Let's take a look at the inhaled anesthetics.
We'll start off with a question.
Which of the following is true regarding anesthetic gases?
A, the more soluble the anesthetic gas,
the faster the induction rate.
B, the slower the ventilation, the faster the rise
in alveolar and blood partial pressure of the anesthetic.
C, the partition coefficient is directly proportional
to induction rate.
D, partial pressures are directly proportional
to inspired partial pressure.
And E, at high pulmonary blood flow,
the gas pressure rises quickly.
I know what you're thinking.
You're very intimidate by this question.
Let's go through the science behind each of these
statements to come up with an answer.
Let's start off with something called
the partition coefficient.
In some place, this is also called the Ostwald Coefficient.
This coefficient is the ratio of the concentration in
blood to the concentration in gas. The more soluble the
anesthetic is in blood, the more it binds to protein in blood,
and the higher the blood gas partition coefficient.
Now, blood gas partition coefficient is inversely related
to induction rate. What does that mean?
Let's take a look at the body. So, we have the airway
where the agent has a concentration of 100 %.
Let me go into the alveoli. And the concentration
might be a little bit less, let's say 99 %.
Now, moving the gas from the alveoli into the blood
requires crossing a barrier.
The partition coefficient of that barrier
allows us to determine or
calculate what the blood concentration will be.
And from the blood to the brain, is another barrier.
Now, the way that I describe partition coefficient
to students is I liken it to a wall,
the higher the partition coefficient, the higher the wall is
that you have to cross over in order to get to the other side.
So, in this particular agent, we have a partition
coefficient of 0.47, and a solubility of 1.5.
This is a low partition coefficient and low solubility.
This is an easy way to get drug into the brain.
Let's take a look at solubility. Now, when I say that there's
low solubility, that means that the blood fills up very quickly.
So, the potential space is small.
Let's compare that to a drug that has high solubility.
Now, here's an example of a partition coefficient that's
actually higher at 2.3, but the solubility is very high.
So, you start off with the concentration in the airway
at 100 %, and then it goes into the alveoli at 50 %.
It does go into the blood fairly easy, that's not
much of a problem, but the problem here is,
is that there's a very high solubility,
so the blood fills up very slowly with this agent.
And so therefore, the amount that goes to the brain
and actually causes anesthesia is quite low.
What is the effect of ventilation rate on induction?
Well, take a look at these two drugs.
There's nitrous oxide and there's halothane.
And I've got 2 different ventilation rates. One is ventilated
at 2 l/min, and the other one is ventilated at 8 l/min.
And you can see that the faster the ventilation, the faster
the induction, and the higher the drug levels are.
And so, you can see in the top with the two red lines, the
person who is ventilated at 8 l/min achieves a high drug
level very quickly. And it doesn't matter what the gas is.
In every gas, the faster the ventilation rate,
the faster the induction.
What about blood flow? If you have blood flow of 3 l/min,
the level of anesthesia is achieved very quickly.
If you have blood flow that is much faster, it's achieved
slower. That doesn't make sense, right?
But think of it this way. Think of blood carrying the drug
away as oppose to to the organ, and then it starts to make sense.
So, when you have a high blood flow, it takes a long time
to fill up the blood because you're trying to fill up
a river that is filling up quickly. It's like pouring dye
in the river in Chicago on St. Patty's Day.
If that river is flowing quickly,
we need a lot of dye to colour the river green.
If that river is flowing slowly, we don't need as much dye.
That's the concept behind cardiac output and anesthesia.