Inhaled Anesthetics - Part 1 Flashcards
(40 cards)
Pharmacokinetics is:
A_______
D_______
M_______
E_______
What the body does to the drug.
Absorption, Distribution, Metabolism, Elimination
Pharmacokinetics of inhalational anesthetic agents.
The depth of general anesthesia depends on the __________ exerted by the inhalational agent in the patient’s _______.
Partial pressure (or gas fraction)
Brain
Pharmacokinetics of inhalational anesthetic agents.
The brain’s partial pressure depends on ________ blood partial pressure which depends on ________ blood partial pressure which depends on partial pressure of ________ in the inspired gas.
arterial (a); alveolar (A); agent
Capacity/volume of circuit
6 L
What is solubility?
Relative affinity of an anesthetic for two phases and therefore the partitioning of that anesthetic between the two phases at equilibrium.
More soluble the gas is → slower induction/slower emergence
(opposite of IV drug (propofol) which is highly soluble means fast induction)
What is equilibrium for inhaled anesthetics?
No difference in partial pressure exists.
(vs. concentration for IV drugs)
In a mixture of gases, each gas has a _____ ______ which is the pressure which the gas would have if it alone occupied the volume.
partial pressure
P = PN2 + PH2 + PNH3
How can we affect PI (Partial Pressure of inspired agent)
- ↑ concentration (2% - 8%)
- ↑ fresh gas flow (FGF), which ↓ time constant
- ↓ volume of the circuit
- ↓ absorption by the machine (plastic, rubber bag absorb some gas and we cannot affect it)
- Wait it out
What is PA/PI?
The ratio of alveolar pressure relative to inspired pressure. (aka in some texts, FA/FI)
Try to get PA as close to PI as fast as possible, esp during inhalational induction
What is PA and how can we ↑ it?
PA = input into the alveoli - uptake into blood
- ↑ ventilation
- ↑ concentration
How do we measure the PBrain?
Look at endtidal concentration of gas being expired.
Effect of ventialtion on anesthetic uptake. If you change the minute ventilation, what happens to the rate of rise in the alveoli?
↑ minute ventilatoin, ↑ rate of rise of alveolus inspired

Two ways to increase ititial concentration and uptake?
Concentration effect
2nd gas effect
What is the impact of the inspired partial pressure of the agent increases the rate of rise of the partial pressure of the alveolus?
Concentration effect
The more we put in, the more that will be there
Concentration effect
Start with 20ml gas A in total of 100ml (20%)
50% of gas A is taken up by pulmonary circulation
This leaves 10mL of gasA in total of 90ml
What is the alveolar concentration?
Now what if the concentration is 80% and 50% of the anesthetic is taken up by pulmonary circulation?
11%
concentration of gas is larger than original
67% (40ml of gas in 60ml total)
Therefore, a higher inspired concentration results in a disproportionaltely higher alveolar concentration (increasing inspired concentratin 4-fold = 6-fold increase in alveolar concentration)
The concentration effect is more significant with ______ gas than with volatile anestetics because it can be used in much higher concentrations.
Nitrous oxide
2nd gas effect
High concentration of _______ will augment the uptake of a concurrently administerd volatile agent.
Nitrous oxide
High volume uptake of one gas accelerates the rate of increase of the PA of the companion gas.
Concentration effect vs. 2nd gas effect figure
Administration of 65% N2O produces a more rapid rise in the alveolar concentration of anesthetic/concentration of inspired anesthetic (Fa/Fi ratio) for nitrous oxide than administration of 5% (i.e., concentration effect, two solid lines).
The Fa/Fi ratio for 4% desflurane rises more rapidly when given with 65% nitrous oxide than when given with 5% (i.e., second gas effect, two dotted lines)

Gas is continually taken up from the lungs for the bloodstream
PA = PI - uptake (from pulmonary blood flow)
Anesthetic uptake is affected by:
S________
C____O____
A_____V_____ Difference
Solubility
Cardiac Output
Alevolar - Venous Difference (higher # in beginning than 12 hours later)
Anesthetic uptake equation
Uptake = Solubility (x) CO (x) (PA - PV)
What is defined as relative affinity of inhaled anesthtic for two phases at equilibrium?
Solubility
Ex: cup of water + sand vs. cup of water + sugar
The cup of water and sugar wants to dissolve, what sinks to the botom of the cup is what the brain sees.
T/F
↑ solubility of an agent ↓ FA/FI, so induction is slower
True
↑solubility = ↑induction time and ↑ emergence
(brain sees less of what settles at bottom of cup because the sugar dissolves)
Halothane is best example
T/F
↓ Solubility of agent, ↑ FA/FI therefore induction is quicker
True
N2O and Desflurane best examples
↓solubility = ↓induction time & ↓ time to emergence



