Density determination of local anaesthetic opioid mixtures for spinal anaesthesia

Purpose: To measure the density of hyperbaric and isobaric local anaesthetics before and after addition of neuroaxial opioids to define a method for calculating any local anaesthetic/opioid mixture density based on individual component densities. Methods: Density was determined using a volumetric pycnometer (25.0281 +/- 0.0013 ml). The density of local anaesthetics (bupivacaine, lidocaine), opioids (fentanyl, morphine) and multiple anaesthetic/opioid mixtures were measured in quadruplicate and expressed in g . ml(-1), at 37 degrees C (mean +/- SD). Regression analysis was used to derive a formula for calculating the density of any anaesthetic/opioid mixture. Results: Individual components:had the following densities (g . ml(-1)): bupivacaine 0.75%; 1.0252 +/- 0.0001, lidocaine 5%; 1.0249 +/- 0.0001, bupivacaine 0.5%; 0.9994 +/- 0.0001, lidocaine 2%; 1.0000 +/- 0.0001, 50 mu g . ml(-1) fentanyl; 0.9936 +/- 0.0001, and 0.5 mg . ml(-1) morphine; 1.0001 +/- 0.0001. Using regression analysis, linear relationships were demonstrated between density (D) of anaesthetic/opioid mixture and the proportion of anaesthetic in the mixture; (fractional volume of anaesthetic) (r = 0.9999, P < 0.001). The following formula was derived; Density(Mixture) = (D-Local anaesthetic - D-Opioid) X Fractional Volume Anaesthetic + D-Opioid Comparison of calculated and measured densities for multiple clinically relevant anaesthetic/opioid mixtures showed a significant degree of correlation (r = 0.9996, P < 0.001). Conclusion: Density of spinal anaesthetic/opioid mixtures can-be calculated from the component densities and the proportion of anaesthetic in the mixture.