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Serum fentanyl levels (ng/mL) at time (h) since placement of a fentanyl patch (50 µg/h). Boxes represent bupivacaine-treated pigs; circles indicate saline controls. Blood sampling occurred at the following times (h; mean ± 1 SD) after fentanyl patch placement: first sampling, 21.1 ± 2.5 h; second sampling, 47.4 ± 3.5 h; and third sampling, 73.2 ± 2.7 h.
