title: Effect of local anesthetic dilution on the onset time and duration of double-injection sciatic nerve block: a prospective, randomized, blinded evaluation.
abstract: BACKGROUND
  Among the various factors influencing the success rate, onset time, and duration of peripheral nerve blocks, the role of local anesthetics concentration remains uncertain. In this prospective, randomized, single-blinded study, we evaluated whether varying the dilution of a fixed dose of mepivacaine solution influenced onset time and duration of sciatic nerve block.
  
  
  METHODS
  Ninety ASA physical status I to II patients scheduled for foot surgery were randomly allocated to receive a double-injection Labat sciatic nerve block with 12 mL mepivacaine 2% (group concentration I = 45 patients) or 24 mL of mepivacaine 1% (group volume II = 45 patients). The nerve stimulator was initially set at 2 Hz, 0.1 millisecond, 1 mA. The total amount of local anesthetic (240 mg) was kept constant and equally divided between the peroneal and tibial nerves. All patients also received an ultrasound-guided popliteal sciatic nerve catheter for postoperative analgesia. Times to readiness for surgery, performance, and offset of local anesthetic were recorded. Our primary end point was to determine a possible difference in offset time between groups. Continuous variables were expressed as median (IQR) and compared with the Wilcoxon-Mann-Whitney U test; WMWodds are reported together with their 95% confidence interval.
  
  
  RESULTS
  The overall success rate of sciatic nerve block was 99%. Time of performance was shorter in group I, 120 seconds (90-150 seconds), than that in group II, 150 seconds (120-180 seconds) (P = 0.0048; WMWodds 2.26 [1.35-4.34]). The onset time of sensory and motor sciatic nerve block was 4 minutes (2-9 minutes) in group I and 6 minutes (4-10 minutes) in group II (P = 0.41; WMWodds 1.21 [0.77-1.95]), while the duration of sensory block was 235 minutes (203-250 minutes) in group I, and 240 minutes (218-247 minutes) in group II respectively (P = 0.51; WMWodds 1.20 [0.69-2.16]).
  
  
  CONCLUSIONS
  We found no evidence that varying volume and concentration while maintaining a fixed total dose of mepivacaine alters the onset time and duration of double-injection sciatic nerve block. Considering our WMWodds results, possible differences in onset time and duration comparable to differences in the performance time between groups cannot be excluded.
authors.affiliation: From the Department of Anesthesia, Azienda Ospedaliera Istituto Ortopedico Gaetano Pini, Milano; Department of Anesthesia, Ospedale di Circolo Varese, Varese; and Department of Anesthesia, Irccs San Raffaele, Milano, Italy.
authors.forename: Gianluca
authors.initials: G
authors.lastname: Cappelleri
authors.forename: Andrea Luigi
authors.initials: AL
authors.lastname: Ambrosoli
authors.forename: Stefania
authors.initials: S
authors.lastname: Turconi
authors.forename: Marco
authors.initials: M
authors.lastname: Gemma
authors.forename: Erika Basso
authors.initials: EB
authors.lastname: Ricci
authors.forename: Gabriele
authors.initials: G
authors.lastname: Cornaggia
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000779:Anesthetics, Local / Q000008:administration & dosage*; D016009:Chi-Square Distribution; D004305:Dose-Response Relationship, Drug; D005260:Female; D005528:Foot / Q000294:innervation* / Q000601:surgery; D006801:Humans; D007267:Injections; D007558:Italy; D053208:Kaplan-Meier Estimate; D016015:Logistic Models; D008297:Male; D008619:Mepivacaine / Q000008:administration & dosage*; D008875:Middle Aged; D009043:Motor Activity / Q000187:drug effects; D009407:Nerve Block / Q000379:methods*; D016017:Odds Ratio; D017288:Pain Threshold / Q000187:drug effects; D011446:Prospective Studies; D012584:Sciatic Nerve / Q000187:drug effects*; D016037:Single-Blind Method; D013997:Time Factors; D016896:Treatment Outcome; D055815:Young Adult
chemical_list: D000779:Anesthetics, Local; D008619:Mepivacaine