Our aim was to establish a plasma concentration curve for ropivacaine following femoral nerve blockade and to ascertain whether the resulting plasma concentrations differ significantly depending on whether neurostimulation (NS) or ultrasound (US) guidance was used.

Sixteen male and female subjects aged 18 to 80 who were scheduled to undergo unilateral total knee replacement or anterior cruciate ligament reconstruction under general or spinal anaesthesia, and for whom a femoral nerve block was indicated for postoperative analgesia, were enrolled in this prospective, randomised study. Patients were randomised to undergo either US or NS-guidance femoral nerve blocks. All blocks were performed with 20mL of 5mg/mL ropivacaine. Blood samples were drawn before the nerve block and 20, 30, 40, 50, 60, 70, and 80minutes after the block. Plasma levels of ropivacaine were analysed by high performance liquid chromatography (HPLC).

All blocks were successful and no patient showed signs or symptoms of local anaesthetic toxicity. The plasma concentration of ropivacaine peaked at 30minutes in both arms. There was no significant difference in peak levels between US and NS-guidance (0.325±0.186 versus 0.356±0.106μg/mL). Cmax and tmax were very similar between groups (0.364±0.177 versus 0.344±0.127μg/mL, 33.75±15.06 versus 31.25±13.56min for US and NS, respectively).

Plasma concentrations of ropivacaine peak around 30minutes after a femoral nerve block regardless of the technique used. No significant difference was found between US- and NS-guidance.