title: A re-evaluation of the ability of thiopental to identify cerebrospinal fluid in epidural catheter aspirate.
abstract: OBJECTIVE
  Sodium thiopental has been used to determine whether fluid aspirated from an epidural catheter is previously injected local anesthetic or cerebrospinal fluid (CSF). The purpose of this study was to test the efficacy of this test in distinguishing opioids from CSF.
  
  
  METHODS
  in vitro study.
  
  
  METHODS
  Laboratory of a university hospital.
  
  
  RESULTS
  Three in vitro studies were performed. The first study tested for precipitation when thiopental was mixed with several commonly used epidural medications. Then, thiopental was mixed in combinations of opioids with local anesthetics to see if the opioid might prevent the precipitation of the local anesthetics. Finally, lidocaine was serially diluted and precipitation with thiopental was assessed. It was found that certain concentrations of opioids as well as normal saline do not precipitate with thiopental. In addition, the ratio of opioids to local anesthetic of 10:1 prevented precipitation when thiopental was added. Local anesthetics combined with cerebrospinal fluid in a 1:10 ratio produced a precipitate on mixing with thiopental.
  
  
  CONCLUSIONS
  Use of thiopental to differentiate opioids from cerebrospinal fluid is unreliable. In addition, in some simulated situations, opioids may mask the presence of local anesthetic.
authors.affiliation: Department of Anesthesiology, Naval Medical Center, San Diego, CA, USA.
authors.forename: J H
authors.initials: JH
authors.lastname: Waters
authors.forename: V L
authors.initials: VL
authors.lastname: Rizzo
authors.forename: S
authors.initials: S
authors.lastname: Ramanathan
mesh_terms: D000767:Anesthesia, Epidural; D002555:Cerebrospinal Fluid / Q000737:chemistry*; D005069:Evaluation Studies as Topic; D006801:Humans; D012680:Sensitivity and Specificity; D013396:Suction; D013874:Thiopental
chemical_list: D013874:Thiopental