Anaesthetic Types Flashcards
Define General Anaesthesia
’ A state of controlled and reversible unconsciousness characterised by a lack of pain sensation, lack of memory and relatively depressed reflex responses’
What the advantages of local anaesthetics?
- Targeted analgesia - e.g. nerve blocks
- Can be used in high risk patients where GA is not an option (they have minimal effects on CNS, CVS and respiratory system)
- Can be part of a multi-modal analgesia approach intra-op so reduces dose of other drugs needed
- Pre-emptive analgesia
How do local anaesthetics work?
Block sodium channels at nerve endings, blocking the conduction of nerve impulses - producing local analgesia.
What sensations are lost when local anaesthetic administered? in order
- Pain
- Cold
- Warmth
- Touch
- Pressure
Name the 5 methods of administration of local anaesthetics
- Injection (local infiltration)
- Regional anaesthesia (injected into the nerve that supplies a region)
- Spinal
- Transmucosal (absorbed via MM)
- Transcutaneous (e.g. EMLA cream)
Name some examples of Local anaesthetic drugs
Lidocaine- IV analgesic (10-20m), especially for superficial pain. Blocks sodium channels. Rapid onset. Short-acting. Excreted via kidney.
Bupivicaine - shouldn’t be given IV as more toxic than lidocaine. Slow onset. Long duration. Excreted via kidney.
Mepivicaine - Rapid onset. Medium duration of action.
How do Local anaesthetics wear off?
They are redistributed to the liver where they are inactivated
Why is adrenaline sometimes added to a lignocaine local anaesthetic?
It causes vasoconstriction which keeps the local anaesthetic in the desired area, decreases the rate of drug absorption therefore increasing its duration of action, and reduces the concentration of drug entering the circulation therefore reducing toxicity.
When would using lignocaine with adrenaline be contraindicated?
In a patient with CV disease where the heart couldn’t cope with an increased HR associated with adrenaline
Name three types of regional anaesthetic techniques
Epidural
Spinal
Topical
What does an epidural do?
Blocks the sensation to the caudal abdomen, pelvis, tail, hind limbs, perineum and obstetrics
Injected between dura and vertebrae
Where is a spinal regional anaesthetic injected
Subarachnoid space
Give some examples of topical anaesthetics
Skin - EMLA cream - lidocaine
Larynx - lignocaine spray
Lignocaine gel for urinary catheterisation
What should be careful with when using topical local anaesthetics?
- Give plenty of time to actually work! 20-30 mins minimum!
- Care not to overdose! E.g. don’t give a tiny kitten a lignocaine throat spray and then lignocaine EMLA cream for their catheter
What are 5 risks of using local anaesthetics?
- May cause nerve damage or permanent loss of function when injected into nerve fibres
- Tissue irritation - slow wound healing, may be painful
- Can chew area following recovery
- Allergic reactions
- Hypotension can occur - especially after epidural
What can an overdose or incorrect route of local anaesthetics cause?
CNS stimulation - may lead toseizures
CNS depression - may lead to respiratory failure
Heart contractility reduced - myocardial depression
What animals should you take extra care with when using local anaesthetics?
Cats and exotics
What are muscle relaxants?
Neuromuscular blocking agents (NMBAs)
What do NMBAs do?
Interrupt the transmission of impulses from motor nerve to muscle synapse. Block the release of Acetyl choline
Name and describe the 2 types of NMBAs
- Depolarising
- Bind to receptors on the muscle side causing an initial surge of muscle activity and then nothing
- e.g. succinylcholine
- have rapid onset and are VERY short acting (a few minutes) - Non-depolarising
- compete with Acetyl choline for receptors so block the receptors at the muscle plates
- no initial surge
- roughly 30-40minutes duration
- can be reversed
- e.g. Vecuronium
Can NMBAs be topped up?
Yes
How are NMBAs administered?
Slow IV
Do NMBAs affect consciousness? How?
They do not affect consciousness as they do not cross the blood-brain barrier
Why are NMBAs not used much in veterinary practice?
The only paralyse skeletal muscle therefore the patient is unable to move or respond to inadequate anaesthesia. A patient on these is very difficult to monitor e.g. eye position doesn’t change
What MUST you do if NMBAs are administered to a patient?
Provide adequate IPPV
What circuits can be used for IPPV?
Bain, Ayres T-piece and Circle (ADE)
What are the disadvantages of using muscle relaxants?
- Hypothermia due to decreased muscle tone (can’t shiver)
- Difficult to assess anaesthetic depth
- Some may cause hypotension
- increased effects if used with aminoglycosides
- Increased susceptibility if used alongside halothane, isoflurane, corticosteroids, some diuretics and epinephrine
Why would we use muscle relaxants?
- To administer prolonged IPPV - e.g. during thoracic surgery
- During intraocular surgery to avoid change in eye position
- Where muscle relaxation required for surgery
- As part of balanced anaesthesia for high-risk patients
Give examples of drugs used to reverse NMBAs
Neostigmine
Edrophorium
What may happen when reversing NMBAs? What’s done to counteract this?
May lead to bradycardia and excess salivation. Reversal agents are often given with atropine or glycopyrrolate to counteract this