Purpose: Epidural anesthesia (EA) is the main anesthesia method for transforaminal percutaneous endoscopic lumbar discectomy (PELD). Reducing the concentration of ropivacaine can help preserve tactile sensation, allowing patients to provide timely feedback to the surgeons when a nerve root is contacted to avoid nerve injury. Therefore, a 90% effective concentration (EC₉₀) that allows for mild pain [visual analog scale (VAS) score ≤ 3] while maximizing tactile sensation must be identified.
Methods: The concentration of ropivacaine for EA was varied for consecutive patients in this study using a two-stage biased-coin design (BCD) according to the response of the previous patient; the concentration used for the first patient was 0.2%. When the previous patient had a negative response (VAS score > 3), the concentration used for the next one was increased by 0.015%. When the previous patient had a positive response (VAS score ≤ 3), the concentration used for the next one had an 89% probability of remaining the same and an 11% probability of being reduced by 0.015%. The EC₉₀ of ropivacaine was estimated using isotonic regression, and the 95% confidence interval (CI) was estimated using the bootstrapping method in R.
Results: A total of 58 patients were included in the study. The calculated EC₉₀ was 0.294% [95% CI (0.271%, 0.303%)]. Among 13 patients who reported unintended nerve root contact during the operation, none were found to have irreversible nerve injury after the operation.
Conclusion: To preserve maximum tactile sensation, the EC₉₀ of ropivacaine was 0.294% for patients with allowed mild pain. This concentration could allow for timely feedback when the nerve root is contacted, to avoid nerve injury.
Keywords: minimum effective concentration, epidural anesthesia, ropivacaine, transforaminal percutaneous endoscopic lumbar discectomy, biased-coin design, isotonic regression