General Anesthetics Flashcards
where do many anesthetics act?
At GABA synapses on postsynaptic membranes by inhibiting propagation of further action potentials
How do barbiturates affect GABAa channels?
Make them stay open longer, allowing more Cl- in and hyperpolarizing cell
what are the three types of glutamate receptors upon which anesthetics act?
- AMPA
- Kinate
- NMDA
what is MAC?
minimum alveolar concentration
50% jump off the table
50% don’t jump off the table
At 1.3 MAC, what happens?
99% of patients are immobile (not jumping off the table)
what is a typical induction dose of volatile anesthetic?
2.0 MACs
what has a faster response, an agent with a low partition coefficient or an agent with a high partition coefficient?
low partition coefficient (low solubility)
does nitrous oxide have a low or high partition coefficient?
low
on and off fast
what are the key things about halothane?
- blood:gas 2.3 - relatively high, induction slow
- soluble in fat
- Cons: immune response to halothane (halothane hepatitis - immune response evoking hepatic necrosis, fever, nausea, rash and vomiting)
what is important to know about methoxyflourane?
- it really is not used much anymore
- very potent - high blood:gas partition, so long induction and emergence
- highly soluble in rubber, so can’t have rubber tubing
- extensive metabolism
- may produce renal failure and nephrotoxicity
What are the key things to know about nitrous oxide?
- can’t acheive unconsciousness with it
- rapid induction and emergence
- analgesic primarily
- 70-80% in 02
what is malignant hyperthermia?
rare hertiable disorder triggered by volatile anesthetics and some neuromuscular blockers caused by an inability of the SR to sequester Ca2+
reaction causes massive pyrexia after exposure
What is key to know about propofol?
Pros:
- rapid metabolism and recovery
- little accumulation
- “milk of amnesia” - hypnotic and forgetful rest
Cons:
- pain on injection
- not water soluble (more difficult handling)
- no antagonist
- no analgesia
- cardiorespiratory depression
what is key to know about etomidate?
- Pros
- antagonist available
- anterograde amnesia
- cardiovascular stability - does not cause hypotension
- Cons
- pain on injection
- no analgesia
- accumulates - slowing recovery
What patient populations must you be extra cautious about with anesthetics?
- Trauma even if the drug shows minimal hypotensive s/e, due to volume depletion in most trauma patients
- Elderly
- Cardiorespiratory Disease
Which preanesthetic medications are used to reduce anxiety and induce amnesia?
Benzodiazepenes
Midazolam (Versed)
Diazepam (Valium)
Which preanesthetic is used to prevent allergic reactions and provide some sedation?
antihistamines (1st gen)
diphenhydramine (Benadryl)
What preanesthetic drug is typically given to prevent aspiration and postsurgical nausea and vomiting?
Ondansetron (5HT3 antagonist) - Zofran
anti-emetic
What preanesthetic is given to provide analgesia?
opioids
morphine
Fentanyl
what preanesthetics are given to prevent bradycardia and to dry up secretions?
antimuscarinics
scopolamine, atropine
What preanesthetic is given as a muscle relaxant to facilitate intubation?
pancuronium
What is key to know about Ketamine?
Pros:
- analgesia
- No respiratory depression - may INCREASE BP
- produces hypnotic state
- adverse effects uncommon in children (favorite in pediatrics)
Cons:
- increased muscle tone and incidence of involuntary tics or jerks
- hallucinations
What is important to know about sodium thiopental (barbiturate)
Pros:
- little post-anesthetic excitement or vomiting
- water soluble
Cons:
- respiratory and CV depression
- no antagonist
- slow recovery
- no analgesia
What are some of the key characteristics of IV anesthetics?
- rapid onset (seconds)
- rapid awakening (minutes)
- danger of overdose due to irrevocability of IV injection
- redistribution determines duration of action
- acts in the vessel-rich group rapidly (CNS and visceral organs)
how do benzodiazepenes act on GABA channels?
they increase the affinity of GABA for its binding site on the GABA(A) receptor
What happens at 0.3 MAC?
mild anesthesia begins
what happens at 0.5 MAC
amnesia begins
What does the dose-response curve with general anesthetics typically look like?
usually, it is very steep (low TI)
What happens at doses higher than 2.0 MAC?
potentially lethal doses
how is MAC often expressed?
as a % needed of the anesthetic in air to get 1 MAC
What is the Meyer-Overton correlation?
The potency of an anesthetic is directly proportional to the Log10(lipid bilayer/water partition coefficient)
in other words, more lipid soluble = more potent
Which anesthetics act primarily by enhancing activity of GABA(A) receptors?
etomidate
propofol
barbiturates
isoflurane
sevoflurane
(i.e., most of them)
Which anesthetics act primarily by inhibiting NMDA glutamate receptors?
ketamine
nitrous oxide (also acts at kainate glutamate receptors)
transfer of anesthetic from the lungs to the arterial blood depends upon _________
the solubility of the drug in the blood
low partition coefficients have low or high solubility?
LOW
the speed of induction is inversely proportional to what ratio?
blood:gas partition coefficient
time course of action of anesthetics depends upon solubility and what else?
distribution of the anesthetic in different tissues
agents more soluble in fat have a longer equilibrium and longer on/off time
a larger fat:blood partition coefficient means that an anesthetic will _________?
remain dissolved in the fat for longer
Is enflurane low or high blood:gas soluble and metabolised?
1.8, relatively high solubility and high metabolism
induction and emergence are slow
what are the disadvantages of enflurane?
- cardiovascular depression due to decreased cardiac contractility
- seizures - no permanent damage
- uterine muscle relaxant
What are key things to know about Isoflurane?
- most commonly used inhalation anesthetic in U.S.
- 1 MAC =1.4%
- blood:gas = 1.4, relatively low, somewhat rapid induction
What are the pros and cons of isoflurane?
Pros
- cardiac output maintained
- systemic vessels dilate causing small decrease in BP
- arrhythmias uncommon
- potent coronary vasodilator
Cons
- more pungent than haloethane
- progressive respiratory depression
what are the properties of sevoflurane?
blood:gas - 0.69
relatively low, fast induction
What are the pros and cons of sevoflurane?
Pros:
- can be used for outpatient because of its rapid on/off
Cons:
- some reports of toxicity
what are the properties of desflurane?
blood:gas 0.42
low, very fast induction and emergence (5-10 minutes)
What are the pros and cons of desflurane?
Pros:
- useful for outpatient surgery
- not very soluble in fat
Cons:
- more irritating to airway - can provoke coughing, salivation and bronchospasm
- low volatility - requires use of specially heated vaporizer
- may evoke tachycardia
