Key factors governing spinal cord opioid bioavailability in the management of acute pain

Abstract

Although there is no either ideal analgesic or route of neuraxial administration, clinicians alike continue to search for compounds with qualities which may approach this idea. However, it´s a demonstrated fact that spinal administration of an opioid drug does not always guarantee segmental and selective analgesia into the spinal cord. This point is valid due to partial reuptake of the drug to systemic blood circulation reaching specific brain opioid receptors, rather than the differences are explained by variations in the clearance rate from the cerebrospinal fluid. Published evidence from either human or animal experimental studies indicates that bioavailability in the spinal cord biophase is negatively correlated with liposolubility. Therefore, opioid spinal cord bioavailability is higher for hydrophilic opioids like morphine, than for lipophilic ones such as fentanyl, sufentanil or alfentanil.