Comparison of the Spread Level During Thoracic Erector Spinae Plane Block Using Two Volumes of Local Anesthetics

The ESPB requires ultrasound guidance which enables visible local anesthetic spread underneath the erector spinae muscles. The spinalis, longissimus thoracis, and iliocostalis muscles comprise the ES muscles, which run vertically along both sides of the vertebral column from the sacrum up to the skull base. The ESPB can be performed in the cervical, thoracic, and lumbar regions. Among them, upper or mid thoracic ESPB has been used more widely compared to cervical and lumbar regions. Previous cadaveric studies on the ESPB at the T5 level using computed tomography (CT) reconstruction or direct dissection demonstrated the extensive craniocaudal distribution of methylene blue ranging from T1 to T8 vertebral segments deep to the ES muscles and variable involvement of epidural, paravertebral, and intercostal spaces. The ESPB performed at the T2 level of the cadaver demonstrated an injected dye distribution ranging from C4 to T10. Also, 36% of cadavers showed the spread of an injected dye to the ventral, dorsal ramus, paravertebral space, and even the contralateral side. The exact mechanism of action of ESPB remains unclear. A recent study suggested that the analgesic effect of ESPB could be obtained by blocking the ventral and dorsal ramus of the spinal nerves by passing through the costotransverse foramen. However, in clinical practice, we can encounter highly variable clinical outcomes or sensory block after the ESPB. The study of the physical spread of the injected agent can be used to predict the clinical result and elucidate the possible mechanism of action of ESPB.