title: The effects of varying local anesthetic concentration and volume on continuous popliteal sciatic nerve blocks: a dual-center, randomized, controlled study.
abstract: BACKGROUND
  It remains unknown whether local anesthetic concentration, or simply total drug dose, is the primary determinant of continuous peripheral nerve block effects. We therefore tested the null hypothesis that providing different concentrations and rates of ropivacaine, but at equal total doses, produces comparable effects when used in a continuous sciatic nerve block in the popliteal fossa.
  
  
  METHODS
  Preoperatively, a perineural catheter was inserted adjacent to the sciatic nerve using a posterior popliteal approach in patients undergoing moderately painful orthopedic surgery at or distal to the ankle. Postoperatively, patients were randomly assigned to receive a 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. Therefore, both groups received 16 mg of ropivacaine each hour with a possible addition of 8 mg every 30 min via a patient-controlled bolus dose. The primary end point was the incidence of an insensate limb, considered undesirable, during the 24-h period beginning the morning after surgery. Secondary end points included analgesia and patient satisfaction.
  
  
  RESULTS
  Patients given 0.2% ropivacaine (n = 25) experienced an insensate limb with a mean (sd) of 1.8 (1.8) times, compared with 0.6 (1.1) times for subjects receiving 0.4% ropivacaine (n = 25; estimated difference = 1.2 episodes, 95% confidence interval, 0.3-2.0 episodes; P = 0.009). In contrast, analgesia and satisfaction were similar in each group.
  
  
  CONCLUSIONS
  For continuous popliteal-sciatic nerve blocks, local anesthetic concentration and volume influence block characteristics. Insensate limbs were far more common with larger volumes of relatively dilute ropivacaine. During continuous sciatic nerve block in the popliteal fossa, a relatively concentrated solution in smaller volume thus appears preferable.
authors.affiliation: Department of Anesthesiology, University of California San Diego, San Diego, CA, USA. bilfeld@ucsd.edu
authors.forename: Brian M
authors.initials: BM
authors.lastname: Ilfeld
authors.forename: Vanessa J
authors.initials: VJ
authors.lastname: Loland
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: Edward R
authors.initials: ER
authors.lastname: Mariano
mesh_terms: D000328:Adult; D000368:Aged; D000556:Ambulatory Surgical Procedures; D000577:Amides / Q000008:administration & dosage*; D016058:Analgesia, Patient-Controlled; D000701:Analgesics, Opioid / Q000627:therapeutic use; D000779:Anesthetics, Local / Q000008:administration & dosage*; D005260:Female; D005528:Foot / Q000601:surgery; D018575:Home Care Services, Hospital-Based; D006801:Humans; D007260:Infusion Pumps; D008297:Male; D008875:Middle Aged; D009407:Nerve Block; D019637:Orthopedic Procedures; D010147:Pain Measurement; D010149:Postoperative Pain / Q000188:drug therapy / Q000517:prevention & control*; D017060:Patient Satisfaction; D000077212:Ropivacaine; D012584:Sciatic Nerve
chemical_list: D000577:Amides; D000701:Analgesics, Opioid; D000779:Anesthetics, Local; D000077212:Ropivacaine