Inhaled anesthetics 1: pharmacokinetics 2 Flashcards
_______ is the tendency of a solute to dissolve in a solvent
Solubility
In the case of inhalation anesthetics, solubility is the ability of the
anesthetic agent to dissolve into the blood and tissues
___________ describes the relative solubility of an inhalation anesthetic in the blood vs. in the alveolar gas when the partial pressures between the two compartments are equal.
The blood:gas partition coefficient
Blood: gas coefficient for sevoflurane.
0.65
Blood: gas coefficient for isoflurane.
1.46
Blood gas coefficient for desflurane.
0.42
Blood gas coefficient for nitrous oxide is
0.46
How easily a solute will dissolve into a solvent depends on the
physiochemical characteristics of both compounds- “like dissolves like”
A polar solute will be more soluble in a
hydrophilic solvent
A nonpolar solute will be more soluble in a
liphophilic solvent
An anesthetic gas with low blood:gas solubility is
less likely to be taken up into the blood
as a result, more of the agent is available to exert a partial pressure in the alveoli and brain
An anesthetic gas with high blood: gas solubility is
more likely to be taken up into the blood;
as a result, less of the agent is available to exert a partial pressure in the alveoli and brain
While administering a volatile anesthetic, we produce a state of anesthesia by
building up a partial pressure of anesthetic agent inside the patient’s brain and spinal cord
These three factors determine anesthetic uptake into the blood:
agent solubility
partial pressure difference between the alveoli and the blood
cardiac output
The amount of anesthetic agent inside the alveoli is a balance between
input (the setting on the vaporizer) and uptake into the blood
The Fa/Fi curve helps us appreciate the speed at which
FA equilibrates with Fi (speed of induction)
The concentration of an agent inside the alveoli is proportional to its
concentration inside the brain
for this reason, we use alveolar partial pressure (FA) as a surrogate for the brain partial pressure
Describe how low solubility affects Fa/Fi.
Faster equilibration of Fa/Fi–> faster onset
Describe how high solubility affects Fa/Fi.
slower equilibration of Fa/Fi–> slower onset
Determinants of alveolar delivery include
setting on the vaporizer
time constant of the delivery system
anatomic dead space
alveolar ventilation
volume of the FRC
The Fa/Fi curve approximates the
speed at which Fa will approximate Fi
Describe which inhalational anesthetic has the highest Fa/Fi in order
N20> des>sevo>iso
Which group is the first group to become saturated with anesthetic?
vessel rich group
Fa is the
partial pressure of the anesthetic inside the alveoli
Fi is the
concentration of the anesthetic exiting the vaporizer
Which factor reduces the rate of rise of Fa/Fi? (select 3)
a. increased cardiac output
b. decreased time constant
c. increased FRC
d. decreased anatomic dead space
e. decreased fresh gas flow
f. increased alveolar ventilation
a. increased cardiac output
c. increased FRC
e. decreased fresh gas flow
Factors that influence the rate of anesthetic delivery to the alveoli:
setting on the vaporizer
time constant of the delivery system
anatomic dead space
alveolar ventilation
functional residual capacity
Factors that influence the removal of anesthetic from the alveoli (uptake):
agent solubility
partial pressure difference between the alveoli and the blood
cardiac output
For Fa/Fi to increase, there must be
greater wash in or reduced uptake
Increased Fa/Fi means that there is
a faster onset or the curve is pushed up
Factors that lead to an increased wash in include
high fresh gas flow
high alveolar ventilation
low FRC
low time constant
low anatomic dead space
Factors that lead to reduced uptake include
low solubility
low cardiac output
low Pa-Pv difference
Decreased Fa/Fi means that
there is slower onset; curve is pushed down
Factors that lead to decreased wash in include
low fresh gas flow
low alveolar ventilation
high FRC
high time constant
high anatomic dead space
Factors that lead to increased uptake include
high solubility
high cardiac output
high Pa-Pv difference
What percentage of body weight is accounted for by the vessel rich group?
a. 20%
b. 50%
c. 10%
d. 75%
C. 10%- the vessel rich group accounts for only 10% of the body weight, yet it receives 75% of the cardiac output
Tissue uptake is dependent on
tissue blood flow
solubility of the anesthetic in the tissue
arterial blood: tissue partial pressure gradient
The vessel rich group consists of the
heart, brain, kidneys, liver and endocrine glands
The muscle/skin group is ____ of the body mass but receives _____ of the cardiac output.
50%; 20%
The fat group is ____ of the body mass but receives ____ of the cardiac output.
20%; 5%
The vessel poor group is_____ of the body mass but receives ____ of the cardiac ouptut.
20%; <1%
The vessel-poor group includes
bone, tendon & cartilage
The uptake of nitrous oxide by any of these groups is
minimal; it partitions nearly the same into all of the compartments
