title: ED(50) and ED(95) of intrathecal bupivacaine in morbidly obese patients undergoing cesarean delivery.
abstract: BACKGROUND
  It has been suggested that morbidly obese parturients may require less local anesthetic for spinal anesthesia. The aim of this study was to determine the effective dose (ED(50)/ED(95)) of intrathecal bupivacaine for cesarean delivery in morbidly obese patients.
  
  
  METHODS
  Morbidly obese parturients (body mass index equal to or more than 40) undergoing elective cesarean delivery were enrolled in this double-blinded study. Forty-two patients were randomly assigned to receive intrathecal hyperbaric bupivacaine in doses of 5, 6, 7, 8, 9, 10, or 11 mg (n = 6 per group) coadministered with 200 μg morphine and 10 μg fentanyl. Success (induction) was defined as block height to pinprick equal to or more than T6 and success (operation) as success (induction) plus no requirement for epidural supplementation throughout surgery. The ED(50)/ED(95) values were determined using a logistic regression model.
  
  
  RESULTS
  ED(50) and ED(95) (with 95% confidence intervals) for success (operation) were 9.8 (8.6-11.0) and 15.0 (10.0-20.0), respectively, and were similar to corresponding values of a nonobese population determined previously using similar methodology. We were unable to measure ED(50)/ED(95) values for success (induction) because so few blocks failed initially, even at the low-dose range. There were no differences with regard to secondary outcomes (i.e., hypotension, vasopressor use, nausea, and vomiting).
  
  
  CONCLUSIONS
  Obese and nonobese patients undergoing cesarean delivery do not appear to respond differently to modest doses of intrathecal bupivacaine. This dose-response study suggests that doses of intrathecal bupivacaine less than 10 mg may not adequately ensure successful intraoperative anesthesia. Even when the initial block obtained with a low dose is satisfactory, it will not guarantee adequate anesthesia throughout surgery.
authors.affiliation: Department of Anesthesia, Stanford University School of Medicine, Stanford, California 94305, USA. bcarvalho@stanford.edu
authors.forename: Brendan
authors.initials: B
authors.lastname: Carvalho
authors.forename: Jeremy
authors.initials: J
authors.lastname: Collins
authors.forename: David R
authors.initials: DR
authors.lastname: Drover
authors.forename: Lindsey
authors.initials: L
authors.lastname: Atkinson Ralls
authors.forename: Edward T
authors.initials: ET
authors.lastname: Riley
mesh_terms: D000759:Adjuvants, Anesthesia; D000293:Adolescent; D000328:Adult; D000773:Anesthesia, Obstetrical; D000775:Anesthesia, Spinal; D000779:Anesthetics, Local / Q000008:administration & dosage* / Q000009:adverse effects; D001794:Blood Pressure / Q000187:drug effects; D002045:Bupivacaine / Q000008:administration & dosage* / Q000009:adverse effects; D002585:Cesarean Section; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D023381:Endpoint Determination; D005260:Female; D005283:Fentanyl; D006801:Humans; D007278:Injections, Spinal; D016015:Logistic Models; D009767:Obesity, Morbid / Q000150:complications*; D020250:Postoperative Nausea and Vomiting / Q000453:epidemiology; D011247:Pregnancy; D011446:Prospective Studies; D017211:Treatment Failure; D055815:Young Adult
chemical_list: D000759:Adjuvants, Anesthesia; D000779:Anesthetics, Local; D005283:Fentanyl; D002045:Bupivacaine