L20 - General anaesthetics Flashcards
Diff. between anaesthesia and analgesia?
Anaesthesia = absence of all sensation (including pain, proprioception, etc.)
Analgesia = no pain
Diff between general anesthesia and regional?
General anaesthesia (GA) = loss of all sensation + CONSCIOUSNESS***
Regional anaesthesia (RA) = absence of all sensation in one part of body (e.g. arm)
What constitutes ‘balanced anesthesia’?
- Unconsciousness (general anaesthesia)
- Analgesia (local anaesthesia)
- Muscle relaxation (neuromuscular blockers)
List the antedotes to NMB, Opioids and anesthetics?
NMB = neostigmine Opioid = naloxone Anesthesia = NO ANTEDOTE
RECEPTOR THAT ANESTHETICS ACT ON?
GABA-A receptors at inhibitory synapses
List the modulators that act on GABA-A receptors at inhibitory synapses? (7) BBAAPPS
Barbiturate site: anesthetics (e.g. pentobarbital)
Benzodiazepine site
Anesthetic / alcohol site
Antagonists (e.g. bicuculine) at GABA site
Picrotoxin site: convulsants (e.g. TBPS)
Propofol site
Steroid site: anesthetics (e.g. alfaxalone)
Compare ED50 to LD50 in general anesthetics?
Very close
ED95 and LD05 overlap
Potentially highly lethal
Explain why general anesthetics can be lethal?
Too deeply asleep:
1) Tongue falls back > blocks nasal, oral passage to trachea > airway obstruction > suffocation
2) Lose protective airway reflex: e.g. vomitus goes to trachea, lungs > aspiration pneumonitis
Metabolism of Thiopentone (thiopental) at physiological pH and storage form?
Storage form in sodium bicarbonate solution = pH >10, ionized sulfide, highly water-soluble
Injection»_space; Unstable non-ionized protonated sulfide»_space; rapid conversion to highly lipid-soluble form at pH7.4 (readily crosses blood-brain barrier)
Describe the distribution of Thiopentone (thiopental) after injection?
Long terminal t1/2 (12h), but almost 0 concentration in brain at 5h due to uptake, fast redistribution to:
- Vessel-rich group: concentration in highly perfused organs (e.g. brain): concentration drops FAST
- Muscle, fat: concentration increase slowly over time
Even if terminal t1/2 is long, patient can wake up in 10 mins due to fast redistribution
Is Thiopental context-sensitive? How is level of thiopental maintained?
Highly context sensitive !!
Rapid redistribution, so constant injection is needed»_space; leads to high accumulation
Time for CNS conc. to drop after repeated admin is much higher than expected
List some Cardiovascular, respiratory, CNS side effects of thiopental?
- Mean arterial pressure decrease, HR increase
- Respiration decrease
- Cerebral blood flow and ICP decrease
What is the most commonly used general anesthetic? How is it stored?
Propofol In lipid solvent
Compare terminal t1/2 of propofol to thiopental? Compare context- sensitivity?
propofol = 4.8 hours, much shorter t1/2 than thiopental (12 hours)
Propofol = very little context sensitivity
Implication of little context sensitivity in propofol? WHy is it good?
infused safely for long surgeries
Predictable wake up time
What is the technique for injecting propofol?
Target controlled infusion (TCI):
Based on body weight, height
varies infusion rate over time to maintain plasma and Brain concentration within therapeutic window
Effects of general anesthesia at increasing dosages?
Increasing dosage = increasing CNS depression:
Sedation > drowsiness > sleep > anesthesia
List some Cardiovascular, respiratory, CNS side effects of propofol? Contraindication?
Systemic vascular resistance, HR, MAP decrease
Respiration decrease
Cerebral Blood flow decrease
Dont use in patients with heavy bleeding or low BP
Advantage of Ketamine over propofol? Disadvantage?
Good for hypotensive patients because of increase in HR, Peripheral resistance, BP
Strong hallucination, strange nightmares»_space; severe mental ADR
Compare terminal t1/2 of ketamine to propofol and thiopental?
Much shorter than propofol
Ketamine t1/2 = 3 hours
List some Cardiovascular, respiratory, CNS side effects of Ketamine?
- INCREASE cardiac contractility, systemic vascular resistance and MAP (not decrease like propofol and thiopental)
- Decrease resp.
Advantage and disadvantage of Etomidate vs propofol?
Good for heart bypass surgery due to no increase in HR or MAP
Significant vomiting and nausea + CNS excitation
List 5 IV general anesthetics?
Thiopental Propofol Ketamine Etomidate Methohexital
Give another preparation of general anesthetic apart from IV GA? Give 5 examples?
Inhaled general anaesthetics via vaporizers (not commonly used now)
halogenated ethers:
- Not gases but very volatile liquids
- E.g. desflurane, sevoflurane, isoflurane, Nitrous oxide, Xenon
Overview of how variable bypass vaporizer works?
Carrier gas > Goes into vaporizing chamber > Bypass channel add set amount of bypass air > exit
Define Minimum alveolar concentration?
Alveolar concentration of the inhaled agent
which prevents movement in response to a standard painful stimulation in 50% of subjects
Relate age to Minimum alveolar concentration?
MAC target changes with age
Younger = harder to anesthesize, need higher conc.
Older = easier to anesthesize
Relate MAC to Oil:gas partition coef. ?
Log is inversely proportional to log of oil:gas partition coefficient
Increase Oil:gas partition coef. = decrease in MAC: Meyer- Overton correlation
Potency is related to lipid solubility
Define blood: gas partition coef.? What determines the wash-in?
Time taken for inhaled gas to reach alveolar and diffuse into circulation at alveolar capillary
» Higher the number, the less time taken to reach target concentration
Wash-in = Alveolar concentration (FA)/ inhaled concentration (FI), modulated by Ventilation rate
» Determines the onset of inhaled agent
Relate ventilation rate to speed of inhaled gas wash-in?
Higher ventilation = increase speed of wash-in = faster build-up of inhaled gas in blood
Relate cardiac output to speed of inhaled gas wash-in?
Increase cardiac output = DECREASE speed of wash in = slower build-up of inhaled gas in blood
List the CVS side effects of inhaled anesthetics?
Decrease Cardiac output, systemic vascular resistance, MAP
Decrease respiration
Increase HR***
Similar to thiopental
Describe the elimination rate of inhaled anesthetics?
Very slow metabolism, mostly remained unmetabolized
Mostly wake up by exhaling the agent to lower the plasma conc
