Local Anaesthesia and Analgesia Flashcards
What is local analgesia?
Use of a drug to produce temporary loss of all sensation in a limited part of the body
What is local anaesthesia?
Any technique to render part of the body insensitive to pain without affecting consciousness
Which groups of drugs can be used to achieve local analgesia?
Opioids, local anaesthetics and alpha-2 agonists
What are the characteristics of lidocaine?
Amide type local anaesthetic with good potency
Short 10-15 min onset of action with up to 2 hrs DOA
Causes vasodilation so often formulated with adrenaline, may cause SC and skin swelling in horses so rarely used
What are the characteristics of procaine?
Ester with poor potency
Long onset of action 15-20min with 45-60min DOA
Only local anaesthetic licensed for food producing animals in EU
What are the characteristics of mepicacaine?
Amide with good potency
Short onset of 10 mins and long DOA of 6-8 hrs
Commonly used for equine nerve blocks
What are the characteristics of bupivacaine?
Amide with strong potency
Long onset of 30-40 mins and long DOA of 6-8 hours
High incidence of cardiotoxicity
What are the characteristics of ropivacaine?
Similar to bupivacaine but less cardiotoxic and may be slightly less potent
What are the characteristics of tetracaine and proparacaine?
Both lipid soluble used topically in the eye with short onset and duration
What are the characteristics of prilocaine?
Lipid soluble for absorption across intact skin
Relatively high potential to cause methaemaglobinaemia
Formulated with lidocaine in EMLA cream
How is EMLA cream used?
Applied to intact skin and left for 30 minutes to take effect, apply a light bandage over the ointment
Useful for IV or arterial catheterisation of nervous animals
What is the dose for lidocaine?
5mg/kg with toxic dose at 10-20mg/kg
What is the dose used for bupivacaine?
2mg/kg with toxic dose of 3.5-4.5mg/kg
What is the dose used for ropivacaine?
1.5-3mg/kg with toxic dose of 5mg/kg
What is the biochemical structure of local anaesthetics?
Weak bases consisting of a lipophilic ring, a link and a hydrophilic amine
Solubilised for injection as strong conjugate acidic hydrochloride salts
What is the classification of amines or esters based on?
The link present between the lipophilic ring an hydrophilic amine
How do local anaesthetics work?
Block the sodium channels from the inner surface of the axonal membrane preventing propagation of axonal action potentials
What is the speed of onset of a local anaesthetic related to?
Dose and proportion of drug in the non-ionised lipid soluble form which is determined by the pKa and ambient pH
What determines the potency of a local anaesthetic?
Lipid solubility with sensitivity of the axon determined by axonal diameter and degree of myelination
What is the duration of action of a local anaesthetic determined by?
Degree of protein binding and speed of absorption from injection site
How are ester local anaesthetics metabolised?
Hydrolysis by plasma cholinesterase
One of the break down products, p-aminobenzoic acid, can cause allergic reactions
How are amide local anaesthetics metabolised?
In the liver
Why are vasoconstrictors added to local anaesthetics?
To reduce local blood flow and therefore reduce absorption and prolong duration of action
How is the caridovascular toxicity of local anaesthetics explained?
Related to reduced membrane conduction resulting in arrhythmias and reduced cardiac contractility
How is the difference in severity depending on different local anaesthetic drugs used explained?
Relative protein binding
Lidocaine binds less strongly than bupivacaine and is used as an anti-arrhythmic
Bupivacaine binds strongly and is difficult to displace from cardiac conducting tissue and myocardium, it also blocks potassium channels
What are the potential CNS toxicity reactions of local anaesthetics?
Mild changes in peripheral sensation to sedation/coma/convulsions and death
How does prilocaine cause methaemoglobinaemia?
Its o-toluidine metabolite can oxidise haemoglobin to methaemaglobin
What are the different routes of local anaesthetic administration?
Topical Infiltrative Regional nerve block Intravenous Regional Analgesia (Bier's Block) Intra-articular analgesia Epidurals
Which local anaesthetic methods are considered topical?
Ophthalmic use, laryngeal prior to intubation, cutaneous and splash blocks
How is local anaesthetic used infiltratively?
Injection of local anaesthetic into the tissues around the surgical site and is commonly referred to as a line/field block
How are local anaesthetics used with soaker catheters?
Soaker catheters are inserted intraperitoneal/interpleural/ intra-articular and slowly release local anaesthetic
Give an example of soaker catheter use
Installation of bupivacaine through chest drains has been shown to improve respiratory function after thoracotomy although pain on initial installation may be seen
What are the indications for intravenous regional analgesia (IVRA)?
Surgery of the distal limb
What are the contraindications for IVRA?
Tumour removal, sensitivity to lidocaine, surgery lasting longer than 2 hours
What is the theory behind IVRA?
The limb is exsanguinated and isolated from the circulation with an Esmarch’s bandage and tourniquet
Local anaesthetic is injected into a vein distal to the tourniquet and bathes the dependent tissues resulting in intense analgesia as long as the tourniquet is in place
What drugs can be used for IVRA?
Lidocaine only as bupivacaine and ropivacaine are CI due to their cardiotoxicity
What dose of lidocaine is used for IVRA?
Dogs and cats with mid-radial tourniquet = 0.5-2ml 2% lidocaine
What are the benefits of using intra-articular analgesia?
Produced profound analgesia of the particular joint with minimal systemic side effects
What must be considered when performing intra-articular analgesia?
Drugs must be preservative free and a sterile injection technique is essential
Which surgeries can epidurals be used with?
Hindlimb, pelvic or abdominal surgery
How are epidural used?
In combination with general anaesthesia or as a sole anaesthetic/analgesic technique
How is an epidural performed?
Analgesic drugs are injected into the epidural space beneath the ligamentum flavum but above the dura mater
SA = lumbosacral space
LA = sacro-coccygeal space between Co1-Co2
What are the contraindications for performing an epidural?
Sepsis or local infection, coagulopathy, hypovolaemia or lumbosacral fractures
Why is there a greater chance of puncturing the dura mater in cats?
The subaracnoid space extends further caudally than in dogs
What combination of drugs are typically used for an epidural?
Local anaesthetic (lidocaine/ropivacaine) and an opioid (morphine) but alpha-2 agonists (xylazine) are used in large animals
What are the potential complications of an epidural?
Failure of block, incompete analgesia, impaired locomotion post-operatively, urinary retention and hypotension
What are the onset and duration of action of some of the drugs commonly used in epidurals?
Lidocaine 2% rapid onset, 2h DOA
Bupivacaine 0.5% 20-30 min onset, 4-8h DOA
Levobupivacaine 0.75% similar to bupivacaine
Ropivacaine 0.75% 15-20 min onset, 4-6h DOA less motor
Morphine 0.1mg/kg onset 1 hour, DOA 12-24 hrs, sensory effects only
Methadone 0.1mg/kg
Alpha-2 agonists xylazine and detomidine have been used but sedation and ataxia at high doses
