Effect of lidocaine phonophoresis on sensory blockade: pulsed or continuous mode of therapeutic ultrasound?

ObjectiveThe optimisation of drug absorption through skin is of great value in modern therapy. Phonophoresis is the use of therapeutic ultrasound to increase percutaneous drug absorption. However, few studies have compared pulsed and continuous modes of therapeutic ultrasound. This study compared these two modes by investigating the effect of lidocaine phonophoresis on sensory blockade. Lidocaine is a common local anaesthetic drug that is used topically to relieve pain and also for minor surgery.DesignPre–post intervention study.ParticipantsNinety-three healthy volunteers, assigned at random to one of three ultrasound groups: pulsed (ultrasound + lidocaine), continuous (ultrasound + lidocaine) and control (sham ultrasound + lidocaine).InterventionLidocaine (approximately 2 cm3) was administered transdermally using a transducer.Outcome measuresTwo-point discrimination, touch and maximum pain thresholds were assessed before and after the intervention in each group.ResultsThere were significant increases in two-point discrimination, touch and maximum pain thresholds after the intervention in all groups (P < 0.05). The mean differences in the threshold changes between the pulsed and continuous groups were significant [two-point discrimination: mean difference 0.66 (standard deviation, SD 0.41) mm, 95% confidence interval (CI) 0.18 to 1.21, P < 0.001; touch: mean difference 1.31 (SD 0.23) mA, 95% CI 0.98 to 1.96, P < 0.001; maximum pain: mean difference 5.59 (SD 4.01) mA, 95% CI 1.01 to 10.11, P < 0.001]. Statistical analysis also revealed significant differences for all senses between the pulsed and control groups [two-point discrimination: mean difference 0.61 (SD 0.29) mm, 95% CI 0.26 to 1.3, P < 0.001; touch: mean difference 1.45 (SD 0.38) mA, 95% CI 0.96 to 2.1, P < 0.001; maximum pain difference 9.17 (SD 4.13) mA, 95% CI 4.32 to 14.1, P < 0.001]. The mean difference in the change in maximum pain threshold between the continuous and control groups was significant [mean difference 3.58 (SD 3.44) mA, 95% CI 0.09 to 8.3, P = 0.001], but two-point discrimination and touch threshold changes revealed no significant difference between the continuous and control groups [two-point discrimination: mean difference 0.05 (SD 0.28) mm, 95% CI −0.31 to 0.48, P = 0.968; touch: mean difference 0.14 (SD 0.19) mA, 95% CI −0.21 to 0.43, P = 0.339].ConclusionPulsed ultrasound with topical lidocaine gel induced greater anaesthetic effect compared with continuous ultrasound with topical lidocaine gel and lidocaine application alone. The mechanical properties of pulsed ultrasound appear to be responsible for greater drug penetration.