Serum Bupivacaine Concentration Levels After Intermittent Bolus Injections Through Erector Spinae Plane Peripheral Catheter

Pain control is an important aspect of recovery for patients who undergo thoracic surgery. Over the past several decades, thoracic surgery has shifted from open thoracotomies to video-assisted thoracic surgery (VATS) with the result of less morbidity, fewer postoperative complications and less postoperative pain compared to open thoracic procedures. However, postoperative acute and chronic thoracic pain remain a main clinical issue with VATS and contributes to patient suffering as well as the ability to meet early rehabilitation goals. Chest wall pain has traditionally been managed with epidural analgesia and/or by narcotic medication administration (such as morphine, hydromorphone and oxycodone). These modalities, however, carry a high risk of serious effects such as epidural hematomas, dural puncture, pneumothorax, hypotension, infection as well as cough suppression, constipation, drowsiness and mental status changes. Additionally, patients who are on anticoagulants or antiplatelet therapy may not be eligible for epidural analgesia. Furthermore, the short- and long-term analgesic benefits of epidural analgesia has been shown to be inconsistent in the setting of VATS.

A relatively new technique used for post-operative pain control in thoracic surgery is the use of fascial plane blocks, specifically the erector spinae plane block (ESPB). The ESPB was first described in 2016 by Forero et al. in 4 case reports: 2 patients with chronic thoracic neuropathic pain and 2 patients with acute postoperative chest wall pain 8. Since that time the ESPB has been widely adopted as a regional technique used in thoracic, breast, abdominal and lumbar surgery. This block is achieved by depositing long-acting local anesthetic medication in the fascial plane deep to the erector spinae muscles and superficial to the transverse process, to achieve a distribution along several vertebral rami of the spinal nerves and intercostal spaces. Radiographic evidence suggests that local anesthetic injected at the ESP spreads both cranially and caudally as the plane is continuous along the vertebral column. These rami carry visceral motor, somatic motor and sensory information to and from the skin and deep muscles of the back.

In thoracic surgery, the ESPB has shown to decrease opioid consumption and pain scores in patients who undergo VATS within the first 24 hours when compared to no block and also when compared to other fascial plane blocks such as the serratus plane block. In the literature thus far, complications are rare.

Although the literature has quickly grown regarding erector spinae blocks over the last four years, specific information regarding dosing regimens for both single injection and intermittent bolus dosing through an erector spinae catheter remains limited. Most of the current information from the erector spinae block lies in case reports. In reviews of the current practices, it appears that ropivacaine and bupivacaine are the most commonly used local anesthetics with injection volumes ranging from 20 mL to 40 mL and concentrations ranging from 0.25%-0.5%. Most case reports describe single injection, while approximately 20% describe dosing either intermittently or continuously through a catheter. It is thought that because this block is a fascial plane block its' success depends, in part, on the volume and concentration of local anesthetic injected in the erector spinae plane to achieve maximal craniocaudal dermatomal distribution. It is in these scenarios that clinicians must balance achieving maximal analgesic benefit from this block with safety concerns for local anesthetic systemic toxicity (LAST).

There are no documented cases described of a patient experiencing LAST from an erector spinae block or erector spinae nerve catheter, however there is some information regarding the safety of similar fascial plane blocks. Convulsions following bilateral ultrasound guided transversus abdominis plane blocks following cesarean analgesia have been described. Trabelsi et al. describe the pharmacokinetics of bupivacaine after bilateral transversus abdominis plane blocks and found that mean peak serum bupivacaine concentrations was 30 minutes from the time of injection. However, during this study, three subjects had serum bupivacaine concentrations above the quoted toxic threshold. Furthermore, data analyzing serum concentration of local anesthetic overtime through the use of catheters is also lacking. In a study of pediatric patients with continuous paravertebral infusions it was found that serum concentration of bupivacaine was >3 mcg/mL in several infants at 30-48 hours. These studies emphasize the need for better understanding local anesthetic uptake from fascial planes blocks.