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Use of Ultrasound to Enhance the Local Anesthetic Effect of Topically Applied Aqueous Lidocaine

Background:Currently available local anesthetics, such as EMLA (eutectic mixture of local anesthetics), have poor skin penetration when administrated topically. As 60 mln is needed for EMLA to be fully effective, attempts to accelerate transdermal delivery of a local anesthetic, such as lidocaine, by use of “electrical” and “physical” energy has previously been reported. The current experiment was undertaken to determine whether ultrasonic energy can increase the local anesthetic effect of lidocaine in the skin of mice. Methods:Hairless mice were immersed in a beaker containing 2% aqueous lidocaine. Ultrasound (48kHz, 0.17 W/cm2) was applied to the beaker by an ultrasound-generating water tank for 5 mln. To examine anesthetic effects, the skin of the legs was stimulated using various voltages ranging from 0 to 50 V before and after treatment. The number of times the hairless mouse reacted out of six stimulations was counted. Results:Stimulation by 15 V at 30 min after lidocaine and ultrasound exposure resulted in positive reaction of 1.3 ± 1.6 (mean ± SD) compared with 5.2 ± 2.0 before treatment. Significant anesthetic effects continued for 2 h. Immersion to lidocaine alone without ultrasound showed no evidence of analgesia after treatment. Ultrasound alone to the legs also caused no anesthetic effects. Conclusions:It was concluded that ultrasound exposure to the legs of hairless mice along with topical 2% lidocaine solution rapidly induced an anesthetic effect.