Welcome to pharmacology by Lecturio.
Today's lecture is about elimination kinetics,
how we get rid of drugs that are already in our body.
My name is Dr. P.J. Shukle, I will take you through
this journey, and hopefully we can learn something together.
Let's talk about kinetics and the different type of
elimination kinetics that occur.
Zero order kinetics is a constant rate of elimination.
You can see that the concentration is very consistently
going down over time. It is irrespective of the
concentration, so the slope is not changing
at all on this graph. First order kinetics is a
changing rate of elimination.
So, you can see that as we get lower and lower in terms
of our concentration, you're getting rid
of less and less of the drug. It is dependent upon
concentration, and there's always a proportional decrease.
Now, we have mixed kinetics in the real world,
so you can see that there's actually
two different slopes to this particular graph. So the
first part of this slope is the initial distribution phase.
That represents drug moving from the blood into the tissues.
This is very rapid and it tends to be what we called
zero order kinetics. The second portion of this curve
is the elimination phase representing
the removal of the drug. This tends to be a bit slower,
and once again, for this particular drug,
it's zero order kinetics. We can have multiple mixtures
depending on the individual drugs.
The half life is a very important concept that we want
to understand when we are looking at drugs.
It is the time whether it's hours, minutes or a day,
at which the mass or total amount of unchanged drug
becomes half of the original concentration.
So, this for example, is an example of a patient
who has a half life of about 2 hours.
Now, let's take a look at this with a question.
Take a look at this graph,
there are three drugs listed here.
Let's start with the dark green.
Question number 1, drug number 1 has what type of kinetics?
Is that a zero order kinetics?
Or is that a first order kinetic kind of elimination?
That's right. It's a zero order kinetics. And that's
because it's a constant rate of elimination over time.
What about drug 2 which is the light green drug?
That's right. First order kinetics.
And similarly, now that you've got it,
drug 3 has first order kinetics as well.
So let's take a look at our answers.
Drug number 2 has first order kinetics because the
elimination is changing over time, the slope is not constant.
But take a look at the concentrations.
At 1 o'clock, the concentration was at 8.
At 3 o'clock, the concentration was at 4.
And at 5 p.m., the concentration was 2.
So every 2 hours, the concentration is halved.
That means that the half life of drug number 2 is 2 hours.
Remember that most drugs are going to follow first order
kinetics, some drugs will follow zero order kinetics.
Let's look at drug 3. So drug 3 also has first order
kinetics. The concentration at 1 o'clock
was 12, at 3 o'clock was 3, and at 5 o'clock
was one half of 1.5, which is 0.0625.
Every 2 hours, the concentration was quartered.
So therefore, the half life is 1 hour cause at
1 hour, the concentration is halved.