Impact of total propofol dose during spinal surgery: anesthetic fade on transcranial motor evoked potentials

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OBJECTIVE

Intraoperative neuromonitoring may be valuable for predicting postoperative neurological complications, and transcranial motor evoked potentials (TcMEPs) are the most reliable monitoring modality with high sensitivity. One of the most frequent problems of TcMEP monitoring is the high rate of false-positive alerts, also called “anesthetic fade.” The purpose of this study was to clarify the risk factors for false-positive TcMEP alerts and to find ways to reduce false-positive rates.

METHODS

The authors analyzed 703 patients who underwent TcMEP monitoring under total intravenous anesthesia during spinal surgery within a 7-year interval. They defined an alert point as final TcMEP amplitudes ≤ 30% of the baseline. Variations in body temperature (maximum − minimum body temperature during surgery) were measured. Patients with false-positive alerts were classified into 2 groups: a global group with alerts observed in 2 or more muscles of the upper and lower extremities, and a focal group with alerts observed in 1 muscle.

RESULTS

False-positive alerts occurred in 100 cases (14%), comprising 60 cases with global and 40 cases with focal alerts. Compared with the 545 true-negative cases, in the false-positive cases the patients had received a significantly higher total propofol dose (1915 mg vs 1380 mg; p < 0.001). In the false-positive cases with global alerts, the patients had also received a higher mean propofol dose than those with focal alerts (4.5 mg/kg/hr vs 4.2 mg/kg/hr; p = 0.087). The cutoff value of the total propofol dose for predicting false-positive alerts, with the best sensitivity and specificity, was 1550 mg. Multivariate logistic analysis revealed that a total propofol dose > 1550 mg (OR 4.583; 95% CI 2.785–7.539; p < 0.001), variation in body temperature (1°C difference; OR 1.691; 95% CI 1.060–2.465; p < 0.01), and estimated blood loss (500-ml difference; OR 1.309; 95% CI 1.155–1.484; p < 0.001) were independently associated with false-positive alerts.

CONCLUSIONS

Intraoperative total propofol dose > 1550 mg, larger variation in body temperature, and greater blood loss are independently associated with false-positive alerts during spinal surgery. The authors believe that these factors may contribute to the false-positive global alerts that characterize anesthetic fade. As it is necessary to consider multiple confounding factors to distinguish false-positive alerts from true-positive alerts, including variation in body temperature or ischemic condition, the authors argue the importance of a team approach that includes surgeons, anesthesiologists, and medical engineers.

ABBREVIATIONS ASA = American Society of Anesthesiologists; AUC = area under the ROC curve; BIS = bispectral index; BMI = body mass index; EBL = estimated blood loss; IONM = intraoperative neuromonitoring; ROC = receiver operating characteristic; SBP = systolic blood pressure; TcMEP = transcranial motor evoked potential.

Article Information

Correspondence Hiroki Ushirozako: Hamamatsu University School of Medicine, Shizuoka, Japan. verisa0808@gmail.com.

INCLUDE WHEN CITING Published online February 8, 2019; DOI: 10.3171/2018.10.SPINE18322.

Disclosures Drs. Oe and Togawa belong to a donation-funded laboratory called the “Division of Geriatric Musculoskeletal Health.” Donations to this laboratory have come from Medtronic Sofamor Danek, Inc.; Japan Medical Dynamic Marketing, Inc.; and the Meitoku Medical Institution Jyuzen Memorial Hospital. Dr. Togawa reports an employee relationship with the Journal of Bone and Joint Surgery.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    A: Control and final TcMEP waveforms in the typical case with global alerts, showing that TcMEP amplitudes decreased to ≤ 30% of the baseline in the right abductor digiti minimi (ADM), bilateral quadriceps femoris (Quad), right hamstring (Ham), bilateral tibialis anterior (TA), and bilateral gastrocnemius (Gc) in the upper and lower extremities. B: Control and final TcMEP waveforms in the typical case with focal alerts, showing that TcMEP amplitudes decreased to ≤ 30% of the baseline in the right quadriceps in the lower extremities.

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    Result of the TcMEP monitoring. FN = false negative; FP = false positive; TN = true negative; TP = true positive; RS = rescue cases.

  • View in gallery

    Total propofol dose for predicting false-positive TcMEP alerts using ROC curve analysis. The value of 1550 mg represents the cutoff point of the total propofol dose with the best sensitivity and specificity.

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