Peripheral nerve blocks as part of a multimodal analgesia approach in orthopaedic patients

Pain pathways

Nociceptors are the peripheral sensory nerve endings that respond to a noxious stimulus (thermal, chemical and mechanical). The nociceptive pathway is made up of four processes that transmit information from the site of injury to the brain. Information relative to the degree of noxious stimulation is converted into electrical impulses (transduction) and transmitted via two different types of afferent sensory nerve; Aδ fibres transmit fast sharp pain in a localized area, and C fibres transmit slow duller pain to a wider area (Flaherty, 2006). These impulses travel to the dorsal horn of the spinal cord and on to the brain (transmission). The nociceptive information is modified (modulation) as it passes through the central nervous system (CNS). Information may be enhanced or inhibited by endogenous systems before reaching the cerebral cortex of the brain where further modulation and perception occurs in the conscious animal (Figure 1).

Sensitization (increased activation and responsiveness) of peripheral nociceptors may occur if tissue damage is extensive or on going; the damaged cells release inflammatory mediators which sensitize the nociceptors further. Nociceptors in the surrounding tissue also become ‘alert’ increasing the area in which pain pathways are activated. Sensitization also occurs centrally within the spinal cord and brain, known as wind up. As the peripheral nociceptors become more sensitive and responsive, the information being transmitted to the spinal cord is increased, this bombardment of the dorsal horn of the spinal cord leads to amplification of the noxious stimulus.

This peripheral and central sensitization leads to misinterpretation of the severity of the noxious stimulation. The patient may respond in an exaggerated manner to normally mildly noxious stimuli (hyperal-gesia) or non-noxious stimuli such as light touch may now be perceived as painful (allodynia). The changes that occur in pain pathways can lead to long-term alterations in pain perception, changes occur within the spinal cord which can lead to the development of chronic pain even after damage to the tissues has healed (Flaherty and MacGillivray, 2003).

Patients will respond differently to similar noxious stimulation and therefore it is important that each patient is treated individually with an appropriate analgesia plan. The prevention and treatment of pain should be of high priory when preparing for orthopaedic surgical procedures. Pain has many detrimental effects on the patient (Table 1), but studies have found pain to be under treated in clinical settings for multi-factorial reasons (Capner et al, 1999). It has been shown that cats in particular receive less analgesic treatment when compared with dogs (Bortolami et al, 2012)

Pre-emptive analgesia

Peripheral and central sensitization is initiated as soon as noxious stimulation is experienced by the patient. The administration of analgesic drugs prior to noxious stimulation (pre-emptive analgesia) will reduce or prevent peripheral and central sensitiza-tion from developing. By reducing the modulation of the peripheral nociceptive impulses (including fewer adjacent nociceptors activated and reduction in in-flammatory mediators) central stimulation will also be reduced, preventing wind up. The level of pain experienced will not be as severe and analgesics administered will be more effective at reducing pain for the patient when it is conscious (Murrell, 2007).

Multimodal analgesia

Due to the complex way that pain pathways transmit and modify nociceptive information, from the periphery to the spinal cord and further onwards to the brain, one class of drug alone could not be effective against the processes in all the multiple sites of the pathway (Murrell, 2007). By using a combination of two or more drugs that act at different sites and on different receptors, more effective pain relief can be achieved for the patient (Wetmore, 2006). By using drugs from different pharmacological classes lower doses of multiple drugs may be used leading to a reduction in side effects (Grint, 2011). Some drugs, when used together have a synergistic effect and enhance the effect of each individual drug; the systemic administration of opioids in combination with an α2 agonist enhances and prolongs the analgesic effect (Thurmon et al, 1999).

Orthopaedic surgery results in pain for multiple reasons. Muscle contracture can make reduction of a fracture technically diffcult for the surgeon and extensive tissue dissection and retraction may be required to visualize and gain access to the fracture site. The integrity of the periosteum (connective tissue covering bones) and endosteum (lining of the inside surfaces of all bones), both of which contain nerve endings is disrupted when a bone is fractured or an elective osteotomy is performed. Fractures often result in damage to adjacent tissues such as nerves, blood vessels and the skin. Inflammation of soft tissue or bleeding from torn periosteal vessels can both cause pressure pain around a fracture or surgical site.

Fracture reduction and repair, corrective osteotomies and joint distraction may all provoke an auto-nomic sympathetic response to the surgical stimulation, such as an increase in heart and respiratory rate from the patient while under anaesthesia. A multi-modal and pre-emptive analgesic approach to pain relief in these patients, specifically designed for each individual patient, can reduce the incidences of au-tonomic response by the patient. This will lead to a ‘smoother’ anaesthetic and less peripheral and central sensitization. It has been demonstrated that pain is easier to prevent than treat, and appropriate timing of analgesic drugs will aid in the prevention of wind up (Robertson, 2009).

A detailed overview of all the classes and mechanisms of action of analgesic drugs that could be used in a multimodal analgesic protocol is outside the scope of this article

The use of a combination of analgesic drugs is advantageous as the mechanism of action differs between analgesic classes, with different drugs acting at different sites within the pain pathway (Table 2).

The use of multiple drugs exerting an effect at different sites can allow a reduction in the dose of individual agents, and therefore can reduce the side effects that could occur when larger doses of one drug are used instead. Opioids (full or partial) are often used in combination with phenoziathines (producing neuroleptanalgesia) for premedication or sedation and bind to specific opioid receptors, primarily within the CNS but also peripherally and at sites of inflammation (Flaherty, MacGillivay 2003). Alpha 2 agonists may also be administered as part of the pre-medication in healthy patients as they are potent sedative hypnotics (Flaherty and MacGillivay, 2003) but they also have the advantage of providing analgesic properties. NSAIDs inhibit the production of pros-taglandins which are associated with inflammation and are useful in the treatment of low to moderate and chronic pain (Thurmon et al, 1999). Ketamine is an antagonist at the NMDA receptor within the CNS which is associated with neuropathic pain; administration of a ketamine constant rate infusion (CRI) will reduce or inhibit central sensitization (wind up). Local anaesthetics can be used as part of a multimodal protocol to block nociceptive information (Table 3).

Wherever possible multimodal analgesia should be administered pre-emptively to limit peripheral and central sensitization from developing. Pain is harder to control once tissue damage has occurred and pain pathways have been activated. Patients will present at the clinic with pre-existing painful conditions and injuries and multimodal analgesia protocols will be advantageous for these patients although effective pain control may be harder to achieve. Rescue analgesia (such as potent opioid CRIs) may be required when analgesia strategies are inappropriate or ineffective, but should not be used in place of good pre-emptive multimodal protocols (Figures 2 and 3)

Regional anaesthesia

Local anaesthetics are the only class of analgesic drug that can block pain sensation completely (Flaherty and MacGillivray, 2003). This complete blockade of peripheral nociceptive information stops transduc-tion, transmission and modulation along pain pathways thus preventing sensitization of the central nervous system. As other analgesic drugs can only modify the pain perceived by a patient, the use of local anaesthetic techniques has an important role in producing a pain-free patient. Local anaesthetics bind to the sodium channels within the neuronal membranes preventing transfer of electrical impulses. Both sensory and motor nerves can be affected by local anaesthetic administration leading to desen-sitization of the area (sensory nerves) and possible temporary loss of movement (motor nerves) (Grint, 2011). Regional anaesthesia is the administration of local anaesthetic close to one or more nerves to desensitize the region or area that is innervated by those nerves. The peripheral nerve blocks commonly used in the forelimb block the radial nerve, ulnar nerve and median nerve, desensitizing the distal forelimb. Peripheral nerve blocks commonly used in the hind limb block the sciatic nerve and femoral nerve, desensitizing the stifle area and distal hind limb.

Local anaesthetic drugs

Local anaesthetic drugs are divided into two groups depending on their chemical structure, metabolism, potency, time of onset and duration of action. They are available in many different preparations for topical and parenteral application. The most commonly used preparations for analgesic use are lidocaine, bupivacaine and ropivacaine.

Lidocaine has a short onset of action (less than 10 minutes) and a relatively short duration of action (approximately 1 hour). Many local anaesthetics cause vasodilation thereby the uptake of the drug into the systemic circulation occurs quickly, reducing the duration of action at the intended site. The addition of a vasoconstrictor such as epinephrine (adrenaline) is used to prolong the concentration of local anaesthetic at the administration site (Flaherty and MacGillivray, 2003). Lidocaine is often used in combination with other local anaesthetic drugs, such as bupivacaine to provide a block that has a fast onset of action (lido-caine) with an increased duration of action (bupi-vacaine) (Grint, 2011).This combination may be used for local infiltration (intradermal or subdermal infl-tration (Flaherty and MacGillivray, 2003)) and blocks of the head area, such as orbital and jaw blocks; it is not routinely used for peripheral nerve blocks. Bupi-vacaine has a slower onset of action (20–30 minutes) but the duration of action is prolonged (reports are between 2.5–6 hours) (Bergadano, 2010). Ropivacaine is a local anaesthetic drug with an onset time similar and possibly shorter than bupivacaine (Bergadano, 2010) with duration of action of up to 6 hours. Human studies have shown that ropivacaine may be less toxic than bupivacaine to cartilage when used as an intra-articular block (Webb and Ghosh 2009) (e.g. post operatively following arthroscopic examination of a joint).

Local anaesthetic doses must be kept within the therapeutic range as toxicity can occur due to absorption into the systemic circulation. The total safe drug dose must be calculated on a patient’s lean body weight with care taken for smaller patients, the total volume must be divided between sites if used in multiple areas or for different types of block, e.g. peripheral nerve block and intra-articular use. Local anaesthetics (with the exception of lidocaine) must not be injected intravenously and aspiration prior to injection must always be performed to check for evidence of venepuncture. Toxic effects of local anaesthetics may be seen in the central nervous and cardiovascular system; initial neurological signs include sedation, disorientation and ataxia. Large over dosage can cause muscle tremors, seizures and respiratory depression. Cardiovascular effects occur clinically at doses approximately four times those required to cause CNS signs. Cardiac toxicity causes a reduction in myocardial contractibility, peripheral vasodilation and profound hypotension (Lemke, 2007). Ventricular arrhythmias may be seen with over dosage of longer acting local anaesthetics. The local anaesthetic solutions themselves can cause tissue irritation and concentrated preparations of local anaesthetic are directly neurotoxic. Solutions used clinically are not neurotoxic when administered extra-neurally, but dilution and avoidance of repeated use of inappropriate concentration solutions is advisable and may reduce the risk of toxicity to local tissues (Lemke, 2007).

Peripheral nerve blocks

Many local anaesthetic nerve blocks are performed without requiring any specialist equipment, using anatomical landmarks, palpation of superficial vessels and a good knowledge of the patient’s anatomy. Electrical nerve stimulators and ultrasound guided techniques use more specialized equipment to accurately locate the position of nerves, improving effcacy and safety (Bergadano, 2010) by indicating the relative location of the needle in relation to the nerve (Figure 4). Once the optimal distance from the nerve is confirmed local anaesthetic can be administered via the needle that is in close proximity to the nerve.

The use of peripheral nerve blocks as part of the multimodal approach to a patient’s analgesic protocol is in most cases performed pre-emptively. The patient is anaesthetized and a surgical clip is performed for the planned surgical procedure. The locations where the nerve block/s is performed are normally within the surgical clip and so no additional clipping of hair is required. Aseptic preparation of the skin is performed around the site where the nerve stimulator needle is introduced. Clinicians that are inexperienced with the technique and landmarks should wear sterile gloves to maintain an aseptic area during palpation of the area. Gloves may be omitted by experienced clinicians when palpation of the area is not necessary, although a thorough hand washing technique is undertaken prior to local anaesthetic administration. A good aseptic technique must be used at all times to avoid contamination and the risk of infection.

Electrical nerve stimulation

By sending a small electrical current through a needle placed in close proximity to a motor nerve, and assessing the corresponding reaction to that stimulation, it is possible to tell how far away from a nerve the tip of a needle is placed. This information allows accurate administration of local anaesthetic, ensuring that the nerve is not traumatized or damaged by the end of the needle and that the calculated dose of drug is placed close enough to ‘bathe’ the nerve extra-neurally and elicit the optimal effect. Electrical impulses that travel along a nerve containing motor fibres will stimulate that nerve to cause movement (twitches) in the corresponding muscle. The strength of the current needed to produce muscle twitches is associated with the distance that the needle tip is from the nerve. If a low electrical current continues to produce muscle twitches, the end of the needle may be too close to the nerve. If the current is higher but no muscle twitches are seen the needle tip is further away from the nerve. The needle used to locate the nerve is specifically designed with an atraumatic bevel end and is insulated with exception of the tip allowing precise electrical stimulation to the area around the nerve. A channel for local anaesthetic administration allows injection of drug close to the nerve fibre.

Hindlimb peripheral nerve block — sciatic and femoral nerves

Surgeries of the hindlimb, including correction of patella luxation, cranial cruciate ligament repair surgery including tibial plateau levelling osteotomy (TPLO), fracture repair of, or distal to the stifle and hock stabilization, are examples where sciatic and femoral nerve blocks can be used to provide analgesia as part of a multimodal protocol (Figures 5 and 6).

Forelimb peripheral nerve block — radial, ulnar and median nerves

Surgeries to the forelimb such as fracture repair, correction for limb deformities, including ulnar osteotomy and ostectomy, and arthroscopy and arthrotomy, are all procedures that can benefit from forelimb nerve blocks (Figure 7).

It is beyond the scope of this article to describe the anatomy of the above nerve blocks but detailed information can be found in Campoy (2008).

Complications of peripheral nerve blocks

Complications can occur when a nerve block is performed. As previously discussed local anaesthetics can cause systemic toxicity if incorrectly dosed or administered. Direct damage to the nerves can be caused by needle trauma due to incorrect placement, intra-neuronal injection, or by damage to the blood vessels associated with the nerve and possible haematoma formation. The use of atraumatic short bevel needles with guidance from a nerve stimulator or ultrasound guided placement will help to reduce incorrect needle placement. Neurological complications (loss of sensory or motor function) that are still present after the duration of action of the local anaesthetic should have ceased may indicate that neurological injury has occurred. Complications resulting from direct nerve damage have no effective treatment and most improve over 1 to 6 months (Campoy, 2008). The supportive care required during this time will depend on the ambulatory ability of the patient, and trauma to limbs with reduced sensation and movement must be prevented. Physical rehabilitation may aid in the prevention of muscle atrophy and treatment protocols are selected depending on the extent of each animal’s neurological injury (Brockstahler et al, 2004).

Intra-articular block

The instillation of local anaesthetic into a joint can be performed pre-emptively prior to surgery but is often performed in the peri-operative period after closure of the joint, to prevent loss of the local anaesthetic into the surrounding surgical area. Examples of application include after arthroscopic examination of a joint, where a calculated dose of ropivacaine is injected into the joint space once flushing of the joint has concluded. Local anaesthetic may also be instilled into the joint space following an arthrotomy (see Figure 2). A good aseptic technique is required if local anaesthetic is to be administered intra-articular prior to surgery, as tracking of bacterial contaminants from the skin may lead to development of infection within the joint (Grint, 2011). There is evidence in human medicine that bupivacaine may no longer be the local anaesthetic drug of choice for intra-articular use due to dose and time dependent chondrotoxicity (Webb and Ghosh, 2009). Ropivacaine is now more widely used for this purpose. If a nerve block has also been administered the total therapeutic dose of local anaesthetic must be divided between all of the sites it has been administered at, regardless of the type, onset and duration times of the drugs used to avoid systemic toxicity.

Local anaesthetic blocks — benefits to the patient

The use of local anaesthetic agents as part of a mul-timodal analgesia protocol has a beneficial effect on the patient under general anaesthesia. By blocking nociceptive input patients have a ‘smoother’ anaesthetic as autonomic sympathetic response to surgery is reduced. Peri-operative interventional analgesia requirement is reduced or not necessary and the amount of inhalational agent required may also be reduced, aiding haemodynamic stability. The reduction in the doses of drugs administered to the patient may help to reduce side effects such as cardiovascular, respiratory and CNS depression and recovery from general anaesthesia may be faster. Comfortable patients will become weight bearing on the affected limb within a shorter period of time and overall spend less time hospitalized in the postoperative period (Figure 8).

Studies have shown that patients that receive local anaesthetic techniques as part of their analgesia protocol require less supplementary post-operative analgesia and/or have an increased time interval to the first required dose (Hoelzler et al, 2005). Patients that were pain scored using the Short Form Glasgow Composite Measure Pain Scale did not show signifi-cant increase in pain scores over a period of 24 hours when treated with bupivacaine after unilateral elbow arthroscopy (Vettorato et al, 2012).

In the author’s experience patients that have had local anaesthetic peripheral nerve blocks as part of their pre-emptive multimodal analgesia protocol have shown less autonomic sympathetic response at the time of joint distraction and osteotomies. Inter-ventional fentanyl administration has been reduced and visual pain assessment in the early post-operative period has demonstrated patients that are more comfortable and have required less opioid analgesia on recovery. The majority of patients have eaten a small meal within 2 hours of recovery from general anaesthesia and are weight bearing on the surgical limb at discharge to the owners.

Conclusion

The use of peripheral local anaesthetic blocks as part of a multimodal analgesic protocol provides superior analgesia, preventing peripheral and central sensitization which has long-term benefits for the patient into the post-operative period. Administration of a nerve block can be a relatively quick procedure, especially when using nerve stimulation equipment. The advantages for the patient (and persons monitoring the anaesthesia) are evident during the surgical procedure and into the recovery period.

Key Points