Local anaesthetic techniques

Transcription

1 Vet Times The website for the veterinary profession Local anaesthetic techniques Author : CARL BRADBROOK Categories : Vets Date : June 30, 2014 CARL BRADBROOK BVSc, CertVA, DipECVAA, MRCVS discusses loco-regional anaesthesia techniques, when they are appropriate, any complications that may be observed and which drugs may be administered Summary Local anaesthesia is the technique of performing nerve blocks using local anaesthetic drugs; most commonly alone, but also in combination with other drugs to manipulate their onset, duration or add to their effect. The term loco-regional anaesthesia is now commonly used to describe this area of anaesthesia and analgesia. Loco-regional anaesthesia is relatively easy to perform with a good knowledge of anatomy and requires little in terms of drugs and equipment. Adding a nerve block into an anaesthetic protocol may aid in reducing intraoperative inhalational anaesthetic requirements, thus reducing the potential adverse effects associated with this class of drugs. This article discusses the techniques that may be performed, emphasising those most applicable to practice. Potential complications and guidelines on how to perform the blocks are highlighted, along with the commonly used drugs. Some advances in loco-regional anaesthesia are also discussed, highlighting those that have are increasingly popular in the veterinary anaesthesia community. Electro-neurolocation and ultrasound guidance are the two areas of clinical research that are considered to offer many advantages over the described blind techniques. 1 / 30

2 Key words anaesthesia, analgesia, loco-regional anaesthesia, local anaesthetics, pain THE term loco-regional anaesthesia is now used to describe the technique of performing nerve blocks using local anaesthetic drugs; most commonly alone, but also in combination with other drugs to manipulate their onset, duration or add to their effect The term is appropriate for describing both local anaesthesia and neuraxial (epidural and spinal) anaesthesia. The objective of loco-regional anaesthesia being to prevent or reduce perception of a painful stimulus (nociception). The techniques described in this article may be successfully incorporated into anaesthetic protocols for use on a daily basis in practice. Loco-regional anaesthesia is used extensively in human medicine to provide intra and postoperative analgesia. The techniques used in veterinary medicine have largely been adapted from those described in human medicine, but relevant species differences in regional anatomy are required for successful block performance. Clinical and cadaver studies now being carried out on a regular basis have allowed for a greater understanding of the most applicable techniques for our veterinary patients. Local anaesthetic techniques are relatively easy to perform with a good knowledge of anatomy and require very little in terms of drugs and equipment. With careful practice they can form an important part of a patient s anaesthetic management. Perhaps most importantly local anaesthetic techniques are the only part of the anaesthetic protocol that completely blocks peripheral nociceptor input, thereby aiding in reducing the development of altered or chronic pain states. Local anaesthesia can reduce intraoperative inhalational anaesthetic requirements, thereby reducing the adverse effects associated with this class of drugs, in particular vasodilation and subsequent hypotension. They can aid in providing a stable level of general anaesthesia and reduce the number of alterations that may be required to the vaporiser setting. They are particularly useful in patients where avoidance of significant hypotension and reduced cardiac contractility are desired. This may be, for example, in patients with cardiac, renal or hepatic disease. Incorporating a local anaesthetic technique into a patient s protocol (alongside the use of opioids and NSAIDs where appropriate) allows for the provision of multimodal analgesia, aids in improving postoperative analgesia and in reducing pain scores post-anaesthesia. Potential risks 2 / 30

3 A number of potential risks are associated with the local anaesthetic techniques described, but they are rare with appropriate training, the correct equipment and a good knowledge of species anatomy. Some of the risks include: inadvertent vascular injection; intra-neural injection; poor efficacy; poor technique; or abnormal anatomy. Commonly used local anaesthetics Local anaesthetics used in small animal practice include lidocaine, bupivacaine, levobupivacaine and ropivacaine. Bupivacaine and levobupivacaine are the most commonly used for loco-regional anaesthesia, primarily due to their longer duration of action compared to lidocaine. Levobupivacaine has superseded bupivacaine due to having a lower risk of cardiotoxicity, otherwise it is similar to bupivacaine. The clinical effect of a local anaesthetic depends on the dose, volume and route of administration. Careful calculation of the maximum dose to be used must be carried out as toxic levels are easily achieved, especially in smaller patients (Table 1). For example, care should be taken when performing a local block in a cat following use of lidocaine topically prior to endotracheal intubation, as the two doses used are cumulative and both must therefore be taken into account. Indicators of toxicity to be aware of include neurological signs such as seizures, which may be followed by cardiovascular signs including rhythm and electrocardiogram (ECG) abnormalities. Under general anaesthesia, neurological signs are not easily observed, therefore cardiovascular abnormalities are often the first sign of toxicity noted. Monitoring the ECG is very important if any suspicion of toxicity is present. Diluting local anaesthetics helps to reduce local tissue toxicity, assist in avoiding excess drug administration and increase the volume potentially facilitating better absorption. On the other hand, over-dilution should be avoided as it may reduce the clinical effect, with less local anaesthetic actually present in the required area. Other drugs may be combined with local anaesthetics for the provision of loco-regional anaesthesia, most commonly for epidural anaesthesia. The most commonly used adjunct with this 3 / 30

4 technique is the opioid analgesics, for example morphine or methadone. Techniques Head blocks There are four commonly performed head blocks. The mandibular nerve (Figure 1) is located on the medial aspect of the vertical ramus of the mandible and may be blocked either from an intraoral approach or through the skin on the ventral aspect of the mandible. To perform this block, the mandibular foramen should be located digitally within the oral cavity and used to direct the needle for correct placement of local anaesthetic. This block allows desensitisation of the entire mandible on the ipsilateral side and is suitable for dental, gingival and mandibular surgery. It is not advised to perform a bilateral block due to the potential for self-trauma to the tongue. The inferior alveolar (mental) nerve (Figure 1) is located on the lateral aspect of the rostral mandible and may be blocked where it exits from the mental foramen, which is easily palpable between the lower canine and first premolar. The needle may be advanced with care into the mental foramen prior to injection. The gingival fold often lies over this region and may make needle placement difficult. This block will only desensitise the most rostral aspect of the mandible and in most cases up to two thirds of the associated canine. It may be blocked bilaterally for rostral dental procedures. The infra-orbital nerve (Figure 2) is located where it exits from the infra-orbital canal on the lateral aspect of the maxilla. The block may be approached through the skin or the gingiva, aiming for the infra-orbital canal, which may be palpated, at a line drawn ventrally from the medial canthus of the orbit. The block may be performed using a hypodermic needle to deposit local anaesthetic at the entrance of the infra-orbital canal or by passage of a shielded-over needle-type cannula to the level of lateral canthus prior to deposition of local anaesthetic. The traditional technique only allows desensitisation to the level of the mid maxilla whereas the modified approach has been described to be an acceptable alternative to the maxillary nerve block. The maxillary nerve block (Figure 3) is performed by palpating the caudal aspect of the zygomatic arch and identifying the most dorsal aspect, this will denote the needle entry point. The needle should be inserted in a cranioventral direction and a spinal needle may be required for sufficient needle length in well-muscled patients until the caudal aspect of the infra-orbital canal is located. Careful aspiration prior to local anaesthetic injection is required as it is possible to puncture the maxillary artery in this location. Use of this block is indicated for dental treatment and surgery to the upper dental arcade, soft palate and maxilla. Ophthalmic blocks 4 / 30

5 The retrobulbar nerve block (Figure 4) is approached dorsolateral to the globe in the orbit, passing through either the skin or conjunctiva. It is only advised for use before enucleation due to risk of trauma to the globe and associated structures. A pre-curved (Figure 5) or a self-curved needle should be walked off the orbital bone until the tip of the needle is caudal to the globe. Care should be taken to aspirate prior to injection and, if excess pressure is encountered during local anaesthetic injection, the needle should be repositioned and the injection repeated. Thoracic limb blocks The brachial plexus (BP) may be blocked by the traditional approach prior to any planned surgery to the forelimb, although it may be less effective for surgery proximal to the elbow. The BP block (Figure 6) may be performed blind or with the aid of electro-neurolocation. Care must be taken during this technique to avoid creating a pneumothorax and it should only be performed unilaterally due to potential for blocking of the phrenic nerve. The BP nerves arise from the spinal nerve roots of C6, C7, C8 and T1 and a successful technique will block the musculocutaneous, radial, ulnar and median nerves. The patient is positioned in lateral recumbency with the affected limb uppermost. A spinal needle is introduced parallel to the thoracic wall, cranial to the acromion and medial to the subscapularis muscle at the level of the scapulohumeral joint and directed in a caudodorsal direction. The first rib should be located, which will mark the caudal aspect of the brachial plexus. Local anaesthetic should be deposited as the needle is advanced, aspirating prior to each injection. Electro neurolocation may be used to identify each nerve individually prior to local anaesthetic injection. The radial, ulnar, median, musculocutaneous (RUMM) nerve block is indicated for surgery distal to the elbow and offers an alternative to the BP block. It requires three separate injection sites and may be approached blind, although electro neurolocation may improve accuracy. The radial nerve (Figure 7) may be palpated on the lateral aspect of the humerus at the junction between the middle and distal thirds. The ulnar nerve (Figure 8) is located on the medial aspect of the humerus and may be palpated over the region of the olecranon, but should be blocked at the level of the distal humerus. The median and musculocutaneous nerves are blocked together on the medial aspect of the distal humerus, proximal to the medial epicondyle of the humerus between the biceps brachii and medial head of the triceps. Pelvic limb blocks 5 / 30

6 The sciatic nerve (SN) block may be combined with either a femoral nerve (FN) or lumbar plexus block (LPB) to provide analgesia for pelvic limb surgery. Electro-neurolocation is recommended for these techniques to improve accuracy, ensure safety and reduce the dose of local anaesthetic required. It is also unlikely satisfactory local anaesthesia will be gained without electroneurolocation. The SN (Figure 9) is blocked at its proximal location caudal to the greater trochanter of the femur. The puncture site is located at the junction of the cranial and middle thirds between a line drawn between the greater trochanter of the femur and the ischial tuberosity. The depth of needle insertion varies depending on the size of patient and may be up to 6cm to 8cm. The FN (Figure 10) is blocked at its location on the medial aspect of the pelvic limb in the femoral triangle. The femoral artery is palpated within the femoral triangle, held in place with light digital pressure and the FN is located and blocked cranial to the artery, usually in a superficial location. An alternative to the FN block is the LPB, which allows for the femoral nerve to be blocked more proximally and avoids the risk of missing the saphenous nerve high within the inguinal region. The saphenous nerve supplies the cutaneous innervation to the stifle and therefore if missed a patient may respond to skin incision. The LPB (Figure 11) is performed with the patient positioned in lateral recumbency with the side to be blocked uppermost. The dorsal process of L7 is palpated and from this the dorsal process of L5 identified. The needle is inserted lateral to L5 (approximately 2cm to 3cm) until the transverse process is contacted. The needle is then walked off bone caudally and a loss of resistance may be felt as it passes through the intervertebral ligament. Local anaesthetic is then injected after aspiration. Neuraxial blockade Epidural and spinal anaesthesia both come under the banner of neuraxial blockade, whereby local anaesthetic, sometimes in combination with other drugs is deposited in the bony vertebral canal. Epidural (extradural) anaesthesia is most commonly employed in veterinary anaesthesia, where the drug is deposited into the space outside of the dura. This technique is indicated for pelvic limb, perineal and abdominal surgery. Epidural injection (Figure 12) is most commonly performed at the lumbosacral junction (L7-S1), although caudal injection may be performed at Co1-Co2 for perineal surgery for example. Table 2 highlights the contraindications and complications of epidural anaesthesia. For epidural anaesthesia the patient should be positioned in either sternal (Figure 13) or lateral recumbency. In sternal recumbency the pelvic limbs should be extended cranially and the wings of 6 / 30

7 the ilium palpated with the thumb and second finger. The dorsal spinous process of L7 is palpated in the midline with the index finger, the lumbosacral space lies caudal to this. The spinal needle (Figure 14) is inserted in the midline perpendicular to the skin. The stylet is removed once in the subcutaneous tissue and saline applied to the needle hub. The needle is then advanced slowly until saline is aspirated into the needle due to the sub atmospheric pressure within the epidural space. The presence of cerebrospinal fluid (CSF) at this point should alert the clinician to withdraw the needle and reconfirm positioning within the epidural space. In the cat the more caudal extension of the spinal cord makes it more likely a spinal injection will be performed. Drug doses should be reduced by a half to three-quarters if a spinal injection is performed instead. A test injection may be performed before aspiration and slow injection of the selected drugs. Preservative-free drugs should be selected to minimise the risk of neurotoxicity. Advances in loco-regional anaesthesia Electro-neurolocation and ultrasound guidance are gaining popularity in veterinary medicine and offer many advantages over the described blind techniques. The use of peripheral nerve locators (Figure 15) is now common in the veterinary anaesthesia community and, once bought, the cost of consumables is not prohibitive if the technique is to be used regularly. Differences in anatomy between individuals and species allows the clinician to improve the accuracy of identifying the correct location for injection and to ensure local anaesthetic is deposited as close as possible to the desired nerve. Ultrasound is widely available in veterinary medicine and with some training and practice may be used to aid in neurolocation. There are a number of techniques now described in the dog and it is considered the gold standard in human anaesthetic practice. Summary Local anaesthetic techniques are easily employed in practice as part of a balanced, multimodal approach to anaesthesia and analgesia and should be considered where appropriate. With a good knowledge of anatomy and careful dose calculation, the various blocks can be carried out with excel- lent success. For the majority of the discussed techniques, no specialist equipment is necessary, requiring only items commonly utilised in practice. The use of local anaesthetic techniques can greatly improve patient analgesia both during anaesthesia and into the recovery period and beyond. There can also be a noticeable improvement 7 / 30

8 in post-operative pain scores and subsequent reduction in analgesic requirements during the recovery period. Further reading Campoy L and Read M R (2013). Small Animal Regional Anesthesia and Analgesia, WileyBlackwell. Dugdale A H A (2010). Veterinary Anaesthesia, Principles to Practice, Wiley-Blackwell. Lemke K A (2007). Pain management II: local and regional anaesthetic techniques. In Seymour C and Duke-Novakovski T (eds) BSAVA Manual of Canine and Feline Anaesthesia and Analgesia (2nd edn): / 30

9 Figure 1. Location of needle insertion site for the mandibular nerve block (top) and mental nerve block (bottom). 9 / 30

10 Figure 2. Location of needle insertion site for the infra-orbital nerve block. 10 / 30

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12 Figure 3. Location of needle insertion site for the maxillary nerve block. 12 / 30

13 Figure 4. Needle insertion site for a retrobulbar nerve block. 13 / 30

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15 Figure 5. Pre-curved needle for retrobulbar nerve block. 15 / 30

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17 Figure 6. Landmarks for brachial plexus nerve block in the dog. 17 / 30

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19 Figure 7. Location of needle insertion site for radial nerve block. 19 / 30

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21 Figure 8. Location of needle insertion site for ulnar nerve block. 21 / 30

22 Figure 9. Landmarks for sciatic nerve block in the dog. 22 / 30

23 Figure 10. Landmarks for femoral nerve block in the dog. 23 / 30

24 Figure 11. Location of needle insertion site for lumbar plexus block. 24 / 30

25 Figure 12. Landmarks for epidural anaesthesia injection. 25 / 30

26 Figure 13. Identification of landmarks for epidural injection can be more challenging in obese patients. 26 / 30

27 Figure 14. Location of needle insertion site and connection of syringe prior to epidural injection. 27 / 30

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29 Figure 15. Example of a peripheral nerve locator used for electro neurolocation techniques. Table 1. Doses of local anaesthetics used in small animal anaesthesia 29 / 30

30 Table 2. Absolute and relative contraindications of epidural anaesthesia 30 / 30 Powered by TCPDF (