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Local anaesthesia in small animal practice

Locoregional anaesthesia can prove to be a valuable tool for both intraoperative and post-operative analgesia

01 May 2020, at 7:55am

The utilisation of locoregional anaesthesia techniques has dual benefits: it provides total anaesthesia to the area undergoing surgery and therefore reduces the depth of inhalational anaesthesia needed, and also reduces systemic analgesic requirements post-operatively. You should therefore see a reduction in both intraoperative and post-operative side effects of anaesthesia, with hypotension, hypoventilation, sedation and bradycardia as prime examples.

Alongside the obvious advantages in patient comfort and care, locoregional anaesthesia has also been shown to have beneficial cost implications (Warrit et al., 2019). The more advanced the technique, the higher the level of skill required and the cost of equipment. Although the cost of performing these techniques means the total cost of anaesthesia may be higher, the reduction in post-operative systemic drug administration means overall costs are low. This may have a beneficial financial impact when procedures are done at set price and costs to the practice are lowered. The average lifespan of the equipment required for more advanced techniques is between 6 and 10 years, with the cost per case to recoup the initial investment being estimated as less than £25.

Locoregional anaesthesia doesn’t need to be complicated and can be incorporated into everyday practice relatively easily. The most basic of techniques involve infiltration of local anaesthetic into the tissues around the surgical site, but this can be used to great effect in even the most delicate and advanced of surgeries. The infiltration of bupivacaine into the epaxial muscles has been shown to reduce the requirement of intraoperative and post-operative opioid in dogs undergoing thoracolumbar hemilaminectomy (McFadzean et al., 2019), despite the infiltration being no more complex than an intramuscular injection. In dogs, pre-emptive incisional infiltration with bupivacaine during celiotomy has been found to reduce pain scores and opioid consumption in the post-operative period (Savvas et al., 2008). Other simple to perform techniques with a low cost and high patient benefit are the infiltration of local anaesthetic into the testes and spermatic cord prior to castration and intraperitoneal splash block of local anaesthetic following ovariohysterectomy. The use of lidocaine block prior to castration has been shown to reduce inhalational agent requirement and responsiveness of the patient during castration (McMillan et al., 2012; Huuskonen et al., 2013), with intraperitoneal splash blocks being effective after both open and laparoscopic ovariohysterectomy (Campagnol et al., 2012; Kim et al., 2012).

Equine and farm clinicians will be familiar with needle positioning based upon anatomical landmarks and this approach can be used to achieve a variety of local blocks in small animal practice too. The efficacy of local anaesthesia when performing dental procedures makes learning these blocks a must for anyone undertaking dentistry on a routine basis. All of the nerve blocks to desensitise the mandible and maxilla can be learnt and performed using anatomical landmarks only. Multiple options for the maxillary nerve block have been reported, with the introduction of a catheter caudally via the infraorbital canal being reported as an easy technique to learn (Viscasillas et al., 2013). A more advanced technique involves the introduction of a needle percutaneously below the zygomatic arch with resultant deposition of local anaesthetic in the pterygoid palatine fossa. This technique has the benefit of more caudal desensitisation of the nerve and should result in a more complete block. The mandibular nerve can be desensitised by introducing a needle perpendicular to the skin from the ventral aspect of the jaw, rostral to the angular process, and advanced until it is in close proximity to the nerve which is identified by palpation in the oral cavity. Once practised these nerve blocks are quick and easy to perform and can be utilised outside of dentistry for other procedures such as rhinoscopy and mandibular fracture repair.

Peripheral nerve stimulator and consumables
FIGURE (1) Peripheral nerve stimulator and consumables ready to perform nerve block

The use of peripheral nerve stimulators (Figure 1) for nerve location comes with increased technical difficulty but should lead to improved results as local anaesthetic can be injected alongside the target nerve. Peripheral nerve stimulators use a low current over a short time period and should only stimulate motor and not sensory nerves; it is, however, advisable to perform these blocks in anaesthetised patients. A current of 1.0 to 2.0mA, with a frequency of 1 to 2Hz, delivered over 0.1 to 0.2 milliseconds is used initially to locate the nerve, with the operator looking for the appropriate motor response, for example stifle flexion when the sciatic nerve is stimulated. Once confident the correct nerve is being stimulated, the current is gradually reduced until the motor response disappears. If this is at a current below 0.6mA close proximity to the nerve is presumed. Aspiration before injection should be performed to check for inadvertent puncture of a blood vessel and the block abandoned should blood be seen. Care should be taken when injecting that no resistance is felt; injections should be smooth and easy. An inline pressure transducer can be used to objectively assess injection pressure, with a reading of over 15psi suggestive of intraneuronal injection. If a motor response is seen below 0.4mA the needle should be repositioned as this could indicate that the needle tip is sitting within the nerve bundle, with injection here leading to damage of the nerve.

Dog leg and peripheral nerve stimulator
FIGURE (2) Performing a femoral nerve block using a peripheral nerve stimulator, using the pre-iliac approach

Nerve stimulator guided blocks are particularly useful when anaesthesia of the limbs is required. The use of a femoral nerve block, via a pre-iliac approach (Figure 2), combined with a sciatic nerve block, will provide anaesthesia to the level of the proximal femur. In the forelimb a brachial plexus nerve block will desensitise the limb to the level of mid humerus, and a paravertebral block will reach as high as the shoulder joint (Lerche et al., 2016). It must be considered that innervation of skin dermatomes does not correspond with the deeper tissues and responses can sometimes be seen to skin incision and suturing despite anaesthesia sufficient to conduct surgery.

The highest level of accuracy is achieved by using ultrasound guidance to perform the block, but this also comes with the highest level of complexity. Ultrasound guidance allows visualisation of the deposition of local anaesthetic in close proximity to the target nerve. The accuracy of this technique means that lower volumes of local anaesthetic are required and thus the chance of systemic side effects such as sedation, nausea and cardiovascular disturbances are reduced. Another advantage is that a differential block may be achieved, meaning that despite total anaesthesia, movement of the limb may still be possible. This has the added advantage of animals being able to walk sooner after surgery.

All of the aforementioned blocks can be achieved with use of ultrasound guidance. An additional block that is fast to perform is the transverse abdominis plane (TAP) block. The TAP block provides anaesthesia to the abdominal cavity and can be used in surgeries as diverse as exploratory laparotomy and mammary mass removals (Figure 3). For the TAP block, the animal is in dorsal or lateral recumbency and the needle advanced until the tip is sitting in the fascial plane of the transverse abdominis muscle (Figure 4A). A small test injection will confirm correct placement as hydro-dissection of the muscle will be seen (Figure 4B). This block has become routine at many hospitals and can easily be implemented in primary care practices.

There are a wide range of techniques available to achieve local anaesthesia of almost all surgeries; once practised these nerve blocks are fast and easy to perform, have clinical benefits to the patient and can have financial benefits to the practice and owner.

References
Author Year Title
Campagnol, D., Teixeira-Neto, F., Monteiro, E., Restitutti, F. and Minto, B. 2012 Effect of intraperitoneal or incisional bupivacaine on pain and the analgesic requirement after ovariohysterectomy in dogs. Veterinary Anaesthesia and Analgesia, 39, 426-430
Huuskonen, V., Hughes, J., Estaca Bañon, E. and West, E. 2013 Intratesticular lidocaine reduces the response to surgical castration in dogs. Veterinary Anaesthesia and Analgesia, 40, 74-82
Kim, Y., Lee, S., Suh, E., Lee, L., Lee, H., Lee, H. and Yeon, S. 2012 Sprayed intraperitoneal bupivacaine reduces early postoperative pain behavior and biochemical stress response after laparoscopic ovariohysterectomy in dogs. The Veterinary Journal, 191, 188-192
Lerche, P., Aarnes, T., Covey-Crump, G. and Taboada, F. 2016 Handbook of Small Animal Regional Anaesthesia and Analgesia Techniques, 1st ed. Wiley Blackwell
McFadzean, W., et al. 2019 Influence of incisional block with bupivacaine pre- or post-surgery on opioid consumption in dogs undergoing hemilaminectomy. In: Association of Veterinary Anaesthetists Spring Meeting 2019. Bristol
McMillan, M., Seymour, C. and Brearley, J. 2012 Effect of intratesticular lidocaine on isoflurane requirements in dogs undergoing routine castration. Journal of Small Animal Practice, 53, 393-397
Savvas, I., Papazoglou, L., Kazakos, G., Anagnostou, T., Tsioli, V. and Raptopoulos, D. 2008 Incisional block with bupivacaine for analgesia after celiotomy in dogs. Journal of the American Animal Hospital Association, 44, 60-66
Viscasillas, J., Seymour, C. and Brodbelt, D. 2013 A cadaver study comparing two approaches for performing maxillary nerve block in dogs. Veterinary Anaesthesia and Analgesia, 40, 212-219
Warrit, K., Griffenhagen, G., Goh, C. and Boscan, P. 2019 Financial impact of ultrasound-guided lumbar plexus and sciatic nerve blocks with electrostimulation for tibial plateau levelling osteotomy surgery in dogs. Veterinary Anaesthesia and Analgesia, 46, 682-688

Will McFadzean, BVetMed, CertAVP(VA), DipECVAA, MRCVS, graduated from the RVC in 2010 and completed a residency in Veterinary Anaesthesia and Analgesia at the University of Bristol. He holds an RCVS Advanced Certificate in Veterinary Anaesthesia as well as a European specialist diploma.

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