Inhaled anesthetics 1: pharmacokinetics 3 Flashcards
Describe the ways in which inhaled anesthetics are eliminated from the body.
elimination from the alveoli (primary mechanism)
hepatic biotransformation (secondary mechanism)
percutaneous loss (minimal and not clinically significant)
Describe how much of sevoflurane is metabolized by the liver.
2% metabolized
Describe how much of isoflurane is metabolized by the liver.
0.2% metabolized
Describe how much desflurane is metabolized by the liver.
0.02%
Describe how much nitrous oxide is metabolized by the liver
0.004% metabolized
Sevoflurane metabolism produces
free fluoride ions (theoretical risk of high output kidney failure)
Desflurane and isoflurane produce
trifluoroacetic acid (small risk of immune-mediated hepatic dysfunction)
Sevoflurane generates ______ when exposed to soda lime
sevoflurane
Desflurane & isoflurane produce ______ when exposed to desiccated soda lime
carbon monoxide
The halogenated anesthetics undergo metabolism by
the P450 system
Up to _____ of halothane undergoes hepatic biotransformation
20%
What is the primary mechanism for immune-mediated hepatic dysfunction (halothane hepatitis)
high trifluoroacetic acid (an important metabolic byproduct of halothane metabolism)
What is the recommendation by the FDA to prevent compound A buildup inside the breathing circuit?
a minimum FGF of 1 L/min for up to 2 MAC-hrs and 2L/min after 2 MAC hrs
FGF rates <1 L/min are not recommended at any time
Which P450 enzyme is chiefly responsible for halogenated anesthetic metabolism in the liver?
CYP2E1
Which concept BEST explains why the rate of rise of FA/FI is faster for nitrous oxide than desflurane?
a. concentrating effect
b. augmented inflow effect
c. second gas effect
d. ventilation effect
a. concentrating effect
The following alter the speed of anesthetic induction and emergence:
concentration effect
ventilation effect
The ______ says that the higher the concentration of inhalation anesthetic delivered to the alveolus (Fa), the faster its onset of action.
concentration effect (also called overpressuring)
What are the two components to the concentration effect?
- concentrating effect
- augmented gas inflow effect
The _______ describes how changes in alveolar ventilation can affect the rate of rise of Fa/Fi
ventilation effect
The greater the alveolar ventilation, the _________ of Fa/Fi
the greater the rate of rise
The concentration effect only applies to
induction
When the patient is breathing room air, _____ is the primary gas in the alveolus
nitrogen
Nitrous oxide is _____ more soluble in the blood than nitrogen
34 x
When nitrous oxide is introduced into the lung, the volume of nitrous oxide going from the alveolus to the pulmonary blood is _______ than the amount of nitrogen moving in the opposite direction. This causes the alveolus to______, and the ________ in alveolar volume causes a relative increase in Fa.
higher; shrink; reduction
The concentrating effect temporarily
reduces alveolar volume
On the subsequent breath, the concentrating effect causes an ______ inflow of tracheal gas containing the anesthetic agent to replace the lost alveolar volume. This _______ alveolar ventilation & augments Fa.
increased; increases
Assuming that the FRC remains constant, which concept explains a temporary increase in alveolar oxygen concentration when using nitrous oxide for an inhalation induction?
A. Concentration effect
B. Diffusion hypoxia
C. Ventilation effect
D. Second gas effect
D. Second gas effect
Explain the meaning of the second gas effect.
Administering one gas during induction (nitrous oxide) hastens the onset of a second gas (halogenated agent)
What is diffusion hypoxia?
Movement of nitrous oxide from the tissue back into the alveoli during emergence. This dilutes alveolar oxygen and carbon dioxide which can lead to transient hypoxemia and hypocarbia
Newer evidence suggests using this during emergence to reduce absorption atelectasis and improve post-operative gas exchange
Lower FiO2
How does the second gas effect work?
When nitrous oxide and the second gas are introduced into the alveolus, the rapid uptake of N2O causes the alveolus to shrink temporarily.
The reduction in alveolar volume and augmented tracheal inflow causes a relative increase in concentration of the second gas.
The partial pressure of alveolar oxygen also increases when the alveolus shrinks transiently.
The end result is that the alveolar concentration of the other gases is higher than if they were administered alone.
The second gas effect produces a more meaningful benefit with agents of
Higher blood:gas solubility
ISO> sevo> des
What is the best way to mitigate diffusion hypoxia after nitrous oxide is discontinued?
Increase the FiO2 for 3-5 minutes after you d/c N2O. It doesn’t have to be 100% though
A patient has a right to left intracardiac shunt. The rate of rise of FA/FI of which drug will be affected MOST?
Desflurane
Agents with low solubility (desflurane) are affected the most
How does a right to left shunt affect the onset of volatile agents?
Slower induction with a volatile agent (less soluble agents are affected to a greater extent)
How does a right to left shunt affect the onset of intravenous anesthetics?
Faster induction with an IV agent
How does a left to right shunt affect the onset of volatile agents?
No meaningful impact on induction with a volatile agent
How does a left to right shunt impact the onset of intravenous anesthetics?
Slower induction with an IV agent
