VNSA11 Flashcards
What does general anaesthesia provide?
Muscle relaxatioopn
Analgesia
Unconsciousness
Define general anaesthesia
The reversible immobile state that induces amnesia
Name anaesthetic types
General anaesthetic
IV induction agents
Inhalation gases
Epidural
Local anaesthetic
Dissociative
Topical
Regional
Local anaesthesia
Used alongside GA
Temporarily blocks conduction in sensory nerve fibres preventing nociceptibe info to be received.
Mode of action - inhibits sodium channels in neuronal membranes, preventing influx of sodium ions into nerve axons.
Nervous function lost in order: pain, cold & warmth sensation, touch, deep pressure, motor function
Pharmacokinetics of local anaesthetics
Absorption: depends on dose, site on inj and if there’s a presence of a vasoconstrictor
Distribution: affected by degree of protein binding. Free drug exerts effects and is metabolised. Metabolism via liver and lungs, excreted via kidneys.
Local anaesthetic can cause vasodilation (often used with adrenaline to delay absorption and lengthen action). Vasoconstrictors can cause localised ischaemia so not used in distal areas as potential necrosis.
Effects: lipid soluble and low molecular weight (cross blood-brain barrier). At therapeutic range these are of great use (anticonvulsants, sedatives and analgesia) at high levels can induce seizures and cause CNS depression. Can slow myocardium conduction causing vasodilation, hypotension, bradycardia and cardiac arrest. Cats and exotics are more susceptible to this than dogs
Local anaesthetic drugs
Lidocaine: rapid onset of activity, good tissue penetration. Short duration of activity (45 mins without adrenaline & 1-2hrs with). Good for regional blocks and general surgery.
Proxymetacaine: topical analgesia of eye, rapid onset but duration only 15 mins. Causes less initial sting than other agents.
Bupivacaine: slow onset of activity (15 mins) but longer duration (6-8 hrs). Potency 4X stronger than lidocaine. Good for epidural analgesia and post-op. Don’t give IV as cardiac toxicity risk.
Topical anaesthesia (local)
Eg sprays, drops, gel or cream
Limited to analgesia of nasal mucous membranes, eye, larynx, penis, vagina, rectum and urethra.
Perineural (regional) block
(Local anaesthetic)
Injected directly around the nerve, reduces motor side effects and risks of systemic absorption and subsequent toxicity.
Nerves must be palpable and easily accessible - use of nerve stimulators and ultrasounds
Eg:
Intercostal nerve block: injected caudal to ribs
Brachial plexus block: analgesia distal to elbow
Maxillary and mandibular nerve blocks: dental and jaw surgery, nerves blocked as exit jaw bones
Intra-articular block (local anaesthetic)
Local anaesthetics or opioids injected into the joint cavity following surgery or arthroscopy.
Used as part as multimodal analgesia
Asepsis to prevent introduction of infection into the joint.
Epidural (local anaesthetic)
Performed by vs only
22G needle, or a 17G/18G tuohy needle
Inserted 90 degrees into skin surface and advanced slowly, stylet removed as it nears epidural space.
Eg; lumbosacral epidural, analgesia to structures caudal to thoracolumbar junction.
Bupivacaine used as long duration of action. Preservative free morphine can be used for analgesia of 18-24hrs
Local infiltration block (local anaesthetic)
Intradermal or subdermal tissue infiltrated by injection of local anaesthetic agent.
Uses: minor procedures used with sedation and or restraint.
Sterile needle injected into area. Draw back on hub to ensure no accidental vascular access. Inject small amount of local, creating a wheel effect.
If each injection is made at the periphery of the previous site the patient will only feel 1 injection.
Adrenaline may be used to delay local absorption and lengthen duration of action
Intravenous regional anaesthetic (Bier block)
(Local anaesthetic)
For surgical procedures in the body’s extremities, eg; toe removal.
Local is injected IV into area below or between two tourniquets (Esmarch bandage) on a limb.
Rapid onset of local in this area
Dissociative anaesthesia
KETAMINE
Has minimal effect on the respiratory system whilst creating analgesia and amnesia.
Patient presents as not anaesthetised as can swallow and eyes remain open.
Used in brief, superficial procedures or diagnostic processes.
Can be used in trauma patients (low bp) or elderly patients.
Recovery symptoms of dissociative anaesthesia
On recovery patient may present with;
Hypersalivation
Hyperventilation
Tachycardia
Muscle twitches
Paddling of the legs
Curling of the tongue
Physiology of anaesthesia
Produces many effects on the autonomic nervous system
Effects the brain by the anaesthetic agents acting on the receptors in the CNS and cell membrane of neuronal cells. Anaesthetic agents cause a reversible depression on the CNS function resulting in loss of consciousness.
Blood-brain barrier: this barrier restricts movement of molecules into brain (water, oxygen and carbon dioxide can cross easily others cross more slowly to not at all). This depends on particles molecular size and lipid solubility.
HIGH lipid solubility and SMALL molecular weight crosses EASILY
The EASIER they cross the barrier the MORE potent they are as anaesthetics.
