Premixed solutions of diamorphine in ropivacaine for epidural anaesthesia: A study on their long-term stability

Background. Local anaesthetics and opioid mixtures are commonly used to provide anaesthesia or analgesia during the perioperative period. In order to facilitate their preparation and storage it is necessary to establish the stability of such solutions. Methods. In our study, diamorphine was added to ropivacaine 0.2% 200-ml polybags to give a concentration of 25 μg ml^-1 and to ropivacaine 1% 50-ml syringes to give a concentration of 45 μg ml^-1. The polybags and syringes were stored at 40°C, 21°C and 4°C for up to 120 days. Samples were taken during this period for measurement of diamorphine and ropivacaine content and pH of the solutions. Results. We found that the storage temperature and the initial concentration influenced the rate of degradation of diamorphine in both the polybags and the syringes. In the syringes, 10% degradation of diamorphine [T (0.9)] was: 6 days at 40°C, 16 days at 21°C and 30 days at 4°C. In the polybags, diamorphine T (0.9) was 6 days at 40°C, 28 days at 21°C and 70 days at 4°C. Conclusions. It is feasible to manufacture such solutions in Pharmacy aseptic units and to store them for up to 1 month for routine use in epidural infusions.