L7 General Anesthetics Flashcards
Why is food and drinks permitted before surgery?
All sensory and autonomic reflexes must be inhibited
What properties should an ideal anaesthetic drug have? (8)
- unconsciousness
- analgesia: not responding to pain
- muscle relaxation, to the point of paralysis
- amnesia, around the procedure
- brief and pleasant
- depth of anaesthesia can be raised or lowered with ease
- minimal adverse effects
- margin of safety, large
Why are INH and IV anaesthetics used in combination for GA?
Synergistic: to ensure that induction is smooth and rapid, and that analgesia and muscle relaxation are adequate
Which inhalant GA gets into brain more quickly? nitrous oxide or halothane
nitrous oxide -> onset depends on distribution
Examaples of volatile liquids that are inhalant GA
halothene, desflurane, enflurane, isoflurane, sevoflurane
Examples of gases that are inhalant GA
nitrous oxide
What is MAC?
Minimum Alveolar Concentration, minimum concentration of drug in the alveolar air that will produce immobility in 50% of patients exposed to a painful stimuli
Volatile liquids GA are administered using ?
agent specific-vaporiser
Which of the volatile liquids inhalant GA is hepatotoxic?
halothane
Which of the metabolites of volatile liquids inhalant GA is nephrotoxic?
inorganic fluorides of isoflurane, sevoflurane and enflurane
Is halothane-associated hepatitis reversible?
Yes, once halothane is completely eliminated
Halothane MOA
- little or no analgesia until unconsciousness supervenes
- relaxes skeletal muscle and potentiates skeletal muscle relaxants
What is a dose-dependent side effect of halothane?
Respiratory depression
Possible ADR of halothane
Decreases bp due to depression of cardiac output
- bradycardia and arrhythmia may also occur leading to hypotension and dysrhythmia
What is the MAC of halothane?
0.75%
What is the MAC of isoflurane?
1.4%
What is the MAC of sevoflurane?
2%
What is a distinct feature of isoflurane?
pungent smell
How does isoflurane decreases bp?
decrease in systemic vascular resistance
Which of the volatile liquid inhalant GA halothane/isoflurane/sevoflurane has the most rapid rate of osnet and recovery?
sevoflurane, lower blood solubility
When is sevoflurane unstable?
when exposed to CO2 in anaesthetic machines
What is the major concern of nitrous oxide?
postoperative nausea and vomiting
Is nitrous oxide commonly used as an adjunct or monotherapy?
Both!
- adjunct: supplement the analgesic effects of primary anaesthetics
- mono: analgesic agent eg dentistry, during labour
Examples of IV GA
thiopentone, etomidate, propofol, ketamine, midazolam
What are the 2 advantages of inhaled + IV anaesthetics?
- permit dosage of the inhalation agent to be reduced
2. produce effects (analgesia, anxiolysis) that cannot be achieved with an inhalation dose
What is the duration of action of thiopentone when injected alone as a single dose?
Without inhaled agents, patient wakes up in 10 minutes
- hence, difficult to use IV GA only, require high dose
What is the duration of action of thiopentone when injected alone as multiple doses/ infusion?
Depends on clearance
What is the MOA of thiopentone?
Cause CNS depression by potentiating the action of the neurotransmitter GABA on GABAa receptor-gated chloride ion channels (binds to a positive regulatory allosteric site, allows receptor to bind better to GABA, usual amount of GABA)
Which drug, thiopentone/propofol, has a more rapid recovery?
Propofol, patients move sooner and feel better
What is the most common IV anaesthetic used in SG?
Propofol: ready-made in injectable form, no need to re-constitute (unlike thiopentone)
Is propofol used for induction or maintenance?
Both!
What is the time of onset and duration of action for propofol?
Unconsciousness develops within 60 seconds, short duration of action of approx 3-5min following single injection
What is propofol extensively used for?
‘Day surgery’
- needs continuous, low-dose infusion for extended effects
What is a huge advantange of using propofol over nitrous oxide?
Propofol has significantly reduced postoperative vomiting, may be related to an anti-ematic action
What is a significant ADR from use of propofol?
Significant cardiovascular effect during induction (decrease bp and negative inotropic) -> hypotension
CI of propofol (or used with caution)
Elderly patients, patients with compromised cardiac function, hypovolemic patients
Does ketamine result in unconsciousness?
No, dissociative anaesthesia (feels dissociated from environment but still some awareness) and responsiveness to pain is lost
What is the only IV anaesthetic that possess analgesic property?
Ketamine, hence very popular in third world countries as the only anaesthetic, due to lack of other anaesthetic agents
Why is ketamine suitable for continuous infusion, without lengthening in duration of action?
Large Vd, rapid clearance
ADR of ketamine, when does it usually happen and how it can be reduced?
Unpleasant psychologic reactions (hallucination, disturbing dreams, delirium) may occur during recovery from ketamine
- risk of psychologic adverse reactions may be reduced with premedication of diazepam or midazolam
Anaesthetic adjuncts (4)
benzodiazepines, a2 adrenergic agonists, analgesics, neuromuscular blocking agents
What are the uses of benzodiazepine: midazolam (IV)?
- used for anxiolytics, amnesia and sedation prior to induction of anaesthesia (perioperative period)
- or used for sedation during procedures not requiring GA eg. endoscopy
Rank the IV GA according to CV and respiratory depressing effects
midazolam
Midazolam SE is compounded by
concurrent usage of other agents
How to minimise ADR of midazolam?
By injecting midazolam slowly (over 2 or more mins) and by waiting another 2 min for full effects to devleop before dosing again if req
How is midazolam metabolised in elderly patients?
In liver, but more sensitive and slower recovery
Can dexmedetomide be used as a GA?
No, sedation and analgesic effects only, does not produce reliable GA even at maximal dose, used as adjunct
Duration of action of dexmedetomide?
short term sedation, <24hr
Dexemedetomidine
highly selective a2 adrenergic receptor agonsist
What are some undesirable side effects of dexmedetomidine?
nausea, dry mouth, hypotension, bradycardia
Relative potency to morphine and duration of action - sufentanil
1000x, intermediate 15min
Relative potency to morphine and duration of action - remifentanil
300x, ultra-short 10min
Relative potency to morphine and duration of action - fentanyl
80x, intermediate 30min
Relative potency to morphine and duration of action - alfentanil
15x, intermediate 20min
Neuromucular blockers - depolarising
succinylcholine
Neuromucular blockers - non-depolarising
Vecuronium
DDI with neuromuscular blockers
Barbiturates - will precipitate when mixed with muscle relaxants, should be allowed to clear from the IV line prior to injection of muscle relaxant
When is neuromuscular blockers administered for anaesthesia?
Inducition, relax muscles of jaw, neck and airway - facilitate laryngoscopy and endotracheal intubation
What is the principal adr of GA?
depression of respiratory and cardiac performance
How are inhalation anaesthetics eliminated?
Through expired air
How does nitrous oxide differ from other gas?
- Very high MAC, cannot be used alone to produce GA
- High analgesic potency, frequently combined with other gas to supplement their analgesic effects
Induction of anaesthesia usually accomplished with
short-acting barbiturates eg thiopentone
