Morphine blood concentrations in elderly postoperative patients following administration via an indwelling subcutaneous cannula

The pharmacokinetics of morphine in venous blood after a 5 mg bolus dose via an indwelling subcutaneous cannula were characterised in 22 elderly patients undergoing elective major surgery. In a subgroup of seven patients, the kinetics were also characterised after a second 5 mg dose of morphine administered 180 min after the first dose. Blood morphine concentrations following the single dose were highly variable - the coefficients of variation of C-max, T-max and the AUC up to 180 min (AUC(180)) were 54, 37 and 39%, respectively, with mean values of 86.6 ng.ml(-1) 15.9 min and 3954 ng.ml(-1), respectively. These mean values for the second dose were not statistically different to those of the first dose but were more variable. It was concluded that the injection of morphine via an indwelling subcutaneous cannula results in blood concentrations that are comparable to, and as variable as, those arising from intramuscular injection.