Systemic ondansetron antagonizes the sensory block produced by intrathecal lidocaine.

In this prospective randomized, double-blind study, we investigated the effect of ondansetron on the lidocaine subarachnoid block. Fifty-four male patients scheduled for transurethral surgery under subarachnoid anesthesia received oral ondansetron 4 mg the evening before surgery and 4 mg IV 15 min before subarachnoid anesthesia (ondansetron group) or placebo (placebo group). Two milliliters of 5% hyperbaric lidocaine was administered intrathecally. Sensory block was assessed 20, 25, and 30 min and motor block 30, 60, and 90 min after lidocaine injection. In two patients in the control group and five in the ondansetron group, sensory block was not assessed for technical reasons. In the ondansetron group, sensory block values differed significantly over the 30-min period of assessments (P = 0.048). Fifteen, 20, 25, and 30 min after subarachnoid lidocaine, the level of sensory block was at T11, T12, T12, and T12 in the control group and T12, T12, T12, and L1 in the ondansetron group and differed between groups at 30 min (P = 0.019). Motor block did not differ between the two groups at any study time. We conclude that, under the conditions of our study, ondansetron antagonizes the sensory block produced by lidocaine.