Local and regional anaesthesia

Local and regional anaesthesia
Dr Andrew Crofton
ED Registrar
Pharmacology
Amides – Bupivacaine, lidocaine, priocaine, ropivacaine
Hepatic microsomal enzymes
Esters – Procaine, tetracaine (amethocaine)
Hydrolysed by plasma cholinesterase enzymes
All reversibly block sodium channels and inhibiting propagation of nerve impulses
Small fibres first (pain and temperature), then touch, deep pressure, proprioception and finally motor function

Local anaesthetics
Local anaesthetics
Lignocaine
3-5mg/kg (5-7mg/kg with adrenaline) up to 300mg maximum in one injection
Lasts 20-60 minutes (2-6 hours with adrenaline)
Nerve block lasts 75-120min
Bupivacaine
Lasts 4-8x longer than lignocaine
1.5mg/kg (3mg/kg with adrenaline)
Max 400mg in 24 hour period
Not recommended for children under 12yo
More cardiotoxic than lignocaine (has lower cardiovascular collapse/CNS toxicity ratio)
Local anaesthetics
Prilocaine
2.5mg/kg (0.5mL/kg of 0.5%)
Least cardiotoxic and is absorbed into tissues prior to deflation of cuff in Bier’s block
Lignocaine is an acceptable alternative for Bier’s block
Pharmacology
Onset of action
Dependent on pKa (pH at which 50% of drug is ionised and 50% non-ionised)
If ambient pH is higher than pKa, a greater proportion of drug will be in non-ionised form, which diffuses more rapidly across lipid membranes, hence faster onset of action
Hence if mixed with bicarbonate get faster onset and less painful injection
Therefore, drugs with lower pKa have more rapid onset of action
Duration of action
Dependent upon receptor affinity
The higher the affinity, the longer the action
Greater risk of systemic toxicity with longer acting agents however
Levopuivacaine and ropivacaine appear to have lower risk of systemic toxicity than bupivacaine
Systemic toxicity
Occurs wth dose-related clinical progression of sodium blockade in non-target tissues (brain and heart)
Can reduce risk by adherence to dose limitations and techniques to minimise systemic absorption
Seizures
Benzodiazepines
Cardiac arrest
Avoid vasopressin, beta-blockers and CCB’s for arrhythmias
IV 20% lipid emulsion is an effective therapy (especially if bupivacaine as very high lipid solubility)
1.5mL/kg infused over 1 minutes with continuous infusion or repeat dosing according to clinical condition
Maximum 10mL/kg over 30 minutes
Prilocaine and benzocaine can cause oxidation of ferric form of Hb to ferrous form, creating methaeoglobin (cyanosis once methaemoglobin concentration >1.5g/dL)
General principles of use
Addition of adrenaline
Reduces systemic absorption
Prolongs duration
Helps to control wound bleeding
Safe for use in end-arterial fields in selected healthy patients but should be avoided in suspected digital vascular injury/PVD/Reynaud’s
Adverse reactions
Most often local reactions to preservatives or adrenaline
True allergic reactions are extremely rare
Alternatives include diphenhydramine and benzyl alcohol
General principles of use
Addition of sodium bicarbonate
Shortens onset of action by raising tissue pH and reduces pain of injection
1mL of 8.4% sodium bicarbonate to 9mL of 1% lignocaine
1mL of 8.4% sodium bicarbonate to 29mL of -0.25% bupivacaine
Can cause precipitation of anaesthetic agent (esp. bupivacaine) and accelerate the degradation of adrenaline so only add immediately before use
Can also reduce pain of injection by:
27- to 30-gauge needle
Slow injection
Warming of solution to body temperature
Injecting through margins of wound
Topical anaesthesia
Probably as effective as local anaesthetic infiltration for many dermal injuries
Alkaline (pH 9) cream mixture of lignocaine and prilocaine (eutectic mixture of local anaesthetics (EMLA)) was first topical anaesthetic formulated to penetrate intact skin
Need to apply for 60 minutes under semiocclusive dressing
As effective as infiltration for IVC and lumbar puncture
Liposome-encapsulated tetracaine (amethocaine – ANGEL), lidocaine and tetracaine gel are all efficacious
Adrenaline (0.1%), Lidocaine (4%), and Tetracaine (0.5%)(amethocaine) – ALA mixture can be used on open wounds (Laceraine)
Onset 20-30 minutes; Duration 30-60min
0.1mL/kg = 4mg/kg lignocaine
Topical anaesthetics
Intradermal and subdermal anaesthesia
Intradermal injection
Leads to wheal
More painful but enhances anaesthetic effect by blocking cutaneous nerves
Subdermal injections
No wheal
Less painful
Slower onset of action
Can perform field block for contaminated wounds or directly into subdermal space for clean lacerations
Regional anaesthesia
Always assess and document peripheral neurovascular function prior to infiltration
For digital nerve blocks, assess 2-point discrimination on volar pads prior (normally <6mm at fingertips and often <2mm. Compare with contralateral uninjured digit Topical anaesthetic prior to nerve block reduces pain of procedure Levobupivacaine and ropivacaine have significantly less cardiotoxicity than lignocaine or bupivacaine and longer duration of action Lignocaine 10-20 minutes onset; Bupivacaine 15-30min US shortens block performance time, reduces number of needle passes and enables lower anaesthetic doses Digital nerve block Less pain with lignocaine 1% + adrenaline vs. bupivacaine 0.25% but block lasts half as long Palm down position Common digital nerves Derive from median and ulnar nerves (divide at 4th digit) In distal palm, common digital nerve divides into paired palmar branches that travel on both sides of the flexor tendon sheath and innervate the lateral and palmar aspect of each digit Dorsal digital nerves Smaller, derivated from radial and ulnar nerves, and travel on the dorsal aspect of each finger to provide sensation to the back of the finger Technique 1mL at volar aspect on one side, 1mL while withdrawing, 1mL across dorsum and 1mL at volar aspect on other side Digital nerve block Transthecal or flexor tendon sheath technique Can be performed in addition to or as an alternative to the prior technique but may not fully anaesthetise the distal fingertip Palm up position At distal palmar crease, identify the flexor tendon and insert needle aiming distally at 45 degrees When feel pop, within flexor tendon sheath insert 2-3mL of anaesthetic solution If feel bone first, withdraw 2-3mm and inject Can also perform at base of digit in metacarpal crease Median nerve block Provides anaesthesia to thumb, index, long and half of ring finger distal to proximal interphalangeal joint (proximal to this is radial nerve) Does not cover dorsum of thumb Palm up position Runs between flexor carpi radialis and palmaris longus tendon at proximal wrist crease Raise a wheal in this position under skin Insert until pop through deep fascia and inject 3-5mL of anaesthetic If bone felt before pop, withdraw 2-3mm and inject To increase success rate, withdraw to skin and inject medially and laterally another 1-2mL of solution at same depth Palmar branch of median nerve runs superficial to the deep fascia and can be blocked by withdawing the needle to the subcutaneous space and injecting another 2-3mL Radial nerve block Covers dorsal thumb and dorsal lateral aspect of hand up to mid-phalanx Superficial branch runs over styloid process of radius Provides sensation to dorsum of thumb, index finger and lateral half of middle finger Other branches run over anatomic snuff box Raise wheal in subcut space just proximal to the anatomical snuffbox Inject 5mL over radial styloid in subcut space Then direct needle across dorsum of wrist in ulnar direction to inject a further 5mL Ulnar nerve block Provides anaesthesia to the entire 5th/half of 4th and medial aspect of hand Can be palpated deep to flexor carpi ulnaris with artery and vein Raise wheal on medial aspect of wrist 1-2cm proximal to the distal wrist crease Insert needle under f.c.u. 5-10mm depth, aspirate and inject 3-5mL To block dorsal branches, inject 2-3mL into subcut space above the tendon of extensor carpi ulnaris Foot and ankle blocks 5 nerves 4 branches of sciatic nerve (deep fibular, superficial fibular, tibial and sural) 1 cutaneous branch of femoral nerve (saphenous nerve) Should block deep nerves (tibial and deep fibular) before field blocks distort anatomy Deep fibular block Deep fibular/peroneal block Provides anaesthesia to first web space and small area just proximal to first and 2nd toe on plantar aspect of foot Found at level of medial malleolus between extensor hallucis longus and tibialis anterior tendon Raise a wheal between the two tendons then insert needle through extensor retinaculum (or to bone and withdraw 2-3mm) and inject 2-3mL Can increase success by injecting medially and laterally a further 1-2mL on each side Posterior tibial nerve block Provides anaesthesia to plantar aspect of foot Misses just proximal to first and 2nd toes on plantar surface and lateral aspect (sural nerve) Nerve lies behind medial malleolus, deep to fascia and superficial/posterior to the artery Palpate posterior tibial artery and insert needle just posterior to bone or deep to fascia Inject 2-3mL an then medially and laterally 1-2mL each side of this Superficial fibular/peroneal nerve block Provides anaesthesia to dorsal lateral aspect of foot Traverses lateral part of ankle in subcut space between lateral malleolus and tibialis anterior tendon Inject 5mL interior subcutaneous space between superior aspect of lateral malleolus and tibialis anterior Sural nerve block Provides anaesthesia to the lateral aspect of the ankle with extension to the plantar aspect of the foot Traverses posterolateral aspect of ankle in subcutaneous space between Achilles tendon and lateral malleolus 5-6mL of anaesthetic in band from superior aspect of lateral malleolus to the Achilles tendon Saphenous nerve block Blocks the medial aspect of the ankle Anteromedial ankle in subcutaneous space between tibialis anterior and medial malleolus 5-6mL in band across this space Facial nerve blocks Supraorbital and supratrochlear nerve block Supraorbital nerve exits at the supraorbital foramen in line with the pupil and above the superior orbital rim Supplies most of the forehead Supratrocheal nerve exists from under the superior orbital rim 5-10mm medial to the supraorbital foramen Supplies the bridge of the nose Inject 2-3mL in subcut space just superior to the eyebrow in line with pupil, then direct needle medially and inject a further 5mL reaching to medial border of eyebrow Infraorbital nerve block Blocks the lower lid, medial cheek, ipsilateral side of nose (not bridge) and ipsilateral upper lip Exits infraorbital foramen 5-10mm inferior to midportion of the orbital rim and just superior to the maxillary canine teeth Provide adequate topical anaesthesia to mucosa at gingiva Dry mucosa, retract upper lip with thumb (fingers on inferior orbital rim) and insert needle at gingival reflection above canine tooth to halfway between entry site and orbital rim and inject 3-5mL of anaesthetic Can feel needle tip with fingers of non-dominant hand to direct needle to site Mental nerve block Covers labial mucosa, gingiva and lower lip adjacent to canines and incisors Inferior alveolar nerve gives rise to the mental nerve that exits at the mental foramen, located inferior to the mandibular canines and first premolars Topical anaesthetic to mucosa, dry and evert the lower lip Insert needle at gingival reflection and direct needle inferiorly 1cm and inject 3-5mL of solution Can palpate mental foramen and direct needle this way Auricular nerve block Auricular block Provides anaesthesia to the entire ear Anteriorly: Auriculotemporal nerve Posteriorly: Greater auricular nerve and mastoid branch of lesser occipital nerve Raise a wheal in subcut space inferior to the auricle Direct needle into subcut space anterior and superior injecting 2-3mL while withdrawing Redirect posterior and superior and deposit a further 2-3mL of local anaesthetic Repeat from superior aspect of ear both anteriorly and posteriorly Intercostal nerve block Clinical observation suggests better pain control and improved lung function as compared to parenteral analgesia A good block will last 8-18 hours Run inferior the artery and nerve in the subcostal space Ribs 1-6 are difficult to block due to the scapula and rhomboids For both anterior and posterior rib fractures, the ideal location is at the rib angle, approximately 6cm lateral to the midline Blocking posterior to the midaxillary line ensures analgesia to the lateral cutaneous and anterior branches of the intercostal nerves Retract skin cephalad at target site, raise wheal in subcut space and insert needle bevel up with syringe lower than entry site (10-15 degrees) Continue to bone, release skin and walk needle caudally until drops off inferior edge of rib then advance 3mm (in subcostal groove) Aspirate and deposit 2-5mL of anaesthetic Monitor for pneumothorax for at least 30 minutes CXR onlyif clinical signs (coughing, SOB, hypoxia) Pneumothorax in 8-9% of patients, or 1.4% for each individual intercostal block Femoral nerve block Can be isolated femoral nerve block OR three-in-one (femoral, obturator and lateral femoral cutaneous nerves) Proven to result in more rapid achievement of lowest pain score with less narcotic dosing USS allows more complete block, lower LA volume and less vascular injury Excellent option for elderly patients with proximal femur or hip fractures If obese, place pillow beneath hip and tract abdominal pannus away Femoral nerve block will cover anterior thigh and medial leg (saphenous) Three-in-one will cover these + obturator and lateral femoral cutaneous nerve distribution 3mg/kg bupivacaine 0.5% (5mg/mL) 0.2mL/kg up to 10mL = 1mg/kg up to 50mg 3mg/kg of ropivacaine 0.75% (= 7.5mg/mL) Typically 20mL (150mg) diluted up to 30-40mL with N/saline is an excellent choice with lower cardiac toxicity and long duration Distal pressure during insertion of 20-30mL of anaesthetic can allow cephalad migration of agent to cover three-in-one Fascia iliaca compartment block Covers femoral nerve and lateral femoral cutaneous nerve Injection of 40mL 0.25% bupivacaine (or ropivacaine 0.75%) 2.5cm distal to point 1/3 of the way from ASIS to pubic tubercle and lateral to the femoral artery pulse Two-pops must be felt (fascia lata and fascia iliacus) Haematoma block Waned in popularity mainly due to misconception of risk of infection Position needle in haematoma (can be difficult and USS may assist this) Aspirate blood Inject 5-15mL of agent into fracture site Ensure maximum dose of LA is not exceeded as could possibly by vascular site Do not perform in open or contaminated fractures Bier’s block Provides dense anaesthesia to a limb for up to 60 min without GA/procedural sedation Provides no post-procedure pain control One small gauge IV in affected extremity (preferably distal/near to injury) and one in unaffected extremity for fluids/sedation if needed Padding beneath specialised double pneumatic tourniquet Exsanguinate affected limb by elevating for 3-4 minutes Inflate distal cuff first, followed by proximal cuff and ALWAYS CHECK FOR PULSE PRIOR TO IV DELIVERY OF DRUG 250-300mmHg in upper limb and 350-400mmHg in lower limb (or 100mmHg above SBP in limb being anaesthetised) Inject Prilocaine 0.5% 3mg/kg (0.6mL/kg) for upper limb or 4mg/kg (0.8mL/kg) for lower limb blocks Lignocaine is a suitable alternative Bier’s block Onset of anaesthesia is usually within 5 minutes Sensation of warmth/cold and skin becomes mottled May retain varying degrees of sensation to touch, deep pressure and motor function Remove IV in affected extremity once adequate anaesthesia obtained If pain at proximal cuff, re-inflate the distal cuff and slowly deflate proximal cuff Release cuff only after 30 minutes has elapsed (to allow tissue distribution of prilocaine), lower for 5-10 seconds, then reinflate for 1-2 minutes 3-5x to prevent any bolusing of LA No more than 1 hour to prevent ischaemic complications Monitor for 30 minutes afterwards