LOCAL ANAESTHETICS AND REGIONAL ANAESTHESIA
Final Objective: Achieve an understanding of local anaesthetic use for analgesia and surgery.
Enabling Objective: To achieve this goal, you should know how to:
Reference Reading:
LOCAL ANAESTHETIC PHARMACOLOGY
Mode of action
Local anaesthetic agents reversibly block nerve conduction by blocking sodium channels. With increasing concentration of local anaesthetic around nerve endings, autonomic, sensory and somatic motor impulses are blocked. Spontaneous return of conduction occurs when the local anaesthetic is removed from the nerve endings. In clinical practice, local anaesthetics are used to block the conduction of pain from different parts of the body.
Local anaesthetics block the propagation of action potentials in nerve axons via the inhibition of the passage of sodium through voltage-gated sodium channels in the nerve membrane. They also block similar voltage-gated channels in cardiac and other neuronal cell bodies (like the brain) and as such are capable of causing toxicity in the brain and heart when high blood concentrations occur after their administration.
Smaller nerve fibres are more sensitive to local anaesthetic agents and fibres of the same size are more readily blocked if they are myelinated than if they do not have a myelin sheath. The most sensitive to least sensitive fibres are myelinated B fibres, small unmyelinated C fibres, small myelinated A-delta fibres, A-gamma fibres A-beta fibres and A-alpha fibres. Pain temperature, light touch, deep pressure and motor function tend to be blocked in that order. Paradoxically, mixed nerves have motor fibres in the outer layer, so paradoxically; they may be blocked by local anaesthetic before the sensory and pain fibres.
Local anaesthetics consist of a lipophilic portion and a hydrophilic chain joined by either an ester or an amide bond. The different local anaesthetics are classified into esters or amides according to this intermediate bond.
Characteristics of different local anaesthetics
Examples of amide local anaesthetics include lignocaine (lidocaine), bupivacaine, prilocaine, mepivacaine, levobupivacaine (the levo-enantiomer of bupivacaine) and ropivacaine. The ester local anaesthetics include cocaine, procaine, chloroprocaine, benzocaine and tetracaine.
The relative potencies of the local anaesthetics in increasing order of magnitude are procaine < cocaine < prilocaine < lignocaine and mepivacaine < bupivacaine and ropivacaine < tetracaine.
The duration of action of a local anaesthetic is proportional to the length of time that is in contact with a nerve. The removal of the local anaesthetic from the site of action determines the duration of action. More lipid soluble drugs and those that are bound to proteins have a longer duration of action. Procaine is short acting, whilst cocaine, lignocaine prilocaine and mepivacaine have and intermediate duration of action. Bupivacaine, ropivacaine and tetracaine have the longest duration of action.
LOCAL ANAESTHETIC TOXICITY
Local anaesthetic toxicity occurs due to the effects of local anaesthetic agents on sodium channels in excitable tissue such as the heart and nervous system. Signs and symptoms of local anaesthetic toxicity include, central nervous system irritability and convulsions followed by loss of consciousness and respiratory arrest. The patient may report tingling around the mouth or a numb tongue before restlessness is observed, followed by vertigo, tinnitus, difficulty focussing, slurred speech and muscle twitching.
Cardiac toxicity causes a depression of cardiac conduction and excitability which may lead to atrio-ventricular block, widening of the QRS complex and cardiac arrest. Myocardial contractility is depressed, leading to a low cardiac output and low blood pressure. Cardiac toxicity occurs after central nervous toxicity for most agents. Bupivacaine is unique in that cardiac toxicity occurs rapidly and is severe due to its slow dissociation from the cardiac sodium channels. If cardiac arrest occurs after intravascular injection of bupivacaine, resuscitation is difficult and prolonged.
Toxicity occurs when the plasma levels are high. The route of administration, site of injection, total dose administered, degree of protein binding, lipid solubility and local tissue blood flow influence the amount of systemic absorption of local anaesthetic agents. Injection of a drug into highly vascular tissue results in more rapid absorption and higher blood levels.
Limiting the total dose administered and carefully avoiding intravascular injection help prevent local anaesthetic toxicity. The administration of large volumes of local anaesthetics as boluses is avoided by fractionating the dose (giving 5 millilitre aliquots every 15 seconds until the total dose has been given). If the patient experiences signs or symptoms of toxicity, the injection is stopped immediately.
Lignocaine has intrinsic vasodilator properties. The addition of a vasoconstrictor such as adrenaline (epinephrine) will reduce plasma uptake of lignocaine and therefore prolong the duration of action and reduce peak plasma concentrations by up to 30%. The addition of adrenaline to other local anaesthetic agents such as bupivacaine and ropivacaine is not useful to prolong their duration of action.
Safe doses of local anaesthetic agents
Lignocaine Plain 4 mg/kg or with adrenaline 7 mg/kg
Bupivacaine 2 mg/kg
Levobupivacaine 2mg/kg
Ropivacaine 3.5 mg/kg
Cocaine 2 mg/kg
Prilocaine 6 mg/kg (iv regional)
Signs of local anaesthetic toxicity The toxicity of local anaesthetics is related to the central nervous system (CNS) and cardiovascular system (CVS) and includes the following:
For example for a 70 kg adult, the dose is = 70 ml x3 via syringe, then 290ml over 15 minutes.
LOCAL ANAESTHETIC TECHNIQUES
Local anaesthetics are administered in order to block pain neurotransmission. This can be achieved in several ways, including topical application to the mucous membranes, local infiltration, nerve blocks, plexus blocks and central neuroaxial blocks.
The amide local anaesthetics with an intermediate duration of action (lignocaine, prilocaine and mepivacaine) are useful for local infiltration and short minor procedures. Nerve and plexus blocks are performed with either agents with an intermediate or long (bupivacaine) duration of action.
Nerve identification:
Loss of resistance
Most nerves will lie in a sheath or tissue plane that can be filled with local anaesthetic and allow the deposition of local anaesthetic near the nerve. Some blocks are suitable for the use of loss of resistance to identify the correct tissue plane where the nerves are. They include the epidural block, ilioinguinal block, penile block and fascia iliaca block. Sometimes it is appropriate to use a short bevelled needle that allows the anaesthetist to feel the tissue planes as it is inserted. A “pop”, or loss of resistance to passage of the needle is used. For the epidural block, it is more appropriate to use a loss of resistance syringe filled with either air or saline attached to the end of a Tuohy or Crawford needle and apply continuous pressure to the plunger to identify the epidural space.
Surface landmarks
All nerve and plexus blocks will require knowledge of surface anatomy to identify the correct insertion point of the needle.
Relation to arteries
Nerves, arteries and veins are commonly closely related and the relevant artery is frequently used to identify the correct tissue plane, or sheath. This is especially useful for the brachial plexus blocks and femoral nerve block. Care must always be taken to avoid the injection of local anaesthetic into a vein or artery, or toxicity may occur.
Paraesthesia
Paraesthesia may be elicited by contacting a nerve with the needle. It is an unpleasant “electric shock” sensation and may be painful for the patient. It is no longer recommended that one should elicit paraesthesia to identify the nerve, but it may occur unintentionally. It does not necessarily mean that there will be a nerve injury, but if it occurs, the needle is repositioned slightly before injection of local anaesthetic.
Electrical stimulation
This requires the use of a nerve stimulator at low current. One lead is attached to an ECG dot on the patient (away from the muscle group that is expected to contract with stimulation) and the other lead is attached to an especially designed needle that is insulated and allows the current to pass from its tip to the grounding electrode on the ECG dot. A stimulus of 0.7 to 1.0 milliamperes is applied at 2 Hertz and a muscle twitch is sought when it estimated that the needle tip is close to the target nerve. When a twitch is detected the current is reduced until a threshold is reached. The threshold should be above 0.2 milliamperes (a level below this is considered too low, and the needle may actually be in the nerve or its endoneurium). If it is low, the tip of the needle is withdrawn one mm and a new threshold is sought. Intraneural injection is avoided as this will damage the nerve.
Ultrasound
The use of ultrasound to identify nerves and their relationship to vascular structures is very useful to increase the success of a block, and it may reduce complications (there is no strong evidence that the complication rate is reduced). This technique is relatively new and requires special equipment and the ability to keep the ultrasound probe sterile.
Safety
Before performing regional anaesthesia, several preconditions must be met. A thorough pre-operative assessment is made just as for general anaesthesia. There is always the possibility of block failure and the need to convert to a general anaesthetic, or local anaesthetic toxicity that will require treatment of seizures and indeed cardiac or respiratory arrest. Intravenous access needs to be secured before performing a major regional nerve block. The procedure is explained to the patient, including the possible complications and side effects. All necessary equipment for the treatment of cardiovascular collapse (intubation, ventilation and defibrillation equipment) is checked and available before the procedure.
The blocks are usually performed close to vessels and other structures such as the pleura (intercostal nerve block) and care must be taking with position of the needle and sudden movement of the patient. Gentle aspiration for blood is performed prior to the injection of local anaesthetic to avoid intravascular injection.
REGIONAL ANAESTHESIA OF THE HEAD AND NECK
Cervical plexus block
The cervical plexus consists of a superficial and deep component and is made up of the anterior primary rami from C1 to C4. It supplies sensation to the ear, anterior neck the shoulder and upper chest wall.
The superficial branches of the cervical plexus are the lesser occipital nerve, greater auricular nerve, transverse cervical nerves and supraclavicular nerves. Blockade of these nerves will provide sensory analgesia of the anterior neck and can be used for carotid endarterectomy (with infiltration of the deeper structures or deep cervical plexus block), superficial neck and ear surgery.
The nerves of the superficial cervical plexus emerge from the posterior border of the sternocleidomastoid (SCM) muscle at the level of the cricoid cartilage (6th cervical vertebral level). A 22 Gauge needle is inserted at this point and is passed through the cervical fascia. 10 ml of local anaesthetic is injected at this point, or it can be a fan-wise injection along the posterior border of the SCM in both cranial and caudal directions.
REGIONAL ANAESTHESIA OF THE UPPER LIMB
Brachial plexus anatomy
The brachial plexus is formed from the anterior primary rami of the 5th to 8th cervical nerves and the first thoracic nerve. It supplies motor and sensory fibres to the arm (apart from the medial aspect of the upper arm, which is supplied by the second thoracic nerve). The roots of the brachial plexus emerge from between the anterior and middle scalene muscles and form trunks (upper, middle and lower) between the sternocleidomastoid and trapezius muscles, divisions (anterior and posterior divisions of each trunk) behind the clavicle, cords (medial, lateral and posterior) around the axillary artery behind the pectoralis minor muscle and finally giving off the terminal branches around the third part of the axillary artery in the axilla. The plexus can be blocked at the interscalene groove, supra or infra clavicular level or in the axilla.
Axillary Brachial Plexus Block
The axillary brachial plexus block is useful for operations on the distal upper arm, lower arm and hand.
The patient is positioned in the supine or semi-recumbent position with the arm abducted, externally rotated and the elbow at 90 degrees. The relevant landmarks are the axillary artery and coracobrachialis muscle.
A 23-gauge needle attached to a plastic extension tube is used with a syringe on the end for aspiration. The axillary artery is palpated and the needle inserted directly over the artery at right angles to it. When arterial blood is seen, the needle is advanced further so as to exit the artery opposite its entry point and where it will be within the perivascular sheath. Using an assistant to aspirate for blood when aspiration is negative 10 ml local anaesthetic injected. This will effectively block the radial nerve. Withdraw the needle almost to skin and angle the needle slightly cephalad, a paraesthesia may be felt and 5-7 ml of local anaesthetic is injected after negative aspiration to blood. This will block the median nerve. The needle is then angled caudad, aspiration check and another 5-7 ml injected. This will block the ulnar nerve.
A distinct click or pop can be felt as the needle enters the neurovascular sheath. A total of 30-40 ml of 1.5% lignocaine with 1:200,000 adrenaline is injected. Bupivacaine 0.5% can also be used, but care is taken not to exceed the maximum dose of 2 mg per kg.
A supplemental injection may be required to anaesthetise the musculocutaneous nerve.
Complications of the axillary approach to the brachial plexus include intravascular injection and local anaesthetic toxicity, haematoma formation and false aneurysm of the axillary artery. It is important to apply at least 5 minutes of firm pressure to the artery to avoid bleeding and aneurysm formation.
Blocks at the elbow:
Radial nerve block
The radial nerve is located in the groove between the biceps and brachioradialis at the elbow. The arm is slightly abducted and the elbow is slightly flexed with the forearm supinated. A 22 G 50mm needle is inserted 2cm above the flexion crease of the elbow towards the lateral epicondyle of the humerus. 5-8 ml of local anaesthetic is injected and a further 5-8 ml is injected subcutaneously along the lateral border of the biceps tendon to block the lateral cutaneous nerve of the forearm, which is the terminal sensory branch of the musculocutaneous nerve that is sometimes missed when performing an axillary brachial plexus block.
Median nerve block
The median nerve is medial and deep to the brachial artery and is partially covered by the biceps tendon. A 22G 5mm needle is inserted medial to the brachial artery and 5-8 ml of local anaesthetic is injected. A further 5-8 ml of local anaesthetic is injected subcutaneously along the medial border of the biceps tendon to block the medial cutaneous nerve of the forearm.
Ulnar nerve block
Block of the ulnar nerve in the ulnar groove of the humerus is not recommended as this can cause neural injury. Ulnar nerve block at the elbow is avoided.
Wrist block
Wrist block is very useful for operations on the hand that do not require the use of a tourniquet. It involves the block of median, ulnar and radial nerves and subcutaneous injection. A block at the wrist will allow the extrinsic muscles of the hand to retain motor function.
The median nerve is deep to the Palmaris longus tendon. A 25 G needle is inserted between the tendons of the Palmaris longus and flexor carpi radialis at the level of the proximal palmar crease. Palmaris longus may be absent in some individuals. If this is the case, the needle is inserted about 5 mm medial to the flexor carpi radialis tendon. There nerve lies deep to the flexor retinaculum at the wrist and 3-5 ml of local anaesthetic is used. Even though the nerve is deep to the tendon and fascial layer at the wrist, the injection is very superficial. On withdrawal of the needle, a subcutaneous injection is made to block the palmar cutaneous branch of the median nerve.
The ulnar nerve may be blocked on the medial aspect of the wrist by passing a 25 G needle underneath the tendon of flexor carpi ulnaris at the level of the wrist crease. 3ml of local anaesthetic is injected at a depth of approximately 1cm. The dorsal cutaneous branch is blocked by infiltrating local anaesthetic in the subcutaneous layer around the ulnar aspect of the wrist.
The radial nerve is performed by infiltration of 5-8 ml of local anaesthetic over the radial aspect of the wrist at a level 2 fingerbreadths proximal the to base of the metacarpal.
Digital nerve block
Digital nerve block of the fingers or toes is relatively simple and can be very useful for operations on fingers and toes that do not require a proximal tourniquet. Adrenaline containing local anaesthetic is contraindicated as their injection around the end arteries may cause ischaemia of the digits.
An injection of local anaesthetic is made after insertion of a 25 G needle perpendicular to the skin just distal to the metacarpophalyngeal joint (in the foot, it is the metatarsophalyngeal joint) and directing the needle to the palmar (plantar in the foot) surface. 2-3 ml is injected on either side of the phalanx. A dorsal injection of 1ml of local anaesthetic across the phalanx completes the block.
REGIONAL ANAESTHESIA OF THE LOWER LIMB
Fascia-iliaca compartment block
The femoral and lateral femoral cutaneous nerves are sensory to skin overlying the anterior and lateral aspects of the thigh respectively. The femoral nerve also supplies the hip and knee joints. In the case of the knee joint, the additional innervation from obturator and sciatic nerves is minor. With the hip these are more significant. The shaft of the femur is predominantly supplied from the femoral nerve.
Place the patient supine and mark the inguinal ligament and the femoral artery. In an adult the injection point is 3-4 cm lateral to the femoral artery and 1 cm inferior to the inguinal ligament or 1 cm below the junction of the middle and lateral thirds of the inguinal ligament (child). Using a short bevel needle, approach the skin at an angle of 45° (bevel up). Two ‘pops’ are felt, the first as the fascia lata is penetrated which is quite a definite pop. The second pop (which is felt as the fascia iliaca is penetrated) is less distinct and is often felt as a series of 2-3 pops. The depth of the needle tip at this stage is usually 4 cm in the adult but may be 5 cm in larger individuals.
I inject 2 mg/kg of bupivacaine 0.25% (plain) after aspiration in doses of 5-7 ml. The total dose is given over 1-2 minutes or 30-40 ml lignocaine 1% in divided doses.
The main complication is the block not working adequately (10% chance) however this risk is lowered with experience. Femoral nerve neuropathy may occur. This usually resolves over weeks, but beware, the neuropathy may be due to the surgery itself. There is a low risk of infection at the injection site if aseptic techniques are applied. There are no reports of local anaesthetic toxicity however potentially it could occur. Motor blockade of the quadriceps occur and the knee extensor can be blocked which can be a problem for walking whilst the block is working.
Ankle block
Five nerves supply the foot: 4 are branches of the sciatic (tibial, superior and deep peroneal, sural) the fifth (saphenous) is a terminal branch of the femoral nerve. Innervation to the foot is highly variable; therefore, aim to block all 5 nerves, except for great toe surgery where a sural nerve block can be excluded. The tibial and deep peroneal nerves, which are blocked beneath the deep fascia supply bones, joints and muscles of the foot. The sensory distribution of the 5 nerves is shown.
Ankle blocks can be used to provide anaesthesia for surgical procedures of the foot. It will not provide a motor block of the foot.
The saphenous, superficial peroneal and sural nerves are superficial, whilst the deep peroneal and posterior tibial nerves are located more deeply and are closely related to the anterior tibial artery and posterior tibial artery.
The posterior tibial nerve lies along the medial aspect of the Achilles tendon, just posterior to the artery. It is blocked with the patient’s foot supported on a pillow with the leg in external rotation. A line drawn from the medial malleoulus to the posterior inferior calcaneum intersects with a point posterior to the posterior tibial artery. A 22G 50mm needle is inserted and if bone is felt, it is withdrawn slightly. 5-7 ml of local anaesthetic is injected after gentle aspiration to ensure a vessel has not been entered.
The deep peroneal block is performed midway between the malleoli lateral to the tendon of extensor hallucis longus and the anterior tibial artery. The needle is advanced toward the tibia and 3-5 ml of local anaesthetic is injected deep to the fascia.
The sural nerve is subcutaneous distal to the middle of the leg near the short saphenous vein behind and distal to the lateral malleolus. A skin wheal is made lateral to the Achilles tendon at the level of the lateral malleolus. The 3 cm needle is inserted to a depth of 1 cm and directed toward the lateral border of the fibula and 3-5 ml of local anaesthetic is injected in a fanwise manner from the lateral border of the Achilles tendon to the lateral border of the fibula.
The superficial peroneal nerve is blocked by subcutaneous infiltration of 5 ml of local anaesthetic from the anterior border of the tibia to the superior aspect of the lateral malleolus.
The saphenous nerve is blocked subcutaneously at the ankle with an injection of 3-5 ml of local anaesthetic immediately proximal and anterior to the medial malleolus to the anterior border of the tibia.
REGIONAL ANAESTHESIA OF THE TRUNK
Intercostal nerve block
The intercostal nerves are the primary rami of the first to 11th thoracic nerves. The nerves from T4 to T11 supply somatic innervation to the area from the nipples to below the umbilicus. The chest wall above the nipples is supplied by the nerves of T2 to T3 and peripheral branches of the cervical plexus (C3 to C4). The intercostal nerve of T12 is not a true intercostal nerve, but a subcostal nerve. Some of its fibres join with fibres from the first lumbar nerve to form the iliohypogastric and ilioinguinal nerves.
Each intercostal nerve has five branches. Two are the paired grey and white rami communicantes, they pass anteriorly to and from the sympathetic chain. The third branch is the posterior cutaneous branch that supplies the skin and muscles of the paravertebral region. The fourth branch is the lateral cutaneous division that is given off anterior to the midaxillary line and supplies fibres to the skin of the chest and abdominal wall. The terminal branch is the anterior cutaneous branch and it supplies the anterior chest wall and anterior part of the abdomen to just below the umbilicus.
Intercostal nerve block is useful for pain control for operations on the chest wall and abdominal wall. It is very useful for control of pain from fractured ribs and herpes zoster.
Intercostal nerve block is performed at the angle of the rib (6-8 cm from the spinous process) where the nerve comes to lie between the internal intercostal muscle and the innermost intercostal muscle. The costal groove of the rib is broadest and deepest in this position. The intercostal veins and artery travel superior to the nerve in the intercostal groove on the inferior border of the rib. The patient is positioned in the lateral or prone position. It is useful to mark the injection points if multiple injections are planned. A line is drawn parallel to the spine at the point where the ribs are easily palpated (6-9 cm from the midline). The correct levels are identified and beginning at the lowest rib to be blocked, the skin is immobilized with one hand whilst the 22 G 3-4 cm short bevelled needle with local anaesthetic in a syringe is inserted onto the rib and the needle is walked off the lower border of the rib and advanced 2-3 mm where a subtle loss of resistance may be felt. An injection of 3-5 ml of local anaesthetic is made and the needle is withdrawn.
The potential complications of intercostal nerve block are pneumothorax and intravascular injection. The dose of local anaesthetic is restricted to avoid the development of toxicity, as its absorption by the intercostal vessels is high.
Inguinal field block
Ilioinguinal and iliohypogastric nerve blocks are used for providing analgesia for inguinal hernia repair and for orchidopexy (where an inguinal incision is made). These blocks alone will not provide sufficient anaesthesia for hernia repair without supplemental local anaesthetic infiltration.
The iliohypogastric nerve originates from branches of the 12th thoracic and 1st lumbar segments. Near the anterior superior iliac spine (ASIS), it is located between the external oblique muscle and the internal oblique muscle. It has cutaneous branches to the inguinal ligament and skin overlying it.
The ilioinguinal nerve originates from the 1st lumbar segment and at the level of the anterior superior iliac spine (ASIS), it lies between the transverse abdominis and the internal oblique muscles and then enters the scrotum to provide sensation to the superior portion of the scrotum and inner part of the thigh.
Block of the ilioinguinal and iliohypogastric nerves is achieved with the patient in the supine position. At a pint 2-3 cm medial and 2-3 cm inferior to the ASIS, a skin wheal is raised and an 8cm 22G needle is inserted in a cephalo-lateral direction to contact the inner surface of the ilium. 10 ml of local anaesthetic is injected as the needle is slowly withdrawn and then the needle is reinserted at a steeper angle (more perpendicular) until it penetrates all three lateral abdominal muscles. A further 10 ml of local anaesthetic is injected as the needle is withdrawn.
In a child, a short bevelled 25 G needle is inserted 1 cm medial and 1 cm superior to the ASIS. It is advanced in an inferolateral direction until it comes into contact with the iliac wing and half the dose is injected as the needle is withdrawn (to anaesthetise the iliohypogastric nerve). With the needle in the same entry point, it is advanced in a posterior and inferior direction toward the inguinal ligament until a fascial pop is felt as it pierces the external oblique fascia. The rest of the dose is injected here to anaesthetise the ilioinguinal nerve.
The main side effect of the block is transient quadriceps femoris weakness due to transient block of the femoral nerve.
Penile block
Penile nerve block is used predominantly for circumcision or any other operation on the body of the penis. This block may not anaesthetise the ventral surface of the penis and will not provide anaesthesia to the penile urethra.
The dorsal nerves of the penis are branches of the pudendal nerves and they emerge from under the bony pubis in a triangular area bounded superiorly by Buck’s fascia and posteriorly by the pubis and inferiorly by the crura of the penis.
Local anaesthetic is injected into this area by passing a 22g needle at the base of the penis at the 10 o’clock and 2 o’clock positions. At each position, the needle strikes the pubis and is then ‘walked’ off in a caudal direction until bone is no longer contacted. As the needle is passed through Buck’s fascia, a fascial click or ‘pop’ may be felt. After gentle aspiration to ensure a vessel has not been entered, 0.1 ml per kg body weight of plain 0.5% Bupivacaine or 1% Lignocaine is injected.
Complications of penile nerve block include ischaemia if adrenaline-containing solutions are used and intravascular injection.
INTRAVENOUS REGIONAL ANAESTHESIA
Prof August Bier first described intravenous regional anaesthesia in 1908. It can be used in the upper or lower limb for procedures on the distal part of the arm. Because local anaesthetic is toxic if injected intravenously, it is important to apply a tourniquet on the proximal part of the limb with the pressure raised to 50-100 mmHg greater than the systolic blood pressure. Typically it is set at 250 mmHg in the arm. The cuff is kept inflated for at least 20 minutes so that a toxic dose of local anaesthetic is not released into the circulation. The amount of local anaesthetic that is immediately released into the circulation of release of the tourniquet cuff is less than with increasing tourniquet time. A smaller proportion of the local anaesthetic is released into the circulation with increasing tourniquet time. This means that the peak plasma concentration of local anaesthetic is less than if would have been if a simple intravenous dose of local anaesthetic was administered without a tourniquet. It is the peak concentration that is important for creating toxicity. After release of the tourniquet, the local anaesthetic will undergo some metabolism in the lungs. The lungs take up Prilocaine to a greater extent than lignocaine. The safest local anaesthetic for intravenous regional anaesthesia is prilocaine (0.5%). Lignocaine 0.5% is also suitable.
A working tourniquet is essential for the safe performance of a Bier’s block. Sometimes a double cuff is used, because tourniquet pain is the limiting factor for the duration of the block. This block is only reliable for procedures that will last less than one hour. Once the tourniquet is released, the block is no longer effective. Mild signs of systemic toxicity may be apparent on release of the tourniquet. It is important to monitor the patient carefully during deflation.
A cannula is placed distally in the arm to be anaesthetised and another is placed in the opposite arm in case of accidental toxicity for the administration of anticonvulsant and other medications.
The upper arm is wrapped in non-irritant fabric to protect the skin from the tourniquet and a double (or two separate) tourniquet is placed. The arm is elevated for 2-3 minutes whilst compressing the axillary artery to exsanguinate it and the distal cuff is inflated after which the proximal cuff is inflated. When it is confirmed that the cuffs are in working order, the distal cuff is deflated and the injection of local anaesthetic is made. For an arm, a dose of 3 mg per kg is used and the injection is made slowly to avoid injecting at high pressure. The onset of anaesthesia is 2-4 minutes. If the procedure lasts more than 20 minutes and the cuff is causing pain, the distal cuff is re-inflated (as this area is theoretically anaesthetised) before the proximal cuff is deflated slowly.
COMPLICATIONS AND SIDE EFFECTS OF REGIONAL ANAESTHESIA
Regional anaesthesia has many benefits, including surgical anaesthesia, postoperative analgesia and the ability to avoid a general anaesthetic. However all techniques have side effects and complications. Drug related complications include systemic toxicity to local anaesthetic or additives, neurotoxicity and allergy. Some side effects relate to the effects of the block itself, including motor block, prolonged block and autonomic block (particularly problematic in spinal and epidural anaesthesia) and unintentional block of adjacent nerves. Unintended complications include injury to adjacent structures (such as the pleura, blood vessels, muscles and viscera) and infection or bleeding after the injection.
Neural injury can result from toxicity from the injected substance (such as additives or preservatives or contamination with alcohol), immunologic reactions to injected substances, infection, direct trauma (injection into a nerve is avoided), compression of the nerve (by blood, an abscess, or incorrect positioning), ischaemia of the nerve or related to the surgical procedure.
Injury due to peripheral nerve block is uncommon, but can be relatively minor and the symptoms may be delayed. Permanent nerve injury after regional anaesthesia is believed to occur at a rate of less than 0.02% to 0.4%. The presentation is usually paraesthesia or dysaesthesia and they tend to resolve over a period of weeks to months.
Injection into the fascicle of the nerve increases intraneural pressure and may induce ischaemia. It should be avoided. In the conscious patient, pain may occur with injection if the needle is in the fascicle. Paraesthesia during the performance of a block indicates that the needle is near to or in contact with the nerve. The needle should be repositioned if it occurs. There is no data to demonstrate that eliciting paraesthesia causes neuropathy.
SELF-ASSESSMENT QUESTIONS
1. Describe the features of the ideal local anaesthetic agent.
2. Lignocaine and bupivacaine are both amide local anaesthetic agents. Outline their major pharmacologic differences.
3. What are the indications and potential applications of fascia iliaca nerve block?
4. What structures does the median nerve supply? Describe three ways it may be blocked and the advantages and disadvantages of each approach.
5. Describe in detail your approach to providing analgesia for a patient who has fractured ribs 6, 7 and 8 on the right side. Outline the advantages and disadvantages of an epidural versus intercostal nerve block for such a patient.
ASSIGNMENT
You are asked to design a consent form for the performance of regional anaesthesia in paediatric patients in your department. Outline what the relevant features of the consent process are important and draft a document that can be given to the parents of a small child. Is asking the parent to sign such a form sufficient to obtain informed consent? Why?
LOCAL ANAESTHETIC CASE STUDIES
Case No 1
Tsahim is a 59 year old male with an inguinal hernia for repair. He is on anticoagulation for atrial fibrillation. He has consented to a general anaesthetic for the repair but is very concerned about the pain after surgery. There is no opioid available for analgesia in your hospital and he has a stomach ulcer so will not tolerate non-steroidal medication.
Describe in detail your approach to providing postoperative analgesia for Tsahim.
What are the advantages of using bupivacaine for this block? What is the safe maximum dose of bupivacaine?
Case No2
Burmaa is a 28 year old woman who has caught her right hand in the door her car. The surgeon will need to repair two metacarpal bones and she has a large area of skin missing from the dorsum of her hand. Several tendons will require repair. She is requesting a regional anaesthetic.
What options are available to you for regional anaesthesia of the arm?
You have injected 30 ml of local anaesthetic into an arm block. After the injection, Burmaa says she has a funny taste in her mouth and blurred vision. What are the other signs of local anaesthetic toxicity you will be seeking? Outline your management of Burmaa.
Final Objective: Achieve an understanding of local anaesthetic use for analgesia and surgery.
Enabling Objective: To achieve this goal, you should know how to:
- Describe the pharmacology of local anaesthetic drugs
- Recognise and manage local anaesthetic toxicity
- Identify the indications for local anaesthetic techniques
- Describe commonly used local anaesthetic techniques including the relevant anatomy, advantages, disadvantages and potential complications
- Discuss the consent process for regional anaesthesia
Reference Reading:
- Developing Anaesthesia Chapters 37, 38
- Oxford handbook of anaesthesia Chapter 2, 41
- Pain Management seminar notes 2008
LOCAL ANAESTHETIC PHARMACOLOGY
Mode of action
Local anaesthetic agents reversibly block nerve conduction by blocking sodium channels. With increasing concentration of local anaesthetic around nerve endings, autonomic, sensory and somatic motor impulses are blocked. Spontaneous return of conduction occurs when the local anaesthetic is removed from the nerve endings. In clinical practice, local anaesthetics are used to block the conduction of pain from different parts of the body.
Local anaesthetics block the propagation of action potentials in nerve axons via the inhibition of the passage of sodium through voltage-gated sodium channels in the nerve membrane. They also block similar voltage-gated channels in cardiac and other neuronal cell bodies (like the brain) and as such are capable of causing toxicity in the brain and heart when high blood concentrations occur after their administration.
Smaller nerve fibres are more sensitive to local anaesthetic agents and fibres of the same size are more readily blocked if they are myelinated than if they do not have a myelin sheath. The most sensitive to least sensitive fibres are myelinated B fibres, small unmyelinated C fibres, small myelinated A-delta fibres, A-gamma fibres A-beta fibres and A-alpha fibres. Pain temperature, light touch, deep pressure and motor function tend to be blocked in that order. Paradoxically, mixed nerves have motor fibres in the outer layer, so paradoxically; they may be blocked by local anaesthetic before the sensory and pain fibres.
Local anaesthetics consist of a lipophilic portion and a hydrophilic chain joined by either an ester or an amide bond. The different local anaesthetics are classified into esters or amides according to this intermediate bond.
Characteristics of different local anaesthetics
Examples of amide local anaesthetics include lignocaine (lidocaine), bupivacaine, prilocaine, mepivacaine, levobupivacaine (the levo-enantiomer of bupivacaine) and ropivacaine. The ester local anaesthetics include cocaine, procaine, chloroprocaine, benzocaine and tetracaine.
The relative potencies of the local anaesthetics in increasing order of magnitude are procaine < cocaine < prilocaine < lignocaine and mepivacaine < bupivacaine and ropivacaine < tetracaine.
The duration of action of a local anaesthetic is proportional to the length of time that is in contact with a nerve. The removal of the local anaesthetic from the site of action determines the duration of action. More lipid soluble drugs and those that are bound to proteins have a longer duration of action. Procaine is short acting, whilst cocaine, lignocaine prilocaine and mepivacaine have and intermediate duration of action. Bupivacaine, ropivacaine and tetracaine have the longest duration of action.
LOCAL ANAESTHETIC TOXICITY
Local anaesthetic toxicity occurs due to the effects of local anaesthetic agents on sodium channels in excitable tissue such as the heart and nervous system. Signs and symptoms of local anaesthetic toxicity include, central nervous system irritability and convulsions followed by loss of consciousness and respiratory arrest. The patient may report tingling around the mouth or a numb tongue before restlessness is observed, followed by vertigo, tinnitus, difficulty focussing, slurred speech and muscle twitching.
Cardiac toxicity causes a depression of cardiac conduction and excitability which may lead to atrio-ventricular block, widening of the QRS complex and cardiac arrest. Myocardial contractility is depressed, leading to a low cardiac output and low blood pressure. Cardiac toxicity occurs after central nervous toxicity for most agents. Bupivacaine is unique in that cardiac toxicity occurs rapidly and is severe due to its slow dissociation from the cardiac sodium channels. If cardiac arrest occurs after intravascular injection of bupivacaine, resuscitation is difficult and prolonged.
Toxicity occurs when the plasma levels are high. The route of administration, site of injection, total dose administered, degree of protein binding, lipid solubility and local tissue blood flow influence the amount of systemic absorption of local anaesthetic agents. Injection of a drug into highly vascular tissue results in more rapid absorption and higher blood levels.
Limiting the total dose administered and carefully avoiding intravascular injection help prevent local anaesthetic toxicity. The administration of large volumes of local anaesthetics as boluses is avoided by fractionating the dose (giving 5 millilitre aliquots every 15 seconds until the total dose has been given). If the patient experiences signs or symptoms of toxicity, the injection is stopped immediately.
Lignocaine has intrinsic vasodilator properties. The addition of a vasoconstrictor such as adrenaline (epinephrine) will reduce plasma uptake of lignocaine and therefore prolong the duration of action and reduce peak plasma concentrations by up to 30%. The addition of adrenaline to other local anaesthetic agents such as bupivacaine and ropivacaine is not useful to prolong their duration of action.
Safe doses of local anaesthetic agents
Lignocaine Plain 4 mg/kg or with adrenaline 7 mg/kg
Bupivacaine 2 mg/kg
Levobupivacaine 2mg/kg
Ropivacaine 3.5 mg/kg
Cocaine 2 mg/kg
Prilocaine 6 mg/kg (iv regional)
Signs of local anaesthetic toxicity The toxicity of local anaesthetics is related to the central nervous system (CNS) and cardiovascular system (CVS) and includes the following:
- CNS
- Initial excitation (via inhibition of inhibitory fibres)
- CNS irritability: tongue tingling, circumoral paresthesia
- Tinnitus, light headedness
- Slurred speech
- Muscle twitching, tremor
- Drowsiness
- Seizures
- Apnoea
- CVS
- BP: initial vasoconstriction followed by hypotension secondary to vasodilatation and depressed cardiac contractility
- Rhythm: increased AV block, QRS widening and extrasystoles, cardiac arrest
- Bupivacaine produces profound cardiac toxicity
- Stop injection
- Supportive measures: avoid hypoxemia, hypercarbia and acidosis (assist ventilation)
- Seizure treatment with benzodiazepines (midazolam), thiopentone
- CVS support: vasopressors, adrenaline, ephedrine, vasopressin, cardioversion, amiodarone
For example for a 70 kg adult, the dose is = 70 ml x3 via syringe, then 290ml over 15 minutes.
LOCAL ANAESTHETIC TECHNIQUES
Local anaesthetics are administered in order to block pain neurotransmission. This can be achieved in several ways, including topical application to the mucous membranes, local infiltration, nerve blocks, plexus blocks and central neuroaxial blocks.
The amide local anaesthetics with an intermediate duration of action (lignocaine, prilocaine and mepivacaine) are useful for local infiltration and short minor procedures. Nerve and plexus blocks are performed with either agents with an intermediate or long (bupivacaine) duration of action.
Nerve identification:
Loss of resistance
Most nerves will lie in a sheath or tissue plane that can be filled with local anaesthetic and allow the deposition of local anaesthetic near the nerve. Some blocks are suitable for the use of loss of resistance to identify the correct tissue plane where the nerves are. They include the epidural block, ilioinguinal block, penile block and fascia iliaca block. Sometimes it is appropriate to use a short bevelled needle that allows the anaesthetist to feel the tissue planes as it is inserted. A “pop”, or loss of resistance to passage of the needle is used. For the epidural block, it is more appropriate to use a loss of resistance syringe filled with either air or saline attached to the end of a Tuohy or Crawford needle and apply continuous pressure to the plunger to identify the epidural space.
Surface landmarks
All nerve and plexus blocks will require knowledge of surface anatomy to identify the correct insertion point of the needle.
Relation to arteries
Nerves, arteries and veins are commonly closely related and the relevant artery is frequently used to identify the correct tissue plane, or sheath. This is especially useful for the brachial plexus blocks and femoral nerve block. Care must always be taken to avoid the injection of local anaesthetic into a vein or artery, or toxicity may occur.
Paraesthesia
Paraesthesia may be elicited by contacting a nerve with the needle. It is an unpleasant “electric shock” sensation and may be painful for the patient. It is no longer recommended that one should elicit paraesthesia to identify the nerve, but it may occur unintentionally. It does not necessarily mean that there will be a nerve injury, but if it occurs, the needle is repositioned slightly before injection of local anaesthetic.
Electrical stimulation
This requires the use of a nerve stimulator at low current. One lead is attached to an ECG dot on the patient (away from the muscle group that is expected to contract with stimulation) and the other lead is attached to an especially designed needle that is insulated and allows the current to pass from its tip to the grounding electrode on the ECG dot. A stimulus of 0.7 to 1.0 milliamperes is applied at 2 Hertz and a muscle twitch is sought when it estimated that the needle tip is close to the target nerve. When a twitch is detected the current is reduced until a threshold is reached. The threshold should be above 0.2 milliamperes (a level below this is considered too low, and the needle may actually be in the nerve or its endoneurium). If it is low, the tip of the needle is withdrawn one mm and a new threshold is sought. Intraneural injection is avoided as this will damage the nerve.
Ultrasound
The use of ultrasound to identify nerves and their relationship to vascular structures is very useful to increase the success of a block, and it may reduce complications (there is no strong evidence that the complication rate is reduced). This technique is relatively new and requires special equipment and the ability to keep the ultrasound probe sterile.
Safety
Before performing regional anaesthesia, several preconditions must be met. A thorough pre-operative assessment is made just as for general anaesthesia. There is always the possibility of block failure and the need to convert to a general anaesthetic, or local anaesthetic toxicity that will require treatment of seizures and indeed cardiac or respiratory arrest. Intravenous access needs to be secured before performing a major regional nerve block. The procedure is explained to the patient, including the possible complications and side effects. All necessary equipment for the treatment of cardiovascular collapse (intubation, ventilation and defibrillation equipment) is checked and available before the procedure.
The blocks are usually performed close to vessels and other structures such as the pleura (intercostal nerve block) and care must be taking with position of the needle and sudden movement of the patient. Gentle aspiration for blood is performed prior to the injection of local anaesthetic to avoid intravascular injection.
REGIONAL ANAESTHESIA OF THE HEAD AND NECK
Cervical plexus block
The cervical plexus consists of a superficial and deep component and is made up of the anterior primary rami from C1 to C4. It supplies sensation to the ear, anterior neck the shoulder and upper chest wall.
The superficial branches of the cervical plexus are the lesser occipital nerve, greater auricular nerve, transverse cervical nerves and supraclavicular nerves. Blockade of these nerves will provide sensory analgesia of the anterior neck and can be used for carotid endarterectomy (with infiltration of the deeper structures or deep cervical plexus block), superficial neck and ear surgery.
The nerves of the superficial cervical plexus emerge from the posterior border of the sternocleidomastoid (SCM) muscle at the level of the cricoid cartilage (6th cervical vertebral level). A 22 Gauge needle is inserted at this point and is passed through the cervical fascia. 10 ml of local anaesthetic is injected at this point, or it can be a fan-wise injection along the posterior border of the SCM in both cranial and caudal directions.
REGIONAL ANAESTHESIA OF THE UPPER LIMB
Brachial plexus anatomy
The brachial plexus is formed from the anterior primary rami of the 5th to 8th cervical nerves and the first thoracic nerve. It supplies motor and sensory fibres to the arm (apart from the medial aspect of the upper arm, which is supplied by the second thoracic nerve). The roots of the brachial plexus emerge from between the anterior and middle scalene muscles and form trunks (upper, middle and lower) between the sternocleidomastoid and trapezius muscles, divisions (anterior and posterior divisions of each trunk) behind the clavicle, cords (medial, lateral and posterior) around the axillary artery behind the pectoralis minor muscle and finally giving off the terminal branches around the third part of the axillary artery in the axilla. The plexus can be blocked at the interscalene groove, supra or infra clavicular level or in the axilla.
Axillary Brachial Plexus Block
The axillary brachial plexus block is useful for operations on the distal upper arm, lower arm and hand.
The patient is positioned in the supine or semi-recumbent position with the arm abducted, externally rotated and the elbow at 90 degrees. The relevant landmarks are the axillary artery and coracobrachialis muscle.
A 23-gauge needle attached to a plastic extension tube is used with a syringe on the end for aspiration. The axillary artery is palpated and the needle inserted directly over the artery at right angles to it. When arterial blood is seen, the needle is advanced further so as to exit the artery opposite its entry point and where it will be within the perivascular sheath. Using an assistant to aspirate for blood when aspiration is negative 10 ml local anaesthetic injected. This will effectively block the radial nerve. Withdraw the needle almost to skin and angle the needle slightly cephalad, a paraesthesia may be felt and 5-7 ml of local anaesthetic is injected after negative aspiration to blood. This will block the median nerve. The needle is then angled caudad, aspiration check and another 5-7 ml injected. This will block the ulnar nerve.
A distinct click or pop can be felt as the needle enters the neurovascular sheath. A total of 30-40 ml of 1.5% lignocaine with 1:200,000 adrenaline is injected. Bupivacaine 0.5% can also be used, but care is taken not to exceed the maximum dose of 2 mg per kg.
A supplemental injection may be required to anaesthetise the musculocutaneous nerve.
Complications of the axillary approach to the brachial plexus include intravascular injection and local anaesthetic toxicity, haematoma formation and false aneurysm of the axillary artery. It is important to apply at least 5 minutes of firm pressure to the artery to avoid bleeding and aneurysm formation.
Blocks at the elbow:
Radial nerve block
The radial nerve is located in the groove between the biceps and brachioradialis at the elbow. The arm is slightly abducted and the elbow is slightly flexed with the forearm supinated. A 22 G 50mm needle is inserted 2cm above the flexion crease of the elbow towards the lateral epicondyle of the humerus. 5-8 ml of local anaesthetic is injected and a further 5-8 ml is injected subcutaneously along the lateral border of the biceps tendon to block the lateral cutaneous nerve of the forearm, which is the terminal sensory branch of the musculocutaneous nerve that is sometimes missed when performing an axillary brachial plexus block.
Median nerve block
The median nerve is medial and deep to the brachial artery and is partially covered by the biceps tendon. A 22G 5mm needle is inserted medial to the brachial artery and 5-8 ml of local anaesthetic is injected. A further 5-8 ml of local anaesthetic is injected subcutaneously along the medial border of the biceps tendon to block the medial cutaneous nerve of the forearm.
Ulnar nerve block
Block of the ulnar nerve in the ulnar groove of the humerus is not recommended as this can cause neural injury. Ulnar nerve block at the elbow is avoided.
Wrist block
Wrist block is very useful for operations on the hand that do not require the use of a tourniquet. It involves the block of median, ulnar and radial nerves and subcutaneous injection. A block at the wrist will allow the extrinsic muscles of the hand to retain motor function.
The median nerve is deep to the Palmaris longus tendon. A 25 G needle is inserted between the tendons of the Palmaris longus and flexor carpi radialis at the level of the proximal palmar crease. Palmaris longus may be absent in some individuals. If this is the case, the needle is inserted about 5 mm medial to the flexor carpi radialis tendon. There nerve lies deep to the flexor retinaculum at the wrist and 3-5 ml of local anaesthetic is used. Even though the nerve is deep to the tendon and fascial layer at the wrist, the injection is very superficial. On withdrawal of the needle, a subcutaneous injection is made to block the palmar cutaneous branch of the median nerve.
The ulnar nerve may be blocked on the medial aspect of the wrist by passing a 25 G needle underneath the tendon of flexor carpi ulnaris at the level of the wrist crease. 3ml of local anaesthetic is injected at a depth of approximately 1cm. The dorsal cutaneous branch is blocked by infiltrating local anaesthetic in the subcutaneous layer around the ulnar aspect of the wrist.
The radial nerve is performed by infiltration of 5-8 ml of local anaesthetic over the radial aspect of the wrist at a level 2 fingerbreadths proximal the to base of the metacarpal.
Digital nerve block
Digital nerve block of the fingers or toes is relatively simple and can be very useful for operations on fingers and toes that do not require a proximal tourniquet. Adrenaline containing local anaesthetic is contraindicated as their injection around the end arteries may cause ischaemia of the digits.
An injection of local anaesthetic is made after insertion of a 25 G needle perpendicular to the skin just distal to the metacarpophalyngeal joint (in the foot, it is the metatarsophalyngeal joint) and directing the needle to the palmar (plantar in the foot) surface. 2-3 ml is injected on either side of the phalanx. A dorsal injection of 1ml of local anaesthetic across the phalanx completes the block.
REGIONAL ANAESTHESIA OF THE LOWER LIMB
Fascia-iliaca compartment block
The femoral and lateral femoral cutaneous nerves are sensory to skin overlying the anterior and lateral aspects of the thigh respectively. The femoral nerve also supplies the hip and knee joints. In the case of the knee joint, the additional innervation from obturator and sciatic nerves is minor. With the hip these are more significant. The shaft of the femur is predominantly supplied from the femoral nerve.
Place the patient supine and mark the inguinal ligament and the femoral artery. In an adult the injection point is 3-4 cm lateral to the femoral artery and 1 cm inferior to the inguinal ligament or 1 cm below the junction of the middle and lateral thirds of the inguinal ligament (child). Using a short bevel needle, approach the skin at an angle of 45° (bevel up). Two ‘pops’ are felt, the first as the fascia lata is penetrated which is quite a definite pop. The second pop (which is felt as the fascia iliaca is penetrated) is less distinct and is often felt as a series of 2-3 pops. The depth of the needle tip at this stage is usually 4 cm in the adult but may be 5 cm in larger individuals.
I inject 2 mg/kg of bupivacaine 0.25% (plain) after aspiration in doses of 5-7 ml. The total dose is given over 1-2 minutes or 30-40 ml lignocaine 1% in divided doses.
The main complication is the block not working adequately (10% chance) however this risk is lowered with experience. Femoral nerve neuropathy may occur. This usually resolves over weeks, but beware, the neuropathy may be due to the surgery itself. There is a low risk of infection at the injection site if aseptic techniques are applied. There are no reports of local anaesthetic toxicity however potentially it could occur. Motor blockade of the quadriceps occur and the knee extensor can be blocked which can be a problem for walking whilst the block is working.
Ankle block
Five nerves supply the foot: 4 are branches of the sciatic (tibial, superior and deep peroneal, sural) the fifth (saphenous) is a terminal branch of the femoral nerve. Innervation to the foot is highly variable; therefore, aim to block all 5 nerves, except for great toe surgery where a sural nerve block can be excluded. The tibial and deep peroneal nerves, which are blocked beneath the deep fascia supply bones, joints and muscles of the foot. The sensory distribution of the 5 nerves is shown.
Ankle blocks can be used to provide anaesthesia for surgical procedures of the foot. It will not provide a motor block of the foot.
The saphenous, superficial peroneal and sural nerves are superficial, whilst the deep peroneal and posterior tibial nerves are located more deeply and are closely related to the anterior tibial artery and posterior tibial artery.
The posterior tibial nerve lies along the medial aspect of the Achilles tendon, just posterior to the artery. It is blocked with the patient’s foot supported on a pillow with the leg in external rotation. A line drawn from the medial malleoulus to the posterior inferior calcaneum intersects with a point posterior to the posterior tibial artery. A 22G 50mm needle is inserted and if bone is felt, it is withdrawn slightly. 5-7 ml of local anaesthetic is injected after gentle aspiration to ensure a vessel has not been entered.
The deep peroneal block is performed midway between the malleoli lateral to the tendon of extensor hallucis longus and the anterior tibial artery. The needle is advanced toward the tibia and 3-5 ml of local anaesthetic is injected deep to the fascia.
The sural nerve is subcutaneous distal to the middle of the leg near the short saphenous vein behind and distal to the lateral malleolus. A skin wheal is made lateral to the Achilles tendon at the level of the lateral malleolus. The 3 cm needle is inserted to a depth of 1 cm and directed toward the lateral border of the fibula and 3-5 ml of local anaesthetic is injected in a fanwise manner from the lateral border of the Achilles tendon to the lateral border of the fibula.
The superficial peroneal nerve is blocked by subcutaneous infiltration of 5 ml of local anaesthetic from the anterior border of the tibia to the superior aspect of the lateral malleolus.
The saphenous nerve is blocked subcutaneously at the ankle with an injection of 3-5 ml of local anaesthetic immediately proximal and anterior to the medial malleolus to the anterior border of the tibia.
REGIONAL ANAESTHESIA OF THE TRUNK
Intercostal nerve block
The intercostal nerves are the primary rami of the first to 11th thoracic nerves. The nerves from T4 to T11 supply somatic innervation to the area from the nipples to below the umbilicus. The chest wall above the nipples is supplied by the nerves of T2 to T3 and peripheral branches of the cervical plexus (C3 to C4). The intercostal nerve of T12 is not a true intercostal nerve, but a subcostal nerve. Some of its fibres join with fibres from the first lumbar nerve to form the iliohypogastric and ilioinguinal nerves.
Each intercostal nerve has five branches. Two are the paired grey and white rami communicantes, they pass anteriorly to and from the sympathetic chain. The third branch is the posterior cutaneous branch that supplies the skin and muscles of the paravertebral region. The fourth branch is the lateral cutaneous division that is given off anterior to the midaxillary line and supplies fibres to the skin of the chest and abdominal wall. The terminal branch is the anterior cutaneous branch and it supplies the anterior chest wall and anterior part of the abdomen to just below the umbilicus.
Intercostal nerve block is useful for pain control for operations on the chest wall and abdominal wall. It is very useful for control of pain from fractured ribs and herpes zoster.
Intercostal nerve block is performed at the angle of the rib (6-8 cm from the spinous process) where the nerve comes to lie between the internal intercostal muscle and the innermost intercostal muscle. The costal groove of the rib is broadest and deepest in this position. The intercostal veins and artery travel superior to the nerve in the intercostal groove on the inferior border of the rib. The patient is positioned in the lateral or prone position. It is useful to mark the injection points if multiple injections are planned. A line is drawn parallel to the spine at the point where the ribs are easily palpated (6-9 cm from the midline). The correct levels are identified and beginning at the lowest rib to be blocked, the skin is immobilized with one hand whilst the 22 G 3-4 cm short bevelled needle with local anaesthetic in a syringe is inserted onto the rib and the needle is walked off the lower border of the rib and advanced 2-3 mm where a subtle loss of resistance may be felt. An injection of 3-5 ml of local anaesthetic is made and the needle is withdrawn.
The potential complications of intercostal nerve block are pneumothorax and intravascular injection. The dose of local anaesthetic is restricted to avoid the development of toxicity, as its absorption by the intercostal vessels is high.
Inguinal field block
Ilioinguinal and iliohypogastric nerve blocks are used for providing analgesia for inguinal hernia repair and for orchidopexy (where an inguinal incision is made). These blocks alone will not provide sufficient anaesthesia for hernia repair without supplemental local anaesthetic infiltration.
The iliohypogastric nerve originates from branches of the 12th thoracic and 1st lumbar segments. Near the anterior superior iliac spine (ASIS), it is located between the external oblique muscle and the internal oblique muscle. It has cutaneous branches to the inguinal ligament and skin overlying it.
The ilioinguinal nerve originates from the 1st lumbar segment and at the level of the anterior superior iliac spine (ASIS), it lies between the transverse abdominis and the internal oblique muscles and then enters the scrotum to provide sensation to the superior portion of the scrotum and inner part of the thigh.
Block of the ilioinguinal and iliohypogastric nerves is achieved with the patient in the supine position. At a pint 2-3 cm medial and 2-3 cm inferior to the ASIS, a skin wheal is raised and an 8cm 22G needle is inserted in a cephalo-lateral direction to contact the inner surface of the ilium. 10 ml of local anaesthetic is injected as the needle is slowly withdrawn and then the needle is reinserted at a steeper angle (more perpendicular) until it penetrates all three lateral abdominal muscles. A further 10 ml of local anaesthetic is injected as the needle is withdrawn.
In a child, a short bevelled 25 G needle is inserted 1 cm medial and 1 cm superior to the ASIS. It is advanced in an inferolateral direction until it comes into contact with the iliac wing and half the dose is injected as the needle is withdrawn (to anaesthetise the iliohypogastric nerve). With the needle in the same entry point, it is advanced in a posterior and inferior direction toward the inguinal ligament until a fascial pop is felt as it pierces the external oblique fascia. The rest of the dose is injected here to anaesthetise the ilioinguinal nerve.
The main side effect of the block is transient quadriceps femoris weakness due to transient block of the femoral nerve.
Penile block
Penile nerve block is used predominantly for circumcision or any other operation on the body of the penis. This block may not anaesthetise the ventral surface of the penis and will not provide anaesthesia to the penile urethra.
The dorsal nerves of the penis are branches of the pudendal nerves and they emerge from under the bony pubis in a triangular area bounded superiorly by Buck’s fascia and posteriorly by the pubis and inferiorly by the crura of the penis.
Local anaesthetic is injected into this area by passing a 22g needle at the base of the penis at the 10 o’clock and 2 o’clock positions. At each position, the needle strikes the pubis and is then ‘walked’ off in a caudal direction until bone is no longer contacted. As the needle is passed through Buck’s fascia, a fascial click or ‘pop’ may be felt. After gentle aspiration to ensure a vessel has not been entered, 0.1 ml per kg body weight of plain 0.5% Bupivacaine or 1% Lignocaine is injected.
Complications of penile nerve block include ischaemia if adrenaline-containing solutions are used and intravascular injection.
INTRAVENOUS REGIONAL ANAESTHESIA
Prof August Bier first described intravenous regional anaesthesia in 1908. It can be used in the upper or lower limb for procedures on the distal part of the arm. Because local anaesthetic is toxic if injected intravenously, it is important to apply a tourniquet on the proximal part of the limb with the pressure raised to 50-100 mmHg greater than the systolic blood pressure. Typically it is set at 250 mmHg in the arm. The cuff is kept inflated for at least 20 minutes so that a toxic dose of local anaesthetic is not released into the circulation. The amount of local anaesthetic that is immediately released into the circulation of release of the tourniquet cuff is less than with increasing tourniquet time. A smaller proportion of the local anaesthetic is released into the circulation with increasing tourniquet time. This means that the peak plasma concentration of local anaesthetic is less than if would have been if a simple intravenous dose of local anaesthetic was administered without a tourniquet. It is the peak concentration that is important for creating toxicity. After release of the tourniquet, the local anaesthetic will undergo some metabolism in the lungs. The lungs take up Prilocaine to a greater extent than lignocaine. The safest local anaesthetic for intravenous regional anaesthesia is prilocaine (0.5%). Lignocaine 0.5% is also suitable.
A working tourniquet is essential for the safe performance of a Bier’s block. Sometimes a double cuff is used, because tourniquet pain is the limiting factor for the duration of the block. This block is only reliable for procedures that will last less than one hour. Once the tourniquet is released, the block is no longer effective. Mild signs of systemic toxicity may be apparent on release of the tourniquet. It is important to monitor the patient carefully during deflation.
A cannula is placed distally in the arm to be anaesthetised and another is placed in the opposite arm in case of accidental toxicity for the administration of anticonvulsant and other medications.
The upper arm is wrapped in non-irritant fabric to protect the skin from the tourniquet and a double (or two separate) tourniquet is placed. The arm is elevated for 2-3 minutes whilst compressing the axillary artery to exsanguinate it and the distal cuff is inflated after which the proximal cuff is inflated. When it is confirmed that the cuffs are in working order, the distal cuff is deflated and the injection of local anaesthetic is made. For an arm, a dose of 3 mg per kg is used and the injection is made slowly to avoid injecting at high pressure. The onset of anaesthesia is 2-4 minutes. If the procedure lasts more than 20 minutes and the cuff is causing pain, the distal cuff is re-inflated (as this area is theoretically anaesthetised) before the proximal cuff is deflated slowly.
COMPLICATIONS AND SIDE EFFECTS OF REGIONAL ANAESTHESIA
Regional anaesthesia has many benefits, including surgical anaesthesia, postoperative analgesia and the ability to avoid a general anaesthetic. However all techniques have side effects and complications. Drug related complications include systemic toxicity to local anaesthetic or additives, neurotoxicity and allergy. Some side effects relate to the effects of the block itself, including motor block, prolonged block and autonomic block (particularly problematic in spinal and epidural anaesthesia) and unintentional block of adjacent nerves. Unintended complications include injury to adjacent structures (such as the pleura, blood vessels, muscles and viscera) and infection or bleeding after the injection.
Neural injury can result from toxicity from the injected substance (such as additives or preservatives or contamination with alcohol), immunologic reactions to injected substances, infection, direct trauma (injection into a nerve is avoided), compression of the nerve (by blood, an abscess, or incorrect positioning), ischaemia of the nerve or related to the surgical procedure.
Injury due to peripheral nerve block is uncommon, but can be relatively minor and the symptoms may be delayed. Permanent nerve injury after regional anaesthesia is believed to occur at a rate of less than 0.02% to 0.4%. The presentation is usually paraesthesia or dysaesthesia and they tend to resolve over a period of weeks to months.
Injection into the fascicle of the nerve increases intraneural pressure and may induce ischaemia. It should be avoided. In the conscious patient, pain may occur with injection if the needle is in the fascicle. Paraesthesia during the performance of a block indicates that the needle is near to or in contact with the nerve. The needle should be repositioned if it occurs. There is no data to demonstrate that eliciting paraesthesia causes neuropathy.
SELF-ASSESSMENT QUESTIONS
1. Describe the features of the ideal local anaesthetic agent.
2. Lignocaine and bupivacaine are both amide local anaesthetic agents. Outline their major pharmacologic differences.
3. What are the indications and potential applications of fascia iliaca nerve block?
4. What structures does the median nerve supply? Describe three ways it may be blocked and the advantages and disadvantages of each approach.
5. Describe in detail your approach to providing analgesia for a patient who has fractured ribs 6, 7 and 8 on the right side. Outline the advantages and disadvantages of an epidural versus intercostal nerve block for such a patient.
ASSIGNMENT
You are asked to design a consent form for the performance of regional anaesthesia in paediatric patients in your department. Outline what the relevant features of the consent process are important and draft a document that can be given to the parents of a small child. Is asking the parent to sign such a form sufficient to obtain informed consent? Why?
LOCAL ANAESTHETIC CASE STUDIES
Case No 1
Tsahim is a 59 year old male with an inguinal hernia for repair. He is on anticoagulation for atrial fibrillation. He has consented to a general anaesthetic for the repair but is very concerned about the pain after surgery. There is no opioid available for analgesia in your hospital and he has a stomach ulcer so will not tolerate non-steroidal medication.
Describe in detail your approach to providing postoperative analgesia for Tsahim.
What are the advantages of using bupivacaine for this block? What is the safe maximum dose of bupivacaine?
Case No2
Burmaa is a 28 year old woman who has caught her right hand in the door her car. The surgeon will need to repair two metacarpal bones and she has a large area of skin missing from the dorsum of her hand. Several tendons will require repair. She is requesting a regional anaesthetic.
What options are available to you for regional anaesthesia of the arm?
You have injected 30 ml of local anaesthetic into an arm block. After the injection, Burmaa says she has a funny taste in her mouth and blurred vision. What are the other signs of local anaesthetic toxicity you will be seeking? Outline your management of Burmaa.