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.
