Uptake and Distribution- Pharmacokinetics of Volatile Agents Flashcards
process of getting the anesthetic agent from the anesthetic machine to the patient’s CNS
- vaporizer
- circuit
- alveoli
- blood/arterial
- brain (CNS)
the series of partial pressure gradients to move agent through barrier to the CNS
PA—–Pa—–Pbr
partial pressure between alveolar and arterial equilibrate
quickly
arterial partial pressure equilibrate __ with the brain
rapidly
alveolar pressure don’t produce __ directly
unconsciousness
we use alveolar concentration as
a stand-in for CNS/brain concentration
fresh gas flow is determined by
the vaporizer and flowmeter settings
Fi (inspired gas concentration) is determined by
FGF rate, breathing-circuit volume, and circuit absorption
FA (alveolar gas concentration) is determined by
uptake, ventilation, and the concentration/second gas effect- concentrating agent and augmented inflow effect
Fa (arterial gas concentration) is affected by
ventilation/perfusion mismatching
a high inspired or inhaled partial pressure (PI) initially offsets
the impact of uptake and speeds induction (rise in PA and thus Pbr)
concentration effect
the higher the PI, the more rapidly the PA approaches the PI
second-gas effect
high volume uptake of one gas to accelerate the rate of increase of the PA of a concurrently administered “companion gas”
as equilibrium is achieved and uptake is slowed, the __ must be reduced to maintain a constant __
PI
Pbr
greater alveolar ventilation promotes
delivery of anesthetic agent to offset uptake
___ induction with greater alveolar ventilation
more rapid
__ induction with decreased alveolar ventilation
slower
the effect of increasing ventilation will be most obvious in raising the FA/FI for
more soluble anesthetics, as the are more subject to uptake
anesthetic agents impact their own uptake due to the
dose-dependent depressant effects on alveolar ventilation
if controlled ventilation is used there is potential for
overdose
time constant is
the time required for flow through a container to equal the volume of the container
time constant =
capacity (L) / flow (L/min)
time constant is the amount of time in minutes required for
a 63% turnover of gas within a container
application of time constant is in
the rate of alveolar rise in anesthetic concentration
time constant multiple 1
63%
time constant multiple 2
86%
time constant multiple 3
95%
time constant multiple 4
98%
time constant multiple 5
99.5%
vessel rich group receives __ of CO
75%
lean muscle receives __ of CO
19%
fat receives __ of CO
6%
vessel poor group receives __ of CO
0%
__ takes away from what is going to the alveoli
the volume of gas in the circuit
___ negates what the machine takes away from the alveoli
higher fresh gas flows (5-10 L/min)
__ in the rubber/plastic components of breathing system slows the rise of the PA initially
solubility of an agent
circle system allows
rebreathing of previously exhaled gas
exhaled gas contains
lower concentration of agent during the initial uptake phase because some is taken out by the body
the agent concentration coming in the fresh gas from the machine is
diluted down by mixing with the exhaled gas
in non-rebreathing circuits, exhaled gas is
no rebreathed and high flow are required
in non-rebreathing circuits, the concentration of agent in the circuit is
very close to the concentration set on the vaporizer dial
in non-rebreathing circuits, is the gas is soluble in the material of the circuit or system, some agent will
be lost to these materials
functional residual capacity is composed of
the residual volume and the expiratory reserve volume
the larger the FRC, the __ the induction of the anesthetic agent
slower
__ have smaller FRC than adults as a percent of TLV, more volume changed with
neonates
each respiration and faster induction with anesthetic agent than adults
solubility is
blood:gas partition coefficient
solubility definition
a distribution ratio describing how the anesthetic distributes itself between two phases at equilibrium (steady state)
the lower the solubility, the __ the rate of rise of the PA toward the PI
faster
partition coefficients are
temperatrue dependent
decreased temperature causes
increased solubility
increased temperature causes
decreased solubility
the initial steep rise of FA/FI is due to
unopposed filling of the alveoli by ventilation
CO (pulmonary blood flow) affects uptake by
carrying away agent from the alveoli and preventing the rise in PA
increased CO leads to
greater uptake/removal of agent from the lungs and slowed induction
the rate of increase of __ is affected more by CO
more soluble agents
the increase in CO will __ induction and the rise in PA
slow
during uptake, __ will remove the anesthetic agent that is delivered
the vessel rich group
the mixed venous blood returning to the lungs has
a lower partial pressure of agent
the gradient between the alveolar partial pressure and the venous blood is __ encouraging __
great
diffusion out of the alveoli
as saturation of the VRG increase, ___ becomes smaller and the __ rises
gradient
PA
arterial partial pressure is __ then ETgas
less
___ enhances the difference in Pa and ETgas
right to left shunt
lower Pa with
right to left shunt, especially with less soluble agents
__ poorly estimates __, with right to left shunt
PA
Pa
V/Q mismatch is seen with
bronchial intubation or right to left intracardiac shunt
factors determining transfer of agent from arterial blood to brain
- brain:blood partition coefficient
- cerebral blood flow
- arterial to venous partial pressure difference
vessel rich group includes
- brain
- heart
- kidney
- splanchnic
- liver
- endocrine
muscle group contains
muscle and skin
vessel poor group contains
- bone
- ligament
- cartilage
rapid increase in FA/FI thats
faster induction
to have faster accumulation/onset/wash-in you need
- higher FGF
- higher alveolar ventilation
- decrease FRC
- smaller time constant
- smaller amount of anatomic dead space
to increase FA/FI decrease amount of uptake
- gas with lower solubility
- low CO
- difference between arterial pressure and venous pressure smaller
to decrease rate of FA/FI meaning slower onset/build up
- decrease FGF
- decrease AV
- higher FRC
- higher time constant
- higher amount of anatomic dead space
decrease FA/FI meaning slower induction with increased uptake
- higher solubility
- higher CO
- higher difference in Pa/Pv
elimination generally =
reverse of uptake
slower awakening with
higher soluble, longer duration of exposure, higher concentration
elimination via
exhalation predominantly so increase alveolar ventilation
factors associated with increased rate of recovery
- elimination of rebreathing
- high FGF
- low anesthesia-circuit volume
- low absorption by anesthesia circuit
- decreased solubility of agent
- high cerebral blood flow
- increased alveolar ventilation
implication for rapid recovery
- airway protection/oxygenation
- rapid movement through the 0.1 MAC concentration that causes enhanced perception of pain
- return to normal CV function
- turnover more rapid
- quicker discharge from PACU
- quicker return to normal activities
changes in compartments in elderly
decreases in lean body mass and increases in body fat
volume of distribution in elderly
of central compartment is smaller (plasma volume)
for volatiles is larger (especially more lipid soluble agents)
elderly clearance
decreased due to impaired pulmonary gas exchange
elderly CO
reduced
reduced CO in elderly leads to
reduced tissue perfusion, increased time constants, altered regional distribution of anesthetics
does switching to a less soluble agent at the end of a case help?
no, it is additive or even synergistic effect regarding return to mental function
