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
