title: Effects of local anesthetic concentration and dose on continuous interscalene nerve blocks: a dual-center, randomized, observer-masked, controlled study.
abstract: OBJECTIVE
  It is currently unknown if the primary determinant of continuous peripheral nerve block effects is simply total drug dose, or whether local anesthetic concentration and/or volume have an influence. We therefore tested the null hypothesis that providing ropivacaine at different concentrations and rates--but at an equal total basal dose--produces similar effects when used in a continuous interscalene nerve block.
  
  
  METHODS
  Preoperatively, an anterolateral interscalene perineural catheter was inserted using the anterolateral approach in patients undergoing moderately painful shoulder surgery. Subjects were randomly assigned to receive a postoperative perineural infusion of either 0.2% ropivacaine (basal 8 mL/h, bolus 4 mL) or 0.4% ropivacaine (basal 4 mL/h, bolus 2 mL) through the second postoperative day. Our primary endpoint was the incidence of an insensate hand/finger during the 24 hours beginning the morning following surgery.
  
  
  RESULTS
  The incidence of an insensate hand/finger did not differ between the treatment groups (n = 50) to a statistically significant degree (0.2% ropivacaine, mean [SD] of 0.8 [1.3] times; 0.4% ropivacaine, mean 0.3 [0.6] times; estimated difference = 0.5 episodes, 95% confidence interval, -0.1 to 1.1 episodes; P = .080). However, this is statistically inconclusive given the confidence interval. In contrast, pain (P = .020) and dissatisfaction (P = .011) were greater in patients given 0.4% ropivacaine.
  
  
  CONCLUSIONS
  For continuous interscalene nerve blocks, given the statistically inconclusive primary endpoint results and design limitations of the current study, further research on this topic is warranted. In contrast, providing a lower concentration of local anesthetic at a higher basal rate provided superior analgesia.
authors.affiliation: Department of Anesthesiology, University of Florida, Gainesville, FL, USA.
authors.forename: Linda T
authors.initials: LT
authors.lastname: Le
authors.forename: Vanessa J
authors.initials: VJ
authors.lastname: Loland
authors.forename: Edward R
authors.initials: ER
authors.lastname: Mariano
authors.forename: J C
authors.initials: JC
authors.lastname: Gerancher
authors.forename: Anupama N
authors.initials: AN
authors.lastname: Wadhwa
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: Brian M
authors.initials: BM
authors.lastname: Ilfeld
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000577:Amides / Q000008:administration & dosage*; D016058:Analgesia, Patient-Controlled / Q000379:methods*; D000701:Analgesics, Opioid / Q000627:therapeutic use; D000779:Anesthetics, Local / Q000008:administration & dosage*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009407:Nerve Block / Q000379:methods*; D010147:Pain Measurement; D017060:Patient Satisfaction; D000077212:Ropivacaine; D012677:Sensation / Q000187:drug effects*; D012782:Shoulder / Q000601:surgery; D016896:Treatment Outcome; D055815:Young Adult
chemical_list: D000577:Amides; D000701:Analgesics, Opioid; D000779:Anesthetics, Local; D000077212:Ropivacaine