Ionic strength influences hemolytic action of dibucaine hydrochloride

BACKGROUND: Little is known about effect of ionic strength on local anesthetic toxicity. Using human erythrocytes, hemolytic action of dibucaine in solutions of various ionic strength was investigated. METHODS: The critical micellar concentration (CMC) of dibucaine and the dibucaine level that causes destruction of half of the red blood cells in vitro (EC50 value) were determined in solutions of various ionic strength. RESULTS: The mean CMC values of the dibucaine solutions adjusted to ionic strength 0.15, 0.30, 0.45 and 0.90 with NaCl, were 35.3, 22.6, 15.9 and 9.6 mM, respectively. The mean EC50 values of these solutions measured at 5 sec were 22.5, 16.0, 12.6 and 8.2 mM, respectively, and those at 30 min were 4.9, 4.5, 4.5 and 2.6 mM, respectively. There was a significant correlation between mean CMC values and mean EC50 values at 5 sec but not at 30 min in the solution of the same ionic strength. CONCLUSIONS: These findings indicated that the mechanism of dibucaine-induced hemolysis within a few seconds is through membrane lysis, whereas dibucaine-induced hemolysis at 30 min is caused by another mechanism. Because each mechanism is enhanced by high ionic strength, dibucaine dissolved in salt solution should not be administered intrathecally.