Local Anaesthesia

Thoughtful use of local anaesthesia can greatly improve the patient’s experience of cutaneous surgery.

Types of Local Anaesthetic

• Esters e.g. procaine are derivatives of para-aminobenzoic acid (PABA) and carry a risk of producing allergic reactions in sensitive individuals.
• Amides e.g. lignocaine, bupivacaine and prilocaine are relatively free from reactions.

Methylparabens is a preservative in multidose vials of lignocaine and may cause severe allergic reactions in PABA-sensitive individuals. The early symptoms include pruritus, urticaria, and nausea, coughing and wheezing. The immediate treatment is adrenaline 0.2 to 0.5mg (i.e. 0.2 to 0.5 mls of 1/1000 solution) by subcutaneous injection. This may be repeated every 5-10 minutes if necessary. It is logical to inject the adrenaline into the site where the anaesthetic was given to delay its absorption.

Differential rates of anaesthesia

Small nerve fibres, which carry pain and temperature sensations, are anaesthetized more rapidly than larger myelinated fibres. As a result: –

• Patients may complain that they do not feel completely anaesthetized because they can still appreciate pressure of vibration sensations in the operative field.
• When operating in the vicinity of a superficial branch of the facial nerve, paralysis of the nerve may develop sometime after the operation has started; giving rise to concern that the nerve has been cut during the procedure.

Lignocaine

Available alone or with adrenaline 1/80,000 or 1/200,000. Its mild vasodilator effect is counteracted by adrenaline. The maximum safe adult dose is 200mg (or 3mg/kg) for plain lignocaine and 500mg (or 7mg/kg) for lignocaine with adrenaline. For children and the elderly these maximum doses should be halved.

Lignocaine Preparation Maximum Safe Volume:

2% plain lignocaine 10mls
1% plain lignocaine 20mls
0.5% plain lignocaine 40mls
1% lignocaine = 10mg/ml
2% with adrenaline 25mls
1% with adrenaline 50mls
0.5% with adrenaline 100mls

Dental Cartridge Syringes.

The finest gauge needles are available for dental syringes and the capsules can be rapidly changed. The disadvantage is that the preparations available (1% and 2% Lignocaine with and without adrenaline 1/80 000) are primarily intended for nerve-block anaesthesia. The common dental syringe cannot be drawn back to check for intravascular placement of the needle although aspirating dental syringes are available.

Ampoules/Multidose Vials.

Plain Lignocaine is available from 0.5% to 2% strengths in glass ampoules and multidose vials. Lignocaine with adrenaline 1/200,000 is available in multidose vials only. Ampoule preparations are preservative-free and stinging is less severe, whereas multidose vials contain methylparabens. If multidose vials are used each patient should have their own vial to exclude the possibility of cross infection

Recommended Preparations.

The least painful is 0.5% lignocaine in an ampoule: the action may be too short for complicated operations. Combined with adrenaline 1 in 200,000 it has a longer action and is the most useful.

Lignocaine Toxicity.

Toxic effects of local anaesthetics should not be encountered as long as the dose limits are adhered to. Inadvertent intravascular injection, however, may produce side effects when only low volumes of anaesthetic have been used. (Patients with impaired liver function may be more susceptible to toxic effects).
The symptoms of lignocaine toxicity include fidgeting, tinnitus, drowsiness, tingling and numbness of lips and tongue, a metallic taste in the mouth, tremors, convulsions and respiratory arrest.

Bupivacaine

Bupivacaine 0.25-0.75% has a prolonged duration of action and is valuable for long procedures and postoperative analgesia. It has a slow onset of action and is not usually used as the sole agent in cutaneous surgery. The maximum safe dose is 150mg or 60mls of 0.25% solution. The higher concentrations of 0.5 and o.75% give a longer duration of action.

‘EMLA’ cream

EMLA cream is a combination of 2.5% Lignocaine and 2.5% Prilocaine in a cream base. It is used to produce topical cutaneous anaesthesia and should be applied under occlusion for approximately 90 minutes. It produces surface anaesthesia sufficient for minor procedures such as removing skin tags or treating vascular blemishes with a tuneable dye laser. It is generally inadequate for incisional surgery, but reduces the discomfort associated with needle puncture. EMLA cream is hazardous if it comes into contact with the surface of the eye and care should be taken to avoid this.

Ametop

Ametop, topical amethocaine 4% gel is a more rapidly acting topical anaesthetic cream for the skin surface. Unlike EMLA, it does not cause vasoconstriction and its onset of action is more rapid, typically in 30 minutes. Patients may develop localized urticaria if it remains in contact with the skin for longer than this time.

Ocular Anaesthesia

Oxybuprocaine

Oxybuprocaine (Benoxinate) 0.4% is reserved for topical ocular anaesthesia. It has a rapid onset of action and is available in 0.5ml single-use dispensers. A drop should be instilled on to the conjunctival surface with the lower eyelid gently retracted and the patient looking upward to avoid direct contact with the sensitive cornea.

Several applications are necessary for maximum effect as reflex lacrimation washes away much of the first instillations. It cross-reacts with ester type local anaesthetics and should not be used in benzocaine sensitive individuals. Proxymetacaine (Ophthaine) 0.5% is also a benzoic acid ester, but due to structural variations does not cross react with benzocaine. It causes less initial sting, but is only available in 15ml bottles.

Following ocular anaesthesia an eye pad should be worn until normal corneal sensation has returned. Patients should be warned not to drive whilst wearing the pad.

Adrenaline

It is added to Lignocaine to promote haemostasis and reduce the rate of Lignocaine absorption. Reactions to adrenaline include tachycardia, elevated blood pressure, tremors, anxiety and palpitations and are most likely to result from inadvertent intravenous injection. The vasoconstrictor action of adrenaline takes a few minutes to develop. Adrenaline should be avoided in patients receiving non-selective beta-blockers (e.g. Propranolol) as there is a risk of causing excessive blood pressure elevation due to the alpha vasoconstrictor action of adrenaline unopposed by Beta-2 receptors that normally dilate vascular smooth muscle.

Adrenaline may lead to ischaemia under certain circumstances. It should be avoided in patients with impaired peripheral circulation (e.g. Raynauds phenomenon, diabetic angiopathy) and in ring blocks around the digits. Some patients appear unduly sensitive to the effects of adrenaline.

The presence of adrenaline, and its antioxidant sodium metabisulphite, contributes to the initial stinging sensation caused by local anaesthetics.

The New York Heart Association state that 0.2mg of subcutaneous adrenaline is safe even in cardiac patients. The maximum dose in healthy adults should not exceed 1mg.

Maximum Safe Doses of Adrenaline
1/1000 solution = 1mg/ml
Adrenaline Strength Maximum Safe Volume
1/1000 1ml
1/80 000 80mls
1/200 000 200mls

Local Anaesthetic Technique

The aim is to deliver local anaesthetic as near to the nerves as possible. Direct infiltration just beneath the dermis is the most efficient way of doing this. Topical anaesthesia, subcutaneous anaesthesia and nerve blocks depend on diffusion of the agent to the nerves and therefore require anaesthetics of higher concentration.

Minimising the Discomfort of Local Anaesthesia

There are two components to the discomfort; the needle puncture itself and a stinging sensation caused by the local anaesthetic agent.

Use the finest needle, stretch or pinch the skin and introduce the needle in a smooth, single movement. In areas of accentuated pores e.g. the nose, inserting the needle through the edge of a pore reduces the discomfort. EMLA cream, refrigerant spray or ice to numb the skin surface can also help.

Injections within the dermis cause more discomfort than subcutaneous injections and rapid injections hurt more than slow ones. Higher concentrations of Lignocaine and those containing adrenaline sting more than weaker ones. The addition of sodium bicarbonate has been advocated to raise the pH of Lignocaine with adrenaline and this reduces the stinging sensation it causes. Warming the ampoules prior to use reduces the pain.

Direct Infiltration – Intradermal or Subcutaneous?

Lignocaine may be injected intradermally producing a wheal but it is painful. It gives instant anaesthesia and also clearly shows the area that is anaesthetic. Subcutaneous infiltration is less uncomfortable but anaesthesia is of slower onset and shorter duration, and a larger volume of more concentrated anaesthetic is required. It is also difficult to see the anaesthetic field and it may help to outline the proposed incision with a skin marker before injecting local anaesthetic under the markings. Subcutaneous infiltration avoids distortion of the operative site, which may be an advantage – for example when performing a shave excision.

Skin biopsies in children

EMLA topical anaesthesia followed by infiltration with 0.5% plain Lignocaine is least likely to upset a young child requiring a skin biopsy.

Field Block Anaesthesia

Local anaesthetic can be infiltrated circumferentially around the operative site blocking all the nerves supplying the area. This is most commonly used on the scalp where the local anaesthetic should be infiltrated into the dermis and subcutaneous fat.

Anaesthesia of the Scalp

Nerves and blood vessels in the scalp lie superficial to the epicranial aponeurosis. Inject superficially and not deep to the aponeurotic fascia.

Anaesthetising the Palm or Sole

Needle puncture directly into the palm or sole is painful. Introduce the needle into the thinner skin on the dorsum of the hand or foot and work around to the palm or sole reinjecting through the anaesthetized skin.

Digital Ring Block

Local anaesthetic may be injected circumferentially around the base of a digit. Each digit is supplied by two dorsal and two ventral nerves on each side. About 2mls of 2% plain Lignocaine is infiltrated both superficially and deeply. The initial injection is made dorsolaterally and from this point down one side of the digit and across its dorsum. The needle is then reinserted into the other side through the already anaesthetized dorsal skin. This block takes 5-10 minutes to work. No more than 4 ml should be given as this may result in compression of the vascular supply. A ring tourniquet is frequently applied after anaesthesia to achieve a bloodless field.

Nerve Blocks

The injection is given in the vicinity of a named nerve supply thus anaesthetising the area supplied by the nerve. It allows a relatively large area to be anaesthetized with a small volume. It avoids distortion of the surgical site and reduced the discomfort. It can provide prolonged postoperative analgesia when Bupivacaine is used. Nerve blocks do not always work fully, do not produce local vasoconstriction and are best used in combination with local infiltration of the operative site. Beware injecting into the nerve or its bony canal. Merely bathe the area with anaesthetic. Avoid inadvertent intravascular injection.

Supraorbital/Supratrochlear Nerve Block

The supraorbital nerve exits its foramen just below the eyebrow in line with the pupil. It supplies sensation to the lateral forehead. The supratrochlear nerve lies between the superior and medial borders of the orbit and supplies the medial forehead. These nerves can be anaesthetized by raising a wheal over the glabella and injecting 2-3mls of 2% Lignocaine along the eyebrow. Both sides of the forehead may be anaesthetized from this entry point.

Infraorbital Nerve Block

The infraorbital nerve exits its foramen 0.5 to 1cm below the inferior orbital rim, in line with the pupil, and passes medially. It may be anaesthetized by a percutaneous or intraoral approach. In both cases it is best to palpate the nerve and guide the approaching needle tip; digital pressure against the orbital rim can be used to direct the local anaesthetic towards the nerve and protect the eye. With the intraoral approach the needle is inserted through the superior oral sulcus in line with the apex of the second bicuspid tooth (a depression in the maxilla can be felt at this point) the nerve lies about 1cm deep to the sulcus. For the percutaneous approach the needle is inserted through the skin at a point 1cm medial and 1cm inferior to the infraorbital foramen.

Mental Nerve Block

This nerve exits the mandible in line with the pupil and second bicuspid tooth. The foramen lies midway between the upper and lower edges of the mandible in the normal adult, nearer the inferior edge in children, and nearer the superior edge in edentulous patients. It may be approached percutaneously or intraorally. Two millilitres of 2% Lignocaine are injected around the foramen. Bilateral blocks are used for lower lip surgery.

Nerve Block of the hand at the Wrist

Anaesthesia of the whole hand can be achieved by nerve blocks of the median, ulnar and radial nerves. With the hand in the anatomical position the median nerve is found running deep on the flexor aspect of the wrist between the tendons of palmaris longus and flexor carpi radialis. The ulnar nerve is found just lateral to the tendon of flexor carpi ulnaris and medial to the ulnar artery. There is also a dorsal cutaneous branch that leaves the main nerve five cm proximal to the wrist this nerve supplies the medial skin of the dorsum of the hand and one and a half digits. The radial nerveusually divides into several branches at the wrist and so it is necessary to infiltrate across the dorsum of the wrist in the subcutaneous plane.

Nerve Block of the foot at the Ankle

The sole can be anaesthetized by a combined tibial and sural block. The posterior tibial nerve runs medial to the Achilles tendon and innervates the anterior and medial parts of the sole of the foot. With the patient prone and the ankle supported, the posterior tibial artery is palpated at the upper border of the medial malleolus. The tibial nerve lies between this and the medial border of the Achilles tendon. A 4cm needle is inserted at this point and directed anteriorly to lie just lateral to the artery. If paraesthesia is elicited (warn the patient of this) the needle should be withdrawn 2-3mm to avoid injection directly into the nerve. 3-5mls of 2% Lignocaine is injected after aspirating to ensure the needle is not within a blood vessel. If the artery is not palpable the needle should be inserted through the skin just medial to the Achilles tendon, at the level of the upper border of the medial malleolus and directed toward the 2nd toe until the nerve or bone is encountered. This block may take 15-20 minutes to take effect.

Further Reading

• Erkisson E (Ed) Illustrated handbook of Local Anaesthesia. Lloyd-Luke, London 1979, ISBN 085325 145 7
• Auletta MK, Grekin RC. Local Anaesthesia for Dermatologic Surgery. Churchill Livingstone, New York 1991. ISBN 0 443 08704.

 

Needles for regional anaesthesia
	Gauge	Length	Bevel (degrees)
Plastic hubbed spinal needles	22G	3 1/2”	30
Steriseal metal hubbed spinal	22-25G	1 ¼ – 3”	30
Braun Plexiflix (with extension)	24G	1-2”	45