Basic plastic surgery techniques and principles: Using local anaesthetics

In the third part of our series, Ben Taylor and Ardeshir Bayat explain local anaesthetics. Which anaesthetic should you use and why? And what are the risks?

Local anaesthetics, when used correctly, enable doctors to do many surgical procedures. However, these drugs can harm patients if care is not taken. When giving local anaesthetics for basic surgery: how, where, which, how much, and what if it all goes wrong?

Local anaesthetics stop the propagation of impulses along nerves by blocking fast sodium channels which are voltage gated.¹ To block a channel, the drug must penetrate the cell membrane and act on the channel from the inside.² Drugs that do this can partition themselves between lipids and water.³ Partitioning is pH dependent and does not occur so readily in acidic conditions in which anaesthetics are less potent.

Toxicity

The potency of local anaesthetics in blocking neurones applies to other excitable cells. Neurones in the brain and conduction pathways in the heart are affected in the same way as peripheral nerves if enough of the drug enters the bloodstream.⁴ Absorption from the tissues into the circulation is dependent on several factors:⁵

• Total dose of drug
• Choice of drug--for example, lignocaine is more readily absorbed than prilocaine⁶
• Site of injection--sites with more blood vessels will absorb blood from the tissues quicker⁷
• Use of vasoconstrictors--some formulations contain additives like adrenaline which shut down the blood vessels in the area of injection and slow absorption.

Once an anaesthetic enters the bloodstream, the body breaks it down. Some drugs contain an ester link which is broken down by plasma pseudocholinesterases.⁸ Most anaesthetics that are used today contain an amide link instead of an ester link, and are broken down in the liver.⁹

What to look for

An early toxic reaction is characterised by mild neurological features, such as acting as if drunk (inebriation), lightheadedness, mood changes, and pins and needles around the mouth and tongue (circumoral parasethesia). However, you will miss these signs if the patient is sedated, anaesthetised, or intoxicated. More severe neurological signs are evident if the toxicity develops, and include seizures and coma. Cardiovascular phenomena are seen in severe toxic reactions. Initial hypertension and tachycardia quickly progress to bradycardia, hypotension, and eventually asystole. Local anaesthetics may induce fatal dysrhythmias such as ventricular fibrillation or tachycardia, which are often resistant to attempts to resuscitate.

Management

This is a medical emergency and must be treated as such. Call for help and stop the procedure straight away. Management is supportive, and should follow the ABCs of medical emergencies (see box).¹⁰

The patient's condition may deteriorate as more drug leaks into the system from tissue. Before any procedure, check that resuscitation facilities are close at hand and that senior help is available. Get intravenous access before procedures in which the toxic dose is likely to be exceeded.

Choice of local anaesthetic

The toxicity of local anaesthetics is proportional to their potency.⁴ But a more potent drug will last longer and may be more appropriate for a longer procedure. It is important to choose the right drug for what you are doing (see the table on studentbmj.com).

Lignocaine--The most frequently used local anaesthetic is a short acting amide-type agent available in many formulations and strengths with or without adrenaline. It can be used for most procedures.

Bupivacaine--A long acting amide type local anaesthetic. It is potent but can be toxic. Its toxicity limits use to blocking nerves or infiltration only.¹¹

Prilocaine--A short acting local anaesthetic with a similar onset and duration to lignocaine. Prilocaine is considered the safest of the amide-type local anaesthetics and is metabolised outside the liver in the blood.⁶ For these reasons, it is often used for intravenous regional anaesthesia. Prilocaine can, however, cause methaemoglobinaemia--where haemoglobin is methylated and becomes ineffective at carrying oxygen--and should not be used in susceptible patients.¹² Prilocaine is available in formulations with the vasopressor felypressin.¹¹ Lignocaine is available as a topical cream--eutectic mixture of local anaesthetics (EMLA). (Eutectic means that the mixture melts at a lower temperature than either of the components.)

Amethocaine--A potentially toxic ester-type local anaesthetic, which is only licensed in the United Kingdom for topical application. It is widely used to prepare venepuncture sites in children, because it acts quicker than a eutectic mixture of local anaesthetics (in 30-45 minutes) and is a potent vasodilator.¹¹

Airway--Clear, open, and protect the patient's airway

Breathing--Give 100% oxygen, ventilate as required, record the respiratory rate and oxygen saturation

Circulation--Monitor pulse, blood pressure, and electrocardiogram. Get intravenous access if not already done. Treat hypotension by initially raising the foot of the trolley. If severe (<90 mm Hg systolic in an adult), give a bolus of colloid, initally 500 ml of gelofusine in a single dose, but aim to maintain urine output at >0.5 mg/kg/min, and monitor the central venous pressure if you can. If the patient has a cardiac arrest, give standard basic and advanced life support

Disability--Monitor and assess the Glasgow coma scale and pupils. Treat convulsions with diazepam 5-10 mg or 0.1 to 0.2 mg/kg by slow intravenous injection through an intravenous cannula over about one minute

Vasopressors

Most local anaesthetics cause vasodilatation. This not only obscures the surgical field, but also increases systemic absorption of the drug.⁵ For this reason, many local anaesthetic solutions contain adrenaline (epinephrine), or a similar drug. This keeps the drug in the tissues for longer because the flow of blood is reduced. Adrenaline is therefore useful to prolong the effect of a local anaesthetic, to assist in haemostasis, and to prevent toxicity. However, adrenaline may precipitate life threatening dysrhythmias in patients with heart disease, diabetes, or thyrotoxicosis. Adrenaline also causes serious problems in patients with cerebrovascular disease, hypertension, and angle closure glaucoma.¹³ Adrenaline may react with some drugs, or cause ischaemic necrosis if given in the digits or penis.¹¹ Adrenaline at a concentration of 1:200 000 is as effective as stronger concentrations, but with fewer side effects.¹³¹⁴

Administering local anaesthetics

Topical anaesthesia is useful to minimise pain before injections. Minor operations, even harvesting a split skin graft can be performed under topical anaesthesia.¹⁵ Topical anaesthesia has the particular advantage of being needle free and is usually relatively non-toxic. As the creams contain a high concentration of drug, however, they can cause serious toxicity if given over broken skin or inflamed areas. Topical anaesthetics should never be used for anaesthetising wounds. One further setback is the time taken for the drugs to act. Creams such as EMLA or tetracaine can be applied to the surface of the area to be anaesthetised. An occlusive dressing should be used to cover the area until ready. See the flow chart in the figure.

Injecting local anaesthetics

Regardless of which anaesthetic you use, your technique should aim to minimise pain on injection, prevent toxicity, and avoid infection. Injecting local anaesthetics is intensely painful. To minimise this pain, use a thin needle; a 27 gauge needle is often used by plastic surgeons. If the drug is injected slowly, the injection is less painful, and the injection can be stopped more quickly if the patient develops signs of toxicity. Warming the solution of local anaesthetic in your hand to near body temperature will make the injection less painful. Some people add 0.1 ml of 1.26% sodium bicarbonate for each millilitre of local anaesthetic solution, which significantly reduces pain.¹⁶ However, you will have to dilute this solution yourself, which is time consuming and may lead to errors in dilution. A reliable way to reduce pain is to use a topical anaesthetic, such as EMLA, before injection. Do not inject into inflamed areas, as they are extremely painful, and highly vascular. The low pH reduces the effectiveness of the drug.

The most common cause of local anaesthetic toxicity is inadvertent intravenous injection.⁷ Pulling back the plunger of the syringe (aspirating) before you inject allows you to check that you are not in a blood vessel and should be standard practice. Always aspirate, every time you move the needle.⁴ If you talk to the patient while you are injecting, you will spot early signs of toxicity and provide some reassurance. To avoid introducing infection when injecting, you should follow aseptic principles. Always prepare the site of injection with antiseptic or alcohol to prevent infection. Wash your hands and wear sterile gloves. Check the ampoule yourself before injecting.

Infiltration anaesthesia

Infiltration of local anaesthetic into the skin, to enable minor surgery or to reduce post-operative pain due to wounds, is a widely used and relatively simple technique. Infiltration is straightforward provided that you remember the general principles above. Insert your needle at a shallow angle and inject enough solution to raise the skin. You can numb a larger area without piercing the skin more than once by moving the needle under the skin. It is an extremely painful route of injection.

Intravenous regional anaesthesia (Bier's block)

One effective way to anaesthetise the arm is a Bier's block. This uses a special tourniquet to allow the drug to circulate around the veins in the arm, but not to enter the main circulation. There is the potential for serious toxicity, so prilocaine is used and an anaesthetist must be present to manage the patient. It is only suitable for healthy adults undergoing a short procedure on one arm.

Tumescent anaesthesia

This procedure, which originated in cosmetic surgery, uses a high volume of infiltrate containing local anaesthetic and adrenaline to help haemostasis and anaesthesia. Tumescent anaesthesia is now widely used in many procedures on subcutaneous tissues, such as the breast or abdominal wall. Indeed, some surgeons use tumescent anaesthesia as the sole means of anaesthesia. Large volumes of infiltrate are quickly injected into the subcutaneous tissues until the area is swollen. (Tumescent is from the same Latin word as tumour.) One formula uses 25 ml of 2% lidocaine and 1 ml of 1:1000 adrenaline for each litre of sodium lactate intravenous infusion. It is possible to use huge volumes of local anaesthetics, up to 35 mg/kg of lignocaine without any apparent ill effects, although the dose depends on the site and the indication.¹⁷

Peripheral nerve blocks

Blocking a nerve is often more efficient than infiltration, and large areas can be blocked with one injection. You need a thorough knowledge of the anatomy of the nerve so that you know exactly where you are putting your needle, and what structures are at risk. You should not aim to directly enter the nerve with the needle because that will cause paraesthesia and may damage the nerve. Try to inject just around the nerve to bathe it in a solution of local anaesthetic. If your patient's clotting is impaired, a nerve block is contraindicated, as you may not be able to stop bleeding if you accidentally damage an artery. The onset of anaesthesia will depend on the size of the nerve trunk to be blocked.

Blocks on the upper limb

Digital nerve block

The common digital nerves run between the metacarpals in the hand, and divide into the digital nerves proper just before reaching the web spaces. In the fingers, they run palmar to the artery. Approach the nerves from the dorsal side. Insert the needle lateral to the phalanx, at a slight medial angle, to approach the nerve. (See fig 1.) When the needle is in the correct position, it should be palpable from the palmar aspect. Aspirate and inject a small volume (1 ml or less) of local anaesthetic, and withdraw while slowly injecting. Never use solutions containing adrenaline.

An alternative approach to the common digital nerve (which will numb half of the adjacent digits) involves inserting your needle from the palmar side at the level of the distal palmar crease, and injecting a larger volume of local anaesthetic.

Median nerve block

This will usually numb the radial three and a half digits, and the corresponding half of the palm. The median nerve is superficial, and is found between the tendon of flexor carpi radialis and palmaris longus at the level of the wrist (but palmaris longus is normally absent in 15% of people). The needle enters at the proximal wrist crease and should almost immediately approach the nerve.

Radial nerve block

The radial nerve splits into several branches just proximal to the anatomical snuffbox to supply skin on the dorsal aspect of the radial half of the hand. It is blocked by infiltration of the area proximal to the anatomical snuffbox, but with caution as the cephalic vein also runs in this area.

Ulnar nerve block

The ulnar nerve gives off a dorsal cutaneous branch in the distal forearm which extends to the dorsal aspect of the ulnar side of the hand. Both this branch and the ulnar nerve can be blocked at the level of the proximal palmar crease, between the ulnar artery and the tendon of flexor carpi ulnaris. As the ulnar artery is at risk, it is wise to check the patency of the radial artery, with an Allen's test.¹⁰

Nerve blocks in the face

Large areas of the face can be blocked with local anaesthetic allowing a wide variety of procedures. Many nerves to the face and forehead emerge from the mid pupillary line, which goes vertically through the pupils when the eyes are looking straight ahead. (See fig 2.)

Forehead block

The supraorbital and supratrochlear nerves supply a substantial portion of the forehead. These are both branches of the ophthalmic division of the trigeminal nerve. They both travel up from the roof of the orbit--the supraorbital nerve travelling through the supraorbital notch and the supratrochlear nerve slightly medial to the supraorbital nerve. They are blocked by infiltration above the line of the eyebrows, from the midline, to just lateral to the mid-pupillary line.

Infraorbital nerve block

The infraorbital nerve is a large branch of the maxillary division of the trigeminal nerve. It enters the face through the infraorbital foramen, in the mid-pupillary line. It provides nerve supply to the lower eyelid, the mid-face, much of the nose, the whole upper lip, and the gum. The best approach is intraoral, as it avoids several painful skin punctures. The approach is a line that runs between the first and second premolars at apex of the superior vestibule of the mouth.

Mental nerve block

A cutaneous branch of the inferior alveolar nerve, which supplies the skin of the lower lip and gum through the mental canal. This can be palpated in the mid-pupillary line, or in the same plane as the space between the first and second premolars.

First do no harm

Since Hippocrates, one of the core teachings in medicine is "first do no harm." This holds true with local anaesthetics, which are useful drugs when used correctly but have the potential to cause great harm to your patient. Always take steps to minimise harm when giving a local anaesthetic by preventing infection, preventing toxicity, and minimising pain.

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