Prolongation of epidural anaesthesia in dogs with bupivacaine in a lipid emulsion

An aqueous solution and a lipid emulsion of bupivacaine were administered epidurally in doses of 1-8 mg/kg to six beagle dogs following a randomised two-phase crossover design. The aqueous solution was absorbed rapidly and the mean (sd) peak venous plasma concentration of bupivacaine, 1-4 (0-4) μg/ml, was detected after five minutes. After administration of the lipid emulsion, the peak plasma concentration of bupivacaine, 0-6 (0-2) μg/ml, was detected after 30 minutes. The mean (sd) t 1/2 (β) of the aqueous preparation was 149-1 (32-G) minutes, and of the lipid emulsion 119-2 (34-0) minutes. Both preparations had a similar bioavailability. The mean time to the onset of motor block after the administration of the aqueous solution, 2-3 (2-2) minutes, was significantly shorter (P=0-028) than after the administration of the lipid emulsion, 9-4 (1-9) minutes, and the duration of the motor block induced by the lipid emulsion, 217-6 (26-2) minutes, was significantly longer (P=0-043) than for the aqueous solution, 158 (48-8) minutes. During anaesthesia, the plasma concentrations of bupivacaine ranged between 1-3 and 0-2 μg/ml. Non-significant changes in systolic blood pressure and heart rate were observed which coincided with the peak plasma concentrations of bupivacaine.