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Canadian Journal of Anesthesia, Vol 45, 341-346, Copyright © 1998 by Canadian Anesthesiologists' Society

ARTICLES

Density determination of local anaesthetic opioid mixtures for spinal anaesthesia

GM Hare and JC Ngan
Department of Anaesthesia, Dalhousie University, Queen Elizabeth II Health Sciences Centre, Nova Scotia, Canada. ghare@compuserve.com

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


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Copyright © 1998 by the Canadian Anesthesiologists' Society.