Basic-Volatile Anesthetics Flashcards
Which of the following factors will lead to the inspired concentration (Fi) of volatile anesthetics to more closely approximate the concentration of volatile anesthetic at the fresh gas outlet (at the anesthesia machine):
What is this question asking?
What does determine that difference?
This question is asking what accounts for the concentration difference between the gas that is leaving the anesthesia machine and that which has reached the lungs
There are three major things: gas flow rate, breathing circuit (circle system) volume, and breathing circuit absorption. A simplified way to think of it is this: imagine that the circle system is a sink for volatile anesthetic, like a sponge. The more sponge you have, the more anesthetic that will be absorbed before you reach the patient, therefore decreasing the size of the circle system (smaller/ shorter tubing) will lead to less absorption.
Now think of increasing the fresh gas flow rate (answer A). When increasing the flow, the sponge absorbs a lower proportion when flows are high than if the flows are low (because the sponge can absorb only so much). Finally, decreasing the absorbency of the circuit (sponge) will lead to less of difference between fresh gas outlet and actual Fi (at the lungs). Increasing the cardiac output (answer C) does not affect this, but does decrease the rise of anesthetic concentration in the alveoli, see below. Increasing tidal volume also has no effect on changing the difference between the two concentrations (answer D).
Is there a difference between anesthetic uptake and speed of induction?
Yes.
What is anesthetic uptake?
Anesthetic uptake describes the process of the volatile gas reaching the alveolus and being taken up by the (mixed) venous blood entering the pulmonary circulation. From there, the anesthetic agent redistributes throughout the body.
What two factors determine uptake of anesthetic gas?
What is blood gas coefficient?
The avidity to which the anesthetic is taken up from the alveolus to the venous blood depends on two major factors: First, the solubility of the agent. The more soluble the agent is in blood, the more anesthetic that will be taken from the alveoli
The partition coefficient describes this in terms of how much agent will be in the blood versus the alveolus (blood / gas partition coefficient). The higher the coefficient, the higher the concentration in the blood as compared to the alveolus (it’s a simple ratio)
Then-difference in partial pressures between the alveolus and the venous blood. The lower the partial pressure of the agent in the blood and the higher the partial pressure in the alveolus results in the greatest pressure difference between the alveoli and pulmonary venous blood (answers C & D). Recall that gas moves from high to low pressure and that as the pressure increases (without changing resistance), so will flow (that is flow of
the anesthetic from alveoli to blood).
So, higher pressure in alveoli and lower pressure in blood-so you want that.
Pulmonary shunt =
Right to left
What constitutes induction?
Induction occurs when the alveoli concentration reaches a certain level (lets say 1 MAC). The speed in which that occurs is described by the alveolar concentration (FA) to inhalation concentration (FI) plot over time. In other words, the faster FA reaches FI, the faster the induction (read that again).
What is the exception to speed of induction being faster with decreased cardiac output?
Shunt.
Why does increased ventilation speed up induction?
Increasing ventilation means that the agent in the alveoli that was taken up by the blood is replaced quicker. This speeds FA (right!). Since FI (the inhalation concentration doesn’t change) the FA/ FI ratio increases, meaning that induction is sped up
Does Hgb have any effect on induction speed?
Very little
KIM that with shunt,
The decreased concentration in mixed venous blood means that there will be a greater concentration difference from alveoli to venous blood, and therefore more anesthetic will be pulled from the alveoli into the blood BUT less blood is being exposed to the lungs with a net effect of decreased uptake. Therefore, altogether FA continues to approximate FI, yet the arterial partial pressure of the volatile agent is decreased.
Are soluble agents more or less affected by shunts than insoluble agents?
But the real reason is essentially this: the shunt fraction blood will increase its concentration of volatile anesthetic over time and as this proportion of blood increases its concentration with continued induction closer to that of the post-capilary blood, the dilutional effects will be less. Soluble agents will saturate this portion of unventilated blood quicker than insoluble agents.
Do right to left shunts slow induction? Why is there less uptake in a left to right shunt?
yea, as the anesthetic gas leaving in the post arterial blood of the lungs is diluted by the unventilated blood from the right heart.
Do left to right shunts slow induction?
NO
