Local anaesthesia and analgesia Flashcards
Define local anaestehsia
Implies that the local anaesthetic is used as the only means of anaesthesia - a cow that has a standing c-section performed under local anaesthesia
Define locla analgesia
to provide extra analgesia as a supplement to GA> Can be used to provide multi-modal analgesia, in association with e.g. opioids and/or NSAIDs
what is the triad of GA?
- narcosis (unconsciousness)
- analgesia (antinociception)
- mm relaxation
How can local analgesia be acheived? 3
- local anaesthetic drugs
- opioids
- alpha 2 agonists
Pharmacology - local anaesthetics
- drugs that reversibly block the transmission of action potentials along an axon
- interfere with the action of sodium channels
- cocaine first to be used in 1800s
What is a differential block?
= not all nn types are equally sensitive to local anaesthetics
- B fibres most sensitive (sympathetic)
- then Adelta fibres (sensory - PAIN)
- least sensitive Abeta and Aalpha fibres (motor and proprioceptive)
- sensitivity of C fibres (unmyelinated overlaps)
D/t differential block, which order are sensations lost?
- pain
- cold
- warmth
- touch
- deep pressure
Name 2 types of local anaesthetic
- esters
- amides
Outline ester pharmacology
- rarely used
- metabolised by plasma pseudocholinesterases
- allergenic
- e.g. procaine (LICENSED), tetracaine
Outlien amide pharmacology
- metabolised in liver by amidases
- allergic reactions rare
- e.g. lidocaine, bupivicaine, ropivicaine (n.b. a drug with two ‘i’s within the word are amides)
What does lipid solubility determine?
potency - axonal membranes are predominantly lipid
What does protein binding determine?
duration of action - bind to protein (sodium channels) within axonal membranes
What does pKa determine?
the speed of onset (must diffuse across axon sheath in uncharged base form)
What is pKa?
the dissociation constant (i.e. when 50% of drug is ionised)
Does lidocaine (pKa 7.9) or bupivicaine (pKa 8.1) work fastest?
Lidocaine because more of the drug is non-ionised nearer blood pH (7.4)
Which local anaesthetics can be used in large animals?
- Farm/food - procaine only licensed (cattle only)
- Horses - all can be used (if licensed or on positive list, reports of skin/SC swelling after lidocaine, mepivacaine mainly used)
Absorption - local aaesthetics
- local anaesthetics are unusual in that they are applied directly to the site of action
- systemic absorption also partly controls duration of action
- vasoconstrictor (adrenaline) can be added to reduce systemic absorption (reduce local BF –> increases duration of action)
When might you have local anaesthetic toxicity?
- accidental intravascular injection (always draw back on syringe before injecting)
- overdose (always check weight and calculate maximum dose)
3 main complications of local anaesthesia
- Nerve damage (low incidence in humans, no intraneural injection as this destroys nn)
- Systemic toxicity (ensure right dose and draw back)
- Local toxicity (very rare)
Features of CNS toxicity d/t local anaesthesia
- sedation
- tremors
- seizures
Outline local anaesthesia effects on CV toxicity
- direct action on heart and peripheral vasculature
- indirect action by blocking autonomic nn fibres
- worst with bupivacaine
- ultimate toxicity is CV arrest
Name 2 miscellaneous toxicities d/t local anaesthetics
- allergy
- methaemoglobinaemia
Describe methaemoglobinaemia
Fe2+ in haemoglobin oxidised to Fe3+ –> cannot bind and carry O2 –> results in cyanosis (especially prolocaine = EMLA cream), also benzocaine, lidocaine and procaine
ROA - local anaesthesia
- topical
- regional
- spinal/ epidural
- intra-articular
- IVRA
- systemic
Name 2 local anaesthetics as eye drops
- Proparacaine (proxymetacaine)
- tetracaine (amethocaine)
- (can slow corneal healing)
Define EMLA cream
= Eutectic mixture of local anaesthetics
- lidocaine and prilocaine
- used for venous catheterisation
- absorbed across intact skin
- apply 30 minutes before (occluded)
What do you use infiltrative anaesthesia for?
- lidocaine +/- adrenaline
- lowest possible concentration
- fine (24g) needles
- don’t use with adrenaline in tissues supplied by end arteries (e.g. tails, ears)
Name 2 field blocks
- line blocks
- inverted L block
(give skin, SC, mm)
What do you block in a paravertebral nn block?
T13, L1, L2 dorsal and ventral branch
Effectiveness - paravertebral anaesthesia
- hyperaemia
- lateral curvature with convexity to blocked side
- test with needle
Problems - paravertebral anaesthesia -4
- fat animals
- mm spasm
- damage to aorta
- infection
How do you do the cornual nn block in cattle and goats?
- CATTLE: cornual branch of lacrimal nn (halfway b/w lateral canthus and horn bud)
- GOATS: also cornual brach of intratrochlear nn (halfway b/w medial canthus and horn bud)
What do you use for a cornual nn block in cattle/goats?
- Sedation: xylazine or detomidine
- Local analgesic: procaine
Name 4 peripheral nn blocks of CN and what they anaesthetise
- Mandibular nn (teeth and lower lip)
- Infraorbital nnn (upper lip and nose)
- Supraorbital and auriculopalpebral nn (eye)
- Retrobulbar block (eye enucleation)
List examples of TL nn blcoks
- Cervical palpebral (shoulder, brachium)
- Axillary approach to brachial plexus (elbow distally)
- Radial, ulnar, median and musculocutaneous (elbow distally)
- Radial, ulnar and median (carpus distally)
- IVRA (Bier’s block, below tourniquet)
List examples of HL nn blocks and what they do
- lumbar and sacral plexus block (whole limb)
- femoral/ sciatic (stifle distally)
- IVRA (Bier’s block, below tourniquet)
What might you use neurostimulation and ultrasound for?
to detect location of nn to align needle to block
Advantages - epidural analgesia
- relatively simple technique
- good analgesia intra and post-op
- decreased stress response to anaesthesia and sx
What drugs can be used for epidural anaesthesia?
- local anaesthetics (SA)
- opioids (SA, LA)
- alpha 2 agonists (LA)
- ketamine, NSAIDs
Indications - epidural analgesia
- abdominal and hind quarter sx in SA under light GA
- standing sx in farm animals and horses
- post-op analgesia for above sx or injuries
Where do you do epidural anaesthesia in small vs. large animals?
- SMALL: lumbosacral site, in association with GA
- LARGE ANIMALS: ‘caudal’ site, sacro-coccygeal or Co1-Co2, standing animal, prevent straining during parturition
Contraindications - Epidural analgesia - 5
- sepsis
- infection at injection site (risk of internalising infection)
- coagulopathy
- hypotension/ hypovolaemia
- distorted anatomy (use ultrasound)
Complications - epidural analgesia
- nn damage
- pruiritus
- urinary retention (ask close observation)
- motor dysfunction
- accidental vascular injury
- haematoma formation
- infection
- hypotension
- respiratory depression d/t cranial spread
Indications - IVRA (Bier’s block)
analgesia for sx of distal limb
Method - IVRA (Bier’s block)
exsanguinate limb distal to sx site - apply tourniquet - inject 2-3ml 1% lidocaine into distal vein - effect dependent on tournique - limit 2 h d/t ischaemia of distal tissue
- don’t use bupivicaine or ropivacaine or your patient will die!!!
- TL or HL
T/F: lidocaine can be used IV
True
Actions of systemic local anaesthetics (lidocaine)
- anti-arrhythmic properties (class 1b - for ventricular arrhythmias)
- analgesia
- prokinetic (equine colic)
- free radical scavenger
What tends to be the first side effect sieen with systemic local anaesthetics (lidocaine)?
usually CNS side effects seen first
When should you be particularly aware of local anaesthetic toxicity in SA?
with repeated blocks/administration
