Short-Term Pharmacokinetics of Ropivacaine in Arterial Plasma During Erector Spinae Plane Block in Thoracic and Cardiac Surgery

: Background/Objectives: Local anesthetics (LAs) are used in a variety of different contexts, from loco-regional anesthesia to analgesia. In cardiothoracic surgery, fascial plane blocks are deserving of attention, including erector spinae plane block (ESPB). In this context, ropivacaine is convenient, due to its peculiar pharmacokinetic/pharmacodynamic properties. Nevertheless, LAs can still cause systemic toxicity (LAST), due to erroneous injection and/or variable systemic adsorption/distribution. This interindividual pharmacokinetic variability can be intensified by the tendency to use a fixed ropivacaine dose in ESPB. The aims of this work were investigating systemic exposure to ropivacaine during ESPB in the context of cardiac and thoracic surgery, comparing it to the literature-reported maximum tolerated concentrations, and identifying potential predictors of exposure. Methods: Patients receiving ultrasound-guided injection of 40 mL of ropivacaine 0.375% solution for ESPB were enrolled. Arterial blood was sampled at 5, 15, 30, 45, 60, 120, and 180 min after the injection, and total and free concentrations of ropivacaine were determined by means of LC-MS/MS analysis of arterial plasma. Results: Concentrations showed wide variability, particularly during the first hour post-dose. Significant differences were observed between patients undergoing cardiac and thoracic surgery, with the latter showing higher concentrations, potentially above the cutoff values predictive of LAST. Pharmacokinetic differences were mainly explained by anthropometric (body weight and BSA) and clinical/hemodynamic (NYHA score) characteristics. Conclusions: This study shows that about 5% of patients could reach arterial plasma concentrations of ropivacaine above the cutoff level for LAST after ESPB with a 150 mg dose. Considering patients' weight could be beneficial to maintain exposure below the toxicity cutoff.