To the Editor:
I read with interest the exchange of letters between Taylor and Cohn . I admit that I was not aware of the fact, highlighted during their exchange, that epidural injection of sterile water produced pain. Nor was I aware that this phenomenon was common to fluids that differed markedly from normal tonicity . This information, however, helps me to explain an interesting clinical observation.
When placing an epidural catheter preoperatively in an awake patient for postoperative pain management, I sometimes tested the patency of the catheter with undiluted fentanyl. This always resulted in severe pain at the site of injection irrespective of the level of the catheter, the speed of injection, or the volume of fentanyl injected. Therefore, I abandoned this practice without being able to explain this phenomenon.
After reading the letters of Taylor and Cohn, I suspected that this phenomenon could be related to the tonicity of fentanyl. Further inquiry into the matter revealed that fentanyl is a hypotonic drug with an osomolality of -10 to + 10 mOsm/kg and a specific gravity of 0.999 (Steven Irvin, Wyeth Laboratories, St. Davids, PA; personal communication, 1994). Based on this information, hypotonicity is most likely responsible for the pain associated with the epidural injection of undiluted fentanyl. Because the drug is usually mixed with normal saline or local anesthetics when injected epidurally, this unpleasant characteristic of undiluted fentanyl is, fortunately, rarely a problem in routine clinical practice.
Medhat Hannallah, MD, FFARCS
Department of Anesthesia, Georgetown University Medical Center, Washington, DC 20007
1. Taylor DR [letter], Cohn AI [response]. Water-saline controversy or deficient regional anesthesia training? Anesth Analg 1994;78:1203-4.
2. Bromage P. Epidural analgesia. Philadelphia: WB Saunders, 1978:200.