A Volunteer Study to Determine the Anesthetic and Analgesic Effect of the Erector Spinae Plane (ESP) Block

Erector spinae plane (ESP) block is a new regional nerve block technique that involves a percutaneous injection of local anesthetic as a bolus through a needle or catheter into the fascial plane between the erector spinae muscle and the transverse processes in the upper or mid back. Injection at the level of the T4-5 transverse process is indicated for chest wall pain relief while injection at the T8-9 level is for abdominal wall pain control. This block aims to relieve postoperative pain but is not intended to provide anesthetic blockade for surgery. Preliminary case reports and some randomized controlled trials show encouraging analgesic effect of ESP block for breast surgery, rib fracture, thoracotomy, sternotomy, epigastric hernia repair, open abdominal surgery, laparoscopic abdominal surgery, radical retropubic prostatectomy and hip arthroplasty. Although effective, controversy remains regarding the accuracy and consistency of analgesic success following ESP block.

Currently, it is unclear whether block inconsistency is due to improper injection technique or the mechanism of action. Presumably, local anesthetic following an ESP block injection will find its way antero-medially through the costotransverse foramen and / or inter transverse ligaments to enter the thoracic paravertebral space or epidural space to block the ventral rami of the thoracic spinal nerves. It may also spread laterally to reach the neighbouring intercostal nerves.

At this time, it is unclear whether cadaver injection studies showing dye spread to these anatomical spaces can fully explain the observed clinical neural blockade. Also a number of cadaver studies show inconsistent and conflicting dye spread results, i.e., failure to reach the thoracic paravertebral space. Although some suggest that an ESP block also provides sympathetic blockade thus may provide visceral pain relief, no study has evaluated the validity of such a claim or the extent of sympathetic blockade.

The mechanisms of action for an ESP block are currently unclear. Although most case reports describe analgesic effects of ESP block for anterior chest and abdominal surgery presumably as a result of blockade of the ventral rami of the spinal nerves in the thoracic paravertebral spaces, it is more likely that local anesthetic injected in the ESP will reliably anesthetize the dorsal rami traversing the erector spinae muscle in the back. This is supported by preliminary clinical analgesic success of ESP block for thoracic spine surgery and lumbar spine surgery. The consistency of blocking the ventral and dorsal rami following an ESP block injection is unknown at this time.

Investigators' clinical experience further confirms that an ESP injection provides variable analgesic results and an objective sign of sensory anesthesia to pinprick or ice is not consistently evident. Currently no study has examined the optimal technique of ESP block injection, peak blood local anesthetic level after injection, the effect of spine movement (i.e., gliding movement of erector spinae muscle on the transverse processes) on neural blockade, and the consistency of this block technique. Also, most case reports on surgical patients did not report the onset, progression or resolution of ESP blockade or the extent of sensory and sympathetic blockade. From a clinical perspective, it would be very helpful to determine the extent and duration of clinical neural blockade following an ESP injection with different local anesthetic doses and such detailed examination is possible only in a carefully conducted volunteer study.