[Influence of the volume of peridural morphine bolus injections on morphine concentration in the cisterna magna of the dog].

14 dogs received randomly a bolus injection of either 2 mg morphine in 10 ml isotonic saline solution or 2 mg morphine in 1 ml isotonic saline solution epidurally at T6. An epidural infusion of 0.16 mg morphine/0.06 ml/h was applied immediately following the 1 ml bolus injection of morphine. Cisternal cerebellomedullary CSF samples were taken at varying intervals to determine free morphine immunoreactivity by radioimmunoassay. Within 20 min after the 10 ml bolus injection the peak CSF morphine concentration of 3594 +/- 910 ng/ml was reached. Following this, CSF morphine levels decreased exponentially and after 48 h 8 +/- 3 ng/ml were measured. The peak CSF morphine concentration of 139 +/- 51 ng/ml was, however, reached only after 2 h following the 1 ml bolus injection plus the infusion treatment. Despite the additional infused amount of 7.68 mg over 48 h the CSF morphine concentrations decreased also exponentially and after 48 h 16 +/- 4 ng/ml were measured. One may conclude, therefore, that the risk of respiratory depression is negligible under epidural infusion treatment following an initial "Low-Volume"-bolus injection of 2 mg morphine, a recommendable method for treating post-operative pain. High-volume bolus injections of morphine should generally be abandoned in epidural treatment of pain.