title: Continuous femoral nerve blocks: decreasing local anesthetic concentration to minimize quadriceps femoris weakness.
abstract: BACKGROUND
  Whether decreasing the local anesthetic concentration during a continuous femoral nerve block results in less quadriceps weakness remains unknown.
  
  
  METHODS
  Preoperatively, bilateral femoral perineural catheters were inserted in subjects undergoing bilateral knee arthroplasty (n = 36) at a single clinical center. Postoperatively, right-sided catheters 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), with the left catheter receiving the alternative concentration/rate in an observer- and subject-masked fashion. The primary endpoint was the maximum voluntary isometric contraction of the quadriceps femoris muscles the morning of postoperative day 2. Equivalence of treatments would be concluded if the 95% CI for the difference fell within the interval -20%-20%. Secondary endpoints included active knee extension, passive knee flexion, tolerance to cutaneous electrical current applied over the distal quadriceps tendon, dynamic pain scores, opioid requirements, and ropivacaine consumption.
  
  
  RESULTS
  Quadriceps maximum voluntary isometric contraction for limbs receiving 0.1% ropivacaine was a mean (SD) of 13 (8) N · m, versus 12 (8) N · m for limbs receiving 0.4% [intrasubject difference of 3 (40) percentage points; 95% CI -10-17; P = 0.63]. Because the 95% CI fell within prespecified tolerances, we conclude that the effect of the two concentrations were equivalent. Similarly, there were no statistically significant differences in secondary endpoints.
  
  
  CONCLUSIONS
  For continuous femoral nerve blocks, we found no evidence that local anesthetic concentration and volume influence block characteristics, suggesting that local anesthetic dose (mass) is the primary determinant of perineural infusion effects.
authors.affiliation: Department of Outcomes Research, Cleveland Clinic, Cleveland, Ohio, USA.
authors.forename: Maria
authors.initials: M
authors.lastname: Bauer
authors.forename: Lu
authors.initials: L
authors.lastname: Wang
authors.forename: Olusegun K
authors.initials: OK
authors.lastname: Onibonoje
authors.forename: Chad
authors.initials: C
authors.lastname: Parrett
authors.forename: Daniel I
authors.initials: DI
authors.lastname: Sessler
authors.forename: Loran
authors.initials: L
authors.lastname: Mounir-Soliman
authors.forename: Sherif
authors.initials: S
authors.lastname: Zaky
authors.forename: Viktor
authors.initials: V
authors.lastname: Krebs
authors.forename: Leonard T
authors.initials: LT
authors.lastname: Buller
authors.forename: Michael C
authors.initials: MC
authors.lastname: Donohue
authors.forename: Jennifer E
authors.initials: JE
authors.lastname: Stevens-Lapsley
authors.forename: Brian M
authors.initials: BM
authors.lastname: Ilfeld
mesh_terms: D000368:Aged; D000779:Anesthetics, Local / Q000008:administration & dosage* / Q000009:adverse effects / Q000378:metabolism; D019645:Arthroplasty, Replacement, Knee / Q000009:adverse effects*; D005260:Female; D005267:Femoral Nerve / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D018908:Muscle Weakness / Q000139:chemically induced / Q000503:physiopathology / Q000517:prevention & control*; D009407:Nerve Block / Q000379:methods*; D010149:Postoperative Pain / Q000517:prevention & control; D011446:Prospective Studies; D052097:Quadriceps Muscle / Q000187:drug effects / Q000502:physiology*
chemical_list: D000779:Anesthetics, Local