Lec 49 General Anesthetics Flashcards
What is mech of action of general anesthetics at molecular level?
d
What are component effects of ideal anesthesia?
- amnesia
- analgesia
- muscle relaxation
- loss autonomic response to noxious stimuli
- loss of consciousness
What is role of barbiturates as anesthesia?
use as induction to anesthesia [not on its own] especially thiopental
What are the stages of anesthesia with increasing anesthetic depth?
stage 1 + 2 = analgesia + stupor
stage 3 = surgical anesthesia
stage 4 = medullary depression = overdose
What is balanced anesthesia?
use combo of drugs to produce effects of an ideal anesthetic
- allows for lower dose of anesthetic
- use adjuncts for amnesia, analgesia, relaxation, suppression autonomic pain
What are the 3 high potency inhalation anesthetics? struct?
sevoflurane
isoflurane
desflurane
– all volatile liquids, derivatives of ether
What is mech of action of inhalation anesthetics?
not well known hypothesized: -- interact with membrane proteins + ion channels -- enhance GABA-A/glycine -- inhibit glutamate + ACh receptors
What is the maximum partial pressure available for an inhaled anesthetic?
at atmospheric p 760 mmHg:
for high potency volatile = the vapor pressure
for NO = 760 mmHg
What is the MAC?
minimal alveolar concentration of inhaled anesthetic require to prevent 50% of subjects from moving in response to noxious stimulus [ie skin incision
= the median across pts of amt required to stop movement
described as % of total pressure
ex 22.8/mmHg/760 = 3%
If you give 1xMac of one drug and 2x Mac of another, what is the combined anesthetic effect?
3x Mac
Why is it that you can talk about anesthetics in terms of MAC but not many other drugs?
little individual variation in the dose response relationship for these drugs
–> at 1.2x Mac movement is suppressed in 99% of individuals
Do inhaled anesthetics suppress ANS response at higher or lower dose than they suppress movement?
need really high dose to suppress ANS effect
What is the relationship lipid solubility and potency?
more lipid solubility (= higher oil:gas partition) –> more potent –> lower MAC
What is the relationship solubility in blood and time frame of action?
less soluble in blood = faster induction and recovery times
Rank in order the oil:gas coefficient from lowest to highest: NO, Isoflurane, Sevoflurane, Desflurane
NO «< Desflurane «_space;Sevoflurane «_space;Isoflurane
thus Iso is most potent
Is NO high or low blood solubility? lipid solubility? what does this mean?
- low lipid solubility –> less potent/higher MAC
- low blood solubility –> faster induction
Rank in order blood:gas coefficient from lowest to highest [high blood gas = higher solubility]?
NO, Isoflurane, Sevoflurane, Desflurane
Desflurane == NO «_space;Sevoflurane «_space;Isoflurane
Iso induces slowest
desflurane/NO induce rapidly
How does ventilation speed affect induction right?
increased ventilation speeds induction
What is typical MAC of anesthesia?
1.3-1.4 x MAC
What is MAC if inhalation anesthetic is used as sole agent to induce anesthesia?
- higher MAC than typical ~3-4 x MAC
- higher MAC for induction with drugs that have high blood solubility [in order to speed up induction more quickly need higher partial P]
Which inhalation anesthetics have fastest recovery time?
those with low blood solubility
What is approximate concentration of NO administered?
~ 70% atmospheric
What is trapped air space?
when inspired, nitrous moves into body and replaces nitrogen
- nitrous way more blood soluble than nitrogen
- nitrous moves in faster than nitrogen moves out
bottom line:
- in trapped air spaces, pressure increases due to NO
What is diffusion hypoxia? how do you prevent?
when NO turned off, floods alveoli as it moves out of the body
causes oxygen to become diluted and thus hypoxic
prevent by giving oxygen when you turn NO off
What is the second gas effect?
- during induction, NO leaves alveoli and creates partial vacuum
- causes increase in ventilation rate
- any other inhaled anesthetic that is co-administered with NO will benefit and thus increase the potency of that co-administered drug
- greater effect for drugs with high blood + tissue solubility
What is the effect of inhalation anesthetics on CV function?
- NO relatively free of CV effects
- high potency ones cause myocardial depression: decrease BP, renal blood flow, GFR
- desflurane and isoflurane have minimal effect on CO
Which inhalation anesthetics effect cardiac output (CO)?
isoflurane, desflurane, NO all no effect
sevoflurane decreases CO
What is effect of high potency inhalation anesthetics on GFR/renal blood flow?
both decreased
What is effect of inhalation anesthetics on respiration?
all except NO: dose dependent resp depression
all: decrease normal ventilatory response to CO2
What is effect of inhalation anesthetics on muscle relaxation?
inhalation anesthetics have some intrinsic muscle-relaxation
decrease dose of NMJ blocker for pts giving these anesthetics
How are inhalation anesthetics metabolize/eliminated?
- eliminated primarily from lung
- minimal metabolism
What are effects of inhaled anesthetics?
- myocardial depression
- respiratory depression
- N/V
- increased cerebral blood flow –> decreased cerebral metabolic demand
What are toxic effects of inhaled anesthetics?
malignant hyperthermia, expansion trapped gas in body cavity,
proconvulsant, nephrotoxicity, hepatotoxicity
What is malignant hyperthermia?
- rare, life threatening hereditary condition triggered when give inhaled anesthetic [except NO] with succinylcholine
signs: muscle contraction/rigidity, fever, tachycardia, potentially fatal
What is treatment for malignant hyperthermia?
dantrolene –> interferes with release of Ca from SR
What is mech of propofol?
activate/potentiate GABA-A
What are kinetics of propofol recovery when given a bolus?
2 phase half life
- initial rapid phase: via distribution to poorly perfused tissues
- then slow phase: via elimination by hepatic metabolism
What is the difference in recovery of propofol when you give bolus injection vs infusion?
bolus: 2 phase kinetics: distribution to poorly perfused tissue then metabolism
infusion: lose distribution phase because the tissue level reaches equilibrium with the blood during the infusion. thus, decline in plasma conc [and waking] takes longer because it depends entirely on metabolism which is the slow phase
What is the context sensitive half-life?
the additional time require for plasma conc to drop by 50% after an infusion
Who is etomidate particularly good for?
pts with CV disease or at risk of hypotension
What is unique about ketamine’s effect?
- causes analgesia even at levels below anesthesia
- causes dissociative anesthesia
How are barbs used as IV anesthetics? example of the one use and its characteristics?
thiopental = high potency, high lipid solubility, rapid entry into brain
- used for induction of anesthesia and short procedures
- terminated by rapid redstrbution into tissue
effect: decreases cerebral blood flow
How are benzos used as IV anesthetics? example of most common used and its characteristics?
midazolam = most common drug used for endoscopy
- use as adjnct with gaseous anesthetics and narctoics
effects: post-op resp depression, decreased BP, anterograde amnesia
