For some readers, much of what is stated here is review – while to others, it is enlightening. This article describes the properties of fentanyl citrate and the appropriate dosing for converting patients from morphine to fentanyl for intrathecal treatment of chronic pain.
Patented by Janssen Pharmaceuticals in 1963, fentanyl is a potent narcotic analgesic primarily used for spinal analgesia. This drug is a congener of meperidine and has poor hypnotic effects. The metabolites of fentanyl are inactive and make an ideal treatment for patients with renal conditions. The drug possesses a high solubility, with a maximum concentration of 25,000 mcg per milliliter. Fentanyl is extremely lipophilic in comparison to morphine sulfate (580:1) that results in a more rapid onset. This property limits its spinal distribution; a fact that must be considered in the treatment of pain at the site of nociception. A review of the literature indicates fentanyl to be 100 times more potent than morphine sulfate for acute pain and 40 to 50 times more potent in chronic pain.
Conversion from Morphine
Often when a physician contacts Hartley Medical for drug conversion assistance, we discuss pertinent literature and then employ mathematical calculations to derive a beginning intraspinal fentanyl dose. We frequently modify our specific calculation of the dosage based on my experience with patients treated with fentanyl. In my many conversations with physicians having extensive experience in fentanyl, these practitioners favored converting patients from morphine to fentanyl utilizing smaller ratios, such as 1/10 to 1/25 of the daily morphine sulfate dose.
With such a broad conversion range, clinicians unfamiliar with intraspinal administration of fentanyl should seek consultation prior to drug conversion. With regard to dosing, the spectrum is also wide for pain providers. Some physicians have reported dosing as low as 4 mcg per day and others, approximately 5,000 mcg per day.
Clinically, fentanyl citrate has been administered intraspinally in combination with many pharmaceuticals. However, there is little published information regarding the stability of fentanyl in combination with such agents. A 1992 study showed fentanyl 20 mcg/mL to be stable with bupivacaine 1.25 mg/mL in normal saline. In a recent publication, David Shields of the former Elan Pharmaceuticals (now Jazz Pharmaceuticals) found fentanyl to be stable for four weeks when combined with ziconotide. His study is consistent with Hartley Medical’s post-infusion analysis indicating that fentanyl citrate is stable when combined with other intraspinal pharmaceuticals.
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