Dewan (Inhalational) Part 1 - 4 key points Flashcards
The greater the uptake of an anesthetic agent…
- the greater the difference between inspired and alveolar concentrations
- slower rate of induction
3 factors that affect anesthetic uptake:
- solubility in the blood (blood:gas partition coefficient)
- pulmonary blood flow / cardiac output
- difference in anesthetic partial pressure b/w alveolar gas and venous blood
Low cardiac output states can lead to…
- overdose with soluble agents
- rate of rise in alveolar concentrations INCREASED since it is not being taken away from the lungs quickly
Factors which speed induction and recovery (7)
- elimination of rebreathing
- high fresh gas flows
- low anesthetic circuit volume
- low absorption by the circuit
- Decreased solubility
- high cerebral blood flow
- increased ventilation
Meyer Overton Rule
- anesthesia occurs from molecules dissolving at specific lipophilic sites and all share a common MOA at a molecular level
In other words, the anesthetic potency of inhalational agents correlates directly with their lipid solubility
What is MAC? What does it mean?
Minimal Alveolar Concentration
alveolar concentration that prevents movement in 50% of patients in response to a standardized stimulus
Complications from prolonged exposure to N2O
Bone marrow suppression and neurologic deficiencies
Halothane can cause ____? (rare) and risks
“Halothane Hepatitis” - rare
Higher risk:
1. middle age obese women
2. familial predisposition to halothane toxicity
3. personal history of toxicity
Isoflurane can ____ _____ arteries
- *dilate coronary** arteries
- not as strongly at NTG or adenosine
- this dilation could divert blood away from fixed stenotic lesions
Desflurane’s ___ solubility causes…
Low solubility causes rapid induction and emergence
(blood solubility)
Rapid increase in Desflurane can cause…?
tachycardia
HTN
catecholamine release
*esp in those with cardiac disease
Sevoflurane is a good agent for ___ and _____
why?
pediatrics and adults
because its nonpungent and allows for rapid alveolar concentration causing rapid and smooth inhalation
What is Fi and factors that affect Fi
Fi = inspiratory concentration
- fresh gas flow rate
- volume of breathing system
- absorption by the machine/circuit
How does Fi oppose FA?
increased Fi decreases uptake, and at 100% inspired concentration, uptake would not oppose ventilation
What is FA?
Alveolar concentration
What is equilibrium?
pressure exerted by gas is equal on both sides of the membrane
NO NET MOVEMENT
not an = amount of molecules
What is the goal of induction?
reach equilibrium between inspired and alveolar concentration of anesthetics
Fi / FA = 1
What does ventilation do for the Fi/FA?
ventilation delivers anesthetics to the lungs and increases FA
Uptake - what does it do for FA/Fi?
Increases FA to match Fi towards = 1
Define uptake and its formula
Uptake = anesthetic in the blood passes through the lung which OPPOSES ventilation by taking the anesthetic away from the lungs
Uptake = solubility x (Pi-PA)/PB x Cardiac Output
3 factors that affect uptake
- solubility
- cardiac output
- difference in partial pressure
Solubility - what is it? how does it affect induction?
- Solubility = blood-gas coefficient, differs one gas from another
- lower solubility = faster onset and emergence
- higher = slower
Solubility temp effects
hypothermia = higher solubility, slower onset
Solubilities of top 4 gasses in order least to greatest
- Desflurane 0.42
- N2O 0.47
- Sevoflurane 0.6
- Isoflurane 1.4
cardiac output effects on induction
Increased CO decreases induction d/t increase in uptake of drug away from lungs
(better perfusion)
What is the difference in anesthetic partial pressure ?
difference between alveolar gas in the lungs and venous blood
order of gas flow
vaporizer → inflow → circuit → alveoli → blood → brain → other tissues
determinants of anesthetic transfer from blood to tissues
- tissue solubility of the agents
- tissue blood flow
- difference in partial pressure b/w arterial blood and tissue
metabolism of agents
metabolism increases uptake
- halothane = significantly metabolized
- sevoflurane = slightly
- desflurane and isoflurane negligible
how does increasing ventilation affect FA/Fi?
increase in ventilation will raise FA/Fi
*especially with soluble agents
how does suppressing ventilation affect FA/Fi?
anything that depresses ventilation (opiates) decreases the rate in rise of FA which is a negative feedback loop
How can Fi be controlled?
- high flow rates (time constant)
- high % concentration
what is minute ventilation and alveolar dead space?
Ve = 6L
Anatomic dead space = 2cc/Kg + circuit dead space
What is FRC and normal value?
Functional residual capacity VA = 4L
Why do different agents produce difference FA/Fi?
different solubilities
What is the time constant?
Capacity / flow rate (L/Lpm)
Time it takes to change a gas over
Time constant %s
1, 2, and 3
½
1 TC = 63%
2 TC = 86%
3 TC = 98%
½ = 50% = 0.7 TC
during short cases, where does the anesthetic go?
only to the vessel rich group due to the long time constant and low flow rate
what is included in the vessel rich group?
% body fat and CO?
brain, heart, liver, kidney, endocrine
10% body fat, 75% CO
first to reach steady state d/t small volume limit and solubility
second highest perfused group
muscle group = muscle and skin
slower uptake yet greater volume capacity which sustains uptake for hours
third vessel group?
fat group, which is fairly similar to muscle group
lowest perfusion group?
bones, ligaments, hair, cartilage
insignificant uptake
Factors that affect the rate of rise in humans: (4)
- Pungency: with inhalation agents
- Overpressure: ventilation with high concentration
- Potency: how many molecules needed for sleep
- Mechanical factors: small airway closures
Oxygenation & Apnea
Oxygenation will continue as long as there is a partial pressure gradient
Fat/Blood partition coefficients - list in order from least to greatest
- Nitrous oxide 2.3 - least fat soluble
- Desflurane 27
- Isoflurane 45
- Enflurane 46
- Sevoflurane 48
- Halothane 67 or 80? – most fat soluble
What is the Meyer-Overton concept?
vapor potency is directly related to fat solubility
why is it harder to wake obsese patients? N2O and sevoflurane equilibrium time?
- fat is a resovoir
- N2O equilibrium ½ time is 70-80min
- Sevoflurane equilibrium is over 30 hours
Alveolar Gas Equation
PAO2 = (760 – 47) x FiO2 – (PaCO2 / 0.8)
What is the concentration effect?
- The higher the inspired concentration the less FA is affected by uptake
- Even if the solubility is the same, the more concentrated the gas, the less uptake
- FA is reached faster
What is the second gas effect?
- 1st gas with high concentration, 2nd gasses uptake is quicker
- Typical with N2O
V/Q mismatch and its affect on anesthetics?
- V/Q mismatching increases the alveolar-arterial difference
- Mismatch redirects bloodflow – raises pressure in front of restriction, lowers pressure beyond the restriction, and reduced flow through the restriction
- AKA increases alveolar partial pressure and decrease in arterial partial pressure
- More with soluble agents
Maintenance of anesthetics (flow rate)
- Flow rates reduced to maintain steady state of anesthetic level and provide constant replenishing to the alveolar concentration
- Higher solubility, higher uptake for long time
What is emergence? Factors?
- recovery = lowering concentration in brain tissue
- cannot overpressure to offset effects of solubility, less adjustable factors, timing is key
- more rapid recovery with insoluble agents
- the longer the anesthesia, the longer the washout, and the more deposits in reservoirs with long time constraints
4 methods of elimination of anesthetics?
- biotransformation
- transcutaneous loss
- exhalation
- ALVEOLUS * most important
Why does diffusion hypoxia occur?
- Large amounts of N2O is released into the alveoli during the first 5-7min of recovery which dilutes the oxygen → hypoxia
- N2O is 32-34x more soluble than nitrogen or oxygen even though it is insoluble
How to prevent diffusion hypoxia?
- If pt is on N2O preoxygenate with 100% supplemental oxygen for 10 min to prevent the hypoxia
- Works better with shorter cases. The longer the case, the longer the fat continues to take up anesthetics
What is MAC?
MAC = minimal alveolar concentration to prevent response with 50% of people
What is MAC awake?
1/3 MAC = open eyes, safe airway
Lower in elderly and with narcotics, temp changes, acute alcohol use, hypotension, pregnancy, hyponatremia, hypercalcemia.
Decreases 6% per decade
When do you get no recall?
0.33 MAC
pt may still move, but there is hypnosis
What patient population would have faster emergence?
Pediatrics
List factors that effect the rate of recovery from inhalation anesthetics
CO, PP gradient, solubility of coefficient
When are tissue solubility coefficients important?
?
Why do the time constants for the VRG, MG, and FG differ?
?
What are atoms? ions? charge?
Atoms = what compose molecules
Ions = charged atoms
Charge = bonds between atoms
Chemical structure of anesthetics?
Ethers
R-O-R’
Density? How temp affects it?
Mass / Volume
IN-Direct relationship b/w temp and density
temp increase → material expands and density decreases
Temp decrease → density increases
2 properties of flow?
Laminar and Turbulent
Laminar R^4 and Turbulent = Reynolds #
Volume
Size of container
gas fills to fit containers shape
Pressure
Force / Unit Area
molecules hitting the membrane
faster movement, higher pressure
4 methods of heat transfer?
Conduction – cold from the table
Radiation – common in the OR
Convection
Evaporation – pt breathes out
What is Vapor?
Vapor = gas that exists as a liquid at room temp and 1 ATM = 760mmHg = 101kPa
Vapor Pressure?
- saturated vapor pressure = pressure gas exerts in ATM above liquid phase
- Dependent on temp and agent… temp increase, VP increases
- maximum pressure exerted by a gas that can also exist as a liquid under standard conditions
- max PP of vapor at a specific temp
What is Latent Heat of Vaporization?
Latent Heat of Vaporization = calories req to change 1mL of gas and break the bonds holding it together
Boyles Law
Temperature Constant
Pressure and Volume INVERSE relationship
V1/V2 = P2/ P1
Charles Law
Pressure is constant
Temp and Volume have direct relationship
Gay-Lussacs Law
Volume is constant
Temp and Pressure direct relationship
Avogadros #
- 6.02 x 10^23
- PV = nRT
- GMW ideal gas occupies 22.4L witih avo # of molecules
Dalton’s Law
- Daltons Law; any gas that exerts a partial pressure in a mix of gasses is proportional to the % concentration of the gas
- 1% of sevoflurane = 7.6 mmHg of 760mmHg
- At sea level, air is 21% O2, PP of O2 in air = 0.21 x 760mmHg = 159mmHg
Henry’s Law
- Henrys Law; constant temp, the amount of a given gas that dissolves in volume is directly proportional to the partial pressure of gas in equilibrium with that liquid
- As temperature drops gasses are more soluble, longer to reach equilibrium
