Concentration- and pH-dependent effects of local anesthetics on onset of epidural anesthesia.

This study was undertaken to determine the effect of concentration of pH-adjusted local anesthetics on the onset time of anesthesia.Lidocaine (0.9% and 1.8%) and bupivacaine (0.25% and 0.5%) were used. In the 0.9 percent lidocaine and 0.25 percent bupivacaine groups, the onset time of analgesia was shortened by about 100 and 200 seconds, when pH was adjusted to 7.4 and 7.0, respectively ( P < 0.05 and P < 0.01). In the 1.8 percent lidocaine and 0.5 percent bupivacaine groups, however, pH adjustment exerted no significant effect on the onset time. Alkalinization of low-concentration local anesthetics shortened the onset time, whereas that of high-concentration did not shorten.