Local anaesthetic pharmacology
Brief review of local anaesthetic pharmacology
Brief review of local anaesthetic pharmacology
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