 2 study days separated by 1 week. On study days, 2 U of blood were withdrawn while maintaining isovolemia, followed by transfusion with either the volunteer's autologous fresh red blood cells (RBCs) removed 2 hours earlier or their autologous stored RBCs (random order). The following week, each volunteer was studied again, transfused with the RBCs of the other storage duration. The primary outcome variable was the change in alveolar to arterial difference in oxygen partial pressure (AaDo(2)) from before to 60 minutes after transfusion with fresh or older RBCs. RESULTS: Fresh RBCs and RBCs stored for 24.5 days equally (P = 0.85) caused an increase of AaDo(2) (fresh: 2.8 mm Hg [95% confidence interval: 0.8-4.8; P = 0.007]; stored: 3.0 mm Hg [1.4-4.7; P = 0.0006]). Concentrations of all measured cytokines, except for interleukin-10 (P = 0.15), were less in stored leukoreduced (LR) than stored non-LR packed RBCs; however, vascular endothelial growth factor was the only measured in vivo cytokine that increased more after transfusion with LR than non-LR stored packed RBCs. Vascular endothelial growth factor was the only cytokine tested with in vivo concentrations that correlated with AaDo(2). CONCLUSION: RBC transfusion causes subtle pulmonary dysfunction, as evidenced by impaired gas exchange for oxygen, supporting our hypothesis that lung impairment after transfusion includes a wide spectrum of physiologic derangements and may not require an existing state of altered physiology. These data do not support the hypothesis that transfusion of RBCs stored for >21 days is more injurious than that of fresh RBCs.

High oxygen partial pressure decreases anemia-induced heart rate increase equivalent to transfusion.

BACKGROUND: Anemia is associated with morbidity and mortality and frequently leads to transfusion of erythrocytes. The authors sought to directly compare the effect of high inspired oxygen fraction versus transfusion of erythrocytes on the anemia-induced increased heart rate (HR) in humans undergoing experimental acute isovolemic anemia. METHODS: The authors combined HR data from healthy subjects undergoing experimental isovolemic anemia in seven studies performed by the group. HR changes associated with breathing 100% oxygen by nonrebreathing facemask versus transfusion of erythrocytes at their nadir hemoglobin concentration of 5 g/dl were examined. Data were analyzed using a mixed-effects model. RESULTS: HR had an inverse linear relationship to hemoglobin concentration with a mean increase of 3.9 beats per min per gram of hemoglobin (beats/min/g hemoglobin) decrease (95% CI, 3.7-4.1 beats/min/g hemoglobin), P < 0.0001. Return of autologous erythrocytes significantly decreased HR by 5.3 beats/min/g hemoglobin (95% CI, 3.8-6.8 beats/min/g hemoglobin) increase, P < 0.0001. HR at nadir hemoglobin of 5.6 g/dl (95% CI, 5.5-5.7 g/dl) when breathing air (91.4 beats/min; 95% CI, 87.6-95.2 beats/min) was reduced by breathing 100% oxygen (83.0 beats/min; 95% CI, 79.0-87.0 beats/min), P < 0.0001. The HR at hemoglobin 5.6 g/dl when breathing oxygen was equivalent to the HR at hemoglobin 8.9 g/dl when breathing air. CONCLUSIONS: High arterial oxygen partial pressure reverses the heart rate response to anemia, probably because of its usability rather than its effect on total oxygen content. The benefit of high arterial oxygen partial pressure has significant potential clinical implications for the acute treatment of anemia and results of transfusion trials.

Increases in voltage may produce false-negatives when using transcranial motor evoked potentials to detect an isolated nerve root injury.

OBJECTIVE: Transcranial Motor Evoked Potentials (TcMEPs) are sometimes used during lumbar spine surgery in order to detect and prevent an intraoperative nerve root injury. Typically, a fixed stimulus is applied, and one monitors for changes in response amplitude from several myotomes. Increased stimulating voltage may or may not alter the response after an acute injury. METHODS: We suture ligated the dominant root innervating the tibialis anterior (TA) muscle in 7 experiments in swine monitored with TcMEPs. Injury to the root was confirmed by an increase in threshold current needed to evoke an EMG response in the TA (from 0.32 ± 0.10 to 2.3 ± 0.9 mA, P < 0.01). We recorded TcMEPs at baseline, after injury, and with incremental 25 V increases in stimulation up to 100 V. RESULTS: After ligation, mean TcMEP amplitude in the TA decreased by 56% from baseline (P < 0.01). Adding voltage progressively restored mean amplitude to within 17% of baseline, but with wide variability in the response. In 1 experiment, there was no augmentation; 3 studies showed partial improvement toward baseline; and in 3 studies, the amplitude was augmented to levels above baseline. CONCLUSION: An acute nerve root injury may be detected by TcMEP monitoring. However, if the stimulating voltage is increased after injury, the response may or may not be affected. In complex spine procedures, adjustments to TcMEP stimulating parameters are often needed to maintain reproducible responses. However, if these changes are made during a period where injury might occur, this could mask the changes and lead to a false-negative interpretation.

Mixed-muscle electrode placement ("jumping" muscles) may produce false-negative results when using transcranial motor evoked potentials to detect an isolated nerve root injury in a porcine model.

INTRODUCTION: Placing EMG electrode pairs that span several muscles is sometimes used to enhance the efficacy of electromyographic recordings. This technique, often referred to as "jumping," has not been studied when using Motor Evoked Potentials (TcMEP) for detecting isolated spinal nerve root injury during spine surgery. METHODS: TcMEPs were obtained in seven pigs under general anesthesia. One pair of recording electrodes was placed entirely within the tibialis anterior (TA) muscle; a second pair had one lead in the TA and the other in the gastrocnemius muscle (TA-GAS). The dominant root innervating the TA was determined using evoked EMG. MEP amplitudes recorded by the TA and TA-GAS electrodes were compared before and after suture ligation of this root in 12 separate experiments. RESULTS: Mean baseline TcMEP amplitude was not significantly different for the TA vs. TA-GAS. After root ligation, mean amplitude dropped from baseline by 72 +/- 13% in the TA vs. 50 +/- 29% in the TA-GAS (p < 0.01). All amplitudes decreased by >50% in the TA group; half of the TA-GAS group had <50% decrease in amplitude. DISCUSSION: Mixed-myotomal recording electrodes did not consistently increase baseline TcMEP amplitude. The decrease in amplitude after ligation was both smaller and more variable in the "jumped" TA-GAS electrodes. Thus, this technique may allow someone relying on TcMEP monitoring to miss an otherwise detectable isolated nerve root injury (i.e., have a false-negative result).

The reliability of motor evoked potentials to predict dorsiflexion injuries during lumbosacral deformity surgery: importance of multiple myotomal monitoring.

STUDY DESIGN: Case-control analysis of transcranial motor evoked potential (MEP) responses and clinical outcome. OBJECTIVE: To determine the sensitivity and specificity of MEPs to predict isolated nerve root injury causing dorsiflexion weakness in selected patients having complex lumbar spine surgery. SUMMARY OF BACKGROUND DATA: The surgical correction of distal lumbar spine deformity involves significant risk for damage to neural structures that control muscles of ankle and toe dorsiflexion. Procedures often include vertebral translation, interbody fusion, and posterior-based osteotomies. The benefit of using MEP monitoring to predict dorsiflexion weakness has not been well-established. The purpose of this paper is to describe the relationship between neural complications from lumbar surgery and intraoperative MEP changes. METHODS: Included were 542 neurologically intact patients who underwent posterior spinal fusion for the correction of distal lumbar deformity. Two myotomes, including tibialis anterior (TA) and extensor hallucis longus (EHL), were monitored. MEP and free-running electromyography data were assessed in each patient. Cases of new dorsiflexion weakness noted postoperatively were identified. Data in case and control patients were compared. There was no direct funding for this work. The Department of Anesthesiology and Perioperative Care provides salary support for authors one and six. Authors two and three report employment in the field of intraoperative neurophysiological monitoring as a study-specific conflict of interest. RESULTS: Twenty-five patients (cases) developed dorsiflexion weakness. MEP amplitude decreased in the injured myotomes by an average of 65 ± 21% (TA) and 60±26% (EHL), which was significantly greater than the contralateral uninjured side or for control subjects. (p < .01) Receiver operator characteristic (ROC) curves showed high sensitivity, specificity, and predictive value for changes in MEP amplitude using either the TA or EHL. Analysis of MEP changes to either TA or EHL yielded a superior ROC curve. Net reclassification improvement analysis showed assessing MEP changes to both TA and EHL improved the predictability of injury. CONCLUSIONS: The use of MEP amplitude change is highly sensitive and specific to predict a new postoperative dorsiflexion injury. Monitoring two myotomes (both TA and EHL) is superior to relying on MEP changes from a single myotome. Electromyography activity was less accurate but compliments MEP use. Additional studies are needed to define optimal intraoperative MEP warning thresholds.

The effect of age on motor evoked potentials in children under propofol/isoflurane anesthesia.

Intraoperative transcranial motor evoked potential (MEP) monitoring may help prevent neurologic injury during spine surgery. This type of monitoring may be difficult in the pediatric population under general anesthesia. We retrospectively reviewed data from 56 children, aged 2 to 18 yr, who were to undergo surgical correction of idiopathic scoliosis with MEP monitoring. Under combined isoflurane-propofol general anesthesia, before incision, we examined the minimum stimulating threshold voltage required to achieve a 50-microvolt or greater MEP response amplitude. Younger age was associated with an increase in the threshold voltage needed to elicit a sufficient MEP response. In addition, younger age was associated with longer stimulating pulse trains and greater need to adjust stimulating scalp electrodes. Body surface area, height, weight, and body mass index were also significant factors, but they were not independent predictors, after adjusting for age. Younger children received significantly lower levels of isoflurane and comparable doses of propofol, compared with older patients. Stronger stimulation needed to produce MEP responses in younger patients may reflect immaturity of their central nervous system, specifically conduction by the descending corticospinal motor tracts. Greater attention must be given to optimizing physiologic variables, limiting depressant anesthetics, and selecting the most favorable stimulating conditions in children, especially those <10 yr old.

Progressive suppression of motor evoked potentials during general anesthesia: the phenomenon of "anesthetic fade".

Transcranial motor evoked potentials (MEPs) are useful for assessing the integrity of spinal cord motor tracts during major spine surgery. Anesthetic agents depress the amplitude of MEPs in a dose-dependent fashion. Anecdotal reports suggest that MEP responses degrade or "fade" over the duration of a surgery, despite unchanged anesthetic levels or other physiologic variables. This phenomenon has not been systematically analyzed. We performed a retrospective study of 418 patients who underwent spine surgery at UCSF using intraoperative MEP monitoring. We excluded patients who experienced variations in physiologic parameters that might affect MEP signals and those who developed new neurologic deficits. We identified 46 neurologically intact patients and 16 myelopathic patients who had surgery performed using a constant desflurane/N2O/narcotic or desflurane/propofol/narcotic anesthetic regimen. The minimum voltage threshold needed to produce an MEP response of at least 50 microV in amplitude was recorded at the beginning ("baseline") and end of surgery. The voltage threshold was higher at the end of the case than at baseline for each patient, regardless of anesthetic regimen. In normal patients, the rate of rise of the threshold was similar for those receiving propofol (11.4 +/- 6.9 V/hr) or N2O (9.7 +/- 5.9 V/hr) (P = not significant). Myelopathic patients demonstrated a larger rate of rise in voltage threshold, 23.4 +/- 12.2 V/hr, versus normal subjects (P < 0.01). The rate of rise of voltage threshold is inversely proportional to anesthetic duration. Prolonged exposure to anesthetic agents necessitates higher stimulating thresholds to elicit MEP responses, separate from the dose-dependent depressant effect. This retrospective study is limited and cannot explain the mechanism for this observed fade in signals. Recognition of anesthetic fade is essential when interpreting changes to the MEP response to avoid false-positive findings.

Strategies for managing decreased motor evoked potential signals while distracting the spine during correction of scoliosis.

Surgical correction of kyphoscoliosis may result in spinal cord injury and neurologic deficits. Monitoring somatosensory evoked potentials (SSEPs) and transcranial motor evoked potentials (MEPs) intraoperatively may allow for early detection and reversal of spinal cord injury. Controlled hypotension and isovolemic hemodilution are often used during these cases to reduce blood loss and transfusion. However, these physiologic parameters may affect the quality of SSEP and MEP signals. Acute reduction or loss of MEP or SSEP signals during spinal distraction presents a crisis for the operative team: should distraction be immediately relieved? The authors describe three patients who showed a decrease in evoked potential signals under hypotensive, hemodiluted conditions at the stage of spinal distraction. Each case illustrates a different strategy for successful management of these patients.

Efficacy of transcranial motor evoked potentials, mechanically elicited electromyography, and evoked electromyography to assess nerve root function during sustained compression in a porcine model.

STUDY DESIGN: This is an animal experiment using transcranial motor evoked potentials (TcMEPs), mechanically elicited electromyographic (EMG) responses, and evoked EMG responses during nerve root compression in a pig model. OBJECTIVE: To compare these 3 electrophysiological measures for compression applied to a lumbar nerve root. SUMMARY OF BACKGROUND DATA: Lumbar nerve root injury may result in motor weakness in up to 30% of spinal deformity cases. Compressive injury may occur during the surgical approach, decompression, and manipulation of the spine. Using an established porcine model, we examined the changes to TcMEPs, mechanically elicited EMG responses, and evoked EMG responses during varied compressive forces. METHODS: TcMEPs, mechanically elicited EMG responses, and evoked EMG responses were recorded for the tibialis anterior muscle in 16 experiments. Precompression TcMEP and nerve root stimulation threshold (NRT) were obtained. The dominant root was compressed at 1 N (n = 8) or 2 N (n = 8) for 10 minutes. TcMEP was recorded every minute during compression, and TcMEP and NRT were recorded after both compression and 10 minutes of recovery. RESULTS: After 10 minutes of 1-N compression, TcMEP amplitude of the tibialis anterior muscle decreased to 69% ± 13% of baseline (P < 0.02 vs. baseline). The mean NRT increased to 645% ± 433% (P < 0.02 vs. baseline NRT). After the recovery period, TcMEP in the 1-N group returned to 98% ± 11% of baseline (P = 0.36 vs. baseline). After 10 minutes of 2-N compression, TcMEPs from the tibialis anterior muscle decreased to 27% ± 15% of baseline (P < 0.02 vs. baseline). After the recovery period, TcMEP in the 2-N group returned to 30% ± 10% of baseline (P < 0.02 vs. baseline). Tonic EMG activity was observed in 3 nerve roots compressed at 2 N. CONCLUSION: Compression at 1 and 2 N produced consistent changes in TcMEPs and EMG responses. TcMEP monitoring is sensitive to an increase in compressive force. TcMEP amplitude change was correlated to the force applied and the ability of the nerve root to recover. Mechanically elicited EMG responses were not sensitive to nerve root compression. LEVEL OF EVIDENCE: N/A.

Relative efficacy of transcranial motor evoked potentials, mechanically-elicited electromyography, and evoked EMG to assess nerve root function during sustained retraction in a porcine model.

STUDY DESIGN: This is an animal experiment using transcranial motor evoked potentials (TcMEP), mechanically elicited electromyography (EMG), and evoked EMG during spinal nerve root retraction in a pig model. OBJECTIVE: To compare the sensitivity of these 3 electrophysiological measures for a constant retraction force applied to an isolated lumbar nerve root for a specific duration of time. SUMMARY OF BACKGROUND DATA: The incidence of nerve root injury during lumbar spine surgery ranges from 0.2% to 31%. Direct retraction of spinal nerve roots may cause these injuries, but the amount and duration of force that may safely be applied is not clear. Using an established porcine model, we examined the changes occurring to multimyotomal TcMEPs, mechanically elicited EMGs, and evoked EMGs during continuous retraction of a nerve root at a constant force applied over 10 minutes. METHODS: TcMEP, mechanically elicited EMG, and evoked EMG responses were recorded from the tibialis anterior (TA) muscle in 10 experiments. The dominant root innervating the TA was determined with evoked EMG; preretraction TcMEP and nerve root stimulation threshold (NRT) was obtained. The dominant root was retracted at 2 Newton (N) for 10 minutes. TcMEP trials were elicited every minute during retraction. NRT was measured immediately after retraction. TcMEP and NRT were measured after 10 minutes of recovery. RESULTS.: During the 10 minutes of retraction at 2 N, the amplitude of the TA muscle progressively decreased in all trials in a highly significant curvilinear fashion. The mean TcMEP amplitude decreased 59% +/- 14% from baseline values. The mean NRT after 10 minutes of retraction at 2 N rose to 1.8 +/- 0.7 mA (P < 0.01 vs. baseline). The NRT increase after retraction strongly correlated with the decrease in motor evoked potentials amplitude in the TA (R = 0.90, P < 0.001). EMG activity was variable; tonic EMG was observed in only 2 nerve roots (20%). CONCLUSION: Three electrophysiologic methods were used intraoperatively to assess neural function during retraction of a single nerve root. Retraction produced consistent changes in TcMEPs and evoked EMG. These 2 methods show promise for assessing the limits on the force and duration of nerve root retraction during spine surgery. Mechanically elicited EMG was not sensitive to the amount and duration of nerve root retraction.

Monitoring of nerve root injury using transcranial motor-evoked potentials in a pig model.

STUDY DESIGN: Animal experiment using transcranial motor-evoked potentials (tcMEPs) in a pig model. OBJECTIVE: To validate measurement of tcMEPs from multiple myotomes in a pig model and determine the capacity to detect injury to a single nerve root. SUMMARY OF BACKGROUND DATA: The ability of intraoperative neuromonitoring methods to give information about a single nerve root remains poorly understood. Reports suggest that tcMEPs may be a reliable and accurate method to detect nerve root injury. An animal model to study the sensitivity and specificity of this technique has yet to be validated. METHODS: Transcranial stimulation was delivered through customized electrodes placed in burr holes over the motor cortex in 7 pigs. Spontaneous and evoked muscle potential activity was recorded in 5 myotomes (rectus femoris, vastus lateralis, vastus medialis, tibialis anterior, and gastrocnemius) bilaterally. After unilateral exposure of the L3-S1 nerve roots, sequential ligations were performed. The tcMEP responses from all myotomes were measured after ligation of each nerve root. RESULTS: Robust MEP responses (range, 37-1165 mV) were achieved in all monitored myotomes. Significant decreases in tcMEP amplitudes occurred in specific myotomes after ligation of the corresponding nerve root. Consistent and substantial decreases were observed after L3 and L5 ligations in rectus femoris (48%) and tibialis anterior (67%), respectively. DISCUSSION: Our results validate monitoring of tcMEPs in multiple myotomes to detect nerve root injury in pigs. This model may be used for further study of the use of tcMEPs to detect predictors and risk factors of nerve root injury during spinal surgery.

The efficacy of motor evoked potentials in fixed sagittal imbalance deformity correction surgery.

STUDY DESIGN: Retrospective analysis of transcranial motor evoked potential (TcMEP) responses and clinical outcome. OBJECTIVE: To determine the sensitivity and specificity of TcMEPs to detect and predict isolated nerve root injury in selected patients having complex lumbar spine surgery. SUMMARY OF BACKGROUND DATA: The surgical correction of fixed sagittal plane deformity involves posterior-based osteotomies and significant changes in the length of and space for the neural elements. The role of transcranial motor-evoked potential (TcMEP) monitoring in osteotomies below the conus has not been established. The purpose of this paper is to describe the relationship between neural complications from surgery and intraoperative TcMEP changes. METHODS: We retrospectively studied 35 consecutive patients in a single center treated with posterior-based osteotomies for the correction of fixed sagittal plane deformity. Transcranial motor-evoked potentials, free-running and evoked electromyography data were assessed for each case. Analysis includes description of the intraoperative changes observed, and a correlation of changes with postoperative clinical findings. RESULTS: Thirty-five consecutive patients underwent surgery for fixed sagittal plane deformity with complete neuromonitoring data. Twenty-five patients (71%) had an episode of greater than 80% reduction in MEP amplitude to at least 1 muscle. Fifteen of 25 had improvement of TcMEPs after repositioning of the legs (1), additional surgical decompression (4), or volume and pharmacologic resuscitation (10). All 15 of these awoke with no detectable neurologic injury. Ten patients (29%) had reduced TcMEP signals that did not improve despite further decompression and manipulation of the osteotomy site. All 10 had a greater than 67% drop in TcMEPs for at least 1 muscle persisting at the end of the case, and all had a postoperative neurologic deficit. The TcMEP changes in patients who demonstrated nerve injury postoperatively were observed most often during osteotomy closure or sustained dural retraction. 9 patients had weakness involving the iliopsoas or quadriceps; 1 patient had isolated unilateral dorsiflexion weakness. Monitoring TcMEPs in multiple muscle groups was both highly sensitive and specific for predicting injury. Nine patients had recovered motor function completely by discharge, and all but 1 patient (grade 4/5) had a normal motor examination at 6-week follow-up. CONCLUSION: The use of TcMEPs is sensitive and specific to change in neural function. No patients had a false negative test. The rate of neural deficits is consistent with previous literature, suggesting that TcMEP monitoring may not prevent neural injury. However, there were several cases in which intraoperative intervention resulted in recovery of TcMEPs, and none of these patients sustained any postoperative neural deficit. The severity of neural deficits in this series was minor and the duration was limited. TcMEPs may contribute to calling attention to the need for intraoperative corrections including widening decompressions, improving perfusion, and limiting deformity correction so that more severe neural compromise may be prevented.

Fresh blood and aged stored blood are equally efficacious in immediately reversing anemia-induced brain oxygenation deficits in humans.

BACKGROUND: Erythrocytes are transfused to treat or prevent imminent inadequate tissue oxygenation. 2,3-diphosphoglycerate concentration decreases and oxygen affinity of hemoglobin increases (P50 decreases) with blood storage, leading some to propose that erythrocytes stored for 14 or more days do not release sufficient oxygen to make their transfusion efficacious. The authors tested the hypothesis that erythrocytes stored for 3 weeks are as effective in supplying oxygen to human tissues as are erythrocytes stored for less than 5 h. METHODS: Nine healthy volunteers donated 2 units of blood more than 3 weeks before they were tested with a standard, computerized neuropsychological test (digit-symbol substitution test [DSST]) on 2 days, 1 week apart, before and after acute isovolemic reduction of their hemoglobin concentration to 7.4 and 5.5 g/dl. Volunteers randomly received autologous erythrocytes stored for either less than 5 h ("fresh") or 3 weeks ("stored") to return their hemoglobin concentration to 7.5 g/dl (double blinded). Erythrocytes of the alternate storage duration were transfused on the second experimental day. The DSST was repeated after transfusion. RESULTS: Acute anemia slowed DSST performance equivalently in both groups. Transfusion of stored erythrocytes with decreased P50 reversed the altered DSST (P < 0.001) to a time that did not differ from that at 7.4 g/dl hemoglobin during production of acute anemia (P = 0.88). The erythrocyte transfusion-induced DSST improvement did not differ between groups (P = 0.96). CONCLUSION: Erythrocytes stored for 3 weeks are as efficacious as are erythrocytes stored for 3.5 h in reversing the neurocognitive deficit of acute anemia. Requiring fresh rather than stored erythrocytes for augmentation of oxygen delivery does not seem warranted.