General Anesthestics Flashcards
What are “general anesthesia” and “balanced anesthesia”
General anesthesia - reversal depression of CNS
Balanced anesthesia - the use of a drug combination to achieve all of the desired effects in the anesthetic state
What does a general balanced anesthetic regimen look like?
- Premedication with sedatives or opioids
- IV anesthetic for induction
- Muscle relaxants to reduce required amount of general anesthesia
- Inhalation anesthetics to maintain anesthesia
What is MAC?
Minimum Alveolar Concentration of anesthetic -> 1 MAC = partial pressure of inhaled anesthetic in alveoli that results in immobility of 50% of patients to noxious stimulus
How does MAC work when multiple anesthetics are used together?
They are additive. That is, if you are using a 0.5MAC concentration of one anesthetic, you will only need 0.5 MACs of another to get to 1 MAC = 50% of patients will be not moving with that.
What is potency of an anesthetic equal to, and what does this correlate with?
Potency = Oil:Gas Partition Coefficient = 1 / MAC
That is, if MAC = 50, potency will be 1/50 (not that potent)
Things with high potency and low MAC are highly LIPOPHILIC -> highly soluble in oil, less soluble in gas phase
What are some factors that will increase and decrease MAC of a given substance for a patient?
Increase -> more will be required = CNS stimulants, young age
Decrease -> less will be required = CNS depressants, alcohol, old age
Give two hypothesizes / theories which were thought to explain why lipid solubility was a perfect proxy for anesthetic potency?
- Critical volume hypothesis - Anesthetics expand volume of cells beyond a critical amount, altering electrical properties
- Fluidization theory - increased membrane fluidity altered electrical properties
What is the new, best theory of how anesthetics work?
Dual Process Model of Anesthesia: Anesthetics bind to hydrophobic pockets of specific proteins that affect ion flux during membrane excitation. Can be presynaptic or postsynaptic. Resulting in:
- Potentiation of inhibitory neurotransmitters
- or- - Inhibition of excitatory neurotransmitters
What proteins are the targets of anesthetics?
- GABA-A - increase
- Glycine - increase
- Potassium channels - increase
- NMDA receptors - decrease
What are the organ systemic effects of inhalation anesthetics (IA) and which one is the exception? Think CV, respiratory, brain, kidney, and liver.
Nitrous oxide is the exception
CV - drop in blood pressure by various mechanism
Respiratory - decreased minute ventilation, hypercapnia
Brain - Increased blood flow (increases ICP)
Kidney - Decreased RBF / GFR
Liver - Decreased hepatic blood flow
What can all inhalation anesthetics trigger, what causes it, and what is the exception?
Malignant hyperthermia
Continual release of Ca+2 from SR in muscle due to defective RyR
Reversed by dantrolene
Exception: Nitrous oxide cannot cause this
What is the primary factor of an IA which determines the rate of induction and recovery?
Blood : Gas Coefficient. Higher coefficient = more soluble in blood = longer time to get to equilibrium and reach anesthetic partial pressure in the brain = lower rate of induction, slower recovery
What type of patient is most likely to take longest to induce and recover?
A person with a high body fat percentage -> more drug will be uptaken by body fat which will have to be metabolized later (these are very lipid-soluble drugs)
What is the order of anesthetic equilibration within the tissues? How do these compartment sizes differ)
Highly perfused tissues equilibrate first -> brain, heart, kidney, liver (small compartment)
Medium blood flow second -> muscles (medium size)
Low blood flow last -> fat (largest size)
What type of IA drug will have the greatest increase in induction speed with increasing ventilation rate?
Drugs with a high blood : gas ratio -> these need to be equilibrated faster by increased delivery of gas to the blood. Low ratio ones will equilibrate very quickly anyway without needing to increase ventilation near as much.
How does pulmonary blood flow impact speed of induction?
Increased pulmonary blood flow = slowed induction. This is due to more blood being delivered which needs to equilibrate, and when blood is moving fast, larger compartments (i.e. fat) will take up a larger proportion of gas
How are most IA’s eliminated?
Eliminated by lungs, although a couple are metabolizes by liver and excreted.
What factors will cause a longer time to recovery from anesthesia?
- High blood : gas PC - more is in blood to expel
- Longer duration of procedure - more anesthetic will have accumulated in muscle / fat
- Slow ventilation in recovery (faster ventilation will make elimination more rapid)
What is the primary toxicity of concern with nitrous oxide?
By inactivating methionine synthase, it can halt DNA production, leading to bone marrow depression and even pernicious anemia (B12-dependent enzyme)
What is the unique risk incurred by usage of halothane?
Hepatotoxicity - “H” - it is metabolized in liver, and can lead to trifuoloracetylated proteins which are hepatotoxic and can cause hepatitis / necrosis
What IA is most likely nephrotoxic and what is its mechanism?
Sevoflurane -> metabolism in liver leads to the formation of inorganic fluoride ions
What is nitric oxide manually used for?
It is a good analgesic - potency is too low as a full anesthetic, used in combination for induction in minor surgery
What extra property do the halogenated IA’s have which can be advantageous in some patients?
They act as bronchodilators (good in asthma)
What is the most used anesthetic in the world and why is it infrequently used in the US?
Halothane - smells gud
Aside from hepatotoxicity:
- Causes hypotension and arrhythmias in adults, sensitizes heart to catecholamines and increases cerebral blood flow
- > bad in strokes and cardiac problems
What is the most widely used anesthetic in adults and why?
Isoflurane
Excellent muscle relaxant, low toxicity, inexpensive
What group is isoflurane not the best for?
Children -> smells really bad
What IA is typically used for obese patients and why?
Desflurane - lowest O:G ratio, does not accumulate in fat as much
What is the IA most commonly used in children and its main issue?
Sevoflurane
Poses a seizure risk in children and has nephrotoxic effects
Also VERY expensive
What are the chemical properties of IV anesthetics and why? When is their action terminated?
Very hydrophobic -> rapidly reach brain / heart and smaller compartments for induction of anesthesia
Action in is terminated when the drug redistributes to the larger compartments -> muscle and fat. Rapid recovery
What is the barbiturate excellent inducer, and why is it rarely used now?
Thiopental - has a “hangover” effect from accumulation in the tissues which is bad for ambulatory surgery. You feel really groggy
When is thiopental still used?
Still used in neurosurgery, it reduces brain oxygen consumption and ischemia-induced brain damage
What is propofol used for and its advantages?
Used for induction and continuous infusion in short procedures, can also be used for sedation in critical care settings
Advantages: Rapid recovery (no hangover), and has anti-emetic properties
What are the disadvantages of propofol?
Its sedative properties make it cause profound respiratory depression
Inhaled anaesthetics often come with a risk of hypotension and cardiac issues (especially halothane). What IV anaesthetic should be given to a patient with these conditions and why?
Etomidate - does not cause significant cardiovascular or respiratory depression
Ketamine is also a valid answer - but causes stimulation
What are the adverse effects of etomidate?
- Post-op nausea / vomiting
2. High pain on injection (co-administer with lidocaine)
What are the advantages and disadvantages of ketamine?
Advantages: Good analgesia, amnesia, and hypnosis. Can be used to induce or sustain anesthesia in patients with CV problems
Disadvantage: Hallucinogen and irrational behavior during recovery
In what “state” are patients in when they take ketamine?
Cataleptic state - it is a dissociative agent - causes nystagmic gaze with eyes open
Why is midazolam given to patients 15-60 minutes prior to induction?
Causes anterograde amnesia and is an anxiolytic
What opioids are given and to what patient populations typically?
Morphine or fentanyl
Given to high risk patients who might not survive full general anesthesia -> produces analgesia
