title: The effects of local anesthetic concentration and dose on continuous infraclavicular nerve blocks: a multicenter, randomized, observer-masked, controlled study.
abstract: BACKGROUND
  It remains unclear whether local anesthetic concentration or total drug dose is the primary determinant of continuous peripheral nerve block effects. The only previous investigation, involving continuous popliteal-sciatic nerve blocks, specifically addressing this issue reported that insensate limbs were far more common with higher volumes of relatively dilute ropivacaine compared with lower volumes of relatively concentrated ropivacaine. However, it remains unknown if this relationship is specific to the sciatic nerve in the popliteal fossa or whether it varies depending on anatomic location. We therefore tested the null hypothesis that providing ropivacaine at different concentrations and rates, but at an equal total basal dose, produces comparable effects when used in a continuous infraclavicular brachial plexus block.
  
  
  METHODS
  Preoperatively, an infraclavicular catheter was inserted using the coracoid approach in patients undergoing moderately painful orthopedic surgery distal to the elbow. Patients were randomly assigned to receive a postoperative perineural ropivacaine infusion of either 0.2% (basal 8 mL/h, bolus 4 mL) or 0.4% (basal 4 mL/h, bolus 2 mL) through the second postoperative day. Both groups, therefore, received 16 mg of ropivacaine each hour with a possible addition of 8 mg every 30 min via a patient-controlled bolus dose. Our primary end point was the incidence of an insensate limb during the 24-h period beginning the morning after surgery. Secondary end points included analgesia and patient satisfaction.
  
  
  RESULTS
  Patients given 0.4% ropivacaine (n = 27) experienced an insensate limb, a mean (sd) of 1.8 (1.6) times, compared with 0.6 (0.9) times for subjects receiving 0.2% ropivacaine (n = 23; estimated difference = 1.2 episodes, 95% confidence interval, 0.5-1.9 episodes; P = 0.001). Satisfaction with postoperative analgesia (scale 0-10, 10 = highest) was scored a median (25th-75th percentiles) of 10.0 (8.0-10.0) in Group 0.2% and 7.0 (5.3-8.9) in Group 0.4% (P = 0.018). Analgesia was similar in each group.
  
  
  CONCLUSIONS
  For continuous infraclavicular nerve blocks, local anesthetic concentration and volume influence perineural infusion effects in addition to the total mass of local anesthetic administered. Insensate limbs were far more common with smaller volumes of relatively concentrated ropivacaine. This is the opposite of the relationship previously reported for continuous popliteal-sciatic nerve blocks. The interaction between local anesthetic concentration and volume is thus complex and varies among catheter locations.
authors.affiliation: Department of Anesthesiology, University of California San Diego, San Diego, California 92037-7651, USA. bilfeld@ucsd.ed
authors.forename: Brian M
authors.initials: BM
authors.lastname: Ilfeld
authors.forename: Linda T
authors.initials: LT
authors.lastname: Le
authors.forename: Joanne
authors.initials: J
authors.lastname: Ramjohn
authors.forename: Vanessa J
authors.initials: VJ
authors.lastname: Loland
authors.forename: Anupama N
authors.initials: AN
authors.lastname: Wadhwa
authors.forename: J C
authors.initials: JC
authors.lastname: Gerancher
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Renehan
authors.forename: Daniel I
authors.initials: DI
authors.lastname: Sessler
authors.forename: Jonathan J
authors.initials: JJ
authors.lastname: Shuster
authors.forename: Douglas W
authors.initials: DW
authors.lastname: Theriaque
authors.forename: Rosalita C
authors.initials: RC
authors.lastname: Maldonado
authors.forename: Edward R
authors.initials: ER
authors.lastname: Mariano
mesh_terms: D000328:Adult; D000577:Amides / Q000008:administration & dosage* / Q000009:adverse effects; D000779:Anesthetics, Local / Q000008:administration & dosage* / Q000009:adverse effects; D001917:Brachial Plexus / Q000187:drug effects*; D002968:Clavicle / Q000294:innervation*; D004305:Dose-Response Relationship, Drug; D004334:Drug Administration Schedule; D005260:Female; D006801:Humans; D007260:Infusion Pumps; D007263:Infusions, Parenteral; D008297:Male; D008875:Middle Aged; D009407:Nerve Block / Q000379:methods*; D009864:Ontario; D019637:Orthopedic Procedures / Q000009:adverse effects*; D010147:Pain Measurement; D017288:Pain Threshold / Q000187:drug effects; D010149:Postoperative Pain / Q000209:etiology / Q000517:prevention & control*; D017060:Patient Satisfaction; D000077212:Ropivacaine; D016037:Single-Blind Method; D014110:Touch / Q000187:drug effects; D014481:United States
chemical_list: D000577:Amides; D000779:Anesthetics, Local; D000077212:Ropivacaine