Pain - Anaesthesia Flashcards
Anaesthesia 3 types
• ANAESTHESIA – is the reversible loss of response to
noxious stimuli;
• GENERAL ANAESTHESIA – when anaesthesia is
associated with loss of consciousness;
• LOCAL ANAESTHESIA – when consciousness is
maintained during anesthesia;
General Anaesthesia
A variety of drugs are given to the patient that have different effects with the overall aim of ensuring unconsciousness, amnesia and analgesia.
General anaesthesia has many purposes, including:
Analgesia (loss of response to pain)
Amnesia (loss of memory)
Immobility (loss of motor reflexes)
Hypnosis (unconsciousness)
Paralysis (skeletal muscle relaxation)
Inhalational Anesthetics
Typically used to maintain unconsciousness
– Advantage of controlling the depth of anesthesia.
– Metabolism is very minimal.
– Excreted by exhalation
Drugs
–historically
»ether, chloroform
–N2O
» analgesia > anaesthesia
–halogenated hydrocarbons
»anaesthesia> analgesia: Relatively little analgesia as awaken!
»halothane, isoflurane, sevoflurane, desflurane
ADR
–bronchodilation
–decreased cerebrovascular resistance →↑perfusion of brain
–hypotension because of decreased cardiac output
–halothane: »malignant hyperthermia
Mechanism of action of inhalational
anaesthetics
• Not fully clear
• Cause generalized depression of CNS
lack of sensation, consciousness often muscle relaxation, loss of autonomic reflex
• Lipid solubility theory
anaesthesia reflects change in neuronal cell lipid bilayer
expands membrane and disrupts protein function
• Membrane receptors
Direct interaction with ion channel?
activation of inhibitory ion channels (eg GABA, Glycine)
inhibition of excitatory ion channels (eg 5HT3 & NMDA)
General Anesthesia
Premedication
–sedation-anxiolytics
»benzodiazepines (diazepam, temazepam, lorazepam)
–Analgesia
»opiates, NSAIDS
–suppress salivation & autonomic reflex (eg bradycardia)
»Cholinergic antagonists –hyoscine
–prophylactic antibiotics
–prophylactic anti-coagulants
–Stop oral hypoglycemics agents, MAOIs, warfarin,
Anaesthetics
–intravenous – for induction and short procedure
–inhalational – for maintenance
Mechanical ventilation may be necessary
–neuromuscular blockade for tracheal intubation and to
facilitate abdominal surgery
Analgesia
Recovery
STAGES OF ANESTHESIA
- loss of pain sensation
- combative behaviour
- surgical anesthesia
- medullary paralysis and death
The Minimum Alveolar Concentration
MAC
• CNS partial pressure to monitor the level of anaesthetics;
• partial pressure of the anaesthetic in alveoli that prevents 50% of patients moving in response to a surgical incision;
• the minimum alveolar anesthetic concentration (% of the
inspired air) at which 50% of patients do not respond to a
surgical stimulus;
• MAC value is a measure of inhalational anesthetic potency;
inversely related to potency, i.e. high MAC equals low potency;
conceptually equivalent to an EC50; MAC values 1.1 to 1.2
used during surgery.
BLOOD : GAS PARTITION CO-EFFICIENT
It is a measure of solubility in the blood.
It determines the rate of induction and recovery of Inhalational anesthetics.
OIL: GAS PARTITION CO-EFFICIENT
It is a measure of lipid solubility
- Higher the Oil: Gas Partition Co-efficient, lower the MAC .E.g., Halothane
-Lipid solubility - correlates strongly with the potency of
the anesthetic.
-Higher the lipid solubility – potent anesthetic
Intravenous Induction Agents
These are used for induction of anesthesia.
• Rapid onset of action.
• Recovery is mainly by redistribution.
• Also reduce the amount of inhalation anesthetic for
maintenance.
Commonly used IV induction agents include
• Commonly used IV induction agents include Propofol, Sodium Thiopental and Ketamine.
(1) Propofol
• Most commonly used IV anesthetic
• They modulate GABAergic neuronal transmission.
• Unconsciousness in ~ 45 sec The duration of action of IV induction agents is generally 5 to 10 minutes, after which
time spontaneous recovery of consciousness will occur.
• Anti-emetic in action
• Non-irritant to airways
• Suited for day care surgery - residual impairment is less marked
• A/E- pain during injection, fall in BP
(2) Sodium thiopental
- Rapid-onset ultra-short acting barbiturate, rapidly reaches the brain and causes unconsciousness within 30–45 seconds.
- The short duration of action is due to its redistribution away from central circulation towards muscle and fat
- The dose for induction is 3 to 7 mg/kg.
- Causes hypotension, apnoea and airway obstruction
(3) Ketamine
- Ketamine is a general dissociative anaesthetic.
- Ketamine is classified as an NMDA Receptor Antagonist.
- Produce - profound analgesia, immobility, amnesia with light sleep.
- Heart rate and BP are elevated due to sympathetic stimulation.
- Respiration is not depressed and reflexes are not abolished
Local Anesthetics
• Decrease or prevent Na+ permeability of the membrane of neurons, which stops the transfer of signals along the peripheral nerves
– Inhibition of action potential in pain fibre
• Classified by duration of action
– Lidocaine is short acting with a rapid on-set
– Bupivicaine is long acting with a slow on-set
– Duration of action can be extended by adding a
vasoconstrictor like epinephrine
• Multiple uses and routes of administration
Topical
most are applied to mucous membranes but some
preparations will be absorbed through skin
0.5% Proxymetacaine and 0.4% Oxybuprocaine are
used for ocular anaesthetic
Lidocaine and benzocaine sprays are used to assist in
intubation (OTC in sore throat and dental pain)
Infiltration
Multiple intradermal or subcutaneous injections of local
anesthetic along proposed incision line
May contain epinephrine (1:200,000) to increase effect and duration
Regional blocks (nerve blocks)
Injection into the connective tissue surrounding a nerve
Can produce loss of sensation and/or paralysis in the region supplied by the nerve
Requires smaller volumes than field blocks, reducing the risk of toxicity
Epidurals
Administered alone or in combination with other analgesics
If combined, smaller doses can be used, decreasing risks of adverse effects
Can cause motor deficits at higher doses
