Morphine is considered the gold standard of therapy for treatment of acute and chronic pain. In regards to intrathecal drug therapies, most physicians will trial their patients on morphine and subsequently institute appropriate measures.
In conditions where morphine is not acceptable or is not yielding the desired clinical effects, clinicians will consider changing to fentanyl citrate. When doing so, it is important to carefully consider both potency and characteristics.
Fentanyl differs from morphine both in regard to clinical response as well as pharmacological characteristics. Morphine is hydrophilic, and fentanyl is very lipophilic, and there is the issue of potency. There is concern among clinicians about changing from morphine to fentanyl because of the fact that fentanyl is demonstrably more potent: in acute pain 100 times more potent and 50 times more potent for chronic pain than morphine.
Due to fentanyl’s lipophilic properties, the distribution of this drug is more focal as related to the catheter tip. Tissues within the intrathecal space are very (dense in fat, fat saturated, fatty), with a great capacity for absorbing fentanyl, consequently, fentanyl does not travel significantly within the intrathecal space.
Once the decision has been made to change therapies, I recommend a 1 to 25 conversion rate. Although the “textbook” conversion recommendation for chronic pain is 1 to 50, I consider that to be too conservative, based upon the comparative characteristics of the drugs themselves. A 1 to 25 rate is still conservative…but I strongly suggest that physicians achieve their own comfort zone as they gain experience with the conversion process. And, of course, weigh the individual differences and concerns of each patient when determining what conversion level will best serve.
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