Inhaled Anesthetics Part 1 (Exam III) Flashcards
What do the pharmacokinetics of inhaled anesthetics describe? (4)
- Uptake from alveoli into pulmonary capillary blood
- Distribution
- Metabolism
- Elimination via lungs
What are the influences of old age on inhaled anesthetics? (5)
- ↓ lean body mass
- ↑ fat
- ↑ Vd for drugs (especially for more fat soluble)
- ↓ clearance if pulmonary exchange is impaired
- ↑ time constraints due to lower cardiac output
What is Boyle’s Law?
What application of this was mentioned in class?
- Pressure and Volume of gas are inversely proportional
- Bellows contract thus increasing circuit pressure → gasses flow from high pressure (circuit) to low pressure (lungs).
- P 1 V 1 =P 2 V 2
What is Fick’s Diffusion Law (as is pertinent to inhaled anesthetics)?
Once air molecules enter alveoli, they move around randomly and begin to diffuse into the pulmonary capillaries.
- The rate at which a gas moves across a membrane is directly proportional to the concentration gradient (difference in partial pressures) across that membrane.
What factors is diffusion dependent on?
- Partial pressure gradient of the gas
- Solubility of the gas
- Thickness of the membrane
What is Graham’s Law of Effusion?
Process by which molecules diffuse through pores and channels without colliding.
- The rate at which a gas escapes from a container (effusion) is inversely proportional to the square root of its molecular weight.
- This means a lighter gas will effuse faster than a heavier gas at the same temperature
Smaller molecules effuse(escape a container i.e. lungs) faster dependent on ________.
solubility
Which diffuses faster CO₂ or O₂ ? Why?
Which would you expect to diffuse faster?
CO₂ is 20x more diffusible due to solubility despite O₂ weighing less.
PA is equal to __ when the inhaled gas concentration equals the exhaled gas concentration and the patient is unconscious.
PBrain
If PBrain is greater than PA, what would we expect to be occurring? Why?
The patient should be waking up. This means the exhaled gas is greater than the inhaled gas and the concentration gradient is moving towards the alveoli away from the brain.
What does the following equation mean?
PA ⇌ Pa ⇌ PBrain
This is comparing the partial pressure of volatile gas in the alveoli to the arterial blood to the brain.
Input
What input factors affect the diffusion of volatile gas from the anesthetic machine to the alveoli? (4)
- Inspired partial pressure
- Alveolar ventilation (More ventilation=More drug)
- Anesthetic system re-breathing
- FRC
Uptake
Which factors affect the uptake of anesthetic gas from the alveoli to the blood?
- Blood:gas partition coefficient (does your gas favor the blood or does it like to be free?)
- Cardiac output (Speed of bloodflow determines amount of gas exchange)
- A-V pressure difference
How would a low cardiac output affect the diffusion of anesthetic gas from the alveoli to the pulmonary capillary blood?
↓CO = more time to diffuse across the alveolus=higher concentrations more quickly
What factors affect the uptake of anesthetic gas from the arterial blood to the brain?
- Blood:Brain partition coefficient
- Cerebral blood flow
- A-V partial pressure difference.
Gas goes from a ____ gradient to a ____ gradient in order to reach a steady state.
high; low
What does PI mean?
Partial pressure of inspired volatile gas.
How can gas be “forced” to the brain quicker?
By increasing PI. This creates a higher gradient for the gas to flow from PA → Pa → PBrain
What does FE/FI mean?
FE/FI is the ratio of expired gas to inspired gas.
What concept is this chart conveying?
Concentration Effect: essentially, ↑concentration inspired gas = ↑PA = increased rate of diffusion
- The higher the concentration of gas (in this case Des) the closer we get to reaching an FE/FI of 1.
What is over-pressurization?
- A large increase in PI so as to force gas from PA → Pa → PBrain much faster.
- Example given in class was 1 vital capacity breath of 7% sevo(high-concentration) may be enough for your patient to lose their eyelash reflex.
This is a very temporary “momentatry” dose. Prolonged over-pressurization leads to quicker/worse S/E and potential overdose.
What would sustained delivery of over-pressurization result in?
Overdose
What gas does the second gas effect always apply to?
N₂O (nitrous oxide)
What is the second gas effect as it relates to anesthesia?
- Uptake of N₂O accelerates a concurrently administered volatile gas.
N₂O gets absorbed very quickly. When left with remaining gas, its % is drastically increased, allowing for more uptake across the alveoli.
How does N₂O create the second gas effect?
N₂O hyper-concentrates volatiles to create a high concentration gradient after it diffuses across the alveoli.
Describe what is being depicted on the graph below.
- This is the concentrating effect of N₂O on halothane.
Notice that the % of administered halpothane does not change but its concentration increases as the N₂O amount increases.
What cases would nitrous oxide not be utilized in?
Why?
- Cases with an air-filled cavity
- N₂O will diffuse into the cavity and fill it. (extent of damage dependent on the compliance of the cavity).
What specific cases are bad for the usage of N₂O?
- Ear (Pain)
- Eye (Retinal artery ischemia)
- Open belly (Can’t put guts back in)
- Lung
What factors affect the degree of pressure N₂O would exert on a cavity that it filled?
- Partial pressure of N₂O
- Blood flow to the cavity
- Duration of N₂O administration
What would nitrous inhalation in a patient with pneumothorax do?
Expand the pneumothorax
Nitrous loves to infiltrate existing air-filled cavities. If the cavity is compliant (lungs) it will increase in volume, thus expanding the existing pneumo.
What could N₂O on an intraocular case do?
- Massively increase retinal artery pressure and cause permanent vision loss.
Decreased PaCO₂ from hyperventilation will decrease cerebral blood flow. This will limit __.
induction speed
If CBF decreases, there is less gas getting to the brain.
Referencing spontaneous vs. mechanical ventilation, how do volatile gases affect the body?
- Volatile gases provide dose dependent depressant effects of alveolar ventilation. In spontaneous ventilation, this creates a negative feedback loop to maintain PA ⇌ Pa ⇌ PBrain
- Mech. Ventilation removes the above response because the anesthesia provider controls all the variables. This puts the patient at increased risk of overdose.
Quoting the Great Corndog, “you gotta be mindful of that.”
What is the definition of solubility for anesthetic gasses?
Ratio of how inhaled gas distribution between two compartments at equilibrium (when partial pressures are equal).
- Solubility is the relative capacity of each compartment to hold a volatile
“Not a volume, not a weight, not a percent…a ratio.” -Dr. K
This ratio is in parts compared to 1
If the temperature of blood increases then solubility ______.
decreases
- This means more gas will leave blood and affect brain
Don’t forget that an increase in temperature also usually means increased metabolism which inevitably means that you must give increased MAC to maintain adequate anestesia (usually)
What does a low blood solubility mean for induction?
Less gas has to be dissolved. This means PA → Pa is rapid = rapid induction.
What does a high blood solubility mean for induction?
More gas has to be dissolved so PA → Pa is slow = slow induction.
What is being described in the graph below?
How quickly the inspired concentration of a gas equals the alveolar concentration of said gas.
What volatile gasses are intermediately soluble?
- Halothane
- Enflurane
- Isoflurane
What is the blood:gas partition coefficient of halothane?
Halothane = 2.54
What is the blood:gas partition coefficient of enflurane?
Enflurane = 1.90
What is the blood:gas partition coefficient of Isoflurane?
Isoflurane = 1.46
What volatile gasses are poorly soluble?
- N₂O
- Desflurane
- Sevoflurane
What is the blood:gas partition coefficient of N₂O?
Nitrous = 0.46
What is the blood:gas partition coefficient of Desflurane?
Desflurane = 0.42
What is the blood:gas partition coefficient of Sevoflurane?
Sevoflurane = 0.69
What are the blood:gas solubilities of all the gasses we have to know for anesthesia pharm? (Thankfully there are only 6)
What occurs (regarding our partial pressure gradients) during emergence from anesthesia?
Concentration gradient reverses.
PA ← Pa ← PBrain
- This washout typically occurs rapidly and depends on the length and type of volatile administration.
- Don’t need to know numbers, but have to understand that higher Fat:Blood coef. results in longer emergence time (especially if cases is longer)
Regarding volatiles, what does it mean when PI is 0?
It means the inhaled agent is turned off silly.
What color coding does isoflurane have?
Purple
What color coding does sevoflurane have?
Yellow
What color coding does desflurane have?
Blue
Why would you want to use a volatile anesthetic with a slower emergence time?
You might have to take your patient to an ICU across the hospital and don’t want them to wake up in transit.
- Be nice to the ICU nurse and give them a sleepy/comfortable patient so they can catch up on their charting. Who knows, maybe they are studing for CRNA school!
Which anesthetic would you anticipate as having the quickest recovery?
Slowest?
Why?
Fastest recovery = desflurane
Slowest recovery = halothane
Who knows? Maybe because Desflurane has the lowest Fat:Blood Coef and Halothane has the highest. This isn’t the end all be all reason though. For example, iso is slower than sevo with a lower Fat:Blood coef but iso does have a much higher blood:gas coef.
What helps decrease concentration of volatile anesthetic in PA and PBrain on emergence?
Continued uptake by Muscle/Fat if not already at equilibrium.
What would ED50 be equivalent to in regards to MAC?
1 MAC
What is 1 MAC?
The concentration at 1 atm that prevents skeletal muscle movement in response to supramaximal, painful stimulation in 50% of patients
At 1 MAC, if the surgeon cuts you with their samurai sword (scalpel), there is a 50/50 chance you wouldn’t move.
What would ED98 be equivalent to in regards to MAC?
ED98 ≈ 1.3 MAC
What is 1.3 MAC?
Concentration at 1atm that prevents skeletal muscle movement in response to surgical stimulation in 98% of patients.
At 1.3 MAC, if the surgeon cuts you with their steak knife (scalpel), there is a 98% chance you wouldn’t move.
What is MACawake?
0.3 - 0.5 MAC: partial awakeness and responsiveness.
What is MACBAR?
Is this MAC safe?
1.7 - 2.0 MAC: Blunts autonomic responses. No SNS response at all, essentially an overdose.
No, an overdose isn’t ever really “safe”
What patient are standardized MAC values based on?
30 - 55 y/o at 37°C at 1atm (760mmHg/torr)
What is the MAC of N₂O?
What does this mean?
N₂O MAC = 104%. Can’t be used as sole anesthetic agent (can’t go above 100%).
What is the MAC of Halothane?
0.75%
What is the MAC of Enflurane?
1.63%
What is the MAC of Isoflurane?
1.17%
What is the MAC of Desflurane?
6.6
What is the MAC of Sevoflurane?
1.8%
What are the two biggest factors that affect MAC?
- Body temperature
- Age
At what age does MAC need peak?
1 y/o
How much does MAC need decrease as one gets older?
6% per decade after 55yr
What factors will increase MAC requirement? (4)
- Hyperthermia
- Excess Pheomelanin (redheads)
- Drug-induced ↑ catecholamines (pheochromocytoma)
- Hypernatremia (More Na+=More membrane depolarization)
What factors will decrease MAC requirement?
Extensive list
Essentially anything that slows metabolism
- Hypothermia
- Pre-op meds
- Intra-op opioids
- α-2 agonists (Dex, clonidine)
- Acute EtOH
- Pregnancy
- Early post-partum
- Lidocaine
- PaO₂ < 38 mmHg
- Mean BP < 40mmHg
- Cardiac Bypass
- Hyponatremia
- ↑ Age
- Renal Failure
- Hypovolemia
- Poor Nutrition
- History of Stroke
What factors have no effect on MAC?
- Chronic alcohol abuse (MEOS Pathway)
- Gender
- Duration of anesthesia
- PaCO2 15-95 mm Hg
- PaO2 > 38 mm Hg
- Blood pressure > 40 mm Hg
- Hyper/hypokalemia (Interesting)
- Thyroid gland dysfunction (Average, not thyroid storm. Memaw is probably taking her synthroid)
How do volatiles cause immobility (Spinal)? (3)
- Depress excitatory AMPA and NMDA (glutamate receptors)
- Enhance inhibitory glycine receptors (Strychnine, glycine antagonist)
- Act on sodium channels (Blocks presynaptic release of glutamate)
How does loss of consciousness occur with the use of volatile anesthetics?
- Potentiation of GABAA transmission in the brain and especially in the reticular activating system (RAS)
- Potentiation of glycine in the brainstem.
Which of these two liquids in enclosed containers has the higher vapor pressure?
Liquid B: more evaporative.
Vapor pressure is the pressure at which vapor and liquid are at equilibirum.
What is Dalton’s law?
- The sum of all partial pressures will equal the total pressure.
- Ptotal = Pgas1 + Pgas2…
Heat will _____ vapor pressure.
increase
Cold temperatures will _____ vapor pressure.
decrease.
A lower vapor pressure gas is inherently more volatile. T/F ?
False. ↑vapor pressure = ↑volatility
- ↑vapor pressure=↑chance of evaporation when exposed to atmosphere
What is the vapor pressure of halothane?
243 torr (mmHg)
What is the vapor pressure of Enflurane?
175 torr (mmHg)
What is the vapor pressure of Isoflurane?
238 torr (mmHg)
What is the vapor pressure of Desflurane?
669 torr (mmHg)
What is the vapor pressure of Sevoflurane?
157 torr (mmHg)
Partial Pressure/Total Pressure=__
Volumes%
- ET and Ins must be equal to have equilibrium of inhaled gas
What is the variable bypass on the anesthetic machine?
A way to dilute/concentrate the amount of anesthetic gas reaching the patient.
What is the splitting ratio?
How much gas is being sent into the vaporizer
What is the purpose of the wicks found in the vaporizing chamber below?
The wicks increase gas-liquid interface (↑surface area) and improve vaporization.
