Background and Objectives The effects of single and fractionated doses of local anesthetic on the extent of thoracic epidural blockade has not yet been determined. This single blinded and randomized study was designed to examine the effects of the initial dose and timing of the additional dose of local anesthetic on the sensory block level of the thoracic epidural anesthesia.

Methods Eighty-nine patients, who received thoracic epidural anesthesia followed by general anesthesia, were randomly divided into 4 groups: Group I received 5 mL of mepivacaine; Group II, 10 mL; Group III, 5 mL twice, with an interval of 5 minutes; and Group IV, 5 mL twice, with an interval of 10 minutes. After 15 minutes of either a single bolus or after the second bolus drug administration, the level of sensory block to coldness and pinprick were determined by an individual who was uninformed of the groups.

Results The median (range) number of spinal segments with sensory block to coldness in Groups I, II, III, and IV were 8 (5 to 12), 12 (7 to 17)^*, 11 (7 to 16)^*, and 9 (6 to 17)# (^* P < .05 v Group I, #P < .05 v Group II), respectively. The number of segments with sensory block to pinprick in the 4 groups were 7 (4 to 11), 11 (6 to 14)^*, 10 (6 to 14)^*, and 9 (4 to 16)^*#, respectively. These differences were mainly due to the differences of the lower sensory block level.

Conclusions We concluded that the timing of the second administration of mepivacaine was one of the factors for the spread of the drug into thoracic epidural space. The more extensive sensory block level occurred by shorter time interval of the second drug administration.