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72 Cards in this Set
- Front
- Back
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FD (and what is it affected by)
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Fraction delivered. The amount delivered from the vaporizer. Affected by over filling/ under filling, tipping, wrong agent, altitude, calibration, calibration to O2 vrs Air
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FI
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The concentration of gas in the circuit. Not the same in both sides of the circuit. Depends on how close the patient is to equilibrium.
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Fe
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Fraction Expired. The expired amount of gas that tells us the concentration in the alveoli.
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FA/FI Ratio
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what is being breathed in (FI) verses what is being breathed out (FA). Tells us how close we are to equilibrium. The numbers should be close to one another once we reach equilibrium.
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FA (big A) & Influenced by what?
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The end title or expired gas. What is at the alveoli. This should reflect what is in the blood and more importantly what is in the brain (Fbr). Ventilation, volume, dial settings, flows.
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Concentration effect
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A physical phenomenon that occurs. When a drug crosses a membrane it pulls more with it. In particular it is related to the Fa/Fi.
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Absorptive Atelectasis
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When you give a patient 100% O2 it quickly transfers across the membrane (and half of its volume leaves) the alveoli shrink because they
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Pa (Little a)
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Partial pressure in the blood. In theory we could take a sample of the blood to get this number but what we really care about is what is in the brain. Fbr
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Metabolism of the agents
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Very little. Sevo has some and so does halothan
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F and P
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Remember these are used interchangeably
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PBr
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Partial pressure at the brain. No real way to measure it. The number at the VA is assumed to be the PBr.
Determined by the input to the alveoli minus the uptake (loss) of the drug in the blood. |
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Uptake
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Bad! Loss! It means the uptake of the gas into the blood. The blood is a pharmacologically inactive reservoir. That being said, its concentration must be met before the brain or other tissues can be affected.
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Negative Feedback fashion of the anesthetic agents
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If the patient is not mechanically ventilated and they don't have NMB on board the patient will hyperventilate when they need more agent and breath themselves down. This works in a negative feedback way.
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Factors that determine alveolar partial pressure
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FA= input (delivery) - uptake (Loss)
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Input is dependent upon
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Inspired Partial Pressure (FI)
Characteristics of the breathing machine (like rubber parts absorbing the anesthetic or longer tubing for head of bed at 180). Alveolar Ventilation (VA) how quickly we are ventilating the patient |
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Uptake is dependent on
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Solubility of the agent
CO Alveolar to venous partial pressure difference (A-vD) |
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Uptake formula
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Uptake = Blood:gas (solubility of the agent) x Q (CO) x A-vD (arterial to venous difference). Increase any one of these and you will increase the uptake.
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A-vD
What is it called? What is it part of? |
Arterial to venous partial pressure difference. It is part of the uptake formula
It basically says that if you have 2% in and then 0% out you have a bunch of space yet to fill. |
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How do we over come uptake or loss?
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Turn up the flows
decrease CO Choose a drug with less solubility Add more flows (like N2O) |
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Concentration effect
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When you give a higher concentration of the drug the patient will become anesthetized more quickly than if you were to give it at a lower dose for a longer period of time.
FI offsets the impact of uptake by accelerating induction by the rate of rise in FA. When a large amount of drug is present at the alveoli it pulls more drug across the membrane. The higher the FI, the faster the FA will reach FI. |
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Absorptive Atelectasis
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When you deliver 100% O2 and the O2 slips across the membrane (and half of the volume leaves) the alveoli shrink because of the loss of volume (no nitrogen to hold it open).
Used to explain the concentration effect. Pulls more across. |
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Overpressure technique
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We give a bunch of agent to the patient because we know we are going to lose a bunch to uptake. In other words, I want the patient at 2% but I give 4% initially to overcome that uptake and more rapidly anesthetize the patient. Ultimately has to do with the FI (circuit).
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How can you overcome a high blood solubility?
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Deliver a higher FI than what is needed to maintain anesthesia. Keep in mind if you continue that high level you will OD the pt.
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When is uptake completed? What is it known as?
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When the Fi and Fa match. This is known as equilibrium.
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What is the Fa/Fi ratio at equilibrium?
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1.0
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Do we want to be fast or slow to reach equilibrium?
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Fast! The faster the better.
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What are we talking about when we talk about Fa/Fi ratios?
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How fast we get to equilibrium
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Daltons law of partial pressure
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When gases mix they will each exert their partial pressure of the total pressure. This is related to the 2nd gas effect.
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Second gas effect
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The ability of a large volume of uptake of a first gas to accelerate the rate of rise of a companion gas (second gas).
The gas moves across the membrane and leaves a void which the companion gas fills. The gas left after N2O moves over will exert more pressure (because the N2O is gone) because of daltons law and increasing the rate of rise of the second gas. |
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What is the benefit of the second gas effect? What gas is this typically related to?
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faster rate of rise of the companion gas. Typically related to N2O.
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What affect does ventilation have on rate of rise? What kinds of gas is this most profound in?
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Hyperventilation causes faster rate of rise
Hypoventilation causes slower rate of rise Highly soluble gases |
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Hyperventilation negative effect
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Cerebral vasoconstriction due to lower CO2. So you might have a rapid FA/FI rate of rise but you have limited or slower perfusion to the brain which negates your intended effect.
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Longer tubing can have what effect on the rate of rise? How can that be overcome?
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Prolong it.
Increase the flows! |
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Uptake =
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Bad!
Loss! Remember the elephants? The prom dress? |
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Partitian Coefficient
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The distribution ratio describing how agents distribute themselves between two phases at equilibrium. Defines the solubility.
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Blood:gas = 2
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Two times as much will be in the blood as in the gas phase.
Example of a partitian coefficient |
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Blood:gas partitian coefficient
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Temperature dependent. Always assume 37 unless told otherwise
Does not mean there is the same amount on each side. It means there is the same amount coming and going. Think of the ladies room example. 200:10 |
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Is a high blood:gas number good or bad
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Bad = it equals a lot of uptake.
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High blood:gas
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Means a large amount must be dissolved into the blood before equilibrium is reached.
Remember the blood is a pharmacologically inactive reservoir. |
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Examples of high and low blood:gas solubilities
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Halothan - high. A lot has to be dissolved into the blood
N2O - low. Very little is dissolved into the blood. |
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Blood:gas solubility tells us what about rate of rise? How are the agents rated?
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High B:G = slow
Low B:G = Fast Rated like this :N2o, des, sevo, iso, halo |
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How does HCT affect B:G?
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Low HCT means less uptake into the blood. Therefore speeds up the rate of rise. Less blood to fill.
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How does a fatty meal affect B:G?
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A fatty meal will slow the B:G rate of rise.
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Blood:Tissue Coefficient
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Time that it takes to reach equilibrium in the tissues with the blood. Keep in mind there are different solubilities between different drugs and different tissues.
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How many time constants necessary to reach equilibrium between the blood and a tissue?
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3
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How do you make sure an equal concentration in the brain
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Maintain pressure in the alveoli for at least 15 minutes (3 time constants).
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Time constant
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A measurement of percentage related to time. The percents don't change but the times do. Can be measured for both going in and going out.
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Oil:Gas partition coefficient
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Parallels anesthetic efficacy.
Inverse relationship to MAC Estimate MAC by 150 divided by oil:gas |
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What is the B:G of N2O important?
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It is 34 times higher than nitrogen and therefore can more quickly get into the tissues and expand them. This is dangerous in any place with air bubbles. Eyes, bowels, cranium, pneumo, etc
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How does CO influence uptake?
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Faster CO means less uptake which means it will take longer to anesthetize the patient.
Slower is better (more time at the filling station). We have to satisfy the blood first then we can have the effects we want. |
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Can CPB affect anesthetics?
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Yes, they dilute them and therefore cause the patient to lighten
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What effect does a low HR have on the anesthetic?
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Allows for more filling of the blood, but if the volatile causes a drop in HR it could exaggerate that and drop the HR even more.
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What should you watch out for with a patient in shock?
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A slow HR. They can have a more profound anesthetic effect and therefore drop the HR even more
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What effect will a right to left shunt have?
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The blood will bypass the lungs and never get any anesthetic therefore significantly prolonging the induction time.
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What effect will a left to right shunt have?
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The agent in the blood will show back up in the lungs and therefore no more uptake will occur.
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What percent of CO do the highly perfused organs get? How much of the body mass do they make up?
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75% of CO
10% body mass |
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How fast do the highly perfused organs reach equilibrium? What happens after they do?
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Fast
The other organs will start to work toward equilibrium. They reach equilibrium in 3 time constants. |
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Skeletal muscles and fat make up how much of the body mass? How much CO do they get?
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70%
25% of the CO. |
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How long does uptake occur in the VPG?
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Days
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What is VPG / VRG?
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The tissues. They are considered an inactive reservoir (like the blood).
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Recovery
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the rate at which the alveolar concentration decreases with time.
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Is recovery faster or slower than induction? Why?
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Faster because the tissues are still taking up the agent (the concentration is still higher) and therefore they will help reduce the amount in the brain. This leaves the patient feeling groggy afterwords sometimes for days. This effect gives you the long tail.
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What happens to the Fa/Fi curve in recovery?
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Its just upside down and has a long tail.
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What is the impact of tissue storage of the anesthetic?
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Small but long tail. The effect depends on the length of the anesthetic delivery and the solubility of the anesthetics
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What is MAC awake dose?
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.3-.5% of MAC. You can also think of it as MAC asleep. This is the point the patient will wake up or stop responding.
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How can we encourage recovery and overcome machine efflux?
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Increase gas flows and ventilation rate.
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Get rid of the Agent or let the CO2 rise?
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Get rid of the agent by hyperventilating the patient and then let your CO2 rise by hypoventilation (to get your CO2 drive to take over).
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How fast does CO2 rise?
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6-10 torr per minute with an apenic patient. It's easier than trying to remove the agent.
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What is context sensitive recovery?
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The amount of time it takes for recovery is dependent upon how long the patient was administered the agent.
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Context sensitivity of N2O
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Is great. We can give it for a long time and it will come out quickly and easily. N2O also aides in the dose of other agents and therefore makes their context sensitivity faster.
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Metabolism of the various agents
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Very little
It is capacity dependent Halo 20-50% (yikes) Iso 0.2% Des 0.02% N2O - none |
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Diffusion Hypoxia
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Similar to the 2nd gas phase (but in reverse). Can happen at the end of N2O administration. As N2O quickly leaves, it pulls O2 with it and leaves the patient hypoxic. This can be over come by providing the patient with 100% O2 when the N2O is discontinued.
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