Thoracic Paravertebral Block

The thoracic paravertebral block is a technique of injecting local anesthetic in the vicinity of the thoracic spinal nerves emerging from the intervertebral foramen with the resultant ipsilateral somatic and sympathetic nerve blockade. The resultant anesthesia or analgesia is conceptually similar to a "unilateral" epidural anesthesia. Higher or lower levels can be chosen to accomplish a unilateral, band-like, segmental blockade at the desired levels without significant hemodynamic changes. This technique is one of the easiest and most time efficient to perform, but more challenging to teach because it requires stereotactic needle maneuvering. A certain "mechanical" mind or sense of geometry is necessary for mastering it. This block is performed in our practice most commonly for surgery in patients undergoing breast (mastectomy and cosmetic breast surgery) and thoracic surgery. A catheter can also be inserted for continuous infusion of local anesthetic, even in patients on anticoagulants.

The thoracic paravertebral space is a wedge-shaped area that lies on either side of the vertebral column. Its walls are formed by the parietal pleura anterolaterally, vertebral body, the intervertebral disk, and intervertebral foramen medially, and the superior costo-transverse process posteriorly. The spinal nerves in the paravertebral space are organized in small bundles submerged in the fat of the area. At this location, a thick fascial sheath does not envelop the spinal nerves. Therefore, they are relatively easily anesthetized by injection of local anesthetic. The thoracic paravertebral space is continuous, with the intercostal space laterally, epidural space medially, and the contralateral paravertebral space via the prevertebral fascia. The mechanism of action of a paravertebral blockade includes direct penetration of the local anesthetic into the spinal nerve, extension laterally along with the intercostal nerve, and medial extension through the intervertebral foramina.

The following surface anatomy landmarks are used to identify spinal levels and estimate the position of the transverse processes:

1. Spinous processes (midline)
2. Tips of scapulae (corresponds to T7)
3. Paramedial line 2.5 cm lateral to the midline

TIPS:

• It should be noted that labeling the position of each individual transverse process at the level to be blocked is, at best, a rough estimation.
• It is more practical to outline the midline instead and simply draw the line 2.5 cm lateral to it. All injections will be along this line. Once two first transverse processes are identified, the rest will follow the same cranial-caudal spacing.

Local anesthetic skin infiltration

After cleaning the skin with an antiseptic solution, 6-8 mL of dilute local anesthetic is infiltrated subcutaneously alongside the line where the injections will be made. The injection should be carried out slowly to avoid pain on injection. New needle reinsertions should be made through already anesthetized skin.

TIPS:

• Addition of a vasoconstrictor helps prevent oozing at the site of injection.
• When more than 5-6 levels are blocked (e.g., bilateral blocks), the use of alkalinized chloroprocaine or lidocaine for skin infiltration is suggested to decrease the total dose of long-acting local anesthetic.

Needle insertion The needle is inserted perpendicular to the skin, while constantly paying attention to the depth of needle insertion and the medial-lateral needle orientation. The utmost care should be paid to avoid medial needle direction (risk of epidural or spinal injection). After the transverse process is contacted, the needle is withdrawn to the skin and redirected superior or inferior to walk off the transverse process. The ultimate goal is to insert the needle to a depth of 1cm past the trans-verse process. A certain "give" occasional can be felt as the needle passes through the costotransverse ligament, however, this is nonspecific and should not be relied on.

TIP: The block procedure essentially consists of three maneuvers (click figure on right): Contact the transverse process of the individual vertebrae and note the depth at which the process was contacted (usually 2-4 cm), needle #1. Withdraw needle to the skin level and reinsert at a 10o caudal or cephalad needle angulation. Walk off the transverse process 1 cm deeper to the transverse process and inject 4-5 mL of local anesthetic, needle #2.

The needle can be redirected to "walk off" the superior or inferior aspect of the transverse process (figure below). At levels of T7 and bellow, "walking off" is recommended to reduce the risk of intrapleural placement of the needle. Proper handling of the needle is important both for accuracy and safety. Once the transverse process is contacted, the needle should be regripped so that the gripping fingers allow 1 cm deeper insertion.

TIPS:

• While some authors suggest using a loss of resistance technique to identify the paravertebral space, such a change of resistance is very subtle and nonspecific at best. For this reason, we do not pay attention to the loss of resistance but carefully measure the skintransverse distance and simply advance the needle 1 cm past the process.
• Never redirect the needle medially because of the risk of intra-foraminal needle passage and a consequent spinal cord injury.
• Use common sense in advancing the needle. The depth at which the transverse processes are contacted vary with a patient's body habitus and the level at which the block is performed. The deepest levels are at the high thoracic (T1,2) and low lumbar levels (L4,5) where the transverse process is contacted at a depth of 6-8 cm in average sized patients. The shallowest depth is at the mid-thoracic levels (T5,10) where the transverse processes are contacted at 2-4 cm in an average sized patient.
• Never disconnect the needle from the tubing or syringe with local anesthetic in while the needle is inserted. Instead, use a stopcock to switch from syringe to syringe during injection.

It is almost always beneficial to achieve longer acting anesthesia/analgesia in thoracic paravertebral blockade by using longer acting local anesthetic. Unless lower lumbar levels (L2-5) are planned to be blocked, paravertebral blocks do not result in motor block of an extremity and do not impair patient's ability to ambulate or take care of themselves. In addition, relatively small volumes injected at several levels do not present a concern for local anesthetic toxicity

TIP: In patients receiving multiple level blockade, consider using alkalinized 3-chloroprocaine for skin infiltration to decrease the total dose of the more toxic long-acting local anesthetic. Chloroprocaine is rapidly metabolized by plasma cholinesterase on its absorption.

Placement of the paravertebral block is associated with moderate patient discomfort. Adequate sedation (midazolam 2-4 mg) is always necessary to facilitate placement of the block. We also routinely administer alfentanyl 250-750 µg just before beginning the block procedure. However, excessive sedation should be avoided because the positioning becomes difficult when patients cannot keep their balance in the sitting position. The blockade depends on anesthetic dispersion within the space to reach the individual roots at the level of the injection. The first sign of the blockade is the loss of pin-prick sensation at the dermatomal distribution of the root being blocked. The higher the concentration and volume of the local anesthetic used, the faster the onset can be expected.

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