Diamorphine and bupivacaine mixtures: an in vitro study of microbiological safety

A small survey confirms that epidural infusions are often prepared by clinicians and solutions may be changed each day because of fears of microbiological contamination. We have assessed the microbiological safety of six mixtures of diamorphine (0.01-1.0%) and bupivacaine (0.1-0.5%) representing the spectrum of clinically useful concentrations for use as extradural infusions.The solutions were studied for antibacterial activity against common contaminants of fluids: Escherichia coli, Pseudomonas aeruginosa, Enterococcus faecalis and a coagulase negative Staphylococcus sp. A saline control was included, Challenge experiments used an inoculum of approximately 5.0 x 10(6) cfu/ml. Mixtures were incubated at 30 degrees C for up to 7 days. Viable counts of all organisms decreased with time for all the formulations tested. Formulations containing 0.5% bupivacaine were rapidly bactericidal, and increasing diamorphine concentrations increased this effect. Formulations with 0.5% bupivacaine had more activity than those with 0.1% bupivacaine. Mixtures of diamorphine and bupivacaine in concentrations used clinically have bactericidal activity against commonly encountered skin organisms. The common practice of changing every day epidural infusions containing these drugs is unnecessary. (C) 1998 International Association for the Study of Pain. Published by Elsevier Science B.V.