Plasma bupivacaine concentrations following orbital injections in cats

Objective: To determine plasma bupivacaine concentrations after retrobulbar or peribulbar injection of bupivacaine in cats. Study design: Randomized, crossover, experimental trial with a 2 week washout period. Animals: Six adult healthy cats, aged 1-2 years, weighing 4.6 ± 0.7 kg. Methods: Cats were sedated by intramuscular injection of dexmedetomidine (36-56 μg kg^-1) and were administered a retrobulbar injection of bupivacaine (0.75 mL, 0.05%; 3.75 mg) and iopamidol (0.25 mL), or a peribulbar injection of bupivacaine (1.5 mL, 0.5%; 7.5 mg), iopamidol (0.5 mL) and 0.9% saline (1 mL) via a dorsomedial approach. Blood (2 mL) was collected before and at 5, 10, 15, 22, 30, 45, 60, 120, 240 and 480 minutes after bupivacaine injection. Atipamezole was administered approximately 30 minutes after bupivacaine injection. Plasma bupivacaine and 3-hydroxy-bupivacaine concentrations were determined using liquid chromatography-mass spectrometry. Bupivacaine maximum plasma concentration (C_max) and time to C_max (T_max) were determined from the data. Results: The bupivacaine median (range) C_max and T_max were 1.4 (0.9-2.5) μg mL^-1 and 17 (4-60) minutes, and 1.7 (1.0-2.4) μg mL^-1, and 28 (8-49) minutes, for retrobulbar and peribulbar injections, respectively. In both treatments the 3-hydroxy-bupivacaine peak concentration was 0.05-0.21 μg mL^-1. Conclusions and clinical relevance: In healthy cats, at doses up to 2 mg kg^-1, bupivacaine peak plasma concentrations were approximately half that reported to cause arrhythmias or convulsive electroencephalogram (EEG) activity in cats, and about one-sixth of that required to produce hypotension.