Isoproterenol is an important topic. It is a three out of five, that means that you should
probably know a little bit about it for your tests, your boards, your exams, things like
that. It's brand or trade name is Isuprel. It is a synthetic non-selective beta-adrenergic
agonist. Now, synthetic means that it is not produced naturally in the body. We do produce
this medicine in a lab through a reaction that is similar to the way that we produce
Epinephrine. You can see that the structures are similar, and lastly, this is a beta-adrenergic
agonist that works on both the Beta 1 and the Beta 2 receptors. Next up is the mechanism
of action, or the effects of Isoproterenol, physiologically speaking. Again this is a
Beta 1 and Beta 2 stimulator, so remember back to what Beta 1 does. Beta 1, I want you
to think about heart effects. That means, Beta 1 is going to cause an increase the heart
rate or the chromotropy and in the force of contraction, which is called inotropy, and
the speed of conduction through the heart, which is called dromotropy. That's going to
lead to an increase in the cardiac output and the systolic blood pressure. Next is the
Beta 2 stimulation. Isoproterenol used to be used as a bronchodilator in cases of asthma.
Now it's not used as much anymore since we have Albuterol. But, the effects would have
been the same. So, why do you think we stopped using Isoproterenol as the go-to drug for
asthma? It's because Albuterol is selective for the Beta 2 receptors, that means that
Albuterol has less side effects. You can imagine that if you gave someone Isoproterenol to
help their asthma, they would have, in addition to bronchodilation, the cardiac effects - which
is really undesireable. What else does the Beta 2 stimulation do? Well, Beta 2 receptors,
when stimulated through the body, cause a dilation in vasculature throughout the body,
such as in skeletal muscle, things that need more blood when you are in the middle of a
fight or flight response. This vasodilation is actually going to cause a decrease in the
peripheral vascular resistance and thus, there's going to be a little bit of a decrease in
the diastolic blood pressure. So, let's go over to our chart and work through this. First
of all, try not to cheat by looking over at the bullet points, Heart Rate: Heart rate
is definitely going to go up, we talked about this already, Beta 1 stimulation from Isoproterenol
is going to cause the heart to beat faster. Blood pressure: What is the systolic blood
pressure going to do again? It's going to go up, again, this is a Beta 1 effect, the
heart is pumping harder and faster. What is the diastolic pressure going to do? That's
actually going to go down, and it's going to go down because of the Beta 2 effects that
are causing the peripheral vasculature resistance to decrease. I hope that makes sense, remember,
if you have questions, you can just let us know and we will explain it better. Lastly,
let's go over the uses and the side effects of Isoproterenol. So, it is used to help mainly
problems with the heart, remember how we talked about how we don't really use it for asthma
anymore, and why? It's because of the heart side effects, remember? There is still some
use for it for things like a complete heart block or bradycardia, which is when the heart
is beating very slowly. And that's because it has very powerful Beta 1 effects: It causes
the heart to beat harder and faster. What about the side effects? The side effects are
actually related to that. The side effects are similar to the effects of Epinephrine
or other very strong stimulants. Bascially, if someone has a heart that is prone to arrhythmias,
maybe an elderly person, Isoproterenol could cause the heart to beat too fast or go into
a disrhythmia, and that would obviously be, in addition, a risk for a possible heart attack.
So let's keep these in mind. Dobutamine is another selective Beta agonist. However, unlike
isoproterenol, which we've just been talking about, which equally stimulates the Beta receptors,
Dobutamine favors Beta 1 over Beta 2. So, what would be the difference in the physiologic
effect between these two drugs? Dobutamine would increase the mean arterial pressure
more or Isoproterenol would increase the mean arterial pressure more. The correct answer
is A. Dobutamine would increase the mean arterial pressure more, and that's because it would
stimulate the Beta 2 receptor less, and remember, Beta 2 is dilating things peripherally, dilating
blood vessels in the skeletal muscles, for example, and also in the heart. So, you could
argue that maybe Isoproterenol would be a little bit safer to give someone who has stenotic
vessels in the heart, just because it would help dilate those while the heart is being
asked to perform more work because of the Beta 1 stimulation. Isoproterenol probably
keeps the MAP about the same, if not bringing it down a little bit - remember, when we talked:
diastolic goes down because peripheral vascular resistance goes down which counterbalances
the increase in systolic blood pressure. Dobutamine is going to increase the pressures across
the board.