ED₅₀ and ED₉₀ of intrathecal hyperbaric 2% prilocaine in ambulatory knee arthroscopy

Hyperbaric 2% prilocaine (HP) is increasingly used for spinal anesthesia in day-case surgery. The aim of this prospective double-blind study was to determine the effective dose (ED)₅₀ and the ED₉₀ of HP for patients undergoing knee arthroscopy.

Doses of HP were determined using an up-and-down sequential allocation technique. Sequences were analyzed by isotonic regression analysis. A subsequent observational study was performed with the calculated ED₉₀ in 50 patients to confirm the initial result and to describe the induced blockade effects and side effects. Times corresponding to onset and duration of sensory and motor block, surgical data, and side effects were recorded.

The ED₅₀ was estimated at 28.9 mg (95% confidence interval [CI]: 26.5 to 35.3) and the ED₉₀ was estimated to be 38.5 mg (95% CI: 35.7 to 39.5). A 40 mg dose of HP provided efficient anesthesia in 46 patients (92%, 95% CI: 82 to 98). The average (SD) time to effective anesthesia was 14.5 (3.9) min. Complete sensory block at level T12 was obtained after ten minutes in 44 of 50 patients. The average (SD) duration of the sensory block was 205 (36.1) min. Maximal level of sensory block was obtained at the T8-T11 levels in 41 of 50 patients without hemodynamic instability. A Bromage 3 score was obtained in 40 of the 46 patients who achieved successful anesthesia after 30 min. Patients did not experience urinary retention, nor were any signs of transient neurologic symptoms observed.

This study determined the ED₅₀ of HP is 28.9 mg and suggests that a 40-mg dose of HP is adequate to provide successful spinal anesthesia for outpatient knee arthroscopy.