title: Importance of volume and concentration for ropivacaine interscalene block in preventing recovery room pain and minimizing motor block after shoulder surgery.
abstract: BACKGROUND
  This three-staged study estimated the volume and concentration of interscalene ropivacaine that would prevent recovery room pain after shoulder surgery under general anesthesia.
  
  
  METHODS
  Stages 1/2: Interscalene catheter administration of ropivacaine was by a 10% incremental up-down sequential manner depending on the presence of recovery room pain in the previous patient. Stage 1: Ropivacaine (0.5% volume) was varied from 30 ml. Stage 2: Ropivacaine (20 ml, the ED(volume)95 estimate from stage 1) concentration was varied from 0.45%. Stage 3: Subjects were randomly assigned to receive 30 ml of ropivacaine, 0.5% ("conventional dose"), or 20 ml of ropivacaine, 0.375% (the estimated ED(volume+concentration)95 from stages 1/2). A postoperative elastomeric infusion of 0.2% ropivacaine (2 ml/h) was administered. Grip strength was measured in the recovery room and time to first pain at 24 h.
  
  
  RESULTS
  Stage 1 (n = 34): Ropivacaine 0.5% ED(volume)50/ED(volume)95 (95% CI) estimates were 2.7/20.5 ml (2.4-9.5/17.3-25.8). Stage 2 (n = 29): Ropivacaine 20 ml ED(concentration)50/ED(concentration)95 (95% CI) estimates were 0.15/0.34% (0.13-0.30/0.29-0.43). The ED(dose)50 was similar for stages 1/2 (13.5 vs. 30 mg), but the ED(dose)95 was higher for stage 1 (102.5 vs. 68 mg). Stage 3 (n = 40): Satisfaction (0-10) was modestly higher for the new/lower dose (median [interquartile range] = 10 [10-10] versus 9 [8-10], P = 0.007). Pooled data regression analysis showed that increasing ropivacaine concentration increased grip weakness but not block duration.
  
  
  CONCLUSIONS
  Ropivacaine interscalene block requires a threshold volume and concentration, with concentration primarily determining motor block. When combined with continuous blockade, suprathreshold ropivacaine doses do not significantly prolong primary block duration but may compromise patient satisfaction.
authors.affiliation: Department of Anaesthesiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand. michaelfredrickson@yahoo.com
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Fredrickson
authors.forename: Katherine R
authors.initials: KR
authors.lastname: Smith
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Wong
mesh_terms: D000328:Adult; D000577:Amides / Q000008:administration & dosage*; D000762:Anesthesia Recovery Period; D004305:Dose-Response Relationship, Drug; D017558:Elective Surgical Procedures / Q000009:adverse effects; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009407:Nerve Block / Q000379:methods; D010149:Postoperative Pain / Q000209:etiology / Q000517:prevention & control*; D011997:Recovery Room; D000077212:Ropivacaine; D012782:Shoulder / Q000601:surgery*; D016896:Treatment Outcome
chemical_list: D000577:Amides; D000077212:Ropivacaine