Membrane electrophysiology

Local anaesthetic solutions exert their primary pharmacological action by interfering with the excitation–conduction process of peripheral nerve fibres and endings, decreasing the rate and degree of depolarisation of the nerve membrane such that the threshold potential for transmission is not achieved and the nerve ceases to conduct (de Jong 1994). There is no effect on the resting or threshold potentials, but prolongation of the repolarisation phase and refractory period may also play a role in anaesthetic action. All excitable tissues possess special voltage-sensitive channels (Morgan & Johnson 2000).

Most of the clinically useful anaesthetic is likely to act by displacement of the calcium ions (Ca²⁺) from a lipoprotein receptor site on the interior of the nerve cell membrane. This will block the sodium ion (Na⁺) channels. Potassium ion (K⁺) channels are also blockable, but are less sensitive than the sodium channels. However, potassium channels are important in regulating the resting potential. The voltage-sensitive channels can be open, closed or inactivated. It is also possible that the local anaesthetic binds at some receptor sites intracellularly, called G-protein receptors (Scholz 2002).

Local anaesthetics are use or state dependent; that is, the degree of block is proportional to the rate of nerve stimulation (i.e. the more rapidly firing neurons are more susceptible than the slower ones) (Wildsmith 2007). Nerve fibres may be of one type or mixed; however, it is generally the smaller C diameter fibres which are more sensitive than larger myelinated A fibres. Thus, a differential block can be achieved where the smaller pain and autonomic fibres are blocked while coarse touch and movement fibres are spared (Strichartz & Ritchie 1987).

Bupivacaine

Bupivacaine, a homologue of mepivacaine and ropivacaine, is a very stable hydrochloride and has been used for its long duration, which may be up to four times that of lidocaine (McClure & Rubin 2005). It is reported to be four times as potent as lidocaine and mepivacaine – a 0.5% solution is approximately equivalent to a 2% solution of lidocaine. It is available in concentrations of 0.25–0.75% and can be acquired with 1 : 200 000 adrenaline added. If the latter agent is used, it would be prudent to retain the same MSD of 150 mg (Mather et al 2005). A disadvantage of bupivacaine is that it does have a slow onset time. There is more likelihood of cardiac toxicity with this drug; for example, cardiac arrhythmias and reduced myocardial contractility, often resulting in a resistant ventricular fibrillation (Royse & Royse 2005), and patients who suffered this were difficult to resuscitate. Bupivacaine may also increase the toxicity of certain drugs that are metabolised by plasma cholinesterases. In podiatry it is more often used in the surgical field.

Levobupivacaine

The great advantage of levobupivacaine over bupivacaine is its reduced cardiotoxicity, while it retains the desirable qualities of being long acting and virtually as potent. Levobupivacaine is the S-enantiomer of racemic bupivacaine (McLeod & Burke 2001). It is available as a 0.25%, 0.5% or 0.75% solution. It is not available in cartridges for dental syringes. The MSD is the same as for bupivacaine (i.e. 150 mg; 2 mg/kg). Levobupivacaine is contraindicated for a Bier’s block in those with severe hypotension, as it has been associated with cardiac shock and serious cardiac arrhythmias. There is little research to show that it is unsafe in other situations or with certain drugs, but it is best avoided in those taking antiarrhythmic drugs such as mexiletine (Burlacu & Bugg 2008). Patients given levobupivacaine should be warned not to drive and/or operate machinery afterwards, and while no time is specified on the data sheet 12 hours seems a sensible recommendation (Foster & Markham 2000).

Etidocaine

This agent is used predominantly in the USA, particularly by the dental profession. It is has a rapid onset and medium to long duration of action of approximately 4 hours. Generally available in concentrations of 1% and 1.5%, it is also available with 1:200 000 adrenaline added (Wynn 1995). Currently, this agent is not available to UK podiatrists, but it has been used for a long time by some American podiatrists (Caputo et al 1982).

Lidocaine

Since its introduction to clinical practice in 1948, lidocaine has become one of the most widely used local anaesthetics (Winstanley & Walley 2002). It is a fairly fast-acting local anaesthetic, with a variable duration of action, which may be as long as 3 hours. It is usually used in plain solutions by podiatrists, but from 1998 was available with adrenaline 1:200 000 added as a vasoconstrictor to certified podiatrists in the UK. The MSD is 200 mg (3 mg/kg). In solutions with adrenaline, the MSD is increased to 7 mg/kg to a maximum of approximately 490 mg. It is available in a number of concentrations from 0.5% to 4%; however, the higher concentrations are usually for intrathecal injection, while the 1% and 2% solutions are the favoured strengths for peripheral nerve block anaesthesia. It is usually supplied in ampoules, but may sometimes be available in a cartridge format that fits into the dental syringes much favoured in podiatry.

There are other medical uses for lidocaine, such as in the management of cardiac ventricular arrhythmia, in this case marketed as Xylogard, this use being due to its marked membrane-stabilising properties. However, for this reason high doses should be avoided in patients with hypovolaemia or heart block problems (Smith, 2007). There are also some pharmacological interactions to be considered, for example: propranolol (Inderal), a beta blocker used in patients with angina, which slows hepatic clearance; cimetidine, which is widely used for peptic ulceration, also interferes with hepatic clearance; and ranitidine, also an ulcer treatment, requires the same consideration. However, these are all considered relative contraindications, and in the main will not be a problem with small dosages (www.astrazeneca.co.uk).

Lidocaine is also used in topical creams, the most relevant of which is EMLA® (eutectic mixture of lignocaine and prilocaine), a cutaneous gel applied to the skin under a dressing for up to an hour to facilitate pain-free injections. This is particularly useful with nervous children.

Mepivacaine

Known under the brand names of Carbocaine, Scandonest and Scandicaine, mepivacaine is related to bupivacaine and ropivacaine and was introduced into clinical practice in 1957 (Ruetsch et al 2001). It is less toxic than lidocaine, particularly to neural tissues. Mepivacaine has a less pronounced vasodilator effect than lidocaine, probably because of its slower clearance. It has a longer duration and quicker onset of action than lidocaine. Because of its slower clearance its use is not advised in obstetric anaesthesia (Kuczkowski 2004). The MSD is 400 mg based on 6 mg/kg, thus enabling a larger dose to be given than with lidocaine. It is available in cartridges for use with dental or disposable syringes. It is available in concentrations of 0.5–4%. In podiatry, solutions up to 3% are used, with Scandonest 3% (plain solution) being the choice in the UK. While this drug is not licensed for the treatment of children, its use in podiatry is indemnified by the Society of Chiropodists & Podiatrists. The law allows podiatrists who hold a certificate of competence in local anaesthesia to administer the substance without any requirement that such administration must be in accordance with the terms of the licence.

Prilocaine

Prilocaine was introduced into clinical practice in 1960, after being synthesised by Löfgren and Tegnér in 1959 (Ruetsch et al 2001). This agent has an MSD of 400 mg, and is most commonly available in concentrations of 1–4%. It is also available with the addition of octapressin or adrenaline for use in dentistry. It is available in ampoules or in dental cartridges in the higher concentrations. While the effectiveness and duration of action are only slightly greater than those of lidocaine or mepivacaine, its safety profile is superior to both due to its rapid clearance (Arthur et al 1987). Being a toluidine derivative its metabolism will produce ortho-toluidine, and in large doses (usually exceeding 600 mg) it may produce methaemoglobinaemia. This is a cyanotic condition and the patient will develop a bluish hue. It occurs because there are ferric rather than ferrous ions in the haematin in the haemoglobin, so the oxygen is unable to combine with ferric ions and transport the oxygen. It is not generally considered a serious condition in otherwise healthy individuals, but some authorities recommend not using prilocaine in the early stages of pregnancy, or in cases where there is severe anaemia or circulatory impairment (Haas & Carmichael 2007).

Ropivacaine

Although synthesised in the late 1950s by Ekenstam, Naropin was not used clinically until 1996 (Whiteside & Wildsmith 2001). It was made available to appropriately qualified UK podiatrists in November 2006 (Order 2807 Department of Health). It is produced as a single-enantiomer drug, unlike bupivacaine and mepivacaine, to which it is closely related. Bupivacaine and mepivacaine are analogues of ropivacaine. However, ropivacaine is marketed as a single stereoisomer, while bupivacaine and mepivacaine are sold as equal mixtures of the two possible stereoisomers. Ropivacaine was created for its long-term efficacy, having a duration of action about 10% less than that of bupivacaine, but with a much safer profile. It also has a more discrete separation between motor and sensory block (McClure 1996). The evidence for the side-effects of and contraindications to ropivacaine is sparse. There is no evidence for dangers in pregnancy and it is used for epidural deliveries. Its use on patients with hypotension should be avoided. There is no direct evidence to indicate that patients should not drive after receiving the drug, but good sense should prevail. Prolonged administration in patients on Faverin (a serotonin reuptake inhibitor/antidepressant) should be avoided (Hansen 2004). It is available in 10 ml polypropylene blister packs, in concentrations of 0.25%, 0.5% and 0.75%. The MSD is not clearly defined by the manufacturer, but the suggested volumes would indicate approximately 4 mg/kg with a maximum dose of 300 mg (Astra-Zeneca 2001). Unlike most other anaesthetic agents, the addition of adrenaline has little effect on the drug’s onset or duration.

Local effects

The following is not exhaustive list, and in most circumstances the risk should be minimised by best clinical practice and attention to detail.

Systemic effects

Local anaesthetics are very safe, and in the doses used in standard podiatric practice are unlikely to cause problems. However, when larger doses are used, as in podiatric surgery, the risks increase (Maher et al 2008). Even larger volumes are used in orthopaedics and other disciplines, where patients may have a line access for continuous input of the anaesthetic (Wiegel et al 2007).

All local anaesthetics can cause systemic toxicity but, as mentioned above, some have a greater potential for specific problems (e.g. bupivacaine and cardiotoxicity). The patient’s ability for hepatic metabolism and renal clearance of amide anaesthetic agents has to be considered. Those agents that have a more rapid clearance will be safer than those that have a longer half-life and slower clearance. Acute toxicity following administration of anaesthetic solution is very rare, but recent advances have seen the introduction of Intralipid as an intravenous treatment for cardiac arrest following local anaesthetic reaction (Clark 2008). This has prompted the suggestion that patients undergoing foot surgery under popliteal blocks and large volumes of local anaesthetic should have an indwelling cannula in case of emergencies arising during the procedure (Maher et al 2008).

It is incumbent on the practitioner to assess the patient’s suitability for the anaesthetic and subsequent procedure (Crausman & Glod 2004). A full medical, medication and social history should be completed, and if there are doubts about the patient’s suitability for local anaesthesia the best practice would be to consult the patient’s medical practitioner or medical consultant.

The following areas are highlighted for particular consideration:

Local infiltration

This is the simplest of the techniques and requires the deposition of the anaesthetic around and subcutaneously beneath the lesion (Fig. 20.5). It may be used for a variety of skin lesions, such as warts, basal cell carcinoma and various minor excrescences, and is ideal where electrocautery, curettage or scalpel excision is to be used (Koay & Orengo 2002).

Figure 20.5 Local infiltration, showing the needle positions around the lesion.

The digital block

Perhaps the most widely used local anaesthetic procedure in podiatry, the digital block is administered to block the hallux and any of the lesser digits (Fig. 20.6). There are a number of variations to this technique, with each practitioner favouring their own. In its basic form the needle is directed from dorsal to plantar at an angle of 80–90° to the skin surface and slightly distal to the webbing line. The needle is advanced and local anaesthetic deposited en route and on the plantar side of the digit. The needle may be withdrawn slightly, without leaving the skin, and redirected so that the points of the needle converge towards the plantar surface, forming a ‘triangular approach’ (Figs 20.7 and 20.8). It is the authors’ practice to keep injecting the solution as the needle advances. This technique does not require aspiration, as the needle is moving and there are no large vessels in this area; however, this remains a decision for the individual practitioner to make. Some podiatrists will also direct the needle laterally across the dorsum of the toe (Uddin & Reilly 2008). In the hallux, the four digital nerves will be anaesthetised. The plantar nerves arise from the medial plantar nerve and these traverse the apex of the toe. Dorsally, the deep peroneal (fibular) will supply the medial border of the hallux and the medial dorsal cutaneous branch of the superficial peroneal (fibular) nerve will have some input along the lateral side. Anaesthesia can normally be achieved with approximately 2–4 ml of solution; occasionally there may be a need to use rather more. Lesser toes rarely require more than 1–2 ml. The main problems with this procedure arise if excessive amounts of solution are forced into an area where distension cannot occur (see earlier reference on personal communication). Digital blocks can be uncomfortable, especially in children or those with low pain thresholds. The use of a needleless system has been considered (Dialynas et al 2003), but has not found widespread use in podiatric practice. Furthermore, each practitioner has their own preferred delivery systems and technique, and novel methods are sometimes used as they may be less painful or use less local anaesthetic (Ouzounov 2005). The use of vasoconstrictors (e.g. adrenaline 1 : 200 000) in end organs and digits is generally advised against (Dollery 1999). However, there is little evidence to support its avoidance when using the amide anaesthetics (Wilhelmi et al 2001), and extensive literature searches have highlighted only 50 cases of digital gangrene since the turn of the last century, and none involving lidocaine with adrenaline (Denkler 2001). It might be that other factors, such as the use of digital tourniquets or infection, heighten the risk of necrosis. It should also be noted that much of the evidence pertains to the fingers, not the toes. Vasoconstrictors may also have adverse systemic effects, such as increased heart rate, tremors and palpitations.

Figure 20.6 Digital nerve block technique.

Figure 20.7 The “triangular approach” showing initial needle position.

Figure 20.8 The “triangular approach” showing secondary needle position.

The ray block

This is usually associated with the first (Mayo block) and fifth rays and is a highly effective technique for anaesthesia of the forefoot, for example for bunion, lesser ray procedures and neuroma. The needle is advanced at a proximal point in the metatarsal interspace and from a dorsal start and is then directed plantarly, where additional anaesthetic is deposited. The needle is then withdrawn and redirected medially and, if required, laterally so that there is a ‘collaring’ of the ray (Fig. 20.9). Anaesthesia of the other rays follows a similar pattern to the Mayo block. This can be a very effective technique, with a low failure rate of less than 1% (Worrell & Barbour 1996).

Figure 20.9 The ray block, showing the needle positions.

More extensive anaesthesia is achieved by blocking the nerves around the ankle. Only rarely is it necessary to block all the nerves at the same time. The nerves involved are the posterior tibial, saphenous, sural, deep and superficial peroneal (Fig. 20.10). All, with the exception of the saphenous nerve, which is a terminal branch of the femoral nerve, are branches of the sciatic nerve. The practicality of these injections is aided by having a relaxed and comfortable patient. All injections may be done with the patient supine, and rotating the leg into an accessible position. However, some practitioners would have the patient in a prone position for the posterior tibial and the sural blocks; it really depends on operator preference and to some extent the patient’s comfort.

Figure 20.10 The anatomical positions of the posterior tibial, saphenous,sural, deep and superficial peroneal nerves.

Posterior tibial nerve

This is the larger of the two branches of the sciatic nerve (root index L4, L5 and S1, S2, S3). At the ankle it runs medially in the neurovascular bundle behind the posterior tibial artery. It passes behind the medial malleolus and flexor retinaculum, and then gives off calcaneal branches before dividing into the medial and lateral plantar nerves. As with all anatomy, there may be individual variations. This may be the reason for failure to achieve anaesthesia in some patients. The most useful indicator for finding the nerve is to palpate the posterior tibial pulse (McGlamry et al 2001). The usual way to approach this injection is to palpate the posterior tibial artery and introduce the needle at the medial aspect of the Achilles tendon. Advance the needle in an anterior direction to the posterior surface of the tibia, and then withdraw very slightly and aspirate before injecting. Whether you choose to actively cause a paraesthesia is again a matter for debate. It is advisable to inform the patient of possible paraesthesia. You should then withdraw slightly and aspirate, and this should result in a good anaesthesia. An alternative method is to go about 4 cm above the medial malleolus, direct the needle deeply to the medial border of the Achilles tendon and then inject into the neurovascular channel. A nerve stimulator may be used to locate the nerve. Although unlikely, putting too much local anaesthetic into the bundle could cause a pressure necrosis, as the nerve lies between the tibial and flexor retinaculum. Aspiration avoids intravascular injection. Approximately 5–10 ml of anaesthetic will be required, and there may be a time lag before full anaesthesia is achieved.

The sural nerve (root index S1/S2)

This nerve, also known as the lateral cutaneous nerve, continues down the posterior calf and runs behind the lateral malleolus with the short saphenous vein. It is a cutaneous nerve supplying the lateral aspect of the foot and the fifth toe. While not particularly difficult to anaesthetise, ultrasound guidance has been used to improve the success rate (MacFarlane & Brull 2009, Redborg 2009).

Insert the needle lateral to the Achilles tendon and at a right angle to the lateral malleolus (Fig. 20.11). Advance needle to lateral malleolus and then withdraw slightly. Aspirate and inject. It is possible to inject, with a fanning action, about 1 cm above the lateral malleolus and this tends to take out the small branches or one can raise a wheal from the Achilles tendon to the lateral malleolus.

Figure 20.11 Sural nerve location and sensory distribution.

Saphenous nerve (root index L3/L4)

This nerve, which is the sensory terminal branch of the femoral nerve, passes down the anteromedial aspect of the leg and enters the dorsum of the foot (accompanying the long saphenous vein) about one finger’s width from the medial malleolus (Fig. 20.12). Enter the needle between the tendon of the tibialis anterior and the long saphenous vein. Advance to the medial malleolus, aspirate and inject. Aspiration is essential due to the close proximity of the vein.

Figure 20.12 Saphenous nerve location and sensory distribution.

The superficial peroneal (fibular) nerves (root index L4/L5, S1)

One of two terminal branches of the common peroneal (fibular) nerve. It becomes cutaneous between the middle and distal thirds of the leg and divides into two branches, medial and lateral although the point of division varies from person to person. The medial branch supplies the skin of the mediodorsal aspect of the foot, medial aspect of the hallux and adjacent sides of the second, third and fourth toes (Dhukaram & Senthil 2004). The lateral branch supplies the intermediate and lateral skin on the dorsum of the foot, the adjacent sides of the third and fourth toes and the medial aspect of the fifth toe.

The medial branch is centrally located between the malleoli as it crosses the extensor digitorum longus (EDL) tendons. If you ask the patient to dorsiflex the foot, and then palpate the EDL tendons, you should be able to roll the nerve with your index finger. Insert the needle on either side of the nerve. It is not vital to aspirate for this procedure (Fig. 20.13).

Figure 20.13 Medial branch of the superficial peroneal nerve and its distribution.

The lateral branch can be palpated lateral to the peroneus (fibularis) tertius muscle between the malleoli. Ask the patient to dorsiflex the foot, and then palpate the peroneus tertius or lateral edge of the EDL muscles. Rolling the nerve will elicit a slight paraesthesia. Inject slowly, there is no need to aspirate. Alternatively, a wheal of anaesthetic may be deposited subcutaneously across the anterior aspect of the ankle.

The deep peroneal (fibular) nerve (root index L4/L5, S1/S2)

This enters the dorsum of the foot between the tendons of the tibialis anterior and the extensor hallucis longus (Kopka & Serpell 2005). It is covered by the superior and inferior retinaculum and runs deep, roughly following the course of the anterior tibial artery. It supplies opposing sides of the hallux and the second toe. Insert the needle between the two tendons and advance until you make contact with the bone, aspirate and inject. Anaesthesia will be over the superolateral foot (Fig. 20.14).

Figure 20.14 Deep peroneal nerve location and sensory distribution.

The popliteal block

The use of this block is expanding in podiatric surgery (Rees & Tagoe 2002). The technique is suitable for more extensive, long-duration forefoot and midfoot operations and for more involved rearfoot procedures where general and spinal anaesthesia is best avoided. In practical terms, both the saphenous nerve on the medial aspect of the knee and terminal portion of the sciatic nerve in the popliteal fossa will need to be blocked (Donohue et al 2004). In an anatomical sense this means that the tibial and common peroneal nerves, which share a common epineural sheath, will be anaesthetised.

This technique does mean that the patient is less mobile for some time following the procedure until motor function returns. However, this block allows a long postoperative anaesthesia, which helps with pain management and allows for a painless application of a midcalf or ankle tourniquet (Reilley et al 2002).

Because it is not possible to palpate these nerves, the use of a nerve stimulator/locator is required. In addition, a Doppler flowmeter may be used to identify the position of the popliteal artery.

Various techniques have been developed to approach the popliteal fossa. Donohue et al (2004) refer to the ‘classic’ approach, which is a posterior injection where the needle is introduced 1 cm lateral to a line drawn from the borders of the semimembranosus medially, the biceps femoris laterally and a line about 5 cm above the popliteal crease. However, they say that this has been superseded by an intertendinous approach that ‘calls for needle insertion midway between the semimembranosus and biceps femoris tendons and 5 cm above the popliteal crease’ (Donohue et al 2004, p. 370) (Fig. 20.15).

Figure 20.15 Deep popliteal nerve location and sensory distribution.

An alternative lateral approach may be used with the patient either sitting with the knee flexed or in a supine position. In this technique, the needle is passed from the lateral side of the thigh along the anterior border of the biceps femoris tendon, about 5 cm proximal to the popliteal crease. The saphenous nerve may also be anaesthetised by injecting along a line drawn from the tuberosity of the tibia to an intersection of the anterior and medial borders of the gastrocnemius muscle. Fairly large volumes (10–20 ml) of 0.5% levobupivacaine, 0.5% bupivacaine or 0.75% ropivacaine may need to be used.

The success of these procedures is likely to see their increased usage in podiatric surgery. However, while the procedure does have significant advantages in some scenarios, as highlighted above, the ankle block remains a very effective technique for most forefoot surgery (Miques et al 2005).