A 79-year-old patient on morphine SR 60 mg twice daily plus breakthrough doses is to receive parenteral diamorphine by syringe driver over 24 hours. What is the most appropriate 24-hour parenteral diamorphine dose?

• A. 60 mg
• B. 80 mg
• C. 100 mg
• D. 120 mg

Answer: (B)

The main idea is converting an existing daily opioid dose from oral morphine to a parenteral diamorphine infusion for a 24-hour syringe driver, aiming for an equianalgesic effect while using a smaller dose because diamorphine is more potent than morphine when given parenterally. In this scenario, the patient is taking oral morphine SR 60 mg twice daily, giving a total 24-hour morphine exposure of 120 mg, plus breakthrough doses. Using typical equianalgesic data for parenteral diamorphine versus morphine, diamorphine is more potent, so the daily diamorphine dose should be less than the daily morphine dose. A commonly used conversion in this context is to divide the daily morphine dose by about 1.5 to obtain the equivalent diamorphine dose. So 120 mg morphine per day ÷ 1.5 equals 80 mg of diamorphine per day for a 24-hour infusion.

Therefore, setting the syringe driver to deliver 80 mg of diamorphine over 24 hours provides an appropriate baseline with room to adjust for breakthrough requests and patient response, especially in an elderly patient where oversedation must be avoided. Choosing a significantly lower dose would risk under-treatment of pain, while a higher dose could lead to excessive sedation or respiratory depression. The key concept is using an equianalgesic conversion that accounts for the higher potency of parenteral diamorphine compared with morphine, then applying it to a 24-hour continuous infusion.