Stability of bupivacaine hydrochloride with diamorphine hydrochloride in an epidural infusion

The stability of diamorphine (0.02 mg/ml as the hydrochloride) in 250 ml bupivacaine hydrochloride (0.15% wt/vol infusion) was studied by high pressure liquid chromatography at temperatures in the range 7 to 45 degrees C. Diamorphine hydrochloride was degraded by approximately 0.13% per day at 7 degrees C. No bupivacaine hydrochloride degradation was detectable during the study. The storage life of the combination at 7 degrees C, based on the lower 95% confidence limit of the time to 5% diamorphine hydrochloride degradation, was 14 days. The stability at 25 degrees C was adequate to allow transport and administration over 24 h at ambient temperature. Stability was also maintained for at least 24 hr at 32 and 45 degrees C. Infusion of the mixture with an ambulatory infusion pump which uses a standard polyvinyl infusion bag is therefore possible. A study of its compatibility with different infusion pump medication reservoirs was not undertaken. The drugs were also stable on frozen storage at -18 degrees C for up to 6 months.