Lower limb anaesthesia
Regional anaesthesia plays an important role in the intra- and post-operative care of trauma patients. The availability and advancement of ultrasound technology as a core skill has allowed the safer delivery of regional anaesthesia, not only by experts, but also by those who are enthusiastic about the field.
1 The benefits and risks of regional anaesthesia in the trauma patient
2 The innervation of the lower limb
3 Understanding the numerous lower limb regional techniques and how to select the correct one
4 Nerve location techniques, including ultrasound
5 Delivery of safe regional anaesthesia.
2G02; 2G03
You are the named anaesthetist for the emergency (CEPOD) list. The orthopaedic surgeon tells you of a case he wishes to do. The patient is a previously healthy 25-year-old man who has suffered a displaced fracture of his distal right tibia and fibula whilst playing rugby. He requires an open reduction and internal fixation under anaesthesia.
The greatest benefit of regional anaesthesia is the post-operative analgesia the patient will be afforded. Regional anaesthesia, as part of a multimodal analgesic strategy, provides improved analgesia, compared with systemic analgesia alone. The use of a regional technique may reduce/avoid the need for opioid analgesia. The benefits of an opioid-free analgesic strategy are numerous, including less sedation, reduced nausea, and a decreased incidence of opioid-induced ileus.
Patients who have a regional anaesthesia technique, as part of their orthopaedic/trauma surgery, are quicker to mobilize than those receiving systemic opioid analgesia. In addition to quicker mobilization, the shorter convalescence leads to a reduced time to discharge.
It has long been thought that regional anaesthesia may impact on mortality. Although there seems to be some improvement in short-term mortality (1 month), this does not continue into the long term. Regional anaesthesia also appears to reduce the incidence of deep vein thrombosis (DVT), although this effect is somewhat masked by the widespread use of thromboprophylaxis.
Regional anaesthesia is a core skill but is highly operator-dependent. If the correct structures are located and the local anaesthesia placed in the correct vicinity, the incidence of serious complications is very low.
Motor nerve blockade is seen in the majority of regional techniques. Although this may not be a desired effect, it should be viewed as a side effect, rather than a complication, and the patient should be made aware of this. The most common complication is analgesic failure of nerve block. This can range from 5% to 25%, depending on the difficulty of the technique.
Familiarity with the anatomy and technique helps to reduce the incidence of ‘failed’ block. During a difficult block, more damage is likely to occur with multiple attempts to locate the target with a needle.
There is an argument that regional anaesthesia masks the pain of limb compartment syndrome, although this has not been shown in evidence.
Less common temporary complications include vascular puncture, nerve damage, local anaesthesia toxicity, and infection. The risk of permanent damage or death is extremely low. All frequently occurring and major complications must be explained to the patient before undertaking regional anaesthesia. A safe and methodical technique must be used to ensure the patient receives maximum benefit and minimal risk.
You see the patient preoperatively. After taking a history and examination, you explain your intraoperative plan. The patient agrees to receive a regional anaesthesia technique as part of his care. Prior to commencing any regional technique, it is important to have detailed anatomical knowledge of the specific region.
See Figure 4.1.
◆ The innervation of the lower limb originates from the sacral plexus (giving rise to the sciatic nerve) and the lumbar plexus (giving rise to the lateral cutaneous nerve of the thigh and the femoral and obturator nerves)
◆ The lumbar plexus arises from the 12th thoracic nerve to the 5th lumbar nerves. The plexus gives rise to the innervation of the lateral thigh (the lateral cutaneous nerve) and the anterior aspect of the thigh (femoral nerve), extending to the anteromedial aspect of the lower leg (saphenous nerve)
◆ The femoral nerve arises from the ventral rami of L2–4. It pierces the psoas muscle, then passes downward between the psoas and iliacus. The nerve enters the thigh, running deep to the inguinal ligament. It lies lateral to the femoral artery and deep to the fascia iliaca. Approximately 4 cm inferior to the inguinal ligament, the femoral nerve splits into anterior and posterior (containing the saphenous nerve) branches
◆ The saphenous nerve is the largest cutaneous branch of the femoral nerve. It initially follows the path of the femoral artery, before descending vertically down the medial part of the knee between the gracilis and sartorius muscles. The nerve continues down the medial aspect of the knee and anteromedially down the lower leg, together with the saphenous vein. The saphenous nerve provides sensation to the anteromedial aspect of the lower leg, including the medial malleolus
◆ The lateral cutaneous nerve of the thigh arises from the dorsal divisions of L2 and L3. It emerges from the lateral border of the psoas muscle, passing laterally and inferiorly towards the anterior superior iliac spine, before coursing underneath the inguinal ligament
◆ The sacral plexus arises from the anterior rami L4–S4. It supplies the motor and sensory innervation for the posterior thigh and the majority of the lower leg and foot. Essentially, the sciatic nerve is two separate nerves, the tibial and common fibular (also known as the common peroneal nerve), which run in unison for the majority of their course in the upper leg
◆ The sciatic nerve exits the pelvis through the greater sciatic foramen, usually inferiorly to the piriformis. It crosses the short external rotator muscles to run posteriorly in the thigh between the biceps femoris and the semimembranous muscles. Just below the level of the mid-thigh, the nerve bifurcates into its tibial and common fibular branches, which innervate the lower leg
◆ After bifurcation, the tibial nerve continues in a straight path down the popliteal fossa, lying close to the tibia as it continues its descent. It runs posterior to the medial malleolus, before entering the foot
◆ The sural nerve is an amalgamation of an articular and a cutaneous branch of the tibial nerve, in addition to some fibres from the common fibular nerve. It descends posterolaterally in the lower leg, running behind the lateral malleolus. The nerve is purely sensory, providing innervation to the posterolateral aspect of the lower leg and the lateral foot
◆ The common fibular nerve descends obliquely to run on the lateral side of the popliteal fossa. The nerve winds round the head of the fibula, soon dividing into superficial (providing sensation to the lateral aspect of the calf and the majority of the dorsum of the foot) and deep branches that supply sensory innervation to the webbing between the 1st and 2nd digits.
Fig. 4.1 The lumbosacral plexus. (Reproduced from Graeme McLeod, Colin McCartney, and Tony Wildsmith, Principles and Practice of Regional Anaesthesia, Fourth Edition, 2012, Figure 18.1, page 188, with permission from Oxford University Press.)
You are in the anaesthetic room, setting up for your list. The orthopaedic surgeon tells you he intends to make two longitudinal incisions, one along each side of the ankle, after which he intends to insert plates and screws to stabilize the fracture. He will also require a tourniquet to be placed.
In essence, regional anaesthesia will be achieved if a suitable amount of local anaesthetic is placed in close proximity to the nerve supplying the area in question. However, there are specific areas in which the nerves are more easily accessible; these sites provide the landmarks for the most commonly used regional blocks. Purists may argue that blocking the nerve at the most immediately proximal point is best (as it will preserve movement of more proximal joints). When undertaking a regional technique, it is best to select one with which the operator is familiar. Operator experience is a major factor in regional technique success.
In this case, to obtain satisfactory regional anaesthesia, the distal branches of both the sciatic (tibial and common fibular) and femoral (or saphenous) nerves need to be blocked. As the surgeons are going to make longitudinal incisions, which will extend above the malleoli, the traditional ankle block is not suitable.
The options for a regional block of the ankle include:
◆ Spinal anaesthesia alone
◆ Lumbar plexus block
◆ Femoral or saphenous nerve block.
In addition to:
◆ Sciatic or popliteal nerve block.
Any of those options listed will successfully block the nervous supply to the ankle. However, there are benefits and limitations to all techniques. It is preferable to preserve proximal limb movement to allow for better mobilization and prevent complications such as pressure sores. As stated, it is advisable to use a familiar technique, but also one which is more simplistic. To this end, the lumbar plexus block does require a significant level of expertise and therefore is best done by those experienced in it.
A femoral nerve block is more likely (than saphenous block) to aid in the tolerance of tourniquet-induced pain, which can be a factor, even in the well-anaesthetized patient. Due to the size of the sciatic nerve, it can be blocked from a number of sites. Proximal approaches include anterior and posterior; more distal approaches include lateral and the popliteal nerve block (which often means blocking the tibial and common fibular separately).
Certain nerve blocks have benefited from the introduction and widespread uptake of ultrasound technology. The target nerves in both the femoral and popliteal nerve blocks are easily visualized with ultrasound. To this end, an ultrasound-guided femoral and popliteal nerve block offers an efficacious and safe option, requiring a moderate level of expertise.
You have requested for the patient to be sent to theatre. The operating department assistant asks you what method you would like to use in locating the target nerves for your regional blocks.
Target-locating methods in regional anaesthesia include the following:
1 Subcutaneous infiltration, e.g. in digital ring blocks
2 Blind injection: as used in peribulbar blocks for cataract surgery
3 Change in resistance: usually reserved for ‘field blocks’ where the target is not an individual nerve, but rather an area in which the nerve supply crosses, e.g. the abdominal wall blocks (ilioinguinal, TAP, and posterior rectus sheath) and neuraxial blocks
4 Nerve stimulation
5 Ultrasound guidance.
Nerve stimulation has long been considered the gold standard for nerve location in regional anaesthesia. A small current (0.5–1.0 mA) passed down a regional block needle produces an impulse when in close proximity of the nerve. This allows the operator to deliver the local anaesthetic agent close to the nerve, whilst reducing the risk of intraneural injection.
Ultrasound has the advantage of allowing the visualization of the needle tip and local solution deposition, thus allowing the detection of inadvertent intraneural injection and optimal local anaesthetic spread. It can be argued that ultrasound is now the gold standard approach to regional anaesthesia. However, success is operator-dependent. Clinicians carrying out ultrasound-guided blocks must have the requisite training and experience. Inexperienced individuals must be supervised to ensure successful regional anaesthesia. As ultrasound is still relatively new, there will be those clinicians who are experienced in the nerve stimulation technique, but not so with ultrasound. An acceptable and safe method for this cohort is to use a combination of nerve stimulation and ultrasound to locate the nerve.
The connective tissue within nerves reflects ultrasound waves. Nerves are best visualized with a high-resolution (8–14 MHz) ultrasound probe, perpendicular to the nerve axis. The disadvantage of high-resolution probes is the shallow penetrative depth they offer. Depending on their structure, nerves can appear hypo- or hyperechoic (dark or bright). The characteristic pattern is of a circular/oval hypoechoic (dark) area surrounded by a brighter hyperechoic rim.
Once the ultrasound probe has successfully located the target nerve, the needle can be introduced and advanced in either the transverse axis (out of plane) or the longitudinal axis (in plane). The out-of-plane approach relies on the operator discerning ‘bouncing’ tissues as the needle is advanced to act as a guide to the needle tip position. The in-plane method is preferable, as it allows the visualization of the entire needle along its course, thus ensuring the needle tip position. The needle has to travel a greater distance in the in-plane method, and it is most probably an unfamiliar approach to those inexperienced with ultrasound.
With the needle in the correct position, in close proximity to the nerve, the injectate can be visualized with ultrasound. This allows greater confidence that the local anaesthetic solution is surrounding the nerve and has not been misplaced. Ultrasound technology allows for fine ‘tweaking’ of the needle position to ensure optimal placement of the local anaesthetic, often leading to a decrease in total volumes required.
You decide upon an ultrasound-guided femoral and popliteal nerve block under general anaesthesia as your intraoperative plan.
Inserting a needle loaded with local anaesthetic is simply one action in what should be a process, beginning with your preoperative visit and sometimes ending up to 72 hours after the procedure (if a catheter technique is used). To deliver safe and effective regional anaesthesia, it is best to adopt a structured approach.
First, it is vital to discover the correct surgical site from the surgical consent form; the patient must also confirm this. It should then be decided if the patient would benefit from a regional technique. Consent for the block must be obtained. The technique, its benefits, side effects, and risks must be explained and documented.
1 Identity and checks.
In theatre, prior to induction of anaesthesia, it is obligatory for the team to pause and conduct a surgical checklist. This allows the confirmation of the patient identity, procedure, correct surgical site, and the marking of the site. The World Health Organization (WHO) surgical checklist has been implemented to reduce the incidence of wrong-site surgery.
2 Prepare the patient.
Two of the major risks of attempting a regional block include inadvertent intraneural injection and intravascular injection, leading to possible local anaesthetic toxicity. Performing a regional block on the awake/lightly sedated patient offers the greatest margin of safety. Blocks are widely performed on asleep patients, but this renders them incapable of alerting the operator to pain/paraesthesiae (intraneural injection) or the early signs of local anaesthetic toxicity.
3 Position and asepsis.
The patient must be positioned correctly to facilitate needle insertion. The distal part of the limb should be exposed, if attempting to discern ‘twitches’ by nerve stimulation. The landmarks should be examined (femoral crease and proximal popliteal fossa) and marked, if required (see Figure 4.2). The block should be performed in aseptic conditions; therefore, the area must be cleaned with a skin disinfectant. It is important to avoid contact of the disinfecting agent with the local anaesthetic solution or equipment. The disinfectant must be allowed to dry (allowing time to prepare equipment), and any remaining fluid discarded.
4 Stop before you block.
The Royal College of Anaesthetists recommends a brief pause before needle insertion to double-check the correct side of block with the anaesthetic assistant. It is also prudent to remind yourself of the maximum volume of solution you are able to give at this point.
5 Nerve location, needle insertion, and injection.
The nerve is visualized by ultrasound (see Figure 4.3). Once the target nerve has been identified, subcutaneous local anaesthesia can be placed under direct vision, in order to make the remainder of the procedure painless. The regional needle is then loaded with local anaesthetic solution and guided (in-plane), until it is in close proximity to the nerve (at this point, nerve stimulation can be used, if required). The needle is attached to the local anaesthetic syringe via a flexible extension. The syringe is aspirated, and 1–2 mL of solution injected; this should result in the body of the nerve being displaced away from the needle. The nerve structure should be seen to remain intact. If the local anaesthetic solution has been deposited in the desired area, the remainder of the local anaesthetic may be carefully injected (with frequent aspiration), at all times under direct vision. The needle can be removed, and the patient prepared for general anaesthesia.
The operator must be vigilant of the signs of local anaesthetic toxicity at all times. Care must be taken in the positioning of the insensate limb. Clues regarding a successful or an unsuccessful block may become apparent during surgery through verbal and non-verbal responses.
Fig. 4.2 Ultrasound images of (a) femoral and (b) popliteal nerves.
Fig. 4.3 Ultrasound images of (a) femoral and (b) popliteal nerves.
In the post-operative period, the patient should be reviewed to ensure the block is working as planned. Care must be taken to protect the insensate limb. The patient should be seen post-operatively and be reviewed regularly by the acute pain service.
Summary
Regional anaesthesia offers an excellent method of analgesia for patients undergoing fracture surgery. Patients must be made aware of the risks and benefits of regional anaesthesia. Operator experience and knowledge are important factors in regional block success. Ultrasound technology has made regional anaesthesia a more accessible modality and has increased its safety. Clinicians must adopt a structured approach to performing regional anaesthesia.
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