Thoracic paravertebral block (TPVB) is a regional anaesthetic technique that produces ipsilateral, segmental, somatic and sympathetic nerve blockade of multiple contiguous thoracic dermatomes. Although it is a safe technique, the needle tip comes close to the pleura during the injection, therefore, increasing the potential for pleural puncture and pneumothorax, especially when performed by inexperienced physicians. With the recently described retro superior costotransverse ligament (SCTL) compartment (using MRI) which has been shown to be in direct continuity with the intervertebral foramen, the investigators propose that the block needle need not pierce the SCTL and lie close to the pleura but can be positioned safely behind the ligament to deposit the local anaesthetic (LA). Therefore, this study aims to describe the sonoanatomy of the retro SCTL compartment and evaluate the block injection technique and sensory dynamics in patients scheduled for video-assisted thoracoscopic surgery (VATS).

During a TPVB the LA is injected into the paravertebral space adjacent to the thoracic vertebra and close to the intervertebral foramen. Traditionally, either landmark or ultrasound-guided, TPVB is performed by depositing the LA anterior to the superior costotransverse ligament (SCTL), i.e. the block needle typically pierces the SCTL to reach the wedge-shaped TPVB space. Although it is a safe technique, the needle tip comes close to the pleura during the injection, therefore, increasing the potential for pleural puncture and pneumothorax, especially when performed by inexperienced physicians.

Recently, with the help of 3D micro computer tomography (CT), Cho T and colleagues have demonstrated in cadavers that the medial part of the SCTL does not end at the base of the transverse process but extends anteriorly to the vertebral body dividing the wedge-shaped paravertebral space into a retro SCTL compartment (posterior to SCTL) and a true paravertebral space (anterior to SCTL). Also, they demonstrated that the SCTL, as it extends to the vertebral body, forms a medial slit near the intervertebral foramen through which the ventral rami enters the true paravertebral space. Furthermore, they highlighted that the retro SCTL space or compartment is in direct communication with the intervertebral foramen and epidural space medially and to the true paravertebral space (through the medial and lateral slits). It is evident from their study that the retro SCTL compartment contains both the divisions of the spinal nerve, i.e., the ventral and dorsal rami, the latter exits the compartment to enter the erector spinae plane. Hence, with this new concept, it appears that the block needle tip needs not to pierce the SCTL as required in a conventional TPVB and LA deposited at the retro SCTL compartment, theoretically can spread to the paravertebral space, intervertebral foramen and the epidural space, to block the spinal nerves and its divisions, and the sympathetic trunks. The investigators believe that the LA injection in the retro SCTL compartment, owing to its close relationship with the intervertebral foramen, thoracic spinal nerve, and the sympathetic trunks, will also produce rapid onset of ipsilateral and or bilateral segmental somatic and sympathetic blockade of the thoracic dermatomes, akin to TPVB. In addition, as the needle tip lies farther away from the pleura (posterior to SCTL) the investigators believe that this technique may also be safer in terms of the potential risk of pleural puncture or pneumothorax than a conventional TPVB.

Inclusion Criteria:

• Phase 1: Healthy volunteers of age 18-60 years
• Phase 2: Patients age 18-80 years scheduled for video assisted thoracoscopic surgery (VATS)

Exclusion Criteria:

• Phase 2: 1. Patient refusal 2. Local skin site infection 3. Coagulopathy 4. History of allergy to local anesthetics