Intrathecal opioid conversions: The importance of lipophilicity

An intrathecal opioid infusion using an implanted programmable pump is frequently used for controlling refractory pain. Morphine, which is the only opioid presently approved by the FDA for use in such pumps, occasionally fails to work or is not tolerated by the patient; therefore other opioids are considered for infusions. When switching from one drug to another, it is important to consider not only equianalgesic dose conversions, but also lipophilicity. We report on three cases that demonstrate the need to use only a fraction of the equianalgesic dose when shifting from lipophilic to nonlipophilic opioids in such infusions. The concept of equianalgesic dosing of opioids is a familiar one to pain specialists. With the increasingly frequent use of intrathecal infusions, pain specialists must be aware of the potential significance of lipophilicity when changing from one opioid to another. Cerebrospinal fluid drug retention time can be prolonged, thus creating the potential for unintended sedation or respiratory depression when switching from fentanyl or sufentanil (highly lipid soluble narcotics) to morphine or hydromorphone (poorly lipid soluble) if the equianalgesic conversion is used without consideration of lipophilicity. To our knowledge, no one has previously reported on these potential dangers. We report on three cases demonstrating the importance of lipid solubility in intrathecal narcotic infusions.