[Patient-controlled analgesia: effect of adding continuous infusion of morphine]

This double blind study aimed to assess the effects of a continuous intravenous (i.v.) infusion of morphine added to an intermittent bolus patient controlled analgesia on morphine demand and related side-effects. Patients scheduled for abdominal and thoracic surgery (ASA 2 or 3) were randomly allocated postoperatively to three groups (n = 10 each): group 1 were given i.v. boluses of 2 mg of morphine (lockout interval = 15 min); the other two groups were given the same boluses as well as a continuous i.v. infusion of either 1 mg.kg-1 of morphine (group 2) or 2 mg.kg-1 (group 3). Pain was assessed with a visual analog scale before starting analgesia, and after 1, 2, 3, 4, 8, 16, 24 and 36 h. Total and bolus morphine doses were recorded at the same time. Breathing rate and the level of sedation were measured every hour and blood gases every time 40 mg of morphine had been consumed. Morphine administration was stopped if breathing rate decreased to less than 10 c.min-1, the patient became too sedated, or PaCO2 rose to more than 45 mmHg. Pain scores were similar in the three groups. Total amounts of morphine were higher in groups 2 (56.8 +/- 23.8 mg) and 3 (116.2 +/- 41.8 mg) compared with group 1 (38.2 +/- 17.8 mg) (p < 0.05). Morphine administration was stopped in 5 patients in group 3 and in 1 in group 2 because PaCO2 had risen to more than 45 mmHg. Therefore, a continuous i.v. infusion is not required in patients receiving PCA, all the more so as this has deleterious respiratory effects.