Serratus plane block: do we need to learn another technique for thoracic wall blockade?

In this issue of the Journal, Blanco and colleagues describe a new block of the thoracic wall, the serratus plane block 1. Using ultrasound, local anaesthetic is injected into the plane either below or above the serratus anterior muscle. The lateral cutaneous branches of the intercostal nerves are blocked as they pass through these planes, before dividing into anterior and posterior branches to supply sensation to most of the chest wall. In all four volunteers, injection of 0.4 ml.kg−1 levobupivacaine 0.125% and 0.1 mmol.kg−1 gadolinium into the superficial plane blocked T2–T9, lasting a mean of 12.5 hours. The authors observed that an injection superficial to the serratus anterior muscle spreads wider and lasts longer than an injection deep to it. It is suggested that this block will therefore be ideal for thoracic wall surgery, particularly breast surgery. However, serratus plane block has not been described in the context of analgesia during or after surgery, either alone or in combination with sedation or general anaesthesia. This promising technique should therefore be compared with established alternative regional anaesthetic methods in the clinical setting The control groups of numerous studies demonstrate that breast surgery with general anaesthesia alone is associated with significant pain, consumption of opioid, nausea and vomiting, compared with regional anaesthetic techniques 2-9. Regional anaesthesia should therefore be actively promoted as an option for patients. Appropriate regional block techniques include epidural 2, intercostal 3, interpleural 4, 5 and paravertebral 6, 7 blocks. Wound infiltration also reduces pain scores 8, 9. Serratus plane block is probably straightforward to perform, with a high success rate and minimal incidence of complications, if the administrator is skilled with ultrasound and has a good understanding of the sonoanatomy of the lateral thoracic wall. However, we are not convinced that serratus plane block can produce “complete paraesthesia of the hemithorax” 1 because the posterior primary rami (posteriorly), the anterior cutaneous branches of the intercostal nerve (close to the sternum) and the supraclavicular nerves (immediately below the clavicle) are not blocked. This is clearly illustrated in Figure 4 of the report by Blanco and colleagues 1. Afferent nociception after thoracic wall surgery is complex and involves both the somatic and autonomic nervous systems 10. Thoracic paravertebral and epidural block produces somatic and sympathetic nerve blockade, including the posterior primary ramus, in multiple contiguous thoracic dermatomes 7, 10, 11. Paravertebral block abolishes intercostal somatosensory evoked potentials at the level of block and adjacent thoracic dermatomes 11. Serratus plane block will not achieve such widespread somatic and autonomic block, such that if complete anaesthesia of the hemithorax is required, an alternative approach will be necessary. In addition, if ultrasound is not available, the block will not be possible and if the administrator is not skilled in the use of ultrasound, failure and/or pneumothorax could easily result. Blanco and colleagues report prolonged numbness over the area supplied by the lateral cutaneous branches of the T2-T9 spinal nerves using only 0.4 ml.kg−1 0.125% levobupivacaine 1. This is remarkable, because alternative techniques such as intercostal 3, interpleural 4, 5 and thoracic paravertebral block 6, 7 require relatively high concentrations and volumes of local anaesthetics to produce similarly prolonged, multi-dermatomal thoracic analgesia. It is unlikely that the additional gadolinium influenced the distribution and/or absorption of the local anaesthetic, but this is possible. Local anaesthetics are very rapidly absorbed from the intercostal 12, interpleural 13, and paravertebral 14 spaces and there is potential for local anaesthetic toxicity. Although Blanco and colleagues provide no pharmacokinetic data, the risk of local anaesthetic toxicity after a serratus plane block is probably lower than for most alternative regional anaesthetic techniques, because a smaller dose of local anaesthetic is injected under ultrasound guidance into a less vascular area. The reduced potential for local anaesthetic toxicity after serratus plane block may be considered an advantage provided that the dose described is clinically effective. However, the latter assumption remains to be proven. The consistency of the distribution of the serratus plane block 1 suggests that local anaesthetic spreads widely and reliably. This may also be considered an advantage over alternative techniques of intercostal nerve block, if confirmed in a larger series. Dermatomal block after a single thoracic paravertebral injection is unpredictable 15, 16 and varies widely; 15 ml bupivacaine 0.5% resulted in a mean unilateral somatic block of 5 (range 1–9) dermatomes 16. Widespread dermatomal block is more reliable when paravertebral injection is at multiple levels 17. Intercostal blocks also produce ipsilateral thoracic analgesia but multiple injections have to be performed as the spread of local anaesthetic after a single injection is limited 18 and there are overlapping innervations from contiguous segments 19. Moreover, multiple intercostal injections are painful, time consuming and associated with a incidence of pneumothorax of up to 5.6% 20. Interpleural block also produces reliable unilateral segmental thoracic analgesia 4, 5 but the technique has not been extensively investigated or compared with alternative methods. Interpleural block is gravity-dependent and the analgesia produced is due to retrograde diffusion of the local anaesthetic to the paravertebral space 4. Therefore a more direct approach to the intercostal nerves may be considered desirable, without a 2% risk of pneumothorax 5. Serratus plane block may have similarities to wound infiltration techniques, which can provide adequate postoperative analgesia, particularly after ambulatory surgery 8, 9. Wound infiltration has also been used as the sole technique for major breast surgery 21. Large volumes of local anaesthetic are usually required, but apart from the risk of toxicity, the limited duration and the discomfort associated with widespread infiltration, there are few other disadvantages 8, 9, 21. Subcutaneous catheters can be placed for continuous infusion 22 and the same technique could be used to extend serratus plane block. In the context of breast surgery, thoracic paravertebral block is probably the gold standard regional anaesthetic technique. A meta-analysis of paravertebral block alone, with sedation, or combined with general anaesthesia concluded that there were significant improvements in pain scores, nausea and vomiting for up to 48 hours, compared with general anaesthesia alone 6. Length of hospital stay was shorter in at least three studies 23-25. A Cochrane review concluded that the odds ratio for chronic pain was 0.37 if a paravertebral block was used, compared with no block 26. In a retrospective study, Exadaktylos et al. showed a significant reduction in cancer recurrence if paravertebral block had been performed in combination with general anaesthesia, compared with general anaesthesia alone 27 and a multicentre prospective trial is nearing completion to assess the influence of paravertebral block on breast cancer 28. Blockade of the central sympathetic afferents may be an important mechanism towards these beneficial effects 11 and if so, they are unlikely to be reproduced by serratus plane block. However, paravertebral block has a failure rate of 6–12% 7, 29, 30 and unreliable block distribution may occur with a single injection 15, 16. Toxicity has been rarely reported 6, 7, 31. Using the multiple-injection caudad approach, no pneumothoraces were reported in a series of over 1000 patients 32 and the relative risk compared with general anaesthesia alone was 0.01 6. In two recent editorials, it has been strongly suggested that there is a significant risk of neurological damage with paravertebral block 30, 33, but there have been no reports in association with breast surgery and overall only five cases have been published, four of which were over 50 years ago, one with alcohol 34 and three with Efocaine 35. Efocaine was subsequently withdrawn because of neurological toxicity 36. The other case was an unpublished anecdote within a literature survey 37 and associated editorial 33. High spinal anaesthesia has been reported twice after thoracic paravertebral block 38, 39 and in both of these reports a catheter was inserted by the surgeon. Blanco and colleagues suggest that the complications of paravertebral block can be catastrophic 1, but although this assertion is true, it is very rare and is commonly associated with avoidable technical factors. Catastrophic complications will still occur with serratus plane block in inexpert hands. Thoracic epidural analgesia has been reported for breast surgery 2. However, bilateral block is rarely necessary and the attendant risks and side-effects make epidural analgesia difficult to justify routinely 40. Paravertebral block has not been specifically compared with other alternatives for breast surgery analgesia, except wound infiltration 9. Paravertebral block was found to be superior in terms of pain scores, nausea and vomiting. It is uncertain whether paravertebral block is actually superior to epidural, intercostal or interpleural blocks in terms of the overall risk/benefit ratio, but this seems likely. In conclusion, Blanco and colleagues are to be commended for presenting an innovative new technique for thoracic wall analgesia. While this holds promise, there is a need for further research to establish the place of this block compared with alternatives, as these authors acknowledge. Based on the data presented by Blanco and colleagues 1 and our understanding of the afferent nociceptive block required for surgical anaesthesia during major breast surgery with axillary dissection, we believe that serratus plane block is unlikely to be as effective as paravertebral block in reducing acute and chronic pain, or as the sole anaesthetic technique. However, If serratus plane block provides similar analgesia and other benefits compared with established methods, with fewer side-effects, it may well be a significant advance. This block may be particularly useful in the context of ambulatory surgery and enhanced recovery. There may also be other applications in thoracic and upper abdominal surgery. So, do we need to learn another new technique for thoracic wall blockade? Perhaps not quite yet, but probably very soon. SQMT is President of Regional Anaesthesia UK. No external funding and no competing interests declared.