Thoracic paravertebral block (PVB) is a well-established technique for perioperative analgesia in patients having thoracic, chest wall, or breast surgery or for pain management with rib fractures. Ultrasound guidance can be used to help identify the paravertebral space (PVS) and needle placement, and to monitor the spread of the local anesthetic. Importantly, interference of the closely related osseous structures with ultrasound imaging and the proximity of the highly vulnerable neuraxial structures make it imperative that all well-described technique precautions are exercised, regardless of the ultrasound imaging. In this chapter, we describe general principles of thoracic PVB, rather than propose a cookbook with specific techniques and step-by-step directions. The reader is advised to use the anatomic information and techniques presented here to devise an approach in line with own clinical experience.
Anatomy and General Considerations
Thoracic PVB is accomplished by an injection of local anaesthetic into the PVS, which contains thoracic spinal nerves with their branches, as well as the sympathetic trunk. Anatomically, the PVS is a wedge-shaped area positioned between the heads and necks of the ribs (Figure 1). Its posterior wall is formed by the superior costotransverse ligament, the anterolateral wall is the parietal pleura with the endothoracic fascia, and the medial wall is the lateral surface of the vertebral body and intervertebral disk. The PVS medially communicates with the epidural space via the intervertebral foramen inferiorly and superiorly across the head and neck of the ribs. Consequently, injection of local anesthetic into the PVS space often results in unilateral (or bilateral) epidural anesthesia. The cephalad limit of the PVS is not defined, whereas the caudad limit is at the origin of the psoas muscle at L1. Likewise, the PVS space communicates with the intercostal spaces laterally, leading to the spread of the local anesthetic into the intercostals sulcus and resultant intercostal blockade as part of the mechanism of action (Figure 2).
Transverse In-Line Technique
Similar to techniques not using ultrasound guidance, the patient can be positioned in the sitting or lateral decubitus position with the site of surgical interest uppermost. Either a linear or phased array (curved) transducer can be used however, latter may be used only in slim patients. A high-frequency (10-12 MHz) transducer is used to obtain images in the axial (transverse) plane at the selected level, with the transducer positioned just lateral to the spinous process (Figure 3). For most patients, the depth of field is set about 3 cm to start scanning. The transverse processes and ribs are visualized as hyperechoic structures with acoustic shadowing below them (Figure 3). Once the transverse processes and ribs are identified, the transducer is moved slightly caudad into the intercostal space between adjacent ribs to identify the thoracic PVS and the adjoining intercostal space. The PVS appears as a wedge-shaped hypoechoic layer demarcated by the hyperechoic reflections of the pleura below and the internal intercostal membrane above (Figure 3). The hyperechoic line of the pleura and underlying hyperechoic air artifacts move with respiration. The goal of the technique is to insert the needle into the PVS and inject local anesthetic, resulting in downward displacement of the pleura, indicating proper spread of the local anesthetic (Figure 3). Although ultrasound-guided thoracic PVB is essentially a superficial, simple technique, visualization of the needle and its tip and control of its path at all times are essential to avoid inadvertent pleural puncture or entry into the intervertebral foramen. For this reason, in-plane needle insertion with direction towards the centroneuraxis is probably best avoided in obese patients. Insertion of a catheter through the needle placed in the PVS carries a risk of catheter (mis)placement into the epidural or mediastinal space, or through the pleura into the thoracic cavity (Figure 4).
Several recommendations are suggested to decrease the risk of potential complications with ultrasound-guided thoracic PVB:
In-plane advancement of the needle should be reserved only for patients who image well; visualization of the needle path at all times is crucial to reduce the risk of needle entry in unwanted locations (pleura, neuraxial space).
Orienting the bevel of the Tuohy needle tip away from the pleura may reduce the risk of penetrating the pleura.
A pop often is felt as the needle penetrates the internal intercostal membrane, alerting the operator of the needle position in the PVS.
Aspiration for blood should always be carried out before injection.
Local anesthetic (15-20 mL) is injected slowly in small increments, avoiding forceful high-pressure injection to reduce the risk of bilateral epidural spread.
Figure 1: A schematic representation of the thoracic paravertebral space and its structures of relevance to paravertebral block.
Figure 2: (A) Three-dimensional magnetic resonance reconstruction image of the spread of local anesthetic (5 mL) within the paravertebral space. (B) A computed topography image of the local anesthetic (LA) spread in the thoracic paravertebral space. The contrast is seen spreading in the medial to lateral and anterior to posterior direction underneath the parietal pleura.
Figure 3: An ultrasound-guided thoracic paravertebral block. (A) Transducer position and needle orientation. (B) Corresponding ultrasound image. (C) Patient is in oblique lateral position. The outlined surface landmarks are: (>) = spinous processes; gray arrow = left scapula; white arrow = paramedian line 3 cm (transverse processes). (D) Needle insertion path and correct injection of local anesthetic. TP = transverse process.
Longitudinal out-of-plane technique
Out-of-plane ultrasound-guided thoracic paravertebral block is the most common approach to PVB in our practice (Figure 5). We feel that this technique is inherently safer then in-line techniques as the needle path is not towards the neuraxis. In addition, this technique is analogous to the true-and-tried surface-based techniques, except that with ultrasound-guided technique, transverse processes can be more accurately identified. The best strategy is to start the scanning process 5-10 cm laterally to identify the rounded ribs and parietal pleura underneath. The transducer is then moved progressively more medially until transverse processes are identified as more squared structured and deeper to the ribs. Too medial transducer placement will yield image of the laminae, at which point the transducer is moved slightly laterally to image transverse processes. Once the transverse processes are identified, a needle is inserted out-of-plane to contact the transfer process and then, walk off the transfer process 1-1.5 cm deeper to inject local anesthetic. While the position of the needle tip may not be seen with this technique, an injection of the local anesthetic will result in displacement of the parietal pleura. The process is then repeated for each desired level. In our opinion, a pragmatic needle insertion 1-1.5 cm past the transverse may be safer then using spread of the injected to displace the pleura as the end-point.
Figure 4: (A) Ultrasound image of the local anesthetic (LA) spread during a thoracic paravertebral block. (B) A three- dimensional magnetic resonance image demonstrating catheter insertion into the thoracic paravertebral space and injection of a small amount of local anesthetic.
Figure 5: Longitudinal, out-of-plane approach to thoracic paravertebral block. The transducer is first placed 5-6 cm lateral to the spinous processes to identify ribs, parietal pleura and intercostal spaces (A1-A3). The transducer is then moved progressively medially to identify transverse processes (B1-B3). Transverse processes (TP) appear square and deeper then ribs (round, superficial). The needle is inserted out-of-plane to contact the TP (C1-C2 and C3, line 1) and then walked off the TP (C3, line 2) inferior or superior to TP to enter the paravertebral space and injection local anesthetic (blue). Proper injection displaces the pleura (blue arrows). PVM – paravertebral muscles.