For pharmacokinetic analysis, a serial of blood samples of patients was collected postoperatively after the extubation from a venous catheter (5mL each) into an EDTA tube (BD, Sao Paulo/Brazil) in the periods as follows: 0, 0.08, 0.25, 0.5, 1, 3, 6, 12, 18, 24 and 36 hrs. Blood samples were centrifuged at 2800 g for 20 minutes, plasma was transferred to labeled polyethylene vials and frozen at -80ºC until drug plasma assay.