Factors influencing peak plasma total ropivacaine concentration after local infiltration analgesia in patients undergoing total knee arthroscopy included age, weight, creatinine level, and female sex, according to the results of recent research published in Regional Anesthesia and Acute Pain.
To evaluate total ropivacaine concentrations and ensure they fell within safe pharmacologic limits in patients undergoing local infiltration analgesia, researchers conducted a prospective observational study of 15 participants undergoing elective total knee arthroplasty.
Local infiltration analgesia consisted of 300 mg ropivacaine and epinephrine 5 μg/mL. Venous blood samples were collected throughout the 24 hours after tourniquet release. Total ropivacaine concentrations were analyzed by liquid chromatography-mass spectrometry and compared with demographic and laboratory values.
The median peak ropivacaine concentration of 0.57 μg/mL was recorded between 6 and 24 hours. Across all patients, peaks ranged from 0.32 μg/mL to 0.88 μg/mL. No peak measurements were close to the documented toxic concentration of 2.2 μg/mL, according to the authors.
In linear regression analysis, female sex (P =.002) and body mass index (P =.011) were found to affect peak ropivacaine concentration.
Age was also found to be moderately positively correlated with peak ropivacaine concentration, and weight was moderately negatively correlated with peak ropivacaine concentration. Peak concentration was higher in women (0.77 μg/mL) compared with men (0.46 μg/mL; P =.029).
Overall, the researchers concluded that, “A fixed dose of 300 mg ropivacaine 0.2% given as a single administration for [local infiltration analgesia] in the setting of [total knee arthroplasty] is a technique that produced peak ropivacaine concentrations less than half of the best available quoted toxicity threshold, suggesting it is a safe technique with an incidence of toxicity of 0% (0%-23.9%).”
Miller RJ, Cameron AJ, Dimech J, Orec RJ, Lightfoot NJ. Plasma ropivacaine concentrations following local infiltration analgesia in total knee arthroplasty: A pharmacokinetic study to determine safety following fixed-dose administration [published online January 24, 2018]. Reg Anesth Pain Med. doi: 10.1097/AAP.0000000000000727