Module 2.1.1 (General Anaesthetics) Flashcards
What are the stages of general anaesthesia?
Premedication
Induction
Intravenous anaesthetics
Maintenance
Inhalation anaesthetics
What is the purpose of GA?
Producing unconsciousness
Loss of sensation and reflexes
Facilitates surgery with much reduced distress to the patient
What are the FOUR stages of anaesthesia?
Premedication = given on the ward before the patient is taken to the operating theatre. What are the aims of premedication?
Relief from anxiety
Reduction of muscarinic effects bradycardia and secretions
Analgesia
Prevention of postoperative emesis
What is given for relief from anxiety? Why is it given? How to reverse ffects?
BZDs: diazepam, lorazepam, midazolam
- To relieve apprehension and anxiety before anaesthesia
- To lessen the amount of general anaesthetic required to achieve and to maintain unconsciousness
- To sedate postoperatively
- Effects reversible by antidote -flumazenil
What is given for reduction of muscarinic effects? Why is it given?
Atropine, Hyoscine, Glycopyrrolate
- Muscarinic antagonists
- Prevent salivation and bronchial secretions
- More importantly to protect the heart from arrhythmias, particularly bradycardia caused by some inhalation agents and neuromuscular blockers
What is used for analgesia?
Fentanyl, Alfentanil, Remifentanil
- Opioid analgesics
- Often given prior to an operation
- Adequate analgesia is important to stop physiological stress reactions to pain
NSAIDs
- Useful alternatives and adjuncts to opiates
- But inadequate for severe postoperative pain when used alone
What is given for postoperative antiemesis?
Ondansetron, Metoclopramide, Prochlorperazine
- Provide postoperative antiemesis
- Nausea and vomiting are common after general anaesthesia
- Often due to opioid drugs peri- and postoperatively
- Antiemetic drugs can be given with the premedication
What is used for the induction stage?
- Intravenous (IV) agents - rapid induction of unconsciousness
- Prevention of acid aspiration in emergency and obstetric operations is crucial
- se either H2-receptor antagonist or a proton pump inhibitor prior to induction
- Muscle relaxation
> Use Vecuronium, suxamethonium after induction
Explain the pharmacology of the following IV anaesthetics:
A) Barbiturates (thiopentone), Benzodiazepines (Midazolam)
B) Thiopentone
C) Midazolam
D) Propofol
E) Ketamine
A)
- Potentiate the action of GABA on GABAA receptors
- Produce a general CNS depression
B)
- ultrashort-acting
- Caution during administration – avoid severe medullary depression
C)
- has anxiolytic, muscle relaxant and anterograde amnesic actions
D)
- Exact mechanism uncertain
- May potentiate the action of GABA on GABAA receptors (differs from barbiturates and benzodiazepines)
- May shorten opening time of nicotinic receptors and sodium channels in CNS
- Produce a general CNS depression
- Rapidly metabolised more rapid recovery
- Can be used for induction and maintenance
E)
- Reduces neuronal excitability by blocking NMDA (N-methyl-D-aspartate ) receptors
- Produce dissociative anaesthesia
- Also stimulates opioid receptors in brain and spinal cord
- Unusual increase in cardiac output via increased sympathetic activity
- Produce unpleasant hallucinations
- Rarely used today
What is used as muscle relaxants? Explain the MOA.
Vecuronium, tubocurarine, pancuronium
- Competitive antagonists - nicotinic ACh receptor
- Cause non-depolarisation of motor end-plat
- Complete paralysis without fasciculation
- Action reversible by increasing Ach levels with anticholinesterase
- Antidote - neostigmine
Suxamethonium
- Cause depolarising blockade at motor end-plate
- Not reversible by anticholinesterase
What is used as maintanence? How to monitor?
Inhalation anaesthetic agents
Maintain a state of general anaesthesia after induction
Intravenous agents can be used via a continuous pump
Requires precise control of depth of anaesthesia in CNS
Level of anaesthesia is denoted by partial pressure of GA in CNS
Monitor using aveolar partial pressure
Ideal general anaesthetic provides a rapid and pleasant induction and maintenance of surgical anaesthesia, followed by a smooth and rapid recovery to a fully functional and conscious state
What is used as inhalation anaesthetics? What is the MOA?
Nitrous oxide, Desflurane, Isoflurane, Methoxyflurane, Sevoflurane
- Typically act at high concentration
- Potency is well correlated with liposolubility
- Cell membranes - hydrophobic domains of proteins are likely sites of action
- Excitatory transmission (glutamatergic and nicotinic) may be inhibited
- Inhibitory transmission at GABAA receptors is potentiated.
- Stereoisomers of some inhalation anaesthetics (e.g. isoflurane) - differences in potency
How do GA in body work?
The Lipid Theory (used to be the only theory!!)
GAs dissolve in membranes of neurons in CNS
Alter membrane function
The Protein Theory
GAs interact with hydrophobic parts of modulatory proteins eg receptors, ion channels
Alter ion conductance
Cellular and Receptor Mechanism
decrease functions of excitatory neurotransmitters
increase functions of inhibitory neurotransmitters
Affect potassium and sodium channels
What does the potency of lipid anaesthetics increase with?
increases as lipid solubility increases
general anaesthetic summary
