Lecture I: Uptake and Distribution – Flashcards
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The goal of induction
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to reach equilibrium bw inspired and alveolar concentration of anesthetic
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The first factor that governs pulmonary inhaled concentration of anesthetic
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ventilation
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Effect of uptake on anesthetic
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uptake by blood opposes the effect of ventilation by drawing anesthetic from the lung
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Uptake is aka:
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concentration effect
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At 100% inspired concentrations of anesthetic, uptake would no longer oppose the effect of ventilation (T/F)
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True
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Metabolism (incr/decr) uptake
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Increase
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Effect of intertissue diffusion on anesthetic uptake
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Uptake may be enhanced as anesthetic moves from highly perfused tissues to poorly perfused tissues with high capacity for anesthetics (fat)
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3 Factors that determine uptake by blood
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solubility (bgp), pulmonary blood flow (CO), and difference in anesthetic partial pressure bw lungs/alveoli and venous blood returning to lungs
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Low solubility translates to _____ recovery from anesthesia
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faster
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Changes in ventilation and distribution of ventilation, cardiac output, and inflow rate, each influence anesthetic concentration in predictable ways (T/F)
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True
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All gases follow the 3 gas laws (T/F)
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True - all gases behave the same
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Density
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mass/volume
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Variables of 3 gas laws. Most significant variable in anesthesia. Explain what it means
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Volume, Pressure, Temperature PRESSURE = molecular hits on a membrane. Force/Surface area
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Temperature refers to
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Average molecular velocity. Molecules move faster when increased temperature - Increased pressure
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Explain pressure in r/t molecular hits
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Pressure is molecular hits. More molecules = more hits = higher pressure. Faster movement = more hits = higher pressure.
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Avagadros number
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all gasses have 6 x 10²³ molecules in 22.4L
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Vapor
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gas phase of a substance that can exist as a liquid at room temp and one atm
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1 Atm = ? kPa
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100kPa
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Vapor pressure
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pressure the gas exerts in an atm above the liquid phase. The max partial pressure a sub can exert at specific temp. When vapor and liquid phase of same substance are in equilibrium
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Vapor pressure depends on (2)
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temperature and the agent
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Latent heat of vaporization
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calories (energy) needed to change 1 cc of liquid to gas. Energy needed to break the bonds that hold liquid together
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What will happen to a gas if it exceeds the saturated vapor pressure
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It will return to liquid phase
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Boyles Law
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P₁V₁=P₂V₂ at constant temperature. Inversely related
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Charles Law
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V₁/T₁=V₂/T₂ at constant pressure. Directly proportional
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Practical example of Charles Law
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on a hot day, it is difficult to keep hot air balloon inflated.
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Gay Lussac's Law
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P₁/T₁=P₂/T₂ at constant volume. Directly proportional
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Clinical example of Gay Lussacs Law
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air in ETT cuff pressure increases with body temp
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General gas law/Avagadros Law
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PV=nrT (r = univeral gas constant, n= avagadro # of particles, T= temp)
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GMW
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gram molecular weight = mass of one mole of a chemical compound = molecular weight in grams
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Daltons Law of Partial pressure
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pressure exerted by a gas is proportional to it's percent concentration in a mixture
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Henry's Law
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At constant temp, amt of gas dissolved is directly prop to the pp of that gas.
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Graham's Law
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Rate of diffusion is inversely proportional to the square root of mw of gas. Heavy sub diffuse more slowly
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Formula for partial pressure
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pp = BP x % gas (whole #)/100
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The maximum partial pressure
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100%
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When could you see a PaO2 >100
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screaming child who is blowing off their CO2 and making more room for O2
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Partial pressure is independent of flow (T/F)
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True
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Effect of hyperbaric chamber on dissolved oxygen
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Increases concentration of dissolved O2. Displaces the CO that is more tightly bound to Hgb
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Fa/Fi
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Alveolar concentration is equal to inspired gas concentration
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Differences in partial pressure to consider in achieving Fa/Fi = 1
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vaporizer to inflow of circuit, inflow to circuit, circuit to alveoli, alveoli to blood, blood to brain/tissues
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In a vaporizer, the area above the liquid gas contains ____
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saturated vapor pressure
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When you want to change over the gases in the alveoli, you should run ____ liter flows. What rate?
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High, at 7-10L/min, OR turn up a high percent concentration
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High percent concentration aka
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overpressure
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Fi can be controlled by
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high flow rates, high percent concentration
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Fi value is reflected in ____
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expired gas
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(Fi/Fa) can be easily controlled and (Fi/Fa) reflects the pp of anesthetic in blood
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Fi, Fa
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Goal of induction
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get inspired and alveolar gas at the same concentration
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How does Cardiac output and ventilation affect anesthetic uptake?
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Increased CO = decr. uptake Increased ventilation = increase uptake
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Normal minute volume
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6-7L adult
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Anatomic dead space volume
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2ml/kg + apparatus
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FRC is
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whats left in lungs after expiration
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Equilibrium of anesthetic gases happens more quickly in more soluble agents (T/F)
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False. Less soluble
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Vessel rich groups get majority of gas (T/F)
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True. In longer cases, muscle may gain some of the gas
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When in equilibrium, the patient is asleep (T/F)
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False. It is a scientific term. Often need more gas than what produces equilibrium
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Rate to equilibrium bw/ inspired and expired fraction of inhaled anesthetics factors (4)
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SOLUBILITY cardiac output fresh gas flow minute ventilation
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Maximal preoxygenation should reach what expired O2 value
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>80%
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How can you manipulate the time constant in achieving equilibrium?
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Increase flow rate
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Formula for determining time constant
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capacity (size of container/tubing+patient)/flow rate.
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When would you manipulate the time constant?
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During gas change over. Increase flow will decrease time. Once in equilibrium, time constant doesn't matter
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What is time constant
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the time it takes to turn over gases
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1 time constant = how much change in gas?
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63% change over
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2 time constants = how much change over in gas?
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86% change over
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3 time constants = how much change over in gas?
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98% change over
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How does size effect time constant?
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larger the container/volume, the longer the time constant
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System volume is 10L, flow rate is 2L/min (low flow). How long will it take to change over gas?
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3 time constants. 3 x (10/2) = 15min.
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partial pressure gradient
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the concentration on both sides of the membrane
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Molecules always try to achieve ____
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equilibrium
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Insoluble agents have (high/low) partial pressure
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High - because they cannot cross the membrane, but try
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What opposes the rise of Fa?
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uptake
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Uptake of O2/min at rest =
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3ml/kg/min
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Soluble agents need (high/low) amounts to increase pp
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high
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Why do uptake curves for anesthetics begin the same then diverge?
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differences in solubility
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At 10L/min flow in a 10L container (average adult), how long is 3 time constants?
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3 minutes
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If uptake removes 2/3 of anesthetic in alveoli then what is the Fa to Fi ratio
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1/3 Fa to 2/3 Fi
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Uptake is the product of (3)
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solubility (BGP), cardiac output, alveolar-pulmonary venous pp gradient
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Gases are (more/less) soluble at low temperature
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more
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There is (more/less) dissolved air in cold water
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more
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Increasing cardiac output and increased solubility causes (quicker/prolonged) induction time
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prolonged
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Blood/gas solubility >1 means
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more molecules in blood phase than dissolved phase
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Blood/gas solubility <1 means
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more molecules in gas phase than blood
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Overpressure
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ventilation with high concentration
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Potency
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how many molecules needed to get in for sleep
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Apneic oxygenation
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O2 is pulled from FRC in alveoli during apnea. Alveoli collapse in this
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Potency is related to ____
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fat solubility
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Where does anesthetic go after equilibrium is achieved?
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uptake by other tissues/fat. Especially in longer cases. Vessel rich - muscle - fat
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The Alveolar to pulmonary venous difference represents ____
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uptake by tissues
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Factors for tissue uptake
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solubility (tissue/blood partition coefficient), blood flow to tissue, arterial-tissue pp gradient
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Define equilibrium
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Equal pp, no net movement of molecules
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How can we attain a more rapid induction with inhalation anesthetics?
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increase flow, increase concentration
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Why are time constants different for VRG, MG, and FG?
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Because the blood flow rate is different (container size/flow rate)
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Concentration effect
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Fa is less affected by uptake with higher inspired concentrations. Equilibrium is achieved quicker.
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Concentration effect is utilized during ____
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induction. When there is large concentration gradient and massive uptake in alveoli
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What drug is used for concentration effect
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Nitrous oxide bc it quickly reaches equilibrium
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Second gas effect
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High concentration of Nitrous Oxide is uptaken and causes an increase in concentration of 2nd gas and quicker induction
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Anesthesia flow rates to maintain steady state should be
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around 2L/min of combined total gas flow (air/O2/gas)
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Why do we lower flows after induction?
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less pollution, saves money, you vaporize less anesthetic
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Inhalation anesthetics are highly metabolized (T/F)
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False. Sevo is very slightly metabolized. Halothane is not longer used bc of high metabolism
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Inter Tissue Diffusion
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fat can uptake anesthetic if adjacent to vessel rich group
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Overpressure does not work in emergence (T/F)
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True
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More rapid recovery occurs with (insoluble/soluble) agents
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insoluble - less absorbed into tissues
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With longer anesthesia time, reservoirs (muscle/fat) have (longer/shorter) time constants
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longer
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DIffusion hypoxia must be considered when using what gas?
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Nitrous Oxide
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Concerns with using N2O r/t gas in body compartments
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N2O will go to wherever there is air in the body. (inner ear, pneumo, GI). N2O will replace Nitrogen in air bc it is more soluble and increase amt of gas
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Diffusion hypoxia. How to treat?
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Nitrous oxide coming out of the blood dilutes oxygen in alveoli during recovery. Give supplemental O2 for atleast 10 min in recovery phase
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MAC awake
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1/3 MAC, opens eyes, safe airway
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MAC awake may be lower/ higher in these conditions
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lower: elderly, other narcs/sedatives used higher: N2O than other agents
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Definition of MAC
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50% of people don't move on incision
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No recall MAC
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0.33 MAC, but may still move
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Receptors thought to be involved in anesthesia
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glycine: likely on spinal cord, most influence on MAC GABA: hypnosis, lipophilic sites. Agonist drugs Both inhibitory NTs
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Factors that effect rate of recovery from inhalation anesthetics
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ventilatory rate, CO (prolongs recovery), fat/muscle content (prolongs recovery in long cases)
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V/Q mismatch has this effect on induction
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slows induction with insoluble agents
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Potency of anesthetic correlates with it's solubility in ____
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oil/fat
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Anesthetics have one target (T/F)
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False
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The immobilizing effect of anesthesia involves a site of action in _____, wheras sedation/hypnosis involve supraspinal mechanisms
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Spinal cord
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Volatile anesthetic effect on inhibitory synaptic transmission
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post synapt: potentiates LOC activated by GABA and glycine Extrasynap:enhancing GABA receptors Presynapt: enhancing GABA release
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Volatile anesthetic effect on excitatory synaptic transmission
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Presynapt: suppress transmission by reducting glutamate postsynapt: inhibit excitatory ionotropic receptors activated by glutamate
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Guedels stages of anesthesia
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I: awake, amnesia, analgesia II: Excitation III: Surgical anesthesia IV: overdose, lose life sustaining reflexes
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Precautions in stage II of anesthesia
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heightened reflexes. Don't extubate. High likelihood of laryngospasm
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Stages of anesthesia not classically seen with ____
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IV agents
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MAC level to prevent movement in 95% of healthy patients
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1.3 MAC in expired concentration
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MAC decreases in these conditions (5)
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age, anemia, other drugs present, intoxication, PaCO2 > 95mmHg, severe Hypotension <40mmHg MAP, cold, hypocalcemia, hyponatremia, pregnancy
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All vapor anesthetics have this effect on Cardiac/Respiratory/Cerebral/Neuromuscular/Renal/Hepatic/Metabolism
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Cardiac: decrease BP, CO, +/- rate Respiratory: Inc rate, decrease TV Cerebral: Increase CBF, decrease CMRO2 NM: relax (better effect with lower muscle mass) Renal: decr. BF and filtration rate Hepatic: decr BF Metabolism: slight decr. All MH triggers
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Avoid inhalation induction in this case
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space occupying lesion/tumor. Increases CBF and will increase ICP
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Lowest MAC in what age group. Highest in this age group. Steady decline at this rate.
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Neonates Infants decrease 6% per decade starting at 1yo
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Caution should be taken when giving inhalation agents to patients with Cerebral vascular disease bc ____
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"vascular stealing" - blood stolen from injured area and gets underperfused. Also, inhalation agents all increase CBF --> ICP
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Halothane: BGP, MAC, solubility, metabolism, SE
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2.3, o.74%, most soluble agent used, high metabolism (10-20%), Hepatic toxicity, sensitizes heart to irritability (arythmia, tachy, HTN)
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Most myocardial depression of inhalation agents from ____
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Halothane
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Contains preservative Thymol
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Halothane
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Halothane is metabolized by this process
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Oxidation
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This anesthetic is an alkane
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Halothane
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Epi max for local anesthesia when using Halothane
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4.5mcg/Kg
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Isoflurane: BGP, MAC
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1.4, 1.15 in O2
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Nonflammable anesthetic
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Isoflurane
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Isoflurane is irritating to the airways (T/F)
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True, so is Des. Sevo and Halothane are not
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Isomer of Isoflurane
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Enflurane
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This biproduct of Enflurane is toxic to the kidneys
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Fluorine
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Formula to calculate % O2
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%O2 = liters O2 x 1 + liters air x 0.21/ total liters flow
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To avoid supporting combustion, keep O2 concentration < ?%
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30%
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Desflurane: BGP, MAC, special considerations (3)
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0.42, 4-6%, needs heated vaporizer bc low boiling point(room temp), tachycardia common, CO formation if absorber is dry
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Desflurane enters the patient in this phase.
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Gas. Others enter as vapor
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Least potent agent
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Desflurane, MAC 4-6%
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Lease soluble in blood (anesthetic). Why is this not used for induction?
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Desflurane, very irritating to airways
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Sevoflurane: BGP, MAC, special considerations
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0.59, 1.71, degrades in Baralyme absorber to compound A (nephrotoxic)
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Sevoflurane is irritating to airways (T/F)
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False. Best for inhalation induction
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What effect does adding N2O to inhalation induction have on MAC
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MAC will decrease of the other agent. You will see the MAC reading on the monitor increase quickly
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What is done to prevent compound A build up?
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Keep flow >2L/min
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Why is N2O not used as sole anesthetic? What is the BGP?
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High MAC (104), not very potent 0.47
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How is N2O stored? How do you know how much gas is remaining?
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as a liquid under pressure. Needs to be weighed
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Critical temperature
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the temp above which liquid will turn to gas. If in tank, it will explode
