**4. The modern era of the tissue plane in regional anaesthesia**

In the modern era, the rules have changed with the introduction of ultrasound. Now we are able to directly visualise tissue planes and at least in theory, manipulate them to our clinical advantage. While we now have a significant advantage with this development, a greater need for understanding the concept and its relevance to regional anaesthesia has arisen. A headlong rush to discover and name new blocks has often preceded the basic scientific work that should have underpinned the practice. Soft endpoints (e.g., 'we have done 20 and they worked well') and generic terms (e.g., 'multimodal analgesia') have the potential to hide the fact that a particular technique does not achieve the intended effect.

Added to this is a confused nomenclature. Surgical specialties have long used the term 'tissue plane' with a clear understanding of its meaning and it would seem odd to borrow this concept and claim it for regional anaesthesia under revised names. That being said, this author would also argue that for historic reasons it is important to retain the original name of individual techniques since these are the names given by authors to their techniques and which have been accepted through a peer-review process.

In the following paragraphs, we examine several of these techniques through the lens of intended clinical application versus anatomic scientific foundations.

PECS block was originally published as a technique to provide analgesia following breast surgery [37]. The name refers to the tissue plane between pectoralis major and pectoralis minor muscles, and the aim is to block the medial and lateral pectoral nerves which derive from the brachial plexus. This creates somewhat of a dilemma as the pectoral nerves do not innervate the breast. There have been modifications since, possibly reflecting that fact and correcting the record somewhat.

The pecto-intercostal fascial block was first published as an analgesic technique for breast surgery [52] and anterior chest wall trauma [53]. It aims to block the anterior cutaneous sensory branches of the intercostal nerves where they penetrate the chest wall near the edge of the sternum. Whilst the cutaneous termination of the intercostal nerves T2-T6 do innervate the skin over the medial aspect of the breast, these same cutaneous sensory nerves do not innervate the bony structures of the anterior chest wall. Further study of the relevant tissue planes is awaited.

By contrast, the superior cluneal nerve block was first published as a very different type of anatomic study [85]. These nerves derive from the first three lumbar dorsal rami and innervate the skin over the buttock area. This technique creates a plane of dissection just deep to the superficial layer of the thoracolumbar fascia in its inferolateral aspect. The authors of that paper first studied a cadaveric model, but then in recognition of the inherent flaws of such modelling repeated the block in volunteers with accompanying mapping of sensory loss against potential surgical incisions. The accompanying editorial suggested that new techniques required similar basic science study so that in the clinical environment, we know exactly what we are doing and what we can expect to achieve [86].

Erector spinae plane block (ESP block), an injection into a tissue plane deep to the erector spinae group of muscles, has quickly become one of the most popular techniques since its first description [60]. It was almost immediately accompanied by multiple case reports with dramatic claims of efficacy [87–90]. There followed several quite different randomised controlled trials with claims of efficacy [91–94], and the indications for the technique have multiplied almost exponentially [95]. The source of greatest debate has seemed only related to the mechanism of action. A cadaveric study [65] disputed the theory that there was adequate spread of local anaesthetic to the ventral rami and instead suggested local anaesthetic spread to the lateral cutaneous branches of the ventral rami through the lateral aspect of the tissue plane. The original authors then followed up with their own cadaveric study [66] showing spread to the ventral rami and hence providing a mechanism of action for their observed results. The tie breaker in this debate was a volunteer study [96] which demonstrated inconsistent spread of injectate to the ventral rami. Hence the enthusiasm to use ESP block has far outrun our understanding of the technique. Can this enthusiasm cause harm? Yes, by lack of effect or failure to use alternative techniques of known efficacy. Some hospitals now run programs using continuous ESP block for rib fractures – one wonders how they work when the ribs are innervated by the ventral rami, and the lateral cutaneous nerves neither pass through the erector spinae group of muscles nor innervate the ribs. Hence the exact role of ESP block remains uncertain.

Continuous rectus sheath block has re-emerged as an option for analgesia post-midline laparotomy. The modern version of placement is a surgical technique, where a plane of dissection is developed between the rectus abdominis muscle and the posterior rectus sheath with a catheter placed in the ensuing compartment for upper abdominal procedures [10], and a plane of dissection developed between the rectus abdominis and the anterior rectus sheath for lower abdominal procedures [97]. It took little time for ultrasound-guided versions of the same technique to emerge [38, 39], although for the majority of placements there is little logic in placing the catheters percutaneously after the wound has been surgically closed [98]. It is suggested that an approach with ultrasound might be indicated if there is intraabdominal sepsis or adhesions making surgical access to the tissue plane unwise or impossible. In this scenario, avoidance of lateral approaches to the rectus sheath is recommended due to the risk of perforation of the epigastric vasculature. There have now been two dye studies confirming the spread of local anaesthetic throughout the developed tissue planes for rectus sheath block [99, 100]. One study demonstrated that inferior to the arcuate line where the posterior rectus sheath is less distinct, a tissue plane containing the relevant abdominal wall nerves lies between rectus abdominis muscle and the transversalis fascia [99], while the other demonstrated the importance of volume to ensure spread of solution across the tissue plane [100]. It is important to note that both of these studies used boluses and this has management implications for continuous systems.

Pericapsular nerve group (PENG) block is a recently published tissue plane block. First described in 2018 [69] it has been accompanied by multiple clinical

#### *The Tissue Plane DOI: http://dx.doi.org/10.5772/intechopen.99533*

reports and recently a randomised controlled study in the hip fracture population [101]. Whilst it appears remarkably effective for analgesia for hip fractures, the reasons for this are yet to be fully answered. It purports to block just the sensory branches to the hip joint from the femoral and obturator nerves [69]. Could there also be spread further posteriorly to include the superior gluteal nerves? A dye study [102] sheds some light on the characteristics of spread in this tissue plane deep to iliopsoas muscle. In this study, the needle was slightly more caudad to the site of PENG block but in the same plane and suggested that restriction of spread of injectate was only possible with small volumes. Larger volumes could spread to reach the femoral nerve, defeating the purpose of the technique. In this respect, is this tissue plane limited anteriorly by iliopsoas muscle fascia or by fascia iliaca, the latter being suggested by the spread characteristics of the larger volume in the study?

The IPACK block (infiltration between the popliteal artery and the capsule of the knee) for analgesia post-total knee joint replacement was introduced in 2019 as a cadaveric study [103] although there had been keen interest in this area in the years prior but without an accepted name for the technique [104–106]. A plane of dissection is developed deep to the popliteal artery with the intention to bathe the articular sensory branches of the knee joint in local anaesthetic as they traverse this area to reach the joint capsule. Since 2019 there have been over 10 randomised controlled trials [107–118] and 3 meta-analyses [119–121] reflecting widespread interest in techniques which might provide pain relief without hindrance to ambulation post-total knee joint replacement. There has also been widespread adoption of the IPACK block as reflected by reports of programmes in the literature [122, 123]. It is therefore somewhat concerning that the meta-analyses are not supportive of the technique, at least in its current format. Has enthusiasm outweighed clinical realities in this particular circumstance? Is this a technique that has yet to find its real indication? Or could the literature somehow not be accurately reflecting current effective clinical practice?
