Comparison between epidural fentanyl, sufentanil, carfentanil, lofentanil and alfentanil in the rat: analgesia and other in vivo effects.

The effects of epidural fentanyl, sufentanil, carfentanil, lofentanil and alfentanil were compared in the rat. All five opiates produced analgesia with the relative order of potency being lofentanil greater than carfentanil greater than sufentanil greater than fentanyl greater than alfentanil, and the relative order of duration at comparative doses lofentanil greater than sufentanil greater than carfentanil greater than fentanyl greater than alfentanil. Because epidural administered opiates still exert some effects within the brain, specificity ratios (defined as the ratio between the ED50s for a prolongation of the tail-withdrawal reaction latency and a blockade of the pinna reflex) were calculated. The relative order of specificity between the five opiates was fentanyl greater than lofentanil greater than sufentanil greater than carfentanil greater than alfentanil. These results are discussed in the context of the search for a safe and sufficiently long-acting lipid soluble epidural analgesic. It is concluded that of these five opiates, sufentanil appears to be the best drug of choice for epidural analgesia.