It is widely accepted that the reduction of pain, or analgesia, assists in a patient’s recovery from surgery. For this purpose, and to be most effective, analgesia should be instigated at the premedication phase, maintained across the surgery and continued well into the recovery phase.
The use analgesia prior to any procedure that is likely to cause pain is known as pre-emptive analgesia.
The administration of several analgesic agents with different modes of action (e.g. opioids, non-steroidal anti-inflammatory drugs (NSAIDs) and local anaesthetics) is known as multimodal analgesia. This technique can target more than one point in the nervous system, thus making the control of pain more effective.
Why Locoregional Anaesthesia?
Local anaesthesia is the reversible loss of sensation in a localised area of the body caused by the depression of excitation in nerve endings or inhibition of the conduction process in peripheral nerves (Covino and Vassallo, 1976). Where local anaesthetic is used to desensitise a region of the patient’s body, the term locoregional anaesthesia (LRA) is often employed.
There has been an increase in the popularity of LRA techniques within the veterinary profession over recent years and in human medicine it is well recognised that the use of local anaesthesia reduces the time to discharge versus the use of systemic analgesia alone. In addition to this, the appropriate application of local anaesthesia is likely to reduce the dose of anaesthetic required for both induction and maintenance. This in turn leads to less cardiopulmonary depression, reduces post-operative pain scores and results in a faster more stable recovery, increasing the likelihood of a safer anaesthetic (Taylor and Robertson, 2004; Caniglia et al. 2012; Romano et al. 2015).
How to Perform Locoregional Anaesthesia and Drugs to Consider
LRA may be performed either via a ‘blind’ technique, where the needle is placed in an appropriate location based on anatomical knowledge of nerve location, or via a ‘guided’ technique using ultrasound or electrical stimulation needles. Whilst a ‘guided’ approach may improve the accuracy of your peripheral nerve block (PNB), if the technique is to be practiced safely, it is vital that the clinician has a thorough appreciation for the associated anatomy. Iatrogenic nerve injury occurs in a reported one in 10,000 cases in human medicine following perineural analgesia and similar injuries are also reported in the veterinary literature (Forterre et al. 2007; Murdoch and Michou, 2015).
The drugs used for LRA mainly fall within three classes, these include:
Local anaesthetics are the primary drug class used for LRA although opioids and alpha2-adrenergic agonists may be added in certain circumstances. In general, preservative free drugs are preferred, especially when used intra-articularly or epidurally to avoid potential preservative related toxic effects (Borer-Weir, 2014).
1. Local Anaesthetics
Local anaesthetics elicit their effects via blockade of sodium ion channels on nerve axons, this prevents sodium ion influx and blocks nerve action potential and propagation of nerve impulse transmission.
Local anaesthetics are an unusual class of drug as their site of application is also their site of action. As a result, systemic absorption controls both the duration and offset of action (Borer-Weir, 2014). Systemic absorption of local anaesthetics is affected by:
- injection site,
- drug dosage,
- presence of added vasoconstrictor,
- or the pharmacological profile of the drug itself.
Common examples include:
|Drug||Lidocaine hydrochloride||Bupivacaine hydrochloride|
|Time to onset||2-5 minutes||5-10 minutes|
|Duration of action||1-1.5 hours||4-8 hours|
|Dose||3-4 mg/kg||1-2 mg/kg|
|Maximum safe dose||10 mg/kg (Dogs)|
6 mg/kg (Cats)
|2 mg/kg (Dogs and Cats)|
|Comments||Licensed formulations available. |
Combined with epinephrine which can prolong the anaesthetic effect.
|No current UK licence for use in veterinary species.|
Table 1. (Adapted from Dugdale, 2001 and Duke-Novakovski, 2016).
Toxic doses (see Table 1.) for these drugs exist and so their judicious use is recommended. Toxicity mainly results from inadvertent intravenous injection and so given the proximity of blood vessels to nerves it is vitally important to aspirate before injecting any local anaesthetic.
Signs of toxicity mainly include depression, sedation, seizure like activity and hypotension through cardiovascular depression.
Commercially available infusions of 20% lipid emulsion can be used at a dose of 1.5-3ml/kg administered over 30 minutes to treat toxicity from overdose of local anaesthetics (O’Brein et al. 2010). The mechanism of action is currently unknown, but theories include:
- By the stimulation of a ‘lipid plasma phase’ which attracts the lipid soluble local anaesthetic therefore reducing the amount of free drug within the plasma (Borer-Weir, 2014).
- By increasing myocardial calcium through fatty acid provision therefore countering the negative inotropic effects of the local anaesthetic.
- By preventing inhibition of the enzyme responsible for the transport of fatty acids, used for oxidative phosphorylation, into myocardial cells (Duke-Novakovski, 2016).
Morphine is the opioid that is most often used in LRA techniques, and there is increasing evidence to suggest that there are opioid receptors on the peripheral terminals of sensory nerves (Borer, 2006). Low doses of morphine are required when used in this manner, typically 0.1mg/kg providing local analgesia when used epidurally (Borer, 2006). The upregulation of opioid receptors that occurs within inflamed joints means that opioids will also exert an analgesic effect when injected intra-articularly (Lindegaard et al. 2010).
3. Alpha2- agonists
The addition of an alpha2- agonist to local anaesthetic when used as a peripheral nerve block has also found to have beneficial effect. For example, Lamont and Lemke, 2008 found that the addition of medetomidine to mepivacaine prolonged both motor and sensory blockade after radial nerve block in dogs.
Please note that not all the drugs discussed in this article are licenced for use in small animals despite their frequent use in veterinary practice. Their use is the responsibility of the administering veterinary surgeon following an appropriate risk/benefit analysis in each case and in line with the cascade.
Please keep an eye out for Part 2 where we will discuss some of the more common practical applications of LRA in veterinary practice.
Originally published: Thursday, 31st May 2018
Last updated: Thursday, 14th June 2018
- Borer, K. 2006. Local analgesic techniques in small animals. In practice. 28(4): 200-207.
- Borer-Weir, K., 2014. Analgesia. In Veterinary Anaesthesia (Eleventh Edition): 101-133.
- Caniglia, A.M., Driessen, B., Puerto, D.A., Bretz, B., Boston, R.C., Larenza, M.P. 2012. Intraoperative antinociception and postoperative analgesia following epidural anesthesia versus femoral and sciatic nerve blockade in dogs undergoing stifle joint surgery. Journal of the American Veterinary Medical Association. 241: 1605–1612.
- Covino, B. G., and Vassallo, H. G. 1976. Local Anesthetics: Mechanisms of Action and Clinical Use. New York, San Francisco, London: Grune & Stratum.
- Duke-Novakovski, T. 2016. Pain management II: local and regional anaesthetic techniques. In: BSAVA manual of canine and feline anaesthesia and analgesia (Third Edition). John Wiley & Sons.
- Dugdale, A., 2011. Veterinary anaesthesia: principles to practice. John Wiley & Sons.
- Forterre, F., Tomek, A., Rytz, U., Brunnberg, L., Jaggy, A. and Spreng, D. 2007. Iatrogenic sciatic nerve injury in eighteen dogs and nine cats (1997–2006). Veterinary surgery. 36(5): 464-471.
- Lamont, L.A. and Lemke, K.A. 2008. The effects of medetomidine on radial nerve blockade with mepivacaine in dogs. Veterinary anaesthesia and analgesia. 35(1): 62-68.
- Lindegaard, C., Gleerup, K.B., Thomsen, M.H., Martinussen, T., Jacobsen, S. and Andersen, P.H. 2010. Anti-inflammatory effects of intra-articular administration of morphine in horses with experimentally induced synovitis. American journal of veterinary research. 71(1): 69-75.
- Murdoch, A.P. and Michou, J.N. 2016. The effect of loco-regional anaesthesia on motor activity induced by direct stimulation of the sciatic nerve in dogs. The Veterinary Journal. 209: 139-143.
- O'Brien, T.Q., Clark-Price, S.C., Evans, E.E., Di Fazio, R. and McMichael, M.A. 2010. Infusion of a lipid emulsion to treat lidocaine intoxication in a cat. Journal of the American Veterinary Medical Association. 237(12): 1455-1458.
- Romano, M., Portela, D.A., Breghi, G. and Otero, P.E. 2016. Stress‐related biomarkers in dogs administered regional anaesthesia or fentanyl for analgesia during stifle surgery. Veterinary anaesthesia and analgesia. 43(1): 44-54.
- Taylor, P.M. and Robertson, S.A. 2004. Pain management in cats—past, present and future. Part 1. The cat is unique. Journal of Feline Medicine and Surgery. 6(5): 313-320.
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