The Optimal Effective Concentration of Spinal Anesthesia for Interlaminar Endoscopic Lumbar Discectomy: An Approach Based on the Biased Coin Design.

PURPOSE: To determine the optimal spinal anesthesia concentration for interlaminar endoscopic lumbar discectomy (IELD), in order to deliver precise and individualized anesthesia for this procedure.

METHODS

Fifty-four patients scheduled for IELD were randomized via a biased coin design to receive spinal anesthesia with different ropivacaine doses. The initial dose was 7.5 mg/3 mL. If the patient reported a negative outcome, the subsequent dose was increased by 0.75 mg. If a positive outcome was reported, subsequent dose remained unchanged (89% probability) or was decreased by 0.75 mg (11% probability). At least 45 positive outcomes were needed to estimate the MEC90. Outcome measures: onset and duration of surgical analgesia, postoperative pain scores, urinary retention incidence, end-of-surgery modified Bromage score, and patient satisfaction.

RESULTS

The isotonic regression estimated the MEC90 of ropivacaine for spinal anesthesia in IELD at 9.474 mg/3 mL (95% CI: 8.242-10.088); Probit regression yielded MEC90 = 9.480 mg/3 mL (95% CI: 8.806-10.154), MEC95 = 9.932 mg/3 mL (95% CI: 8.957-10.907), and MEC99 = 10.779 mg/3 mL (95% CI: 9.122-12.436). Analgesia onset was 5.0 min (IQR: 5.0-7.0), lasting 100.0 min (IQR: 90.0-110.0). Postoperative pain scores (VAS, cm) at 2, 6, 12, and 24 hours were 2.0 (1.0-3.0), 2.0 (2.0-2.0), 2.0 (2.0-3.0), and 2.0 (2.0-2.0), respectively. Bromage score at surgery end was 4.5 (IQR: 3.0-5.0). Mild urinary retention occurred in 4 patients (7.4%); no other complications were observed. Patient satisfaction: excellent (53.7%), good (20.4%), fair (13.0%), poor (5.6%), very poor (7.4%). Ropivacaine concentration was negatively correlated with analgesia onset time (r = -0.228, P = 0.049) and Bromage score (r = -0.235, P = 0.044), but not associated with analgesia duration (r = 0.105, P = 0.224).

CONCLUSION

The MEC90 of ropivacaine for spinal anesthesia in IELD was 9.474 mg/3 mL, the clinically recommended dose was 9.5 to 10.5 mg/3 mL.