Comparison of pH-adjusted lidocaine solutions for epidural anesthesia

One hundred forty-eight adult patients having epidural anesthesia for cesarean section, postpartum tubal ligation, lower extremity orthopedic procedures, or lithotriptic therapy were assigned to five groups. Group 1 patients were given a commercially prepared 1.5% lidocaine solution with 1:200,000 epinephrine plus 1 ml of normal saline per 10 ml of lidocaine; the solution pH was 4.6. Group 2 patients were given commercially prepared 1.5% lidocaine solution plus 1:200,000 epinephrine, with 1 mEq (1 ml) NaHCO₃ per 10 ml of lidocaine; the solution pH was 7.15. Group 3 patients received the commercial solution of 1.5% lidocaine with 1:200,000 epinephrine; the solution pH was 4.55. Group 4 patients were given a mixture of 18 ml of 2% lidocaine with 30 ml of 1.5% lidocaine, both commercially packaged with 1:200,000 epinephrine, plus 1 mEq (1 ml) of NaHCO₃ added per 10 ml of solution; the solution pH was 7.2. Group 5 patients received 1.5% plain lidocaine to which epinephrine was added to a final concentration of 1:200,000; the solution pH was 6.35. Times of onset of analgesia (time between the completion of the anesthetic injection and loss of scratch sensation at the right hip (L-2 dermatome) and of surgical anesthesia (time between completion of injection and loss of discomfort following tetanic stimulation produced by a nerve stimulator applied to skin on the right hip) were significantly more rapid in the groups that received the pH-adjusted solutions (group 4 and 2). Group 4 had the fastest mean onset time, 1.92 ± 0.17 min, followed by group 2, 3.31 ± 0.23 min. Onset times were progressively longer in group 5 at 4.27 ± 0.51 min, group 3 at 4.73 ± 0.37 min, and group 1 at 7.11 ± 0.82 min. The spread of sensory blockade was also significantly more rapid in the pH-adjusted groups 5, 10, and 15 min after epidural injection. In patients having cesarean sections in groups 1 and 2, plasma lidocaine levels in the maternal peripheral venous and in umbilical cord blood and Apgar scores were similar in both groups.