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°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, 50μg·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) × 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.