General Anaesthesia Flashcards
Requirements for general anaesthetics?
- Intravenous access: to give anaesthetic agents
- Start process
- Induction of anaesthesia with induction agents
- Analgesia and muscle relaxation – and anti-sickness drugs
- Maintain process
- Agents for amnesia, analgesia, muscle relaxation
- Replace fluid/blood loss
- Reverse process
- Reverse muscle relation
- Maintain post-operative analgesia
The first step in GA is amnesia induction. How quickly do amnesia induction agents induce loss of consciousness?
Induce loss of consciousness in one-two arm-brain circulation times (10-20 seconds).
What is an ‘arm-brain circulation time’?
the time it takes for the drug to travel from the injection site to the brain
Describe the properties of IV agents used for GA
- Quick onset
- Short duration (redistribution to fat and muscle)
- Commonly used as a bolus for induction, but increasingly as an infusion for maintenance
You must be aware of potency in IV agents in GA. What is Cp50?
The concentration of the agent in the blood that will prevent movement after a skin incision in 50% of the patients to whom the agent is given.
Mechanism of action for GAs?
- GA’s stimulate inhibitory receptors (such as GABA receptor) and inhibit excitatory receptors
- Produces overall inhibition
Ideal properties of IV induction agents:
- Simple preparation
- Compatible with other agents
- Painless on administration
- High potency and efficacy
- Predictable action within one circulation time
- Minimal cardiovascular effects or other toxicity
- Depression of airway reflexes for intubation
- Rapid and predictable offset of effect
- Rapid metabolism for minimal hangover
What are the 4 standard drugs used in IV induction?
- Propofol
- Thiopentone
- Ketamine
- Etomidate
what is the most common drug for IV induction?
Propofol
What are the advs/disadvs of propofol?
Advs:
- Excellent suppression of airway reflexes; patient falls asleep and stop breathing –> need to put in an airway!
- Decreases incidence of PONV (postoperative nausea and vomiting)
Disadvs:
- Marked drop in HR+BP
- Pain on injection (as lipid based)
- Involuntary movements
what are the advs/disadvs of Thiopentone?
Advs:
- Faster than propofol, used for rapid sequence induction
- Antiepileptic properties and protects brain
Disadvs:
- Drops BP, rise in HR
- Rash/bronchospasm
- Accidental intra-arterial infection: thrombosis and gangrene
- Contraindicated in porphyria
How does ketamine differ from the other drugs for IV induction?
- Anterograde amnesia and profound analgesia (different to others)
- Sole anaesthesia for short procedures
- Slow onset (90 seconds)
- Only used in a small number of cases
Disadvs of ketamine?
- Nausea, vomiting
-
Emergence phenomenon (depressed or hyperexcitable, vivid dreams)
- Anxiety or distress on awakening from ketamine sedation, which settles spontaneously
What are the advs/disadvs of Etomidate?
Advs;
- Rapid onset (0.3mg/kg)
- Haemodynamic stability, lowest incidence of hypersensitivity reaction
Disadvs;
- Pain on injection
- spontaeneous movements
- adreno-cortical suppression
- highest incidence PONV (Postoperative nausea and vomiting)
How long do induction agents last?
Induction agents all only last 4-10 mins BUT want patient to be asleep for duration of operation
What are the 2 ways in which anaesthesia can be maintained after induction?
- Propofol infusion
- Inhalational agents
How is anaesthesia maintained using propofol infusion?
- Continuing infusion of induction agents; propofol infusion (total intravenous anaesthesia)
- TIVA is a technique of GA which uses a combination of agents given exclusively by the IV route without the use of inhalation agents
What is inhalational anaesthesia?
Can also give the patient volatile gases which are absorbed into the blood and keep the patient asleep for the duration of the operation
What are the 2 routes of administration of inhalational anaesthesia?
- Vaporisers
- Breathing circuits
What are the requirements for inhalational anaesthetics?
- Non-flammable
- Stable with material (plastics, metal), long shelf life
- Environmentally friendly
- Cheap and easy to manufacture
- Pleasant to inhale, non-irritant
- Fast onset
- High potency
- Minimal effects to other systems
- No biotransformation
- Non-toxic to theatre personnel
Which receptor does propofol and etomidate target?
GABAA receptors
When are inhalational agents started?
Immediately after induction
What are the 3 inhalational agents used for amnesia?
- Isoflurane
- Sevoflurane
3.
How des the solubility of the inhalational agent affect the onset and recovery?
- Low solubility = fast equilibration
- Agents with low solubility= fast onset and quick recovery
- Highly fat-soluble agents: GA given for a long time accumulates in fat, which can result in a hangover effect for hours
Inhalational agents are most commonly used for maintenance, but can be used for induction. When would this be the case?
if no IV access can be established
What are the 3 neurological effects of anaesthetics?
1.
Anaesthetics and the receptors they target
Which inhalational agent would be used if no IV access can be established?
Sevoflurane
Desflurane has a low lipid solubility. What does this mean?
- Rapid onset and offset
Which inhalational agent is used for organ transplant? Why?
Isoflurane –> has the least effect on organ blood flow
Which inhalational agent is sweet smelling?
Sevoflurane
What are the potential adverse effects of inhalational agents?
- Vasodilation
- Decrease cardiac contractibility
- Potentially affect organ perfusion
- Malignant hyperthermia
- Hepatotoxicity (Halothane, mostly withdrawn)
What would be the best inhalational agent for a long, 8-hour finger re-implantation?
Desflurane –> low lipid solubility therefore rapid onset and offset
What is the preferred inhalational agent for a chubby child with no IV access?
Sevoflurane –> sweet smelling
What is minimum alveolar concentration (MAC)?
The minimum alveolar concentration (MAC) is the minimum concentration of an inhaled anesthetic at 1 atm of pressure that prevents skeletal muscle movement in response to a surgical incision in 50% of patients. (stimulus is a set depth and width of skin incision)
What is one MAC of:
a) nitrous oxide
b) sevoflurane
c) isoflurane
d) desflurane
a) 104%
b) 2%
c) 1.15%
d) 6%
if 2% MAC for Sevoflurane is achieved, what does this mean?
if 2% MAC for Sevoflurane is achieved, 100% of patients are asleep but 50% of patients will not react to a surgical stimulus
What is the 2nd step in GA?
Analgesia
What are analgesic drugs in GA required for?
- Insertion of airway
- Laryngeal mask airway
- Intubation
- Intraoperative pain relief
- Postoperative pain relief
Examples of analgesic drugs used in GA:
- Short acting: Remifentanil > Alfentanil > Fentayl
- Long acting: Intra-op and post-op analgesia e.g. Morphine and Oxycodone
- Weaker: Tramadol, Dihydrocodeine
- Post op: Paracetamol (most common) and NSAIDS (if not contraindicated)
What is the 3rd step in GA?
Muscle relaxation
What is muscle relaxation required for in GA?
What are the 2 types of muscle relaxants used in GA?
- Depolarising
- Non-depolarising
What is the depolarising muscle relaxant?
Suxamethonium
Examples of non-depolarising muscle relaxants:
- Short acting: Atracurium, mivacurium
- Intermediate acting: Vecuronium, rocuronium
- Long acting: Pancuronium
How are muscle relaxants reversed at the end of surgery?
Reversal at end of procedure to allow breathing: Neostigmine and Glycopyrrolate
How can a patient’s consciousness be monitored during surgery?
- Clinical signs
- Measure level MAC
- BIS monitor
- Isolated forearm
- Evoked potentials
Which of the following ion-channels are not affected by general anaesthetic agents:
- GABA channels
- Na K ATPase
- 5HT3 channels
- NMDA receptors
Na K ATPase; General anaesthetics block GABA dependent chloride, Glycine sensitive, 5 HT3 sensitive and Acetylcholine sensitive and Glutamate sensitive channels.
What is the best induction agent for a patient requiring a burn dressing change?
- Propofol
- Thiopentone
- Etomidate
- Ketamine
Ketamine; Ketamine causes dissociative anaesthesia and profound analgesia and can be useful to perform small procedures on ward in low doses.
What is the best induction agent for a patient with a history of heart failure requires a general anaesthetic?
- Propofol
- Thiopentone
- Etomidate
- Ketamine
Etomidate; Etomidate causes least haemodynamic changes of all four commonly used induction agents.
Best Induction agent for a patient undergoing arm operation under GA with an laryngeal mask airway (LMA)?
- Propofol
- Thiopentone
- Etomidate
- Ketamine
Propofol; Propofol the most common induction agent used for general anaesthesia as it cases suppression of airway reflexes.
MAC for an inhalational agent stands for…?
Minimum alveolar concentration
What is the MAC of Sevoflurane?
2%
Best inhalational agent for induction is a chubby child who has no intravenous access ?
- Desflurane
- Isoflurane
- Sevoflurane
- Nitrous oxide
Sevoflurane; Sevoflurane is sweet smelling and is used for inhalational induction.
Most commonly used non-opiod analgesic agent in anaesthesia?
Paracetamol; due to its safety profile is the most common oral/iv analgesic used for pain.
Which of the following is a depolarising muscle relaxant?
- Suxamethonium
- Mivacurium
- Vecuronium
- Atracurium
Suxamethonium
Which of the following is used for reversal agent for muscle relaxation?
- Neostigmine and glycopyrrolate
- Phenylephrine
- Metaraminol
- Rocuronium
Neostigime and glycopyrrolate
Which of the following is not used to assess awareness under anaesthesia?
- Clinical signs
- MAC
- BIS
- EMG (Electromyography)
EMG
What does BIS stand for?
- Bilateral index of Sleep
- Bilateral index of Somnolence
- Bispectral Index
- Biomedical Index of Sleep
Bispectral index
