Ropivacaine Pharmacokinetics in ESP Blocks

The erector spinae plane (ESP) block was first described in 2016 as a novel fascial plane block that provided analgesia for thoracic neuropathic pain. Since then hundreds of articles have been published that have reported use of the ESP block for indications such as rib fractures, breast surgery, abdominal surgery, and even shoulder surgery. It has also been studied in thoracic surgery and our clinical experience confirms that patients undergoing video-assisted thoracoscopic surgery (VATS) or robot-assisted thoracic surgery experience satisfactory analgesia with ESP blocks. Because the block location is further from the neuraxis than both epidural and paravertebral blocks, ESP blocks have been suggested as a safer alternative to these older blocks but safety data have not yet been generated. In particular, the risk of local anesthetic systemic toxicity (LAST) has not been studied in ESP blocks. While the pharmacokinetics of ropivacaine used for thoracic paravertebral blocks have been established, similar studies have yet to be performed for the newer ESP block. Of particular concern for ESP blocks are two factors not present in some other blocks with established safety: 1). significant intercostal spread has been noted in anatomical studies, which could put patients at risk for LAST and 2). some of the proposed dosing regimens involve the intermittent injection of large bolus doses of local anesthetic. While measurement of arterial plasma levels is useful and necessary to study the safety of ropivacaine given in ESP blocks, the measurements alone do not allow for prediction of plasma levels that would occur in populations as a whole. Nonmem is a population pharmacokinetic application that provides estimates of mean parameters and residual variability in pharmacokinetic values across populations and has been shown to generate better estimates than the two-stage approach. Nonmem will be used in this study to predict pharmacokinetics in populations with different characteristics than the one being studied here, which would create generalizable results.