Addition of opioids alters the density and spread of intrathecal local anesthetics?: An in vitro study

Purpose: To determine whether the addition of opioids alters the density and spread of intrathecal local anesthetics in vitro. Methods: In Part I, the densities of hyperbaric bupivacaine 0.75% (HB), hyperbaric lidocaine 5% (HL) and isobaric bupivacaine 0.5% (IB) with and without morphine (M), and fentanyl (F) were measured at 22 degrees C. In Part II a model was constructed utilizing a column containing a solution similar in composition to cerebrospinal fluid (CSF) at 37 degrees C. The various local anesthetic-opioid solutions, coloured with crystalline methylene blue dye, were injected at 22 degrees C into the column at a controlled rate through a spinal needle. The direction and extent of spread of the injectates were compared. Results: The relative densities of the five solutions were: HB = HL > IB > M > F. The addition of fentanyl to IB reduced the density of the final solution (P < 0.05). In the model, IB alone and IB with morphine showed mainly downward spread, with the addition of fentanyl to IB resulting in upward movement (P = 0.004). The hyper baric local anesthetics moved downward with or without opioids. Conclusion: The addition of fentanyl reduces the density of IB in vitro and alters its movement in simulated CSF: This may prove to be important in predicting the level of spinal block in clinical practice.