title: Continuous peripheral nerve blocks: is local anesthetic dose the only factor, or do concentration and volume influence infusion effects as well?
abstract: BACKGROUND
  The main determinant of continuous peripheral nerve block effects--local anesthetic concentration and volume or simply total drug dose--remains unknown.
  
  
  METHODS
  We compared two different concentrations and basal rates of ropivacaine--but at equivalent total doses--for continuous posterior lumbar plexus blocks after hip arthroplasty. Preoperatively, a psoas compartment perineural catheter was inserted. Postoperatively, patients were randomly assigned to receive perineural ropivacaine of either 0.1% (basal 12 ml/h, bolus 4 ml) or 0.4% (basal 3 ml/h, bolus 1 ml) for at least 48 h. Therefore, both groups received 12 mg of ropivacaine each hour with a possible addition of 4 mg every 30 min via a patient-controlled bolus dose. The primary endpoint was the difference in maximum voluntary isometric contraction (MVIC) of the ipsilateral quadriceps the morning after surgery, compared with the preoperative MVIC, expressed as a percentage of the preoperative MVIC. Secondary endpoints included hip adductor and hip flexor MVIC, sensory levels in the femoral nerve distribution, hip range-of-motion, ambulatory ability, pain scores, and ropivacaine consumption.
  
  
  RESULTS
  Quadriceps MVIC for patients receiving 0.1% ropivacaine (n = 26) declined by a mean (SE) of 64.1% (6.4) versus 68.0% (5.4) for patients receiving 0.4% ropivacaine (n = 24) between the preoperative period and the day after surgery (95% CI for group difference: -8.0-14.4%; P = 0.70). Similarly, the groups were found to be equivalent with respect to secondary endpoints.
  
  
  CONCLUSIONS
  For continuous posterior lumbar plexus blocks, local anesthetic concentration and volume do not influence nerve block characteristics, suggesting that local anesthetic dose (mass) is the primary determinant of perineural infusion effects.
authors.affiliation: Department of Anesthesiology, University of California San Diego, UCSD Center for Pain Medicine, 9300 Campus Point Drive, La Jolla, California 92037-7651, USA. bilfeld@ucsd.edu
authors.forename: Brian M
authors.initials: BM
authors.lastname: Ilfeld
authors.forename: Lisa K
authors.initials: LK
authors.lastname: Moeller
authors.forename: Edward R
authors.initials: ER
authors.lastname: Mariano
authors.forename: Vanessa J
authors.initials: VJ
authors.lastname: Loland
authors.forename: Jennifer E
authors.initials: JE
authors.lastname: Stevens-Lapsley
authors.forename: Adam S
authors.initials: AS
authors.lastname: Fleisher
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Girard
authors.forename: Michael C
authors.initials: MC
authors.lastname: Donohue
authors.forename: Eliza J
authors.initials: EJ
authors.lastname: Ferguson
authors.forename: Scott T
authors.initials: ST
authors.lastname: Ball
mesh_terms: D000328:Adult; D000368:Aged; D000577:Amides / Q000008:administration & dosage / Q000493:pharmacokinetics; D000762:Anesthesia Recovery Period; D000765:Anesthesia, Conduction; D000775:Anesthesia, Spinal; D000779:Anesthetics, Local / Q000008:administration & dosage* / Q000493:pharmacokinetics*; D019644:Arthroplasty, Replacement, Hip; D023381:Endpoint Determination; D005260:Female; D005267:Femoral Nerve / Q000187:drug effects; D006801:Humans; D007262:Infusions, Intravenous; D007537:Isometric Contraction / Q000187:drug effects; D008297:Male; D008875:Middle Aged; D053580:Muscle Strength / Q000187:drug effects; D018482:Muscle, Skeletal / Q000187:drug effects; D009407:Nerve Block; D010147:Pain Measurement; D011300:Preoperative Care; D016059:Range of Motion, Articular; D000077212:Ropivacaine; D016896:Treatment Outcome; D016138:Walking / Q000502:physiology
chemical_list: D000577:Amides; D000779:Anesthetics, Local; D000077212:Ropivacaine