Basics of local anaesthesia


The first reported use of local anaesthetic for surgical purposes was by a Peruvian army surgeon, that drug being cocaine, derived from Erythroxylon coca, an Andean shrub. In 1884 cocaine was used in Germany by Carl Koller to anaesthetise the cornea by instillation into the conjunctival sac.


In the 21st century local anaesthesia remains a key modality in the prevention and management of pain. Advances in our knowledge of techniques has been facilitated by research and technology, namely nerve locators and ultrasound guidance.



An action potential is propagated down a nerve because of activity in sodium channels. Local anaesthetics block sodium channels and therefore block transmission of the stimulus via A and C fibres. Local anaesthetics act on nerves in the periphery, depending on where they are injected. Local anaesthetics reversibly block sodium channels in the nerve membrane which are necessary for membrane depolarisation. The conduction of the action potential is halted and thus transmission of the stimulus is prevented.


Ideal properties of a local anaesthetic

  • Rapid in onset

  • Long duration

  • Highly potent

  • Small volume

  • Low toxicity

  • No pain upon injection

  • Cost effective

Local anaesthesia prevents nociceptive impulse transmission therefore minimises central sensitisation which reduces the requirement for post-operative systemic analgesics.


The focus of current research is on developing new veterinary techniques, which are often modifications of techniques in humans, documenting new approaches to well-established techniques and improving the efficacy of techniques. In conjunction with opioids and NSAIDs local anaesthetics form the basis of multimodal analgesia and enable us to offer a preventive approach to analgesic management.


The basic principle of regional anaesthesia was stated in an editorial in the British Journal of Anaesthesia:

Regional anaesthesia always works – provided you put the right dose of the right drug in the right place’ (Denny & Harrop Griffiths, 2005).


Techniques to improve accuracy and therefore efficacy include ultrasound guidance and the use of a nerve locator, both of which are reported for blockade of nerves innervating the pelvic limb in dogs. Our aim is deposition of local anaesthetic surrounding the nerve – not intraneural injection which may cause nerve damage.


How can local anaesthetics be used?

The most common use of local anaesthetics is in nerve blocks whereby local anaesthetic is infiltrated around the nerve in anaesthetised patients. This is particularly useful for dentistry as the nerve blocks are easy and quick to perform and the patient is much more comfortable after surgery. Evidence from human studies document better patient satisfaction where first day pain is more effectively controlled. Work in dogs clearly demonstrates superiority of local anaesthesia over a fentanyl infusion for stifle surgery. Further studies have shown a reduction in the stress response to anaesthesia and surgery.


Local anaesthetic techniques should be considered in all cases undergoing a surgical procedure;

  • Reduced volatile agent requirements & associated cardiopulmonary depression

  • Superior pain control

  • Decrease requirement for other analgesics

  • Key to multimodal analgesia

What are the benefits of using local anaesthesia?

With practice and a basic knowledge of anatomy local analgesic techniques are quick and simple to perform and add greatly to balanced anaesthesia. The benefits to the animal are reduced post-operative pain and reduced intra-operative pain. As a result of this the anaesthetic will be much smoother and the requirements for maintenance agents will be much reduced.


Which agents are available in practice?

Lidocaine, bupivacaine and ropivacaine are all available in preparations suitable for infiltration and are most commonly used in small animal practice. Neither bupivacaine nor ropivacaine are licensed for veterinary use. Lidocaine without adrenaline should be used.


The ideal properties of a local anaesthetic

The ideal local anaesthetic should be rapid in onset so that it is working by the time the surgeon incises but should have a long duration so that it is working for the duration of the surgery and well into the post op period. Ideally the chosen drug would be a small volume that does not cause pain on injection. Individual properties of local anaesthetics are listed here.

Local anaesthetic agents

Unfortunately none of these properties are found in one preparation, however can be partly achieved by combining products. Lidocaine and bupivacaine combination is thought to provide a rapid onset with a good duration of action whereas lidocaine/ropivacaine gives a similar duration of action with less risk of cardio-toxicity: this however has not been evaluated in dogs or cats.


Additives

Various additions have been reported. Adrenaline causes vasoconstriction and keeps the local anaesthetic near the site – or also prevents diffusion away and absorption. Bicarbonate alters the pH and increases the speed of onset.

Of most interest is the addition of dexmedetomidine to the local anaesthetic to prolong action – although a recent metanalysis in humans failed to demonstrate a benefit. Exact dose required is yet to be determined in dogs.


Extended action

A liposome encapsulated bupivacaine (Nocita) is currently marketed in the US and may be released with a vet license soon in the UK. The lipid bilayer of the liposomes releases over time to give an anaesthetic effect up to 96hrs (Lascelles & Kirkby 2016).


Toxicity

Local anaesthetics reversibly block sodium channels in excitable tissues which as well as nerves includes the myocardium. This blockade affects the cardiac action potential. Bupivacaine is ten times slower to dissociate away from sodium channels than either lidocaine or ropivacaine and this may lead to arrhythmias and ventricular fibrillation. For this reason neither bupivacaine nor ropivacaine should be used IV.

Levobupivacaine, an isomer of bupivacaine has a better CV side effect profile than bupivacaine however should still not be administered IV. The dose for bupivacaine and levobupivacaine is the same at 1-2mg/kg.


The key to reducing toxic potential is to use only as much local anaesthetic as is needed. In the human field with an awake patient they can be asked when sensation disappears, however we do not have this luxury in our patients! In some situations the use of a peripheral nerve stimulator allows for more accurate infiltration. A needle is used that a current can be passed down to stimulate the motor nerve being blocked. Following infiltration the nerve is again stimulated to check correct deposition of local. In veterinary patients this technique may be used for a brachial plexus nerve block, however would be ineffective for a maxillary nerve block because the maxillary nerve contains only sensory fibres in this region.


Contraindications to local anaesthesia

Infection

Neoplasia


Next steps

Keen to get started with local anaesthesia? View some of the videos here – perhaps try the incisional block first for an easy to practice technique offering substantial benefit to your patients.


References

Denny, N. M., & Harrop-Griffiths, W. (2005). Editorial I: Location, location, location! Ultrasound imaging in regional anaesthesia. British Journal of Anaesthesia.

Lascelles, B. D. X., & Kirkby Shaw, K. (2016). An extended release local anaesthetic: potential for future use in veterinary surgical patients? Veterinary Medicine and Science, 2(4), 229–238. http://doi.org/10.1002/vms3.43

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