Research PaperEvaluation of the effect of ultrasound guidance on the accuracy of intercostal nerve injection: a canine cadaveric study
Introduction
Regional anesthesia describes a variety of techniques for providing analgesia to a specific anatomic focus. Unlike systemic analgesics, which suppress pain by modulating neural pathways in the central nervous system, regional anesthesia techniques act peripherally, providing complete analgesia by preventing transmission of nociceptive signals (Strichartz 1976). Because of their superior analgesic effect, regional anesthesia techniques are associated with a greater attenuation of the surgical stress response and a reduction in opioid requirements when compared with opioid-based analgesic protocols (Portela et al. 2008; Mosing et al. 2010; Romano et al. 2016; Congdon et al. 2017). However, the efficacy of regional anesthesia depends on the ability of the practitioner to accurately and reliably inject local anesthetics around the target nerves. Anatomic landmarks and electrical nerve stimulation can be used to estimate the location of nerves, but ultrasound guidance improves accuracy by reducing the incidence of off-target, intraneural and intravascular injections (Sala Blanch & De Andrés 2004; Marhofer et al. 2005; Costa-Farré et al. 2011).
Thoracotomy is a common procedure in dogs (Moores et al. 2007) where the transection of muscles, spreading of ribs and compression of intercostal nerves are associated with significant somatic pain (Popilskis et al. 1991, 1993; Thompson & Johnson 1991; Pascoe & Dyson 1993; Conzemius et al. 1994; Rooney et al. 2004; Yoon et al. 2015). Intercostal nerve blocks have been used to desensitize the thoracic body wall in dogs for thoracotomy and thoracic trauma (Thompson & Johnson 1991; Pascoe & Dyson 1993; Duke 2000). The intercostal nerves run along the caudal border of each rib, parallel to an intercostal artery and vein and in between the endothoracic fascia medially and intercostales interni muscle laterally (Evans & de Lahunta 2013). The intercostal nerves supply the parietal pleura and lateral thoracic body wall, and they give rise to the lateral and ventral cutaneous branches, which supply the skin of the mid to ventral thorax (Bailey et al. 1984). A blind technique is often used by practitioners, whereby a needle is inserted caudal to the rib and advanced cranially until the rib is contacted, presumably placing the tip of the needle in close proximity to the intercostal nerve (Duke 2000). However, the success rate of this technique in dogs has not been described, and the incidence of inaccurate injection is unknown. Given the proximity of the intercostal nerves to the pleural space and vascular structures, intramuscular, intravascular and intrapleural injections are possible complications (Shanti et al. 2001; Portela et al. 2018). Block failure may result from inaccurate deposition of local anesthetic. Ultrasound guidance allows visualization of relevant anatomical landmarks, which are essential to improve the accuracy of intercostal nerve block in humans (Bhatia et al. 2013), but an ultrasound-guided technique has never been described in dogs.
The aims of this study were: 1) to describe the ultrasound anatomy of the intercostal nerves and their surrounding structures to develop an ultrasound-guided technique for intercostal nerve block; 2) to compare the success rate of the blind versus ultrasound-guided technique; and 3) to evaluate the effect of practitioner experience on the success rate with each of these techniques.
Section snippets
Materials and methods
This study used 14 previously frozen mid-sized adult canine cadavers with body condition scores 3–7/9. Prior to use, 13 cadavers were thawed at room temperature for 48 hours and the hair on the thorax was clipped. One cadaver remained frozen for cryosections. Cadavers were excluded from the study if they had lesions of the thoracic body wall, subcutaneous emphysema or if the skin was easily torn while clipped, suggesting excessive autolysis. Cadaver use was reviewed and approved by the
Anatomical study
Intercostal nerves were identified arising from the ventral branch of each thoracic spinal nerve and coursing slightly medial to the caudal border of each rib. The thirteenth intercostal nerve was not apposed to the rib, instead running more caudally between the transversalis fascia and transversus abdominis muscles after crossing the quadratus lumborum muscle. A lateral cutaneous branch diverged from each nerve at the level of the mid-axillary line, emerging through the intercostal muscles to
Discussion
The present study demonstrates that ultrasound guidance increases the success rate of intercostal nerve injection, probably by more specifically identifying anatomical landmarks and dynamic signs such as displacement of the parietal pleura. Historically, intercostal nerve blocks in dogs have been performed blindly. However, the present study found that when a previously published technique for blind intercostal nerve block (Read & Schroeder 2013) was used, staining the target nerves was only
Conclusions
Intercostal nerve injections performed using the blind technique resulted in 58.6% successful staining in canine cadavers, regardless of whether they were performed by an experienced or inexperienced practitioner. Failure to deposit local anesthetic in the correct location may result in block failure. Ultrasound-guided technique significantly increased the accuracy of intercostal nerve injections to 91.4% independently of the practitioner’s expertise in regional anesthesia; therefore, it may be
Acknowledgements
Funding for this study was provided by the 2018-2019 Resident Research Grant Competition from the University of Florida College of Veterinary Medicine.
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