Pharmacokinetics of Morphine and Oxycodone in Frail Elderly Undergoing Cardiac Surgery

* Single centre observational cohort study. * 20 older patients undergoing cardiac surgery who receive standard of care pain treatment. * Postoperative ICU: blood sampling to determine morphine and metabolite (M3G, M6G, noroxycodone and oxymorphone) pharmacokinetics * After ICU discharge on general ward: blood sampling to determine morphine, oxycodone and metabolite (M3G, M6G, noroxycodone and oxymorphone) pharmacokinetics * During study monitoring of pain scores, total opioid consumption, side effects (e.g. nausea, vomiting, pruritus), sedation and delirium scores.