Local Anaesthesia Techniques in Dogs and Cats: A Review Study

Abstract

The use of multimodal anaesthesia and analgesia is desirable as part of a complete analgesic plan. Analgesic strategies for perioperative pain treatment include combinations of drugs with different means of action to increase their efficacy and to reduce the required doses and adverse effects. Local anaesthetics prevent the transduction and transmission of painful stimuli through their action on neuronal cell membranes. They undergo minimal systemic absorption and are therefore ideal alternatives to drugs that could result in systemic toxicity. Numerous benefits have been recognised for the use of local anaesthesia, such as a decreased need for systemic analgesics and decreased hospitalisation periods. Local anaesthetics have been used in veterinary medicine in several ways. Anatomical landmarks can be used to identify the target nerves and the clinician can employ an electrical nerve stimulator or ultrasound guidance to perform a more accurate injection. Local anaesthetic techniques can implement other drugs, apart from or in combination with local anaesthetics, such as opioids, α₂−adrenergic agonists or vasoconstricting agents. This review article presents and discusses the most common techniques of local anaesthetic use in small animals, with the aim of providing the clinician with further and comprehensive information regarding the analgesic options during the perioperative period.

1. Introduction

The first reported use of a local anaesthetic in veterinary medicine was a cocaine solution used for desensitisation of the eye for the removal of foreign objects in cattle, guinea pigs, rabbits, and dogs in 1884 [1]. Procaine was the next local anaesthetic agent used in 1905 [2,3]. In 1948, lidocaine, an amino-amide drug, was formulated with a longer duration of action than procaine, which was used for local and regional nerve blocks, epidural, and infiltration anaesthesia [4,5]. Bupivacaine was formulated in 1957 [6], while the first use of the drug was reported in 1963 [7,8]. Evidence of cardiotoxicity by bupivacaine [9] led to the formulation and introduction of levobupivacaine in the early 1990s, which is a less toxic stereoisomer. In 1996, after extensive toxicological studies, ropivacaine was introduced in medicine, a drug with limited toxicity and adverse effects [10,11,12]. Local anaesthetic techniques have been used for many years, but the need for advanced techniques and adaptation of human medical techniques is imperative to improve pain management in veterinary medicine.

Even during deep anaesthesia, the pain and stress produced during surgery stimulates the sympathetic system and activates the inflammatory response [13,14]. These physiologic responses consist of metabolic, haemodynamic, neuro-endocrine and behavioural modifications, which can lead to serious consequences concerning the animal’s health. Therefore suitable and timely pain management is imperative for veterinary patients [13,14].

This review aims to present commonly used locoregional anaesthetic techniques used in veterinary practice, in dogs and cats. The collection of local techniques and their comprehensive examination can provide the clinician with easily accessible information regarding the analgesic options during the perioperative period.

The websites that were used for our study were PubMed (https://pubmed.ncbi.nlm.nih.gov/), Google Scholar (https://scholar.google.com/) and the Wiley Online Library (https://onlinelibrary.wiley.com/). Specific keywords that were used in the research included the following: local anaesthesia techniques, regional analgesia and anaesthesia, local anaesthesia blocks, dog, cat, ultrasound guided local anaesthesia, lidocaine, ropivacaine, bupivacaine, as well as specific local anaesthetic techniques such as lumbosacral epidural, sacrococcygeal epidural, intraarticular, intraperitoneal, dental and ocular techniques, quadratus lumborum block, etc.

In this review article, the following sections are analysed and investigated:

• Pharmacology–Mechanism of action–Pharmacokinetics;
• Anaesthesia of the skin and open wounds;
• Local anaesthesia of the ocular region;
• Local anaesthesia of the oral region;
• Local anaesthesia of the auricular region;
• Local anaesthesia of the forelimb;
• Local anaesthesia of the hindlimb;
• Local anaesthesia of the thoracic region;
• Local anaesthesia of miscellaneous nerves.

2. Pharmacology–Mechanism of Action–Pharmacokinetics

Local anaesthetics consist of an aromatic ring (lipophilic group) and a tertiary amine (hydrophilic group) linked by either an ester or an amide link (intermediate link). The ester group (procaine, tetracaine) is hydrolysed by plasma cholinesterases and tissue esterases, causing these local anaesthetics to typically last shorter. On the other hand, the amide group (lidocaine, bupivacaine, ropivacaine) is metabolised in the liver and lungs, so these local anaesthetics typically last longer but have a higher chance of accumulation and toxicity [7,15,16,17].

Local anaesthetics act by blocking the voltage-gated sodium channels (VGSCs) of the neuronal cell membranes [18,19]. The binding site of local anaesthetics is located in the α-subunit of the VGSC, or the functional ion channel, while the β-subunit of the VGSC is responsible for the kinetic function and the voltage dependence of activation and inactivation [18,20]. The primary α-subunits of the peripheral nervous system are the Na_V 1.7, the Na_V 1.8 and the Na_V 1.9 [20]. Sodium channels normally remain in a resting state and sodium ions cannot infiltrate them. When a neuron is stimulated, the channel is activated, so ions can infiltrate the cell, starting the depolarization [21]. Local anaesthetics enter through the cell membrane or open sodium channels and reach their binding site. Therefore, the drug acts by inhibiting the rapid entry of sodium ions through nerve axons and ceasing the transmission of the nerve impulse [17,22].

The lipid solubility of local anaesthetics affects the duration of action and the degree of protein binding [23]. Additionally, local anaesthetics with greater lipid solubility enter the cellular membrane more easily and are, therefore, faster acting, more potent, and more toxic compared to less soluble local anaesthetics. On the other hand, the degree of systemic absorption and the multitude of local vascular tissues inversely affect the duration of action of the local anaesthetic [7,15]. The duration of action of lidocaine 2% is approximately 60–90 min, while ropivacaine and bupivacaine demonstrate similar onset of action (15–30 min depending on the site of injection) and duration of action of 4 to 6 or even up to 8 h [24,25,26,27].

Local anaesthetics used in inflamed tissues may not cause a successful blockade because of the more acidic pH of the tissue and faster absorption caused by the vasodilation [18].

Large doses or continuous infusions of local anaesthetics can cause local tissue inflammation and dose-dependent systemic toxicity (myotoxicity and neurotoxicity) [28,29,30]. High concentrations can cause central nervous system (CNS) depression and induce convulsive seizures [24] leading to coma, respiratory, and cardiovascular collapse, immediately or up to 30 min after injection [18]. Bupivacaine appears to be more toxic than lidocaine [28,31] and is considered the most cardiotoxic local anaesthetic. Additional side effects of local anaesthetics are rare allergic reactions caused by metabolites or additional substances in the local anaesthetic solution [32], tachyphylaxis reported in laboratory animals [33] and methaemoglobinaemia [34].

Additional actions of local anaesthetics, and specifically lidocaine, have been reported in several studies. Lidocaine could potentially inhibit metastasis or suppress the development of tumours in humans [18,35,36]. Specifically, lidocaine appeared to suppress cell metastasis and exhibited proliferation of gastric tumour cells in a study by Guan et al. (2021) [35], and epithelial ovarian cancer cells in a study by Sun et al. (2021) [36]. Furthermore, lidocaine has demonstrated anti-inflammatory effects [37,38] in addition to antimicrobial activity [39]. The combination of local anaesthetics with antibiotics, opioids, preservatives or epinephrine has revealed antimicrobial properties through antagonistic or synergistic action in in vitro and in vivo studies [39,40,41,42].

Vasoconstrictors, such as epinephrine, can be used in combination with local anaesthetics to prolong their duration of action [43]. It has been reported that the addition of 1–5 μg/mL of epinephrine in a lidocaine solution prolongs its action for up to one hour through vasoconstriction, and can cause some analgesic effects, through its action on α–adrenergic receptors [17,44]. Dexmedetomidine can also be used as an additive to prolong the block of local anaesthetics, at a dose of 3–100 μg [45], though it can also cause transient bradycardia [46]. The anti-inflammatory agent dexamethasone, has also been used as an adjunct to lidocaine [47,48]. In a study by Movafegh et al. (2006), lidocaine was combined with dexamethasone to perform an axillary brachial plexus block in humans. The duration of action was significantly prolonged in the lidocaine–dexamethasone group compared to the control group (lidocaine alone), while the times to achieve sensory and motor blockade were similar between groups [48]. Dexamethasone possibly prolongs the action of local anaesthetics through its systemic effects, or through local action on the nerve, and not due to vasoconstriction, as it occurs with epinephrine [48]. Accordingly, the analgesic effect of this combination is not yet clear and may originate from the systemic effects of dexamethasone [49]. Moreover, sodium bicarbonate can be combined with local anaesthetics to reduce pain during injection by adjusting the pH of the solution [50,51,52].

Regional nerve blocks are performed using anatomical landmarks in anaesthetised patients in a sterile environment [53]. They can also be performed using an electrical nerve stimulator or under ultrasound guidance for a more accurate injection of the local anaesthetic and with a smaller dose and volume [18,54]. When ultrasound guidance is used the incidence of systemic toxicity is reduced, as is the number of complications, while the efficiency of the technique is increased [55,56,57]. Ultrasound guidance for regional nerve blocks has been used for many techniques including cervical paravertebral block, intercostal block, brachial plexus block, sciatic and femoral nerve blocks, and peribulbar block [57,58,59,60].

3. Anaesthesia of the Skin and Open Wounds

3.1. EMLA Cream

A mixture composed of lidocaine (2.5%) and prilocaine (2.5%) (EMLA cream AstraZeneca, UK) is used as a eutectic emulsion to decrease pain and discomfort during minimal procedures, such as intravenous catheter placement or blood sampling in veterinary patients. EMLA cream is well absorbed through the skin at body temperature and targets local cutaneous and subcutaneous pain receptors (Aδ myelinated, fast-acting fibres and C unmyelinated, slow-acting fibres) and nerve endings [61,62,63]. The skin should be intact, free of hair and the cream should remain on for at least 30–60 min for better absorption and desensitisation of the area [61]. The recommended dose of EMLA cream is approximately 1–1.5 g of cream per 10 cm² of skin [64]. Neurotoxicity or methaemoglobinaemia rarely occur because of the slow absorption from the epidermal and dermal layers of the skin [62,64]. Nevertheless, application on mucous membranes should be avoided. EMLA emulsion applied for 60 min provided analgesia and less struggling for venipuncture in dogs [64] and jugular catheterisation in cats and dogs [61,62]. EMLA cream applied on the ears of rabbits prior to tattooing them caused a decreased pain response, based on minimal behavioural and cardiovascular changes compared to rabbits with no EMLA [65].

3.2. Lidocaine Dermal Patches

A 5% lidocaine adhesive patch is licensed for use in dogs and cats and provides skin analgesia with an onset of action at 30 min and a duration of more than 60 h [66,67,68,69]. Lidocaine is absorbed by the skin at a very slow rate reaching a peak concentration in plasma at 24–36 h [68]. Adverse reactions include erythema and edema, which resolve after the patch is removed. The patch should be applied as close as possible to the surgical incision or the afflicted site, due to its local action. Studies in dogs and cats have measured the plasma concentration after the use of lidocaine patches, showing that lidocaine concentration was higher in the skin in comparison to plasma and that the plasma concentration was nowhere near a toxic dose [67,69].

3.3. Splash Incisional Block

In the splash block technique, a local anaesthetic (most commonly lidocaine or bupivacaine) is applied and remains on the wound before suturing. This method is used for desensitisation of small wounds or surgical incisions. The use of lidocaine (2%) or bupivacaine (0.75%) as a splash block just prior to the incision’s closure provided postoperative analgesia and reduced the demand for rescue analgesia in dogs [70]. Additionally, in a study by Yilmaz et al. (2014), bupivacaine solution (2 mg/kg) poured on a sterile gauze, which remained on the surgical wound for the entirety of the time required for suturing, induced postoperative analgesia in cats undergoing bilateral mastectomy. The cats in the bupivacaine group had better recovery, increased appetite, decreased anxiety, faster wound healing, and lower pain scores, compared to the cats of the control group. Furthermore, no symptoms of systemic neurotoxicity or heavy sedation were reported, indicating that wound splash with bupivacaine (2 mg/kg) is a safe method of local anaesthesia in cats [71].

3.4. Infiltration Local Anaesthesia

In infiltration anaesthesia, local anaesthetic solutions are used for desensitisation of an area of skin without targeting any specific nerve group (Figure 1 and Figure 2). It involves multiple injections of local anaesthetic into and around the area of interest and is usually performed for the repair of small wounds, the removal of skin lesions [72] or for perioperative analgesia in surgical wounds [71,73,74]. A lidocaine or bupivacaine solution can be used for intradermal and/or subcutaneous injections, with small gauge hypodermic needles (23− to 25−gauge) [72,74]. The dose of lidocaine used is around 2–5 mg/kg, which can be diluted with saline (NaCl 0.9%), to increase the volume and decrease the concentration, and/or sodium bicarbonate, to reduce pain in the site of injection. Injections around neoplastic or infected lesions (i.e., abscesses) are contraindicated, to avoid the spread of neoplastic cells or bacteria [72,74]. In a study by Savvas et al. (2008) multiple injections of bupivacaine (2 mg/kg) were administered subcutaneously and intramuscularly, covering the entire length of the incision in dogs undergoing celiotomy, and preoperative administration of bupivacaine was associated with lower pain scores and reduced requirements of rescue analgesia [75]. On the other hand, the pre- or post-incisional local infiltration with bupivacaine offered no additional analgesia in dogs undergoing ovariohysterectomy, in a study by Fitzpatrick et al. (2010) [74].

The need to provide long-term local anaesthesia and analgesia in patients for the first few days of the postoperative period or during prolonged hospitalisation cannot be sufficiently met with the currently used local anaesthetics, because of their short duration of action (up to 8 h) [26]. In that respect, a long-acting liposomal suspension of bupivacaine has been released for use in humans [76,77,78], dogs, rabbits [79,80,81], and more recently, cats [82]. This liposomal injectable suspension of bupivacaine can be safely used as a single dose for infiltration of the surgical site and provide prolonged release of the drug and effective postoperative analgesia [26,80,81]. This product utilizes the technology of soluble bupivacaine that is encapsulated within multivesicular liposomes. The liposomes are slowly released in tissues and therefore, bupivacaine is also gradually released and is distributed to the local tissues and ultimately has a systematic action [26]. The injection is performed using a moving needle technique to guarantee even distribution to local tissues. In this technique, the needle is inserted subcutaneously, is aspirated and then the injection is performed as the needle is withdrawn [26]. In a study by Richard et al. (2011), the repeated administration of large bupivacaine doses in dogs produced no apparent CNS symptoms or cardiovascular depression [80,81]. Additionally, the use of liposomal bupivacaine has not been associated with higher rates of infection in dogs and cats [82]. Nevertheless, a few cases of granulomatous inflammation have been observed in dogs but were not considered adverse effects as it was a normal reaction to the liposomes of the solution [80,81].

3.5. Diffusion Catheter

Diffusion catheters, also referred to as fenestrated or wound soaker catheters, can be inserted during surgery to provide perioperative analgesia by continuous administration of local anaesthetics in the deeper layers of the surgical incision [83]. This method targets the cutaneous nerves of the afflicted area, and it is used for large surgical wounds such as in auricular surgery, tumour removal, or thoracic surgery [84,85]. A flexible (usually polyurethane) catheter of small diameter with multiple, evenly distributed orifices is inserted along the incision before its closure and it is secured externally with sutures on the skin to prevent its dislodgement [83]. The catheter can remain for a few days postoperatively for long-term analgesia and it is usually well tolerated by the patient. It reduces stress and pain levels, provides improved patient comfort and relatively reduces the need for opioids or other adjunct analgesia [73]. Lidocaine is used as a continuous infusion at a rate of 2 mg/kg/h mixed with saline for a total volume of 5 mL/h in dogs although the volume used should not be dependent on the size of the patient, but the length of the incision [84,86]. Bupivacaine is administered as a slow infusion bolus (1–2 mg/kg). The use of diffusion catheters has minimal and rare complications, such as dislodgement of the catheter or seroma development [84]. Infection of the surgical site has been reported in the same rate as in patients without a diffusion catheter [84].

4. Local Anaesthesia of the Ocular Region

4.1. Retrobulbar Block

This technique desensitises the ophthalmic branch of the trigeminal nerve (cranial nerve V), oculomotor (III), trochlear (IV) and abducens (VI) causing analgesia, central position of the eye and mydriasis [87]. In dogs, a small amount of local anaesthetic is injected into the extraocular muscle cone (intraconal injection) behind the lower eyelid. A 22–25 G curved needle (or a retrobulbar needle) is inserted through the lower eyelid, at the junction of the middle and temporal thirds, advanced until the piercing of the orbital fascia (indicated by a slight popping sensation), and then advanced further dorsally–caudally where the injection is made [88]. In cats, the needle is inserted through the upper eyelid and advanced in the direction of the caudal pole of the globe, where the injection is made. Nevertheless, this technique is not suggested in cats, because the different anatomy increases the chances of complications [88] and reduces the success rate [89].

The use of bupivacaine 0.5% and/or lidocaine 2%, is suggested for ocular surgery such as enucleation [90,91], cataract surgery (phacoemulsification) or conjunctival flap [90,92,93,94]. The retrobulbar block is considered a difficult technique, with a low percentage of successful intraconal distribution adequate for analgesia, with many potential adverse effects, such as seizures, nausea, vomiting, haemorrhage, cardiac or respiratory depression or optic nerve damage and neuropathy (blindness of the contralateral eye, hemiplegia, aphasia or convulsions) [91,93,95]. Similarly, the retrobulbar injection can increase the intraocular pressure and activate the oculocardiac reflex [88,89]. Thus, this technique must be performed with caution and by experienced personnel.

4.2. Peribulbar Block

A volume of local anaesthetic is injected around the extraocular muscle cone (extraconal injection) which diffuses into the orbit causing immobility, corneal desensitisation and centring of the pupil [60]. This technique is used as an alternative to the retrobulbar block and has fewer adverse effects. It is used for intraocular surgeries such as cataract surgery (phacoemulsification) and enucleation [96]. One study describes a single injection at the ventrolateral eyelid with a 22 G, 2.5 cm needle with and without ultrasound guidance [60], while another study suggests a double injection first into the ventrolateral orbit and second into the dorsomedial orbit both using a 22 G, 2.5 cm needle [96]. Complications associated with the peribulbar block include blepharospasm, ecchymosis and subconjunctival haemorrhage, that usually resolve without treatment in a few hours [60]. This technique can be performed successfully in veterinary and human patients, using ultrasound guidance to reduce complications [60,89,97].

4.3. Auriculopalpebral Block

This technique desensitises the auriculopalpebral branch of the facial nerve and provides effective eyelid akinesia [98]. A needle is inserted dorsally to the zygomatic process and caudally to the tympanic bulla and the injection is made subcutaneously [99,100]. For this technique, lidocaine, bupivacaine and ropivacaine have been used [98,99,101]. It is suggested to combine this block with other regional techniques to provide adequate analgesia for ocular surgery [99,101]. Adverse effects include pain on injection, occasional itching [98] and inability to close the eyelids, therefore eye ointment should be applied until normal eyelid movement returns [100].

4.4. Sub-Tenon’s (Episcleral) Block

In this technique, instead of a needle, a blunt end, sterile canula is inserted into the Tenon’s space and the local anaesthetic desensitises the short and long ciliary nerves [102]. A small snip incision (4–5 mm) is made with conjunctival scissors in the bulbar conjunctiva and the cannula is inserted above the sclera and along its curve and local anaesthetic (0.25–0.5% bupivacaine) is injected into the sub-Tenon’s space [102]. The sub-Tenon’s technique provides mydriasis, akinesia, central position of the globe and analgesia without undesirable systemic effects and a low rate of complications in humans [103]. In a study by Bayley et al. (2018), the sub-Tenon’s technique has been successfully used in dogs undergoing cataract surgery (phacoemulsification), with similar desired effects as systemic neuromuscular blockade [102].

4.5. Splash Block

In a study by Chow et al. (2015) in dogs undergoing eye enucleation, bupivacaine was administered intraoperatively with a splash block technique, before suturing of the wound and remaining in contact with the wound for at least 30 s, or preoperatively using the retrobulbar block. The splash block technique provided similar analgesia to the retrobulbar block and no adverse effects were reported [104].

4.6. Absorbable Gelatine Haemostatic Sponge Infused with Local Anaesthetic

The placement of a gelatine haemostatic sponge soaked with local anaesthetic is suggested as an alternative to the retrobulbar block [91], with no requirement to remove the sponge postoperatively, as it is absorbable. The volume of local anaesthetic that would be injected via the retrobulbar technique was infused in the gelatine sponge and the sponge was placed into the orbital cavity intraoperatively, following the enucleation. This technique provided adequate postoperative analgesia in dogs, with a longer duration due to the slow release of the anaesthetic from the sponge, while providing additional haemostasis and no serious complications were observed [91].

5. Local Anaesthesia of the Oral Region

The trigeminal nerve (cranial nerve V) is the main nerve that innervates the teeth. Dental nerve blocks in veterinary practice are used to desensitise five major nerves; the maxillary, infraorbital, major palatine, inferior alveolar (mandibular) and middle mental nerves [105]. These blocks are used mostly as pre-emptive, adjunct analgesia for procedures such as tooth extractions, mass excisions and dental biopsies, cleft palate repair and mandibulectomy [105,106]. The same main principles should be followed for all the techniques of the dental nerve blocks. The technique should be performed under sterile conditions, with sterile equipment, aspiration should always be negative prior to injection to avoid intravascular injection and the anaesthetic should be administered slowly, without any sense of resistance [105,107]. After the injection, the site is pressed for 30 s to avoid escape or diffusion of the agent [108]. Adverse effects from the local anaesthetics used in dental blocks include hematoma formation, infection, paresthesia from nerve injury and accidental intravascular injection [108,109]. The recommended volume of local anaesthetic used per injection site for dental nerve blocks is 0.1–0.5 mL for dogs and 0.1–0.3 mL for cats, depending on the body size. When bupivacaine is used, the dose should not exceed 2 mg/kg for dogs and 1 mg/kg for cats [105,109].

5.1. Maxillary Nerve Block

This block is used to provide desensitisation of the bones and teeth of the maxilla, the hard and soft palates, the upper lip, the nasopharynx and the nasal mucosa [107,110]. Two techniques are reported for this block, the intraoral and the extraoral−intraoral technique. For the intraoral technique in the dog, the needle is inserted caudally towards the centre of the last molar, then it is advanced dorsally just past the tooth’s root tips [105]. In the cat the injection site can be palpated as a depression just behind the last molar, slightly towards the palate [105]. For the extraoral−intraoral technique, the needle is inserted through the skin, perpendicular to its surface, just below the ventral border of the zygomatic arch and caudal to the lateral canthus of the eye, then it is guided in the direction of the maxillary foramen (Figure 3) [107,109]. The recommended dose for this block is 0.25 mg/kg of lidocaine and it can be combined with 0.25 mg/kg of bupivacaine at each site of injection [107].

5.2. Infraorbital Nerve Block

The infraorbital nerve block provides anaesthesia and analgesia foremost of the molar teeth, the associated soft tissues, as well as the upper lip [109]. There are two approaches for the infraorbital nerve block, the cranial and the caudal approach [105]. Desensitisation of the incisor, canine, and premolar teeth rostral to the fourth premolar, together with the ipsilateral soft tissues, is achieved by means of cranial infraorbital nerve block. The injection site can be approached intraorally or extraorally through the skin and it can be found at the opening of the infraorbital foramen [105,108], which can be palpated as a depression of the mucosa above the third premolar (Figure 4). On the other hand, the tissues desensitised with the caudal approach are the same as with the cranial approach and all the bone, the soft tissue and the teeth cranial to the upper first molar. The caudal approach is different to the cranial one, in that the head is kept elevated and pressure is applied to help the local anaesthetic advance caudally [111,112]. A small amount of local anaesthetic, such as bupivacaine, should be administered, usually 0.25–0.5 mL [111,112]. The infraorbital canal is relatively short in cats and brachycephalic dogs; thus, caution should be exercised not to puncture the globe by inserting the needle beyond the median canthus [105,111].

5.3. Infraorbital or Maxillary Block for Rhinoscopy

Common adverse reactions that occur during rhinoscopy include sneezing, gagging and head twitching, which can interfere with the procedure or injure the patient [113]. The infraorbital and the maxillary nerve block have been tested to minimise these adverse effects, without increasing the dose of general anaesthetics. The use of lidocaine for a bilateral maxillary block decreased the incident of adverse effects, whereas a bilateral infraorbital block did not provide adequate desensitisation for dogs undergoing rhinoscopy [113].

5.4. Inferior Alveolar (Mandibular) Nerve Block

Local anaesthetics can be administered with this technique in each mandibular side to provide desensitisation of the respective mandibular teeth, the associated soft tissues (including the tongue) and the bone of the hemimandible [105]. There are two approaches to block the mandibular nerve, either intraorally or extraorally (Figure 5) [108]. For the extraoral technique the patient is placed in dorsal recumbency. The needle is inserted just cranial to the angular process of the mandible (0.5–1 cm) and is then advanced dorsally until the mandibular foramen is palpated. A finger can be inserted into the mouth to assure the proper placement of the needle. For the intraoral technique the patient is placed in lateral recumbency, with the target area facing upwards. A mental line is drawn between the mandible, just caudal to the last molar and the angular process of the mandible. The injection site can be found approximately two-thirds of the distance caudally from the aforementioned line [105,107,108].

5.5. Mental Nerve Block

Performing this block provides analgesia of the mandibular incisors, canine, first premolar and the associated soft tissues and bone [108]. The foramen’s location varies between breeds and is palpated with ease in larger dogs [105]. The injection site is the middle mental foramen, the largest of three foramina, and is located ventral to the rostral root of the second premolar, found by palpating the sub-mucosa between the first and second premolars (Figure 6) [105]. The site is found more easily in the dog, while in cats the procedure is the same but the foramen is very small and difficult to palpate [105]. If the foramen cannot be located, then the needle is placed near the foramen and the agent can be injected in its vicinity [108]. In a study by Krug et al. (2011), the administration of bupivacaine in anesthetised dogs, at the middle mental foramen, resulted in inadequate desensitisation of the target area [114].

5.6. Infiltration Local Anaesthesia

A small amount of a local anaesthetic is administered near the root canal and is diffused near the nerve of the target tooth [100,110,115]. The local anaesthetic is injected into the gingiva at both the palatal and the lingual aspects of the tooth (bleb technique). This technique can be used in all teeth, except for the premolars and molars of the lower mandible, where the injection is not easily performed [110,115]. Specifically, a dental syringe and needle should be used, and 0.25–1 mL should be administered per site of injection, always keeping in mind the total dose administered to avoid toxicity. The infiltration technique is a safe and useful alternative to the regional nerve blocks, which can cause nerve damage during injection. In this technique, local anaesthetics, such as lidocaine (≤4 mg/kg), mepivacaine and ropivacaine can be administered, but bupivacaine or levobupivacaine (1–2 mg/kg) are considered the drugs of choice, due to their long duration of action [115]. It is important to note that the infiltration block could not have the desired effects in inflamed tissues, infection or severe periodontal disease; therefore, other techniques should be used [100,110].

6. Local Anaesthesia of the Auricular Region

Auricular and Auriculotemporal Block

The sensory innervation of the ear is provided by two nerves, the great auricular (cervical nerve II) and the auriculotemporal (cranial nerve V) [116]. This block is performed for auricular surgeries, in particular total ear canal ablation and lateral bulla osteotomy, which is indicated in chronic otitis externa [116]. The great auricular nerve can be located ventrally to the wing of the atlas and caudally to the tympanic bulla, and the local anaesthetic is injected just rostral to this site, at the vertical ear canal. The auriculotemporal nerve can be located rostral to the vertical ear canal, on the most caudal part of the zygomatic arch [116]. A study suggested no apparent advantage of this technique in dogs compared to the use of opioids alone or a splash block technique [117]. The use of a diffusion catheter to provide postoperative analgesia for auricular surgeries is a useful alternative to the auricular and auriculotemporal block [85,118].

7. Local Anaesthesia of the Forelimb

7.1. Cervical Paravertebral Block

The paravertebral brachial plexus block (PVBP or cervical paravertebral block) targets the ventral branches of the spinal nerves (C6−T1) that assemble to form the brachial plexus, providing anaesthesia and analgesia for the entire forelimb, including the shoulder joint [119]. Therefore, any surgical procedure regarding the shoulder joint and the area distally consists of an indication for this block. The technique is difficult to perform and requires experience, necessitating the use of a nerve locator or an ultrasound. If a blind technique is used, less than one-third of the blocks are successful [58]. Complications of this technique include displacement of the drug, Horner’s syndrome, hematoma formation and phrenic nerve paralysis [120,121].

7.2. Brachial Plexus Block

Eleven nerves compose the brachial plexus, which extends under the area medial to the scapula and five of them are spinal nerves (C6−T2) [122].

The patient is positioned in lateral recumbency with the target limb uppermost, which is then aseptically prepared [123]. The site of injection is located dorsomedially to the scapula–humeral joint and cranially to the acromion. The needle is directed vertically to the line connecting the acromion with the cranial border of the greater tubercule and is inserted medial to the shoulder joint, towards the costochondral junctions [122]. A spinal needle can be used for this technique. To avoid penetration of the thorax, the needle is advanced until its tip is palpated caudally [124]. In the axillary brachial plexus block the patient is positioned in dorsal recumbency with their forelimbs pulled caudally [59]. Indications to perform the brachial plexus block include surgery of the shoulder, and upper arm, arthroscopic shoulder procedures and humeral fracture repair.

The use of an ultrasound is recommended to locate the injection site, which can be located dorsal to the cranial edge of the pectoralis superficialis muscle and lateral to the jugular vein (Figure 7) [59,125]. A 5–10 cm 21–22 G needle can be used depending on the patient’s size. The ultrasound provides real-time observation of the position of the needle as it is advanced cranio-caudally; therefore, no potential complications were reported [59]. Additionally, the utilisation of an electric nerve stimulator for this technique can provide similar results requiring lower doses of anaesthetics [124,125].

7.3. Radial, Ulnar, Median and Musculocutaneous Nerves Block (RUMM)

The RUMM block provides desensitisation for the radial, ulnar, median and musculocutaneous nerves of the forelimb, thus providing anaesthesia and analgesia for the area distal to the elbow, and it is considered safer to perform than the brachial plexus block [126]. Two injections are made for this block and, depending on the size of the animal, 0.5–2 mL can be used at each injection site. One is made on the lateral side of the mid-humeral region, between the brachialis muscle and the lateral head of the triceps, targeting the radial nerve and the other injection is made on the medial side of the mid-humeral region, by palpating the brachial pulse just proximal to the elbow joint, targeting the ulnar, musculocutaneous and medial nerves (Figure 8) [127]. A nerve stimulator [126] or an ultrasound [128] can be used to improve the accuracy of this block. The ultrasound is used on the lateral aspect of the forelimb to provide a view of the superficial and deep branches of the radial nerve and then on the medial aspect of the forelimb between the elbow and the shoulder, to provide a view of the three other nerves. Likewise, the peripheral nerve stimulator can be used to target the aforementioned nerves as described above [128]. This technique can be used for surgery distal to the elbow [126,128].

8. Local Anaesthesia of the Hindlimb

8.1. Femoral Nerve

The femoral nerve block can be a useful alternative of the epidural block, anaesthetising the femoral nerve and its saphenous branch providing analgesia to the distal femur, for fracture repairs or stifle joint surgeries [129,130]. To perform this block the animal is positioned in lateral recumbency with the target limb uppermost and pulled caudally and abducted 90^o [59]. The landmarks to perform this block are the femoral artery (pulse) and vein inside the femoral triangle, which are located next to the femoral nerve [129]. The injection is performed through the skin and medially to the femoral artery [130]. Additionally, an ultrasound or a peripheral nerve stimulator can be used to help locate the femoral artery, vein and nerve (Figure 9) [130,131].

8.2. Sciatic Nerve

The sciatic nerve block provides analgesia for stifle and tarsus surgery [132]. The landmarks to perform this block are the greater trochanter of the femur and the ischiatic tuberosity; however, using an electric nerve stimulator or an ultrasound enhances accuracy (Figure 10) [54]. The sciatic nerve passes between these two landmarks in the gluteal region and can be reached with a trans-gluteal approach described by Mahler and Adogwa (2008), who used a peripheral nerve stimulator [122], a parasacral approach described by Portela et al. (2010) also with a peripheral nerve stimulator [133], or a lateral approach described by Campoy et al. (2010) using an ultrasound-guided technique [59]. Complications include direct damage to the nerve (intraneural injection) and puncture of the caudal gluteal vessels, including the femoral artery, resulting in hematoma formation [122,132].

The combination of the femoral and sciatic nerve blocks has been described in humans [134] and dogs [135,136], to provide analgesia for surgery of the stifle joint, such as stifle arthrotomy/arthroscopy or patella luxation surgery [130] and in healthy cats to evaluate antinociception and motor blockade [137]. In a study by Caniglia (2012), the combination of femoral−sciatic nerve blocks had similar antinociceptive and analgesic results with a lumbosacral epidural [136]. For this technique, bupivacaine has been used in combination with dexmedetomidine or buprenorphine in cats [137] and lidocaine has been used in combination with bupivacaine or dexmedetomidine in dogs [135,136].

8.3. Tibial and Common Peroneal Nerve

The tibial nerve is located between the biceps femoris and the semimembranosus muscle, and more distally between the heads of the gastrocnemius. The common peroneal nerve can be palpated and runs laterally over the gastrocnemius and across the lateral surface of the fibular head [132]. The tibial and common peroneal nerves can be desensitised collectively near the mid-region of the caudal thigh. The landmark to perform the injection is the depression between the biceps femoris and semimembranosus/semitendinosus muscles [138]. The animal is placed in lateral recumbency with the treated leg facing upwards, a spinal needle should be used and be positioned perpendicular to the long axis of the femur, parallel to the table surface. The needle is advanced cranially through the skin until the caudal aspect of the femur is contacted and is withdrawn again caudally before injecting a part of the dose. The rest of the dose can be divided into parts injected as the needle is withdrawn. Before each part is injected, the needle should be aspired to avoid intravascular injection [138]. The tibial and common peroneal nerve block provides desensitisation of the leg distally to the injection site, specifically for surgery distal to the mid-tibial shaft [132].

8.4. Lumbosacral Epidural Block

The lumbosacral epidural is a common technique used for anaesthesia and analgesia of the pelvis, the pelvic limbs, the tail and the abdomen as a part of a multimodal anaesthesia and analgesia plan [139,140]. Additionally, a lumbosacral epidural can provide analgesia of the caudal abdominal tissues and can be used as part of a multimodal analgesic plan for ovariohysterectomy and ovariectomy [141,142,143]. The administration of local anaesthetics in the epidural space has been used in various surgeries, such as caesarean section, orthopaedic surgeries of the pelvic limbs and soft tissue surgeries, with minimal cardiopulmonary depression [144,145,146]. Epidural analgesia suppresses the stress response, by decreasing cortisol and norepinephrine secretion [147,148], and provides better postoperative pain management [129]. This technique blocks both the pelvic limbs, as it targets the spinal nerve roots and affects the afferent (sensory) and efferent (motor) nerve fibres [149].

The anatomical landmarks to perform this block are the cranial points (wings) of the ileum and the lumbar vertebras [72]. The injection site is the lumbosacral junction (L7−S1) (Figure 11a) [150]. In most adult dogs the extradural injection is safely performed at the lumbosacral junction. The subarachnoid puncture occurs rarely because the spinal cord and dura mater end cranial to this site in most animals. The opposite applies to cats and young dogs, where the dura mater extends beyond L7; therefore, the puncture of subarachnoid tissue may occur more frequently [150]. Proper positioning of the patient is very helpful for palpation of the injection site and ensuring the correct placement of the needle [140]. There are two options for the epidural block, the sternal and lateral recumbency, with the hindlimbs flexed cranially [151]. The sternal recumbency is more commonly used and easily performed, although the lateral position can be used if the sternal position causes discomfort [152].

The specific gravity of solutions classifies them to either isobaric (similar specific gravity to the CSF), hyperbaric (higher specific gravity) or hypobaric (lower specific gravity). When hyperbaric solutions are administered in the epidural space, they diffuse in the direction of the lower spinal tissues, while the opposite applies for hypobaric solutions [153]. The solutions used for epidural anaesthesia are usually hypobaric or isobaric [153], although hyperbaric bupivacaine and ropivacaine have also been administered in dogs [154,155], as well as humans [156,157,158]. In a study by Sarotti et al. (2012), hyperbaric bupivacaine with morphine was administered intrathecally in dogs. The dogs then remained in lateral recumbency for 10 min with the afflicted limb lowermost, to allow the hyperbaric solution to diffuse unilaterally [155]. The aforementioned administration provided effective intraoperative analgesia for 70 min and the adverse effects presented were no different compared to the use of an isobaric solution [155,159].

The technique requires sterile and aseptic conditions, whether a single injection is made or an epidural catheter is inserted, to avoid complications such as epidural abscess formation or discospondylitis [150]. The area around the L7−S1 intervertebral space must be clipped and aseptically prepared and surgical gloves should be used [150]. Spinal or Tuohy needles are preferable because they have a stylet. The sizes most commonly used are 20 G, 1.5–2.5 inches (3.8–6.35 cm) for larger dogs and 22 G, 1.5 inches (3.8 cm) for small dog breeds, young dogs and cats [153]. Additionally, an epidural catheter can be used to provide long-term or repeated administrations of analgesics in the epidural space [72,150].

Technique: Under sterile conditions, the thumb and middle finger of the non−dominant hand are placed on the dorso-cranial points of the iliac crests, so the index finger can be used to locate the midline and the lumbosacral junction (L7−S1) and the dominant hand can perform the injection [150]. The spinous process of the L6 vertebra is more prominent and easier to palpate, thus it is used to locate the L7. Caudally to the L7, the lumbosacral junction can be located as it is felt as a subtle depression by the index finger [150]. The needle is advanced slowly vertically to the spinal cord. The penetration of the dura mater and entry to the epidural space is usually felt as a popping sensation (when the needle passes through the flavum ligament) [150,153]. To ensure the right placement of the needle, the techniques of the hanging drop and the loss of resistance are combined [140], exploiting the sub-atmospheric pressure of the epidural space. The first technique commences with the removal of the stylet, placing a drop of sterile saline (or local anaesthetic) in the hub of the spinal needle, resulting in the aspiration of the hanging drop on entry into the epidural space [160]. In the second technique, a small amount of either sterile saline or air is injected with a low-friction syringe, and the injection is performed without resistance indicating the entry to the epidural space [160].

If, during insertion, the needle touches bone, the needle can be withdrawn slightly and the injection can be continued [72]. If, after removal of the stylet, cerebrospinal fluid (CSF) is observed in the needle hub, the technique is either reattempted or the doses can be reduced and the injection can be continued intrathecally [150]. Intrathecal injections provide larger bioavailability of drugs and require reduction of the dose by 75% or up to 90% [153] of the originally calculated dose of local anaesthetics.

The local anaesthetics used for lumbosacral epidural are preservative free amino-amides such as 2% lidocaine, 0.5% bupivacaine, 0.75% ropivacaine or 0.5% levobupivacaine [140]. Other drugs that can be used in combination with local anaesthetics include opioids (morphine, buprenorphine, tramadol, fentanyl), ketamine and α₂−adrenergic agonists (dexmedetomidine, medetomidine) [140,150,153,161,162,163,164,165]. The volume used is 0.2 mL/Kg [72] and, because the epidural space is a standard-volume space, to avoid more cranial blockade, the total volume used must not exceed 6 mL [150]. The recommended drugs and the doses used for lumbosacral epidural is presented in the following table (

Table 1. Drugs and recommended doses used for lumbosacral epidural.

Drug	Dose (mg/kg)	Onset (min)	Duration (h)	Reference
Lidocaine 2%	4–5	4–6	1–1.5	[150,153,166]
Bupivacaine 0.5%	0.5–1.65	5–15	2–6	[150,153,166]
Levobupivacaine 0.5%	0.5–1	5–15	1–1.5	[166]
Ropivacaine 0.75%	1–1.65	7–15	1.5–4	[153,166]
Morphine	0.1–0.3	45–90	10–24	[150,153,166]
Hydromorphone	0.02–0.04	<30	6–12	[150,153]
Buprenorphine	0.004–0.005	<60	≥24	[153,166]
Tramadol	1	−	6–8	[162]
Fentanyl	0.004–0.01	−	0.3	[150,163]
Medetomidine	0.005–0.01 (cat)0.015 (dog)	<20	4–6	[153]
Dexmedetomidine	0.003–0.006	<15	≤4.5	[166]
Ketamine	3.0	5–15	≤0.5	[165]

).

8.5. Sacrococcygeal Epidural Block

This technique targets the nerves of the sacral plexus [167] and provides desensitisation of the perineum, the tail and the sacrum [100] by blocking the pudendal, pelvic, and caudal nerves. It is mostly used in large animals for standing procedures, as it does not affect the hindlimbs, and in companion animals for procedures including urinary catheter placement, tail amputation, assisted delivery of neonates, perineal and urogenital procedures of the lower urinary tract, such as perineal urethrostomy [100].

The site of injection is the space between the caudal aspect of the sacrum (S3) and the first caudal (coccygeal) vertebra (Figure 11b) [167]. The spinal cord ends cranially to the target area in cats and small dogs, so the possibility of spinal cord injury or accidental intrathecal injection is minimised [100,168]. Therefore, the sacrococcygeal epidural is considered a useful alternative and a safer technique than the lumbosacral epidural in cats, with less risk during injection [168,169]. Similarly, the sacrococcygeal epidural is occasionally preferred in large dogs in comparison to the lumbosacral epidural [151].

A spinal or a regular hypodermic needle can be used [100]. The needle is placed in the midline in the S3−C1 intervertebral junction, advanced until it penetrates the ligamentum flavum, and the injection is slowly made in the sacrococcygeal space [100,168]. The movement or bending of the tail helps identify the coccygeal vertebra, which will maintain mobility, in contrast to the sacral crest which remains immobile. When the needle penetrates the ligament, a «popping» sensation may be felt, especially if a spinal needle is used, while no resistance should be felt during the injection [100]. The adequate volume for sacrococcygeal epidural block has been reported to be less than half of the dose used to perform a lumbosacral epidural in dogs, that is 0.1 mL/kg of body weight [100,167]. Injecting a larger volume could cause more cranial blockage. However, after injection of 0.1–0.2 mL/kg in cats, no pelvic limb paralysis occurred [100]. The maximum volume that is used in cats is approximately 0.5 mL per cat. The use of this block has also been reported with ultrasound guidance in cadavers and live dogs [167] and with a nerve stimulator [168] and an ultrasound [169] in cats with adequate results.

8.6. Forelimb and Hindlimb—Digital Nerve Block

This technique can be used for both the hindlimb and the forelimb, desensitising the digital nerves, which run across either side of the metacarpal bones. It is used for surgery on the digits and associated pads [132]. A 22–25 G needle can be used injecting 0.2–1 mL of local anaesthetic on either side of the chosen metacarpal bone. The needle is aspired before each injection to avoid intravascular injection [72].

9. Local Anaesthesia of the Thoracic Region

9.1. Intercostal Block

This technique is used for surgery or injuries of the chest wall [57] and more commonly for thoracotomy or rib fractures [170]. The intercostal nerve block desensitises the ventral branch of the intercostal nerve [171]. Because the innervation of the intercostal spaces overlaps, at least three consecutive intercostal nerves must be blocked to provide anaesthesia and analgesia [170]. The nerves on both sides of the incision are targeted and three or five nerves are selectively blocked [57,172,173]. The local anaesthetic is injected at the proximal third of each pleura, near the intervertebral foramen and the agent is gradually diffused across the pleura and blocks the intercostal nerves (Figure 12) [174,175]. When the tip of the needle crosses the caudal margin of the pleura, it is then gently redirected cranio-medially and the drug is administered after aspiration [176].

This block does not depress respiratory function but improves it, by reducing pain on the targeted area [170,174]. When using anatomical landmarks to perform this block, the technique has a small probability of success (50%) compared to the ultrasound-guided technique (95%) [57]. Complications include unsuccessful injection, pneumothorax, laceration of vessels, injury of the lung and total spinal anaesthesia [57]. Ropivacaine can be used, at a maximum dose of 1.5 mg/kg [174] or bupivacaine at 0.5–1 mg/kg [173], divided in equal volumes for each site of injection.

9.2. Interpleural Block

This technique is usually performed by thoracocentesis or after thoracotomy and placement of a thoracic catheter, which is used to inject local anaesthetics into the thoracic cavity [170,174]. It is a safe and easy technique and can be performed repeatedly, as it is well tolerated by patients. The use of long-acting local anaesthetics, such as bupivacaine or ropivacaine, is suggested at a dose of 1.5–2 mg/kg diluted in saline and applied every six hours. After the injection, the patient should be placed in a manner to assist the diffusion of the drug towards the most affected area [174]. Complications of this technique include local anaesthetic toxicity caused by high systemic absorbance, Horner’s syndrome, or phrenic/sympathetic neural blockade [174,177].

9.3. Serratus Plane Block

This compartmental block is used to provide desensitisation of the thoracic wall for thoracotomy, usually under ultrasound guidance [178,179]. The ultrasound is used at the level of the shoulder, to visualise the area over the fourth and fifth pleurae, specifically the latissimus dorsi and ventralis thoracis muscles. The needle is inserted in a caudocranial direction and the local anaesthetic is injected between these two muscles [179]. This technique has been described in a dog cadaveric study using methylene blue stained ropivacaine [179] and in four dogs undergoing thoracotomy, using levobupivacaine, as part of a multimodal analgesic plan [178]. The aforementioned studies show promising results for this plane block and support that it is an easily performed block, when an ultrasound is used, with adequate analgesic effect [178,179].

9.4. Transversus Thoracic (TTP) Plane Block

This technique has been described in humans [180,181] and only once in veterinary medicine, in a dog cadaveric study, under ultrasound guidance [182]. It was used to provide analgesia for the anterior—ventral thoracic region and desensitise the local intercostal nerves [182]. In the study by Alaman (2021), several intercostal nerves have been successfully and straightforwardly stained—desensitised with the TTP plane block, providing promising results for this technique [182].

9.5. Erector Spina Plane (ESP) Block

This technique has recently received interest in human medicine and has been used to provide analgesia for acute and chronic pain [183]. In veterinary medicine, this technique has been described in 2 cadaveric studies in dogs [184,185] and in a study including 42 dogs undergoing hemilaminectomy surgery [186]. In the aforementioned study, bupivacaine was used with this technique alone or in combination with dexmedetomidine. The ESP block provided analgesia, reduced perioperative anaesthetic and analgesic drug requirements, while no adverse effects regarding the technique were observed [186]. The ultrasound is used to visualise the area between the longissimus thoracis muscle and the ninth thoracic transverse process [184,185].

10. Local Anaesthesia of the Abdominal Region

10.1. Quadratus Lumborum Block (QLB)

This block, under ultrasound guidance has been initially described to provide analgesia for abdominal surgery in humans [187]. The approach to perform the QLB block has been thoroughly described by Garbin et al. (2020) [188]. With this technique local anaesthetics are injected into an intramuscular plane and targets the abdominal lumbar spinal nerves (Figure 13). Furthermore, the administered local anaesthetic can reach the extrapleural area of the thoracic paravertebral space and result in visceral analgesia [189]. Ultrasound-guided QLB has been used as part of a multimodal anaesthetic plan, in cats undergoing ovariectomy and dogs undergoing ovariohysterectomy or ovariohysterectomy, resulting in reduced rescue analgesia requirements and reduced postoperative pain. The administrations above consisted of either 0.3–0.4 mL/kg of 0.25% bupivacaine or 0.3 mL/kg of ropivacaine 0.5% [190,191,192]. Nevertheless, further studies are required to evaluate the analgesic properties of the QLB in veterinary patients.

10.2. Transversus Abdominis (TAP) Plane Block

This is a technique that is used to provide unilateral analgesia of the abdominal wall, under ultrasound guidance, for patients undergoing abdominal procedures, such as mastectomy, caesarean section or retropubic prostatectomy [193,194,195,196,197]. The ultrasound is used to identify the transversus abdominis muscle and a large volume of the local anaesthetic (approximately 1 mL/Kg per hemiabdomen) is injected into the fascial tissue of the target area (Figure 14) [193,195]. Lidocaine has been used for this block, as well as ropivacaine and bupivacaine providing adequate and long-lasting analgesia [194,196,197,198,199].

The TAP block has been successfully combined with the serratus plane block, using diluted bupivacaine 0.25% (0.6 mL/kg of the solution) in four dogs undergoing unilateral radical mastectomy [194]. Moreover, the TAP block has been combined with intercostal nerve local analgesia (targeting the T4−T11 intercostal nerves), using bupivacaine 0.25%, in dogs undergoing unilateral radical mastectomy, producing sufficient analgesic results [199].

10.3. Rectus Abdominis Sheath Block

This is an ultrasound-guided technique that targets the abdominal wall and the fascial nerves that has been described in a cadaveric study in dogs [200]. It has also been described in a study including cats undergoing ovariectomy, using lidocaine or bupivacaine [201]. The technique is easily performed with the utilisation of an ultrasound and provides adequate analgesia and general anaesthetic-sparing effects [201]. The ultrasound is placed between the last pleura and the umbilicus, or 1 cm cranial to the umbilicus, and provides visualisation of the rectus abdominis muscle bilaterally, where the injections of local anaesthetic are made [200,201].

10.4. Intraperitoneal Technique

The intraperitoneal administration of local anaesthetics is recommended to reduce early postoperative pain and analgesic requirements after abdominal surgery, such as ovariohysterectomy, in dogs [70] and cats [202]. For the intraperitoneal injection the recommended dose of bupivacaine or ropivacaine is 1–2 mg/kg in cats and 2–4 mg/kg in dogs, while the recommended dose of lidocaine is 2–4 mg/kg in cats and 4–6 mg/kg in dogs [100], although higher doses have been used without signs of toxicity [70,100]. The administration can be performed after the procedure and before the closure of the linea alba, in the intraperitoneal space [70]. Alternatively, the local anaesthetic can be administered right after the abdominal incision [100]. The local anaesthetic should be left in the abdomen to lavage/infuse the peritoneal cavity for at least 10–30 min [100,203]. Combining ropivacaine (1 mg/kg) with dexmedetomidine (4 μg/kg) intraperitoneally after ovariohysterectomy in cats did not reduce rescue analgesia requirements [204]. In another study bupivacaine (2 mg/kg) was administered in combination with dexmedetomidine (1 μg/kg), and rescue analgesia was required in only 1 of 16 cats [202]. The use of bupivacaine 2.5 mg/kg or levobupivacaine 2.5 mg/kg intraperitoneally after laparotomy, in dogs undergoing ovariohysterectomy, resulted in lower intraoperative anaesthetic and postoperative analgesic requirements, in comparison to the control group [205].

Intraperitoneal local anaesthetic injection has been used in combination with opioids to treat severe abdominal pain caused by pancreatitis in dogs [203,206]. In this technique, an intraperitoneal injection of a mixture of bupivacaine with saline and sodium bicarbonate [203] or repeated doses of 1.5 mg/kg lidocaine [206] have been used, via an intravenous catheter (18–20 G) inserted into the abdomen.

11. Local Anaesthesia of Miscellaneous Nerves

11.1. Intratesticular Block

The intratesticular administration of local anaesthetics, as part of a balanced anaesthetic protocol, has been widely reported. A 2% lidocaine solution was administrated intratesticularly, using a 23 or 25 G needle, five min before surgery at a dose of 2 mg/kg, in cats [207] and dogs [208], resulting in signs of decreased intraoperative pain. A 1% ropivacaine solution was injected (1–2 mg/kg) intratesticularly and subcutaneously in dogs resulting in reduced anaesthetic requirements. Finally, the use of a lidocaine (1 mg/kg) and bupivacaine (1 mg/kg) mixture injected intratesticularly caused no postoperative analgesic benefit or postoperative complications compared to the use of saline [209]. The use of bupivacaine, which is a cardiotoxic drug, should be avoided intratesticularly because of the tissue’s high perfusion [208].

11.2. Mesovarium Splashing during Ovariohysterectomy

For this technique, the local anaesthetic is splashed directly on the ovaries and the mesovarium prior to their manipulation [210]. Using lidocaine for this technique (2 mg/kg on each ovary) combined with incision splash block and incisional infiltration, in cats undergoing ovariohysterectomy, resulted in adequate analgesic effect [210]. On the contrary, a study in dogs undergoing ovariohysterectomy, reported that splashing 0.5 mL of lidocaine (2%) on the mesovarium had no analgesic benefit [211].

11.3. Intra-Articular Block

Intra-articular local anaesthesia is used to provide postoperative and intraoperative anaesthesia and analgesia for various joints in small animals, including the elbow [212], the wrist [213] and the stifle joint [214].

The patient should be sedated to tolerate the injection and is usually placed in lateral recumbency with the targeted joint uppermost; the area is then clipped and disinfected and aseptic conditions should be used (gloves, syringes, needles). To ensure the right placement of the needle before the injection, synovial fluid should be aspirated and no resistance to the injection should be observed [215].

The approaches for the individual puncture sites are listed below, as described by Van Vynckt [215]. The puncture of the shoulder joint is performed craniolaterally, between the acromion and the greater tubercule; the site for the elbow joint can be approached by directing the needle in the supratrochlear foramen proximal and parallel to the anconeal process; for the carpal joint, the carpus is flexed to 90° and the needle is inserted laterally or medially to the common digital extensor tendon. For the hindlimb, the approach to the hip joint is performed dorsally to the greater trochanter and perpendicularly to the long axis of the limb; for the stifle joint the needle is inserted to the centre of the intercondylar joint space, between the patella and the tibial tuberosity; lastly, for the tarsal joint the needle is inserted between the distal part of the tibia and the calcaneus [215].

Several studies showed that intra-articular administration of local anaesthetics results in no signs of neurotoxicity [212,216,217]. A single dose of bupivacaine intra-articularly in dogs resulted in a slow systemic absorption, did not reach toxic plasma concentration, and no neurological symptoms were reported after the injection [216]. After intra-articular administration of lidocaine in dogs, no signs of neurotoxicity were reported [212], although in some cases the blood plasma concentration exceeded the neurotoxic dose (2.7 μg/Kg), as suggested by Lemo et al. (2007) [218].

While intra-articular administration of local anaesthetics is a common technique, its use is controversial, because local anaesthetics are believed to have a toxic effect on articular chondrocytes [219,220]. In vitro and ex vivo studies suggest that local anaesthetics have a time- and dose-dependent toxic effect on articular chondrocytes [219,221]. One in vivo study in rats suggested that a single intra-articular bupivacaine administration in the stifle joint caused no change to the chondral surfaces, although six months after the injection, chondrocyte density was reduced up to 50%, compared to the control group [222].

11.4. Intravenous Regional Anaesthesia (Bier’s Block)

This technique is used for short procedures in the distal part of the limb, such as foreign object removal, digit amputation, fracture repair, mass removal, wound suturing, and others. The local anaesthetic is administered intravenously after a compression bandage/tourniquet is placed on the limb, proximal to the surgical site, which is either a thoracic limb distal to the elbow or a pelvic limb distal to the tarsus [223]. The cephalic or saphenous vein should be catheterised beforehand to provide access for the local anaesthetic [123] and the catheter can be removed after the injection if needed. The correct tourniquet placement presupposes obstruction of both arterial and venous blood flow (loss of peripheral pulse feeling) and offers less bleeding and better visualisation of the surgical site [223]. If an inflated tourniquet is used, a pressure of 250–300 mmHg has been recommended in humans to achieve the desired result [224,225]. Alternatively, the lower occlusion pressure could be measured, as the lowest pressure required to prevent arterial blood flow by the tourniquet (loss of pulse). The tourniquet is then placed using a pressure 50–100 mmHg above the previously measured lower occlusion pressure [223]. Nevertheless, the tourniquet should remain in place for up to 60–90 min to avoid tissue ischaemia and ischemic pain [226]. Analgesia is provided by the diffusion of the local anaesthetic to the local nerve endings, because of the tourniquet, that prohibits the systemic absorption of the local anaesthetic [123]. The local anaesthetic should be injected slowly (over two to three min) and the analgesic effect begins 5–10 min after the injection [123]. The anaesthetic of choice is preservative free lidocaine (0.25–2%) at a dose of 2.5–5 mg/kg and a total dose of 0.6 mL/kg [226]. Ropivacaine (0.2–0.25%) has also been proven effective in humans, providing longer analgesia than lidocaine (even after tourniquet removal), and fewer complications than bupivacaine [227], although animal studies should also support this claim before ropivacaine is used in companion animals. Common complications include ischaemia and nerve injury, hematoma formation, subcutaneous haemorrhage or extremity engorgement because of incorrect tourniquet placement and local anaesthetic systemic toxicity because of overdose or leakage from the tourniquet [223].

The combination of dexmedetomidine with lidocaine for Bier’s block has been used for faster onset of action, prolonged duration of action and reduced rescue analgesic requirements in humans [228,229]. Additionally, the administration of fentanyl in combination with lidocaine or prilocaine for Bier’s block, demonstrated limited clinical relevance in humans [230,231]. Similar results were observed after the administration of morphine alone or in combination with prilocaine for Bier’s block in humans [232,233]. On the other hand, the combination of buprenorphine and lidocaine for Bier’s block, in humans, produced faster onset and longer duration of action in comparison to lidocaine alone [234]. The research concerning the combination of local anaesthetics with opioids to perform Bier’s block is limited in veterinary medicine, but several results from human medicine are promising.

11.5. Combination of the QLB with the Greater Ischiatic Plane Block

The caudal approach of the QL block has been recently combined with a greater ischiatic plane block to provide desensitisation of the hindlimb in a study in dogs (healthy Beagles and dog cadavers) [235]. In the aforementioned study, an ultrasound was used in combination with a nerve stimulator to provide visualisation of the target areas. This combination of local anaesthetic techniques (or GIN-CQLB technique), provided adequate analgesia of the femur, while preserving the motor function and therefore it is considered a promising novel technique for hindlimb analgesia [235].

12. Conclusions

Local anaesthetics have been used in veterinary practice for many years. Further incorporation of advanced techniques and the utilization of the ultrasound and the electric nerve stimulator is imperative to perform the local anaesthesia techniques safely, with more ease and with an increased success rate. The use of local anaesthesia in veterinary practice, as part of a multimodal analgesic plan is beneficial for the patient, the veterinarian, as well as the client and the practice facility. Local anaesthetic techniques reduce the perioperative pain and stress of the patient. At the same time, lower doses of systemic analgesic and anaesthetic agents can be used, reducing possible adverse effects. Finally, recovery is smoother and discharge from the hospital occurs earlier.