Material and Method: Forty-three patients scheduled for unilateral total knee arthroplasty (TKA) were enrolled in the prospective observational study. The dose of bupivacaine for spinal anesthesia was adjusted by the attending anesthesiologist. The single-shot femoral nerve block (FNB) and periarticular injection (PAI) were performed with 20 ml of 0.5% bupivacaine and 20 ml of 0.25% bupivacaine respectively. Venous blood samples from antecubital vein were collected at 60 minutes after femoral nerve block and at the time before periarticular injection, then at 15, 30, 45, and 60 minutes afterwards. Plasma bupivacaine concentrations were analyzed, using a high performance liquid chromatography with tandem mass spectrometry.

Results: Ten males and 32 females, ASA I-II were included. The highest median plasma concentration was 586.22 ng/ml (min = 245.39, max = 1,614.36) at 45 minutes after periarticular injection. The maximum plasma bupivacaine concentration was 1,709.71 ng/ml at 60 minutes after periarticular injection. No clinical toxicity was encountered.

Conclusion: The plasma concentration of bupivacaine in patients performed periarticular injection with 20 ml of 0.25% bupivacaine after spinal anesthesia and single-shot femoral nerve block with 20 ml of 0.5% bupivacaine is below the plasma toxic level.

Keywords: Plasma bupivacaine level, Femoral nerve block, Periarticular block