• J H Waters, V L Rizzo, and S Ramanathan.
• Department of Anesthesiology, Naval Medical Center, San Diego, CA, USA.
• J Clin Anesth. 1995 May 1; 7 (3): 224-7.

Study ObjectiveSodium 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.Designin vitro study.SettingLaboratory of a university hospital.Measurements And Main ResultsThree 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.ConclusionsUse of thiopental to differentiate opioids from cerebrospinal fluid is unreliable. In addition, in some simulated situations, opioids may mask the presence of local anesthetic.

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