Density determination of local anaesthetic opioid mixtures for spinal anaesthesia

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.

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.

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 micrograms.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; DensityMixture = (DLocal anaesthetic-DOpioid) x Fractional Volume Anaesthetic + DOpioid. 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).

Density of spinal anaesthetic/opioid mixtures can be calculated from the component densities and the proportion of anaesthetic in the mixture.