Plasma morphine concentrations and clinical effects after thoracic extradural morphine or diamorphine

Twenty-seven patients undergoing thoracotomy received either morphine sulphate 2 mg or diamorphine hydrochloride 2 mg by thoracic extradural injection for postoperative analgesia. Arterial plasma morphine concentrations were measured by specific radioimmunoassay, and the analgesic, respiratory and biochemical effects noted. The plasma morphine concentrations were significantly greater after extradural diamorphine than after extradural morphine in the first 30 min after injection. The maximum increase in plasma morphine concentration was significantly (P less than 0.02) greater after extradural diamorphine, and mean peak values occurred at 5 and 10 min for diamorphine and morphine, respectively. There were significant decreases in respiratory rate and plasma cortisol concentration with maximum effects between 90 and 180 min after the extradural injection. The analgesia produced by these doses was inadequate. The role of lipophilicity is discussed.