Nitrous oxide enhances the level of sensory block produced by intrathecal lidocaine

We examined the effect of nitrous oxide (N2O) administration on the level of sensory block produced by intrathecal lidocaine in patients undergoing transurethral procedures. Twenty minutes after subarachnoid injection of 100 mg (5%) hyperbaric lidocaine, the level of block to pressure sensation was assessed. After establishing the baseline sensory block, patients were randomly assigned to receive either 50% nitrogen (control group) or 50% N2O in oxygen for 10 min, and the sensory level was reassessed. All patients then received 35% oxygen for 5 min, and the level of block to pressure was assessed again. Changes were measured in centimeters and standardized by dividing those results by the height of patients (in centimeters). Ten minutes after nitrogen or N2O administration, a 3.8-cm regression of sensory block was found in the control group, and a 1.8-cm cephalad increase was found in the treatment group (P < 0.0001). Discontinuation of N2O for 5 min resulted in a rapid regression of the level of sensory block (4 an in the N2O group versus 1.9 an in the control group, P < 0.0001). However, 5 min after discontinuation of N2O, the overall regression of the sensory block in the control group, when measured from the baseline, was 5.7 an versus 2.2 an in the N2O group (P < 0.001). The differences between the two groups before standardization are consistent with those after standardization (t = 9.02 at 10 min, t = 4.24 at 15 min, and t = 3.97 for the overall change at 15 min). The results suggest that inhalation of 50% N2O enhances the level of sensory block produced by intrathecal lidocaine. Implications: We measured the level of sensory block produced by subarachnoid anesthesia with lidocaine before and after inhalation of 50% nitrous oxide for 10 min. Nitrous oxide enhanced the level of subarachnoid anesthesia with minimal hemodynamic effects. These findings are of clinical importance when subarachnoid anesthesia subsides before the completion of surgery.